What’s Eating You? Lone Star Tick (Amblyomma americanum)

The lone star tick (Amblyomma americanum) is distributed throughout much of the eastern United States. It serves as a vector for species of Rickettsia, Ehrlichia, and Borrelia that are an important cause of tick-borne illness (Table). In addition, the bite of the lone star tick can cause impressive local and systemic reactions. Delayed anaphylaxis to ingestion of red meat has been attributed to the bite of A americanum.¹ Herein, we discuss human disease associated with the lone star tick as well as potential tick-control measures.

Tick Characteristics

Lone star ticks are characterized by long anterior mouthparts and an ornate scutum (hard dorsal plate). Widely spaced eyes and posterior festoons also are present. In contrast to some other ticks, adanal plates are absent on the ventral surface in male lone star ticks. Amblyomma americanum demonstrates a single white spot on the female’s scutum (Figure 1). The male has inverted horseshoe markings on the posterior scutum. The female’s scutum often covers only a portion of the body to allow room for engorgement.

Figure 1. The female lone star tick demonstrates a single white spot on the scutum, leading to the common name lone star tick. A local inflammatory reaction has surrounded the site of attachment.

Patients usually become aware of tick bites while the tick is still attached to the skin, which provides the physician with an opportunity to identify the tick and discuss tick-control measures as well as symptoms of tick-borne disease. Once the tick has been removed, delayed-type hypersensitivity to the tick antigens continues at the attachment site. Erythema and pruritus can be dramatic. Nodules with a pseudolymphomatous histology can occur. Milder reactions respond to application of topical corticosteroids. More intense reactions may require intralesional corