Kelsey D. Wilson, MD

Tick-borne diseases are increasing in prevalence, likely due to climate change in combination with human movement into tick habitats.^1-3 The Ixodes genus of hard ticks is a common vector for the transmission of pathogenic viruses, bacteria, parasites, and toxins. Among these, Lyme disease, which is caused by Borrelia burgdorferi, is the most prevalent, followed by babesiosis and human granulocytic anaplasmosis (HGA), respectively.⁴ In Europe, tick-borne encephalitis is commonly encountered. More recently identified diseases transmitted by Ixodes ticks include Powassan virus and Borrelia miyamotoi infection; however, these diseases are less frequently encountered than other tick-borne diseases.^5,6

As tick-borne diseases become more prevalent, the likelihood of coinfection with more than one Ixodes-transmitted pathogen is increasing.⁷ Therefore, it is important for physicians who practice in endemic areas to be aware of the possibility of coinfection, which can alter clinical presentation, disease severity, and treatment response in tick-borne diseases. Additionally, public education on tick-bite prevention and prompt tick removal is necessary to combat the rising prevalence of these diseases.

Coinfection

Risk of coinfection with more than one tick-borne disease is contingent on the geographic distribution of the tick species as well as the particular pathogen’s prevalence within reservoir hosts in a given area (Figure). Most coinfections occur with B. burgdorferi and an additional pathogen, usually Anaplasma phagocytophilum (which causes human granulocytic anaplasmosis [HGA]) or Babesia microti (which causes babesiosis). In Europe, coinfection with tick-borne encephalitis virus may occur. There is limited evidence of human coinfection with B miyamotoi or Powassan virus, as isolated infection with either of these pathogens is rare.

In patients with Lyme disease, as many as 35% may have concurrent babesiosis, and as many as 12% may have concurrent HGA in endemic areas (eg, northeast and northern central United States).^7-9 Concurrent HGA and babesiosis in the absence of Lyme disease also has been documented.^7-9 Coinfection generally increases the diversity of presenting symptoms, often obscuring the primary diagnosis. In addition, these patients may have more severe and prolonged illness.^8,10,11

In endemic areas, coinfection with B burgdorferi and an additional pathogen should be suspected if a patient presents with typical symptoms of early Lyme disease, especially erythema migrans, along with (1) combination of fever, chills, and headache; (2) prolonged viral-like illness, particularly 48 hours after appropriate antibiotic treatment; and (3) unexplained blood dyscrasia.^7,11,12 When a patient presents with erythema migrans, it is unnecessary to test for HGA, as treatment of Lyme disease with doxycycline also is adequate for treating HGA; however, if systemic symptoms persist despite treatment, testing for babesiosis and other tick-borne illnesses should be considered, as babesiosis requires treatment with atovaquone plus azithromycin or clindamycin plus quinine.¹³

A complete blood count and peripheral blood smear can aid in the diagnosis of coinfection. The complete blood count may reveal leukopenia, anemia, or thrombocytopenia associated with HGA or babesiosis. The peripheral blood smear can reveal inclusions of intra-erythrocytic ring forms and tetrads (the “Maltese cross” appearance) in babesiosis and intragranulocytic morulae in HGA.¹² The most sensitive diagnostic tests for tick-borne diseases are organism-specific IgM and IgG serology for Lyme disease, babesiosis, and HGA and polymerase chain reaction for babesiosis and HGA.⁷

Prevention Strategies

The most effective means of controlling tick-borne disease is avoiding tick bites altogether. One method is to avoid spending time in high-risk areas that may be infested with ticks, particularly low-lying brush, where ticks are likely to hide.¹⁴ For individuals traveling in environments with a high risk of tick exposure, behavioral methods of avoidance are indicated, including wearing long pants and a shirt with long sleeves, tucking the shirt into the pants, and wearing closed-toe shoes. Wearing light-colored clothing may aid in tick identification and prompt removal prior to attachment. Permethrin-impregnated clothing has been proven to decrease the likelihood of tick bites in adults working outdoors.^15-17

Topical repellents also play a role in the prevention of tick-borne diseases. The most effective and safe synthetic repellents are N,N-diethyl-meta-toluamide (DEET); picaridin; p-menthane-3,8-diol; and insect repellent 3535 (IR3535)(ethyl butylacetylaminopropionate).^16-19 Plant-based repellents also are available, but their efficacy is strongly influenced by the surrounding environment (eg, temperature, humidity, organic matter).^20-22 Individuals also may be exposed to ticks following contact with domesticated animals and pets.^23,24 Tick prevention in pets with the use of ectoparasiticides should be directed by a qualified veterinarian.²⁵

Tick Removal

Following a bite, the tick should be removed promptly to avoid transmission of pathogens. Numerous commercial and in-home methods of tick removal are available, but not all are equally effective. Detachment techniques include removal with a card or commercially available radiofrequency device, lassoing, or freezing.^26,27 However, the most effective method is simple removal with tweezers. The tick should be grasped close to the skin surface and pulled upward with an even pressure. Commercially available tick-removal devices have not been shown to produce better outcomes than removal of the tick with tweezers.²⁸

Conclusion

When patients do not respond to therapy for presumed tick-borne infection, the diagnosis should be reconsidered. One important consideration is coinfection with a second organism. Prompt identification and removal of ticks can prevent disease transmission.

Tick-borne diseases are increasing in prevalence, likely due to climate change in combination with human movement into tick habitats.^1-3 The Ixodes genus of hard ticks is a common vector for the transmission of pathogenic viruses, bacteria, parasites, and toxins. Among these, Lyme disease, which is caused by Borrelia burgdorferi, is the most prevalent, followed by babesiosis and human granulocytic anaplasmosis (HGA), respectively.⁴ In Europe, tick-borne encephalitis is commonly encountered. More recently identified diseases transmitted by Ixodes ticks include Powassan virus and Borrelia miyamotoi infection; however, these diseases are less frequently encountered than other tick-borne diseases.^5,6

As tick-borne diseases become more prevalent, the likelihood of coinfection with more than one Ixodes-transmitted pathogen is increasing.⁷ Therefore, it is important for physicians who practice in endemic areas to be aware of the possibility of coinfection, which can alter clinical presentation, disease severity, and treatment response in tick-borne diseases. Additionally, public education on tick-bite prevention and prompt tick removal is necessary to combat the rising prevalence of these diseases.

Coinfection

Risk of coinfection with more than one tick-borne disease is contingent on the geographic distribution of the tick species as well as the particular pathogen’s prevalence within reservoir hosts in a given area (Figure). Most coinfections occur with B. burgdorferi and an additional pathogen, usually Anaplasma phagocytophilum (which causes human granulocytic anaplasmosis [HGA]) or Babesia microti (which causes babesiosis). In Europe, coinfection with tick-borne encephalitis virus may occur. There is limited evidence of human coinfection with B miyamotoi or Powassan virus, as isolated infection with either of these pathogens is rare.

In patients with Lyme disease, as many as 35% may have concurrent babesiosis, and as many as 12% may have concurrent HGA in endemic areas (eg, northeast and northern central United States).^7-9 Concurrent HGA and babesiosis in the absence of Lyme disease also has been documented.^7-9 Coinfection generally increases the diversity of presenting symptoms, often obscuring the primary diagnosis. In addition, these patients may have more severe and prolonged illness.^8,10,11

In endemic areas, coinfection with B burgdorferi and an additional pathogen should be suspected if a patient presents with typical symptoms of early Lyme disease, especially erythema migrans, along with (1) combination of fever, chills, and headache; (2) prolonged viral-like illness, particularly 48 hours after appropriate antibiotic treatment; and (3) unexplained blood dyscrasia.^7,11,12 When a patient presents with erythema migrans, it is unnecessary to test for HGA, as treatment of Lyme disease with doxycycline also is adequate for treating HGA; however, if systemic symptoms persist despite treatment, testing for babesiosis and other tick-borne illnesses should be considered, as babesiosis requires treatment with atovaquone plus azithromycin or clindamycin plus quinine.¹³

A complete blood count and peripheral blood smear can aid in the diagnosis of coinfection. The complete blood count may reveal leukopenia, anemia, or thrombocytopenia associated with HGA or babesiosis. The peripheral blood smear can reveal inclusions of intra-erythrocytic ring forms and tetrads (the “Maltese cross” appearance) in babesiosis and intragranulocytic morulae in HGA.¹² The most sensitive diagnostic tests for tick-borne diseases are organism-specific IgM and IgG serology for Lyme disease, babesiosis, and HGA and polymerase chain reaction for babesiosis and HGA.⁷

Prevention Strategies

The most effective means of controlling tick-borne disease is avoiding tick bites altogether. One method is to avoid spending time in high-risk areas that may be infested with ticks, particularly low-lying brush, where ticks are likely to hide.¹⁴ For individuals traveling in environments with a high risk of tick exposure, behavioral methods of avoidance are indicated, including wearing long pants and a shirt with long sleeves, tucking the shirt into the pants, and wearing closed-toe shoes. Wearing light-colored clothing may aid in tick identification and prompt removal prior to attachment. Permethrin-impregnated clothing has been proven to decrease the likelihood of tick bites in adults working outdoors.^15-17

Topical repellents also play a role in the prevention of tick-borne diseases. The most effective and safe synthetic repellents are N,N-diethyl-meta-toluamide (DEET); picaridin; p-menthane-3,8-diol; and insect repellent 3535 (IR3535)(ethyl butylacetylaminopropionate).^16-19 Plant-based repellents also are available, but their efficacy is strongly influenced by the surrounding environment (eg, temperature, humidity, organic matter).^20-22 Individuals also may be exposed to ticks following contact with domesticated animals and pets.^23,24 Tick prevention in pets with the use of ectoparasiticides should be directed by a qualified veterinarian.²⁵

Tick Removal

Following a bite, the tick should be removed promptly to avoid transmission of pathogens. Numerous commercial and in-home methods of tick removal are available, but not all are equally effective. Detachment techniques include removal with a card or commercially available radiofrequency device, lassoing, or freezing.^26,27 However, the most effective method is simple removal with tweezers. The tick should be grasped close to the skin surface and pulled upward with an even pressure. Commercially available tick-removal devices have not been shown to produce better outcomes than removal of the tick with tweezers.²⁸

Conclusion

When patients do not respond to therapy for presumed tick-borne infection, the diagnosis should be reconsidered. One important consideration is coinfection with a second organism. Prompt identification and removal of ticks can prevent disease transmission.

Tick-borne diseases are increasing in prevalence, likely due to climate change in combination with human movement into tick habitats.^1-3 The Ixodes genus of hard ticks is a common vector for the transmission of pathogenic viruses, bacteria, parasites, and toxins. Among these, Lyme disease, which is caused by Borrelia burgdorferi, is the most prevalent, followed by babesiosis and human granulocytic anaplasmosis (HGA), respectively.⁴ In Europe, tick-borne encephalitis is commonly encountered. More recently identified diseases transmitted by Ixodes ticks include Powassan virus and Borrelia miyamotoi infection; however, these diseases are less frequently encountered than other tick-borne diseases.^5,6

As tick-borne diseases become more prevalent, the likelihood of coinfection with more than one Ixodes-transmitted pathogen is increasing.⁷ Therefore, it is important for physicians who practice in endemic areas to be aware of the possibility of coinfection, which can alter clinical presentation, disease severity, and treatment response in tick-borne diseases. Additionally, public education on tick-bite prevention and prompt tick removal is necessary to combat the rising prevalence of these diseases.

Coinfection

Risk of coinfection with more than one tick-borne disease is contingent on the geographic distribution of the tick species as well as the particular pathogen’s prevalence within reservoir hosts in a given area (Figure). Most coinfections occur with B. burgdorferi and an additional pathogen, usually Anaplasma phagocytophilum (which causes human granulocytic anaplasmosis [HGA]) or Babesia microti (which causes babesiosis). In Europe, coinfection with tick-borne encephalitis virus may occur. There is limited evidence of human coinfection with B miyamotoi or Powassan virus, as isolated infection with either of these pathogens is rare.

In patients with Lyme disease, as many as 35% may have concurrent babesiosis, and as many as 12% may have concurrent HGA in endemic areas (eg, northeast and northern central United States).^7-9 Concurrent HGA and babesiosis in the absence of Lyme disease also has been documented.^7-9 Coinfection generally increases the diversity of presenting symptoms, often obscuring the primary diagnosis. In addition, these patients may have more severe and prolonged illness.^8,10,11

In endemic areas, coinfection with B burgdorferi and an additional pathogen should be suspected if a patient presents with typical symptoms of early Lyme disease, especially erythema migrans, along with (1) combination of fever, chills, and headache; (2) prolonged viral-like illness, particularly 48 hours after appropriate antibiotic treatment; and (3) unexplained blood dyscrasia.^7,11,12 When a patient presents with erythema migrans, it is unnecessary to test for HGA, as treatment of Lyme disease with doxycycline also is adequate for treating HGA; however, if systemic symptoms persist despite treatment, testing for babesiosis and other tick-borne illnesses should be considered, as babesiosis requires treatment with atovaquone plus azithromycin or clindamycin plus quinine.¹³

A complete blood count and peripheral blood smear can aid in the diagnosis of coinfection. The complete blood count may reveal leukopenia, anemia, or thrombocytopenia associated with HGA or babesiosis. The peripheral blood smear can reveal inclusions of intra-erythrocytic ring forms and tetrads (the “Maltese cross” appearance) in babesiosis and intragranulocytic morulae in HGA.¹² The most sensitive diagnostic tests for tick-borne diseases are organism-specific IgM and IgG serology for Lyme disease, babesiosis, and HGA and polymerase chain reaction for babesiosis and HGA.⁷

Prevention Strategies

The most effective means of controlling tick-borne disease is avoiding tick bites altogether. One method is to avoid spending time in high-risk areas that may be infested with ticks, particularly low-lying brush, where ticks are likely to hide.¹⁴ For individuals traveling in environments with a high risk of tick exposure, behavioral methods of avoidance are indicated, including wearing long pants and a shirt with long sleeves, tucking the shirt into the pants, and wearing closed-toe shoes. Wearing light-colored clothing may aid in tick identification and prompt removal prior to attachment. Permethrin-impregnated clothing has been proven to decrease the likelihood of tick bites in adults working outdoors.^15-17

Topical repellents also play a role in the prevention of tick-borne diseases. The most effective and safe synthetic repellents are N,N-diethyl-meta-toluamide (DEET); picaridin; p-menthane-3,8-diol; and insect repellent 3535 (IR3535)(ethyl butylacetylaminopropionate).^16-19 Plant-based repellents also are available, but their efficacy is strongly influenced by the surrounding environment (eg, temperature, humidity, organic matter).^20-22 Individuals also may be exposed to ticks following contact with domesticated animals and pets.^23,24 Tick prevention in pets with the use of ectoparasiticides should be directed by a qualified veterinarian.²⁵

Tick Removal

Following a bite, the tick should be removed promptly to avoid transmission of pathogens. Numerous commercial and in-home methods of tick removal are available, but not all are equally effective. Detachment techniques include removal with a card or commercially available radiofrequency device, lassoing, or freezing.^26,27 However, the most effective method is simple removal with tweezers. The tick should be grasped close to the skin surface and pulled upward with an even pressure. Commercially available tick-removal devices have not been shown to produce better outcomes than removal of the tick with tweezers.²⁸

Conclusion

When patients do not respond to therapy for presumed tick-borne infection, the diagnosis should be reconsidered. One important consideration is coinfection with a second organism. Prompt identification and removal of ticks can prevent disease transmission.

The Ixodes tick is prevalent in temperate climates worldwide. During a blood meal, pathogens may be transmitted from the tick to its host. Borrelia burgdorferi, a spirochete responsible for Lyme disease, is the most prevalent pathogen transmitted by Ixodes ticks.¹ Borrelia mayonii recently was identified as an additional cause of Lyme disease in the United States.²

The Ixodes tick also is associated with several less common pathogens, including Babesia microti and the tick-borne encephalitis virus, which have been recognized as Ixodes-associated pathogens for many years.^3,4 Other pathogens have been identified, including Anaplasma phagocytophilum, recognized in the 1990s as the cause of human granulocytic anaplasmosis, as well as the Powassan virus and Borrelia miyamotoi.^5-7 Additionally, tick paralysis has been associated with toxins in the saliva of various species of several genera of ticks, including some Ixodes species.⁸ Due to an overlap in geographic distribution (Figure) and disease presentations (eTable), it is important that physicians be familiar with these regional pathogens transmitted by Ixodes ticks.

Human Granulocytic Anaplasmosis

Formerly known as human granulocytic ehrlichiosis, human granulocytic anaplasmosis is caused by A phagocytophilum and is transmitted by Ixodes scapularis, Ixodes pacificus, and Ixodes persulcatus. The incidence of human granulocytic anaplasmosis in the United States increased 12-fold from 2001 to 2011.⁹

Presenting symptoms generally are nonspecific, including fever, night sweats, headache, myalgias, and arthralgias, often resulting in misdiagnosis as a viral infection. Laboratory abnormalities include mild transaminitis, leukopenia, and thrombocytopenia.^9,10 Although most infections resolve spontaneously, 3% of patients develop serious complications. The mortality rate is 0.6%.¹¹

A diagnosis of human granulocytic anaplasmosis should be suspected in patients with a viral-like illness and exposure to ticks in an endemic area. The diagnosis can be confirmed by polymerase chain reaction (PCR), acute- and convalescent-phase serologic testing, or direct fluorescent antibody screening. Characteristic morulae may be present in granulocytes.¹² Treatment typically includes doxycycline, which also covers B burgdorferi coinfection. When a diagnosis of human granulocytic anaplasmosis is suspected, treatment should never be delayed to await laboratory confirmation. If no clinical improvement is seen within 48 hours, alternate diagnoses or coinfection with B microti should be considered.¹⁰

Babesiosis

The protozoan B microti causes babesiosis in the United States, with Babesia divergens being more common in Europe.¹³ Reported cases of babesiosis in New York increased as much as 20-fold from 2001 to 2008.¹⁴ Transmission primarily is from the Ixodes tick but rarely can occur from blood transfusion.¹⁵ Tick attachment for at least 36 hours is required for transmission.¹³

The clinical presentation of babesiosis ranges from asymptomatic to fatal. Symptoms generally are nonspecific, resembling a viral infection and including headache, nausea, diarrhea, arthralgia, and myalgia. Laboratory evaluation may reveal hemolytic anemia, thrombocytopenia, transaminitis, and elevated blood urea nitrogen and creatinine levels.¹⁶ Rash is not typical. Resolution of symptoms generally occurs within 2 weeks of presentation, although anemia may persist for months.¹³ Severe disease is more common among elderly and immunocompromised patients. Complications include respiratory failure, renal failure, congestive heart failure, and disseminated intravascular coagulation. The mortality rate in the United States is approximately 10%.^10,16

A diagnosis of babesiosis is made based on the presence of flulike symptoms, laboratory results, and history of recent travel to an endemic area. A thin blood smear allows identification of the organism in erythrocytes as ring forms or tetrads (a “Maltese cross” appearance).¹⁷ Polymerase chain reaction is more sensitive than a blood smear, especially in early disease.¹⁸ Indirect fluorescent antibody testing is species-specific but cannot verify active infection.¹⁰

Treatment of babesiosis is indicated for symptomatic patients with active infection. Positive serology alone is not an indication for treatment. Asymptomatic patients with positive serology should have diagnostic testing repeated in 3 months with subsequent treatment if parasitemia persists. Mild disease is treated with atovaquone plus azithromycin or clindamycin plus quinine. Severe babesiosis is treated with quinine and intravenous clindamycin and may require exchange transfusion.¹⁰ Coinfection with B burgdorferi should be considered in patients with flulike symptoms and erythema migrans or treatment failure. Coinfection is diagnosed by Lyme serology plus PCR for B microti. This is an important consideration because treatment of babesiosis does not eradicate B burgdorferi infection.¹⁹

Powassan Virus

Powassan virus is a flavivirus that causes encephalitis. It is transmitted by Ixodes cookei (Powassan virus, lineage I) in the Great Lakes region and by I scapularis (Powassan virus, lineage II, or deer tick virus) in the northeastern United States. Transmission can occur within 15 minutes of tick attachment.^6,20,21

Patients typically present with fever, headache, altered mental status, seizures, and focal neurologic deficits. Gastrointestinal symptoms and rash also have been reported.²¹ The diagnosis is made based on clinical presentation and laboratory testing with PCR or enzyme-linked immunosorbent assay (ELISA). Cross-reactivity on ELISA exists, necessitating confirmation with a neutralizing antibody or PCR. Treatment is supportive. Corticosteroids and intravenous immunoglobulin have been proposed as treatment modalities, but evidence of their efficacy is limited.²²

Tick-borne Encephalitis

Tick-borne encephalitis is caused by the flavivirus tick-borne encephalitis virus in Europe and Asia. The tick-borne encephalitis virus is transmitted by Ixodes ricinus in Europe and by Ixodes persulcatus in eastern Russia, China, and Japan. It also has been associated with consumption of unpasteurized milk.^23,24

Tick-borne encephalitis presents in a biphasic pattern. The initial viremic phase can persist for as long as 8 days with headache, nausea, myalgia, and fever. One-third of patients then enter an asymptomatic phase, followed by virus penetration into the central nervous system. The neurologic phase produces continued headache and fever with photophobia, focal neurologic deficits, seizures, respiratory depression, or coma. Neurologic sequelae persist in 10% to 20% of patients.^25,26

In the viremic stage, diagnosis is made with PCR or culture. During the latent phase or neurologic phase, serologic testing for tick-borne encephalitis virus antibodies is indicated. Neutralizing antibody evaluation may be necessary due to cross-reactivity among flaviviruses.²⁷ Treatment is supportive. An inactivated vaccine is available for high-risk populations.²⁸

Borrelia miyamotoi Disease

Borrelia miyamotoi is a symbiont of the Ixodes tick formerly believed to have no pathogenic significance; however, B miyamotoi was isolated in febrile patients in Russia in 2011⁷ and was identified as a pathogen in both North America²⁹ and Europe in 2013.³⁰ Disease presentation includes nonspecific symptoms of fever, fatigue, headache, arthralgia, myalgia, and nausea. Rash is uncommon. Laboratory abnormalities include leukopenia, thrombocytopenia, and transaminitis.^31,32 Meningoencephalitis may occur in immunocompromised patients.^29,30

The diagnosis of B miyamotoi disease is confirmed by PCR or serology. An ELISA that is positive for B burgdorferi IgM but negative with confirmatory immunoblot suggests B miyamotoi disease. Seroconversion using a glpQ protein ELISA also can be assessed.³¹ If ELISA is positive, Lyme disease can be excluded because B burgdorferi does not possess g1pQ. Treatment is with doxycycline.³²

Tick Paralysis

Tick paralysis is an intoxication with holocyclotoxin from the saliva of gravid hard ticks. In the United States, intoxication is associated with ticks of various species of Amblyomma, Dermacentor, and Ixodes in the Northwest, Southeast, and Northeast. In Australia, intoxication is associated with Ixodes.³³ Patients present with weakness and fatigue, progressing to ascending flaccid paralysis with sensory sparing. The treatment is tick removal.^8,33

Conclusion

Arthropods carry many regional pathogens. Physicians outside of those regions should seek a travel history and be alert for imported disease.

The Ixodes tick is prevalent in temperate climates worldwide. During a blood meal, pathogens may be transmitted from the tick to its host. Borrelia burgdorferi, a spirochete responsible for Lyme disease, is the most prevalent pathogen transmitted by Ixodes ticks.¹ Borrelia mayonii recently was identified as an additional cause of Lyme disease in the United States.²

The Ixodes tick also is associated with several less common pathogens, including Babesia microti and the tick-borne encephalitis virus, which have been recognized as Ixodes-associated pathogens for many years.^3,4 Other pathogens have been identified, including Anaplasma phagocytophilum, recognized in the 1990s as the cause of human granulocytic anaplasmosis, as well as the Powassan virus and Borrelia miyamotoi.^5-7 Additionally, tick paralysis has been associated with toxins in the saliva of various species of several genera of ticks, including some Ixodes species.⁸ Due to an overlap in geographic distribution (Figure) and disease presentations (eTable), it is important that physicians be familiar with these regional pathogens transmitted by Ixodes ticks.

Human Granulocytic Anaplasmosis

Formerly known as human granulocytic ehrlichiosis, human granulocytic anaplasmosis is caused by A phagocytophilum and is transmitted by Ixodes scapularis, Ixodes pacificus, and Ixodes persulcatus. The incidence of human granulocytic anaplasmosis in the United States increased 12-fold from 2001 to 2011.⁹

Presenting symptoms generally are nonspecific, including fever, night sweats, headache, myalgias, and arthralgias, often resulting in misdiagnosis as a viral infection. Laboratory abnormalities include mild transaminitis, leukopenia, and thrombocytopenia.^9,10 Although most infections resolve spontaneously, 3% of patients develop serious complications. The mortality rate is 0.6%.¹¹

A diagnosis of human granulocytic anaplasmosis should be suspected in patients with a viral-like illness and exposure to ticks in an endemic area. The diagnosis can be confirmed by polymerase chain reaction (PCR), acute- and convalescent-phase serologic testing, or direct fluorescent antibody screening. Characteristic morulae may be present in granulocytes.¹² Treatment typically includes doxycycline, which also covers B burgdorferi coinfection. When a diagnosis of human granulocytic anaplasmosis is suspected, treatment should never be delayed to await laboratory confirmation. If no clinical improvement is seen within 48 hours, alternate diagnoses or coinfection with B microti should be considered.¹⁰

Babesiosis

The protozoan B microti causes babesiosis in the United States, with Babesia divergens being more common in Europe.¹³ Reported cases of babesiosis in New York increased as much as 20-fold from 2001 to 2008.¹⁴ Transmission primarily is from the Ixodes tick but rarely can occur from blood transfusion.¹⁵ Tick attachment for at least 36 hours is required for transmission.¹³

The clinical presentation of babesiosis ranges from asymptomatic to fatal. Symptoms generally are nonspecific, resembling a viral infection and including headache, nausea, diarrhea, arthralgia, and myalgia. Laboratory evaluation may reveal hemolytic anemia, thrombocytopenia, transaminitis, and elevated blood urea nitrogen and creatinine levels.¹⁶ Rash is not typical. Resolution of symptoms generally occurs within 2 weeks of presentation, although anemia may persist for months.¹³ Severe disease is more common among elderly and immunocompromised patients. Complications include respiratory failure, renal failure, congestive heart failure, and disseminated intravascular coagulation. The mortality rate in the United States is approximately 10%.^10,16

A diagnosis of babesiosis is made based on the presence of flulike symptoms, laboratory results, and history of recent travel to an endemic area. A thin blood smear allows identification of the organism in erythrocytes as ring forms or tetrads (a “Maltese cross” appearance).¹⁷ Polymerase chain reaction is more sensitive than a blood smear, especially in early disease.¹⁸ Indirect fluorescent antibody testing is species-specific but cannot verify active infection.¹⁰

Treatment of babesiosis is indicated for symptomatic patients with active infection. Positive serology alone is not an indication for treatment. Asymptomatic patients with positive serology should have diagnostic testing repeated in 3 months with subsequent treatment if parasitemia persists. Mild disease is treated with atovaquone plus azithromycin or clindamycin plus quinine. Severe babesiosis is treated with quinine and intravenous clindamycin and may require exchange transfusion.¹⁰ Coinfection with B burgdorferi should be considered in patients with flulike symptoms and erythema migrans or treatment failure. Coinfection is diagnosed by Lyme serology plus PCR for B microti. This is an important consideration because treatment of babesiosis does not eradicate B burgdorferi infection.¹⁹

Powassan Virus

Powassan virus is a flavivirus that causes encephalitis. It is transmitted by Ixodes cookei (Powassan virus, lineage I) in the Great Lakes region and by I scapularis (Powassan virus, lineage II, or deer tick virus) in the northeastern United States. Transmission can occur within 15 minutes of tick attachment.^6,20,21

Patients typically present with fever, headache, altered mental status, seizures, and focal neurologic deficits. Gastrointestinal symptoms and rash also have been reported.²¹ The diagnosis is made based on clinical presentation and laboratory testing with PCR or enzyme-linked immunosorbent assay (ELISA). Cross-reactivity on ELISA exists, necessitating confirmation with a neutralizing antibody or PCR. Treatment is supportive. Corticosteroids and intravenous immunoglobulin have been proposed as treatment modalities, but evidence of their efficacy is limited.²²

Tick-borne Encephalitis

Tick-borne encephalitis is caused by the flavivirus tick-borne encephalitis virus in Europe and Asia. The tick-borne encephalitis virus is transmitted by Ixodes ricinus in Europe and by Ixodes persulcatus in eastern Russia, China, and Japan. It also has been associated with consumption of unpasteurized milk.^23,24

Tick-borne encephalitis presents in a biphasic pattern. The initial viremic phase can persist for as long as 8 days with headache, nausea, myalgia, and fever. One-third of patients then enter an asymptomatic phase, followed by virus penetration into the central nervous system. The neurologic phase produces continued headache and fever with photophobia, focal neurologic deficits, seizures, respiratory depression, or coma. Neurologic sequelae persist in 10% to 20% of patients.^25,26

In the viremic stage, diagnosis is made with PCR or culture. During the latent phase or neurologic phase, serologic testing for tick-borne encephalitis virus antibodies is indicated. Neutralizing antibody evaluation may be necessary due to cross-reactivity among flaviviruses.²⁷ Treatment is supportive. An inactivated vaccine is available for high-risk populations.²⁸

Borrelia miyamotoi Disease

Borrelia miyamotoi is a symbiont of the Ixodes tick formerly believed to have no pathogenic significance; however, B miyamotoi was isolated in febrile patients in Russia in 2011⁷ and was identified as a pathogen in both North America²⁹ and Europe in 2013.³⁰ Disease presentation includes nonspecific symptoms of fever, fatigue, headache, arthralgia, myalgia, and nausea. Rash is uncommon. Laboratory abnormalities include leukopenia, thrombocytopenia, and transaminitis.^31,32 Meningoencephalitis may occur in immunocompromised patients.^29,30

The diagnosis of B miyamotoi disease is confirmed by PCR or serology. An ELISA that is positive for B burgdorferi IgM but negative with confirmatory immunoblot suggests B miyamotoi disease. Seroconversion using a glpQ protein ELISA also can be assessed.³¹ If ELISA is positive, Lyme disease can be excluded because B burgdorferi does not possess g1pQ. Treatment is with doxycycline.³²

Tick Paralysis

Tick paralysis is an intoxication with holocyclotoxin from the saliva of gravid hard ticks. In the United States, intoxication is associated with ticks of various species of Amblyomma, Dermacentor, and Ixodes in the Northwest, Southeast, and Northeast. In Australia, intoxication is associated with Ixodes.³³ Patients present with weakness and fatigue, progressing to ascending flaccid paralysis with sensory sparing. The treatment is tick removal.^8,33

Conclusion

Arthropods carry many regional pathogens. Physicians outside of those regions should seek a travel history and be alert for imported disease.

The Ixodes tick is prevalent in temperate climates worldwide. During a blood meal, pathogens may be transmitted from the tick to its host. Borrelia burgdorferi, a spirochete responsible for Lyme disease, is the most prevalent pathogen transmitted by Ixodes ticks.¹ Borrelia mayonii recently was identified as an additional cause of Lyme disease in the United States.²

The Ixodes tick also is associated with several less common pathogens, including Babesia microti and the tick-borne encephalitis virus, which have been recognized as Ixodes-associated pathogens for many years.^3,4 Other pathogens have been identified, including Anaplasma phagocytophilum, recognized in the 1990s as the cause of human granulocytic anaplasmosis, as well as the Powassan virus and Borrelia miyamotoi.^5-7 Additionally, tick paralysis has been associated with toxins in the saliva of various species of several genera of ticks, including some Ixodes species.⁸ Due to an overlap in geographic distribution (Figure) and disease presentations (eTable), it is important that physicians be familiar with these regional pathogens transmitted by Ixodes ticks.

Human Granulocytic Anaplasmosis

Formerly known as human granulocytic ehrlichiosis, human granulocytic anaplasmosis is caused by A phagocytophilum and is transmitted by Ixodes scapularis, Ixodes pacificus, and Ixodes persulcatus. The incidence of human granulocytic anaplasmosis in the United States increased 12-fold from 2001 to 2011.⁹

Presenting symptoms generally are nonspecific, including fever, night sweats, headache, myalgias, and arthralgias, often resulting in misdiagnosis as a viral infection. Laboratory abnormalities include mild transaminitis, leukopenia, and thrombocytopenia.^9,10 Although most infections resolve spontaneously, 3% of patients develop serious complications. The mortality rate is 0.6%.¹¹

A diagnosis of human granulocytic anaplasmosis should be suspected in patients with a viral-like illness and exposure to ticks in an endemic area. The diagnosis can be confirmed by polymerase chain reaction (PCR), acute- and convalescent-phase serologic testing, or direct fluorescent antibody screening. Characteristic morulae may be present in granulocytes.¹² Treatment typically includes doxycycline, which also covers B burgdorferi coinfection. When a diagnosis of human granulocytic anaplasmosis is suspected, treatment should never be delayed to await laboratory confirmation. If no clinical improvement is seen within 48 hours, alternate diagnoses or coinfection with B microti should be considered.¹⁰

Babesiosis

The protozoan B microti causes babesiosis in the United States, with Babesia divergens being more common in Europe.¹³ Reported cases of babesiosis in New York increased as much as 20-fold from 2001 to 2008.¹⁴ Transmission primarily is from the Ixodes tick but rarely can occur from blood transfusion.¹⁵ Tick attachment for at least 36 hours is required for transmission.¹³

The clinical presentation of babesiosis ranges from asymptomatic to fatal. Symptoms generally are nonspecific, resembling a viral infection and including headache, nausea, diarrhea, arthralgia, and myalgia. Laboratory evaluation may reveal hemolytic anemia, thrombocytopenia, transaminitis, and elevated blood urea nitrogen and creatinine levels.¹⁶ Rash is not typical. Resolution of symptoms generally occurs within 2 weeks of presentation, although anemia may persist for months.¹³ Severe disease is more common among elderly and immunocompromised patients. Complications include respiratory failure, renal failure, congestive heart failure, and disseminated intravascular coagulation. The mortality rate in the United States is approximately 10%.^10,16

A diagnosis of babesiosis is made based on the presence of flulike symptoms, laboratory results, and history of recent travel to an endemic area. A thin blood smear allows identification of the organism in erythrocytes as ring forms or tetrads (a “Maltese cross” appearance).¹⁷ Polymerase chain reaction is more sensitive than a blood smear, especially in early disease.¹⁸ Indirect fluorescent antibody testing is species-specific but cannot verify active infection.¹⁰

Treatment of babesiosis is indicated for symptomatic patients with active infection. Positive serology alone is not an indication for treatment. Asymptomatic patients with positive serology should have diagnostic testing repeated in 3 months with subsequent treatment if parasitemia persists. Mild disease is treated with atovaquone plus azithromycin or clindamycin plus quinine. Severe babesiosis is treated with quinine and intravenous clindamycin and may require exchange transfusion.¹⁰ Coinfection with B burgdorferi should be considered in patients with flulike symptoms and erythema migrans or treatment failure. Coinfection is diagnosed by Lyme serology plus PCR for B microti. This is an important consideration because treatment of babesiosis does not eradicate B burgdorferi infection.¹⁹

Powassan Virus

Powassan virus is a flavivirus that causes encephalitis. It is transmitted by Ixodes cookei (Powassan virus, lineage I) in the Great Lakes region and by I scapularis (Powassan virus, lineage II, or deer tick virus) in the northeastern United States. Transmission can occur within 15 minutes of tick attachment.^6,20,21

Patients typically present with fever, headache, altered mental status, seizures, and focal neurologic deficits. Gastrointestinal symptoms and rash also have been reported.²¹ The diagnosis is made based on clinical presentation and laboratory testing with PCR or enzyme-linked immunosorbent assay (ELISA). Cross-reactivity on ELISA exists, necessitating confirmation with a neutralizing antibody or PCR. Treatment is supportive. Corticosteroids and intravenous immunoglobulin have been proposed as treatment modalities, but evidence of their efficacy is limited.²²

Tick-borne Encephalitis

Tick-borne encephalitis is caused by the flavivirus tick-borne encephalitis virus in Europe and Asia. The tick-borne encephalitis virus is transmitted by Ixodes ricinus in Europe and by Ixodes persulcatus in eastern Russia, China, and Japan. It also has been associated with consumption of unpasteurized milk.^23,24

Tick-borne encephalitis presents in a biphasic pattern. The initial viremic phase can persist for as long as 8 days with headache, nausea, myalgia, and fever. One-third of patients then enter an asymptomatic phase, followed by virus penetration into the central nervous system. The neurologic phase produces continued headache and fever with photophobia, focal neurologic deficits, seizures, respiratory depression, or coma. Neurologic sequelae persist in 10% to 20% of patients.^25,26

In the viremic stage, diagnosis is made with PCR or culture. During the latent phase or neurologic phase, serologic testing for tick-borne encephalitis virus antibodies is indicated. Neutralizing antibody evaluation may be necessary due to cross-reactivity among flaviviruses.²⁷ Treatment is supportive. An inactivated vaccine is available for high-risk populations.²⁸

Borrelia miyamotoi Disease

Borrelia miyamotoi is a symbiont of the Ixodes tick formerly believed to have no pathogenic significance; however, B miyamotoi was isolated in febrile patients in Russia in 2011⁷ and was identified as a pathogen in both North America²⁹ and Europe in 2013.³⁰ Disease presentation includes nonspecific symptoms of fever, fatigue, headache, arthralgia, myalgia, and nausea. Rash is uncommon. Laboratory abnormalities include leukopenia, thrombocytopenia, and transaminitis.^31,32 Meningoencephalitis may occur in immunocompromised patients.^29,30

The diagnosis of B miyamotoi disease is confirmed by PCR or serology. An ELISA that is positive for B burgdorferi IgM but negative with confirmatory immunoblot suggests B miyamotoi disease. Seroconversion using a glpQ protein ELISA also can be assessed.³¹ If ELISA is positive, Lyme disease can be excluded because B burgdorferi does not possess g1pQ. Treatment is with doxycycline.³²

Tick Paralysis

Tick paralysis is an intoxication with holocyclotoxin from the saliva of gravid hard ticks. In the United States, intoxication is associated with ticks of various species of Amblyomma, Dermacentor, and Ixodes in the Northwest, Southeast, and Northeast. In Australia, intoxication is associated with Ixodes.³³ Patients present with weakness and fatigue, progressing to ascending flaccid paralysis with sensory sparing. The treatment is tick removal.^8,33

Conclusion

Arthropods carry many regional pathogens. Physicians outside of those regions should seek a travel history and be alert for imported disease.

Ticks are ectoparasitic hemophages that feed on mammals, reptiles, and birds. The Ixodidae family comprises the hard ticks. A hard dorsal plate, scutum, and capitulum that extends outward from the body are features that distinguish the hard tick. ¹Ixodes is the largest genus of hard ticks, with more than 250 species localized in temperate climates.² It has an inornate scutum and lacks festoons (Figure 1).¹ The Ixodes ricinus species complex accounts for most species relevant to the spread of human disease (Figure 2), with Ixodes scapularis in the northeastern, north midwestern, and southern United States; Ixodes pacificus in western United States; I ricinus in Europe and North Africa; and Ixodes persulcatus in Russia and Asia. Ixodes holocyclus is endemic to Australia.^3,4

Life Cycle

Ixodes species progress through 4 life stages—egg, larvae, nymph, and adult—during their 3-host life cycle. Lifespan is 2 to 6 years, varying with environmental factors. A blood meal is required between each stage. Female ticks have a small scutum, allowing the abdomen to engorge during meals (Figure 3).

Larvae hatch in the early summer and remain dormant until the spring, emerging as a nymph. Following a blood meal, the nymph molts and reemerges as an adult in autumn. During autumn and winter, the female lays as many as 2000 eggs that emerge in early summer.⁵ Nymphs are small and easily undetected for the duration required for pathogen transmission, making nymphs the stage most likely to transmit disease.⁶

The majority of tick-borne diseases present from May to July, corresponding to nymph activity. Fewer cases present in the autumn and early spring because the adult female feeds during cooler months.⁷

Larvae have 6 legs and are about the size of a sesame seed when engorged. Nymphs are slightly larger with 8 legs. Adults are largest and have 8 legs. Following a blood meal, the tick becomes engorged, increasing in size and lightening in color (Figure 3).¹

Ticks are found in low-lying shrubs and tall grass as well as on the forest floor. They search for a host by detecting CO₂, warmth, the smell of sweat, and the color white, prompting attachment.⁸ Habitats hospitable to Ixodes have expanded in the wake of climate, environmental, and socioeconomic changes, potentially contributing to the increasing incidence and expansion of zoonoses associated with this vector.^9,10

Local Reactions

A tick bite may induce local hypersensitivity, leading to a red papule or plaque at the bite site, followed by swelling, warmth, and erythema. A cellular immune reaction induces induration and pruritus. Hard ticks are less likely than soft ticks to cause a serious local reaction.^11,12

A variety of clinical and histologic features are observed following an arthropod bite. Histologically, acute tick bites show a neutrophilic infiltrate with fibrin deposition. Chronic reactions demonstrate a wedge-shaped, mixed infiltrate with prominent endothelial swelling. Eosinophilic cellulitis, or Wells syndrome, reveals tissue eosinophilia and flame figures.¹³ Tick mouthparts may be identified in the tissue. B-cell hyperplasia is seen in Borrelia lymphocytoma and is more common in Europe, presenting as erythematous to plum–colored nodules on the ear and areola.¹⁴

Lyme Disease

Disease manifestations vary by location. Lyme disease is associated with Borrelia burgdorferi and the recently identified Borrelia mayonii in the United States¹⁵; in Europe and Asia, acrodermatitis chronica atrophicans is associated with Borrelia afzelii and neuroborreliosis, with Borrelia garinii. Lyme disease is the most common tick-borne illness in the United States.¹⁶ The I ricinus species complex is the most common vector harboring Borrelia species.¹⁷ At least 36 hours of tick adherence is required for disease transmission.¹⁸ The incubation period is 3 to 20 days (median, 12 days).¹⁹

Clinical Findings
Erythema migrans is the most characteristic sign, seen in 80% of cases of Lyme disease. The typical rash is a centrifugally spreading, erythematous, annular patch with central clearing at the site of the tick bite.²⁰ Atypical rashes include vesicular, indurated, ulcerated, and follicular variants.²¹ Histopathology commonly shows a superficial and deep perivascular lymphocytic infiltrate with plasma cells, histiocytes, and eosinophils.²² Typically, the rash resolves in 3 to 5 weeks.¹⁸

Early disseminated Lyme disease can present with any of the following findings: multiple erythema migrans; neurologic involvement, including cranial nerve palsy and meningitis; and Lyme carditis, which may result in atrioventricular block.^23,24 Late findings include arthritis, encephalopathy, and polyneuropathy. A late cutaneous manifestation, acrodermatitis chronica atrophicans, is rare in the United States but occurs in as many as 10% of Lyme disease cases in Europe. An initial inflammatory response manifests as blue-red erythema and edema of the extensor surfaces of the extremities, commonly on the dorsal hands, feet, elbows, and knees. Firm fibrotic nodules may develop later over the olecranon and patella.^23,24

The term chronic Lyme disease has been used to describe the persistence of symptoms after treatment; however, large clinical trials have not detected a difference in symptom frequency between patients with a history of Lyme disease and matched controls.^25,26 Many patients with chronic Lyme disease may instead have posttreatment Lyme disease syndrome, described as nonspecific symptoms including fatigue, arthralgia, and decreased mental acuity following treatment of confirmed Lyme disease. Symptoms generally improve within 1 year.²⁷

Laboratory Testing
The gold standard for laboratory diagnosis of Lyme disease is 2-tiered serologic testing. First, an enzyme immunoassay or immunofluorescence assay is used to screen for antibodies. A Western blot follows if the result of the screen is positive or equivocal. Western blot testing for IgM and IgG is used when illness duration is less than 4 weeks; after 4 weeks, a Western blot for IgG alone is sufficient.^27,28 The 2-tiered test has 99% specificity. Sensitivity increases with duration of disease (29%–40% with erythema migrans; 42%–87% in early disseminated disease; 97%–100% in late disease).^29,30 A false-positive result can occur in the presence of infectious mononucleosis, an autoimmune disorder, and syphilis. If serologic testing is negative and suspicion remains high, testing should be repeated in 2 to 4 weeks.³¹ When a patient in a Lyme-endemic area presents with typical erythema migrans, serologic testing is unnecessary prior to treatment.³²

Management
Treatment of Lyme disease centers on antibiotic therapy (Table). First-line treatment of early disseminated disease is doxycycline for 14 days (range, 10–21 days).²⁷ In pregnant women, children younger than 8 years, and tetracycline-allergic patients, amoxicillin or cefuroxime axetil for 14 days (range, 14–21 days) may be used.³³ For erythema migrans without complications, doxycycline for 10 days is effective. Complications that require hospitalization are treated with intravenous ceftriaxone.²⁷ Re-treatment in patients with posttreatment Lyme disease syndrome is not recommended.³⁴ Prophylaxis with a single dose of doxycycline 200 mg may be indicated when all of the following conditions are met: (1) the patient is in an area where more than 20% of Ixodes ticks are infected with B burgdorferi, (2) the attached tick is I scapularis, (3) the tick has been attached for more than 36 hours, and (4) treatment is begun within 72 hours of tick removal.²⁷

Ticks are ectoparasitic hemophages that feed on mammals, reptiles, and birds. The Ixodidae family comprises the hard ticks. A hard dorsal plate, scutum, and capitulum that extends outward from the body are features that distinguish the hard tick. ¹Ixodes is the largest genus of hard ticks, with more than 250 species localized in temperate climates.² It has an inornate scutum and lacks festoons (Figure 1).¹ The Ixodes ricinus species complex accounts for most species relevant to the spread of human disease (Figure 2), with Ixodes scapularis in the northeastern, north midwestern, and southern United States; Ixodes pacificus in western United States; I ricinus in Europe and North Africa; and Ixodes persulcatus in Russia and Asia. Ixodes holocyclus is endemic to Australia.^3,4

Life Cycle

Ixodes species progress through 4 life stages—egg, larvae, nymph, and adult—during their 3-host life cycle. Lifespan is 2 to 6 years, varying with environmental factors. A blood meal is required between each stage. Female ticks have a small scutum, allowing the abdomen to engorge during meals (Figure 3).

Larvae hatch in the early summer and remain dormant until the spring, emerging as a nymph. Following a blood meal, the nymph molts and reemerges as an adult in autumn. During autumn and winter, the female lays as many as 2000 eggs that emerge in early summer.⁵ Nymphs are small and easily undetected for the duration required for pathogen transmission, making nymphs the stage most likely to transmit disease.⁶

The majority of tick-borne diseases present from May to July, corresponding to nymph activity. Fewer cases present in the autumn and early spring because the adult female feeds during cooler months.⁷

Larvae have 6 legs and are about the size of a sesame seed when engorged. Nymphs are slightly larger with 8 legs. Adults are largest and have 8 legs. Following a blood meal, the tick becomes engorged, increasing in size and lightening in color (Figure 3).¹

Ticks are found in low-lying shrubs and tall grass as well as on the forest floor. They search for a host by detecting CO₂, warmth, the smell of sweat, and the color white, prompting attachment.⁸ Habitats hospitable to Ixodes have expanded in the wake of climate, environmental, and socioeconomic changes, potentially contributing to the increasing incidence and expansion of zoonoses associated with this vector.^9,10

Local Reactions

A tick bite may induce local hypersensitivity, leading to a red papule or plaque at the bite site, followed by swelling, warmth, and erythema. A cellular immune reaction induces induration and pruritus. Hard ticks are less likely than soft ticks to cause a serious local reaction.^11,12

A variety of clinical and histologic features are observed following an arthropod bite. Histologically, acute tick bites show a neutrophilic infiltrate with fibrin deposition. Chronic reactions demonstrate a wedge-shaped, mixed infiltrate with prominent endothelial swelling. Eosinophilic cellulitis, or Wells syndrome, reveals tissue eosinophilia and flame figures.¹³ Tick mouthparts may be identified in the tissue. B-cell hyperplasia is seen in Borrelia lymphocytoma and is more common in Europe, presenting as erythematous to plum–colored nodules on the ear and areola.¹⁴

Lyme Disease

Disease manifestations vary by location. Lyme disease is associated with Borrelia burgdorferi and the recently identified Borrelia mayonii in the United States¹⁵; in Europe and Asia, acrodermatitis chronica atrophicans is associated with Borrelia afzelii and neuroborreliosis, with Borrelia garinii. Lyme disease is the most common tick-borne illness in the United States.¹⁶ The I ricinus species complex is the most common vector harboring Borrelia species.¹⁷ At least 36 hours of tick adherence is required for disease transmission.¹⁸ The incubation period is 3 to 20 days (median, 12 days).¹⁹

Clinical Findings
Erythema migrans is the most characteristic sign, seen in 80% of cases of Lyme disease. The typical rash is a centrifugally spreading, erythematous, annular patch with central clearing at the site of the tick bite.²⁰ Atypical rashes include vesicular, indurated, ulcerated, and follicular variants.²¹ Histopathology commonly shows a superficial and deep perivascular lymphocytic infiltrate with plasma cells, histiocytes, and eosinophils.²² Typically, the rash resolves in 3 to 5 weeks.¹⁸

Early disseminated Lyme disease can present with any of the following findings: multiple erythema migrans; neurologic involvement, including cranial nerve palsy and meningitis; and Lyme carditis, which may result in atrioventricular block.^23,24 Late findings include arthritis, encephalopathy, and polyneuropathy. A late cutaneous manifestation, acrodermatitis chronica atrophicans, is rare in the United States but occurs in as many as 10% of Lyme disease cases in Europe. An initial inflammatory response manifests as blue-red erythema and edema of the extensor surfaces of the extremities, commonly on the dorsal hands, feet, elbows, and knees. Firm fibrotic nodules may develop later over the olecranon and patella.^23,24

The term chronic Lyme disease has been used to describe the persistence of symptoms after treatment; however, large clinical trials have not detected a difference in symptom frequency between patients with a history of Lyme disease and matched controls.^25,26 Many patients with chronic Lyme disease may instead have posttreatment Lyme disease syndrome, described as nonspecific symptoms including fatigue, arthralgia, and decreased mental acuity following treatment of confirmed Lyme disease. Symptoms generally improve within 1 year.²⁷

Laboratory Testing
The gold standard for laboratory diagnosis of Lyme disease is 2-tiered serologic testing. First, an enzyme immunoassay or immunofluorescence assay is used to screen for antibodies. A Western blot follows if the result of the screen is positive or equivocal. Western blot testing for IgM and IgG is used when illness duration is less than 4 weeks; after 4 weeks, a Western blot for IgG alone is sufficient.^27,28 The 2-tiered test has 99% specificity. Sensitivity increases with duration of disease (29%–40% with erythema migrans; 42%–87% in early disseminated disease; 97%–100% in late disease).^29,30 A false-positive result can occur in the presence of infectious mononucleosis, an autoimmune disorder, and syphilis. If serologic testing is negative and suspicion remains high, testing should be repeated in 2 to 4 weeks.³¹ When a patient in a Lyme-endemic area presents with typical erythema migrans, serologic testing is unnecessary prior to treatment.³²

Management
Treatment of Lyme disease centers on antibiotic therapy (Table). First-line treatment of early disseminated disease is doxycycline for 14 days (range, 10–21 days).²⁷ In pregnant women, children younger than 8 years, and tetracycline-allergic patients, amoxicillin or cefuroxime axetil for 14 days (range, 14–21 days) may be used.³³ For erythema migrans without complications, doxycycline for 10 days is effective. Complications that require hospitalization are treated with intravenous ceftriaxone.²⁷ Re-treatment in patients with posttreatment Lyme disease syndrome is not recommended.³⁴ Prophylaxis with a single dose of doxycycline 200 mg may be indicated when all of the following conditions are met: (1) the patient is in an area where more than 20% of Ixodes ticks are infected with B burgdorferi, (2) the attached tick is I scapularis, (3) the tick has been attached for more than 36 hours, and (4) treatment is begun within 72 hours of tick removal.²⁷

Ticks are ectoparasitic hemophages that feed on mammals, reptiles, and birds. The Ixodidae family comprises the hard ticks. A hard dorsal plate, scutum, and capitulum that extends outward from the body are features that distinguish the hard tick. ¹Ixodes is the largest genus of hard ticks, with more than 250 species localized in temperate climates.² It has an inornate scutum and lacks festoons (Figure 1).¹ The Ixodes ricinus species complex accounts for most species relevant to the spread of human disease (Figure 2), with Ixodes scapularis in the northeastern, north midwestern, and southern United States; Ixodes pacificus in western United States; I ricinus in Europe and North Africa; and Ixodes persulcatus in Russia and Asia. Ixodes holocyclus is endemic to Australia.^3,4

Life Cycle

Ixodes species progress through 4 life stages—egg, larvae, nymph, and adult—during their 3-host life cycle. Lifespan is 2 to 6 years, varying with environmental factors. A blood meal is required between each stage. Female ticks have a small scutum, allowing the abdomen to engorge during meals (Figure 3).

Larvae hatch in the early summer and remain dormant until the spring, emerging as a nymph. Following a blood meal, the nymph molts and reemerges as an adult in autumn. During autumn and winter, the female lays as many as 2000 eggs that emerge in early summer.⁵ Nymphs are small and easily undetected for the duration required for pathogen transmission, making nymphs the stage most likely to transmit disease.⁶

The majority of tick-borne diseases present from May to July, corresponding to nymph activity. Fewer cases present in the autumn and early spring because the adult female feeds during cooler months.⁷

Larvae have 6 legs and are about the size of a sesame seed when engorged. Nymphs are slightly larger with 8 legs. Adults are largest and have 8 legs. Following a blood meal, the tick becomes engorged, increasing in size and lightening in color (Figure 3).¹

Ticks are found in low-lying shrubs and tall grass as well as on the forest floor. They search for a host by detecting CO₂, warmth, the smell of sweat, and the color white, prompting attachment.⁸ Habitats hospitable to Ixodes have expanded in the wake of climate, environmental, and socioeconomic changes, potentially contributing to the increasing incidence and expansion of zoonoses associated with this vector.^9,10

Local Reactions

A tick bite may induce local hypersensitivity, leading to a red papule or plaque at the bite site, followed by swelling, warmth, and erythema. A cellular immune reaction induces induration and pruritus. Hard ticks are less likely than soft ticks to cause a serious local reaction.^11,12

A variety of clinical and histologic features are observed following an arthropod bite. Histologically, acute tick bites show a neutrophilic infiltrate with fibrin deposition. Chronic reactions demonstrate a wedge-shaped, mixed infiltrate with prominent endothelial swelling. Eosinophilic cellulitis, or Wells syndrome, reveals tissue eosinophilia and flame figures.¹³ Tick mouthparts may be identified in the tissue. B-cell hyperplasia is seen in Borrelia lymphocytoma and is more common in Europe, presenting as erythematous to plum–colored nodules on the ear and areola.¹⁴

Lyme Disease

Disease manifestations vary by location. Lyme disease is associated with Borrelia burgdorferi and the recently identified Borrelia mayonii in the United States¹⁵; in Europe and Asia, acrodermatitis chronica atrophicans is associated with Borrelia afzelii and neuroborreliosis, with Borrelia garinii. Lyme disease is the most common tick-borne illness in the United States.¹⁶ The I ricinus species complex is the most common vector harboring Borrelia species.¹⁷ At least 36 hours of tick adherence is required for disease transmission.¹⁸ The incubation period is 3 to 20 days (median, 12 days).¹⁹

Clinical Findings
Erythema migrans is the most characteristic sign, seen in 80% of cases of Lyme disease. The typical rash is a centrifugally spreading, erythematous, annular patch with central clearing at the site of the tick bite.²⁰ Atypical rashes include vesicular, indurated, ulcerated, and follicular variants.²¹ Histopathology commonly shows a superficial and deep perivascular lymphocytic infiltrate with plasma cells, histiocytes, and eosinophils.²² Typically, the rash resolves in 3 to 5 weeks.¹⁸

Early disseminated Lyme disease can present with any of the following findings: multiple erythema migrans; neurologic involvement, including cranial nerve palsy and meningitis; and Lyme carditis, which may result in atrioventricular block.^23,24 Late findings include arthritis, encephalopathy, and polyneuropathy. A late cutaneous manifestation, acrodermatitis chronica atrophicans, is rare in the United States but occurs in as many as 10% of Lyme disease cases in Europe. An initial inflammatory response manifests as blue-red erythema and edema of the extensor surfaces of the extremities, commonly on the dorsal hands, feet, elbows, and knees. Firm fibrotic nodules may develop later over the olecranon and patella.^23,24

The term chronic Lyme disease has been used to describe the persistence of symptoms after treatment; however, large clinical trials have not detected a difference in symptom frequency between patients with a history of Lyme disease and matched controls.^25,26 Many patients with chronic Lyme disease may instead have posttreatment Lyme disease syndrome, described as nonspecific symptoms including fatigue, arthralgia, and decreased mental acuity following treatment of confirmed Lyme disease. Symptoms generally improve within 1 year.²⁷

Laboratory Testing
The gold standard for laboratory diagnosis of Lyme disease is 2-tiered serologic testing. First, an enzyme immunoassay or immunofluorescence assay is used to screen for antibodies. A Western blot follows if the result of the screen is positive or equivocal. Western blot testing for IgM and IgG is used when illness duration is less than 4 weeks; after 4 weeks, a Western blot for IgG alone is sufficient.^27,28 The 2-tiered test has 99% specificity. Sensitivity increases with duration of disease (29%–40% with erythema migrans; 42%–87% in early disseminated disease; 97%–100% in late disease).^29,30 A false-positive result can occur in the presence of infectious mononucleosis, an autoimmune disorder, and syphilis. If serologic testing is negative and suspicion remains high, testing should be repeated in 2 to 4 weeks.³¹ When a patient in a Lyme-endemic area presents with typical erythema migrans, serologic testing is unnecessary prior to treatment.³²

Management
Treatment of Lyme disease centers on antibiotic therapy (Table). First-line treatment of early disseminated disease is doxycycline for 14 days (range, 10–21 days).²⁷ In pregnant women, children younger than 8 years, and tetracycline-allergic patients, amoxicillin or cefuroxime axetil for 14 days (range, 14–21 days) may be used.³³ For erythema migrans without complications, doxycycline for 10 days is effective. Complications that require hospitalization are treated with intravenous ceftriaxone.²⁷ Re-treatment in patients with posttreatment Lyme disease syndrome is not recommended.³⁴ Prophylaxis with a single dose of doxycycline 200 mg may be indicated when all of the following conditions are met: (1) the patient is in an area where more than 20% of Ixodes ticks are infected with B burgdorferi, (2) the attached tick is I scapularis, (3) the tick has been attached for more than 36 hours, and (4) treatment is begun within 72 hours of tick removal.²⁷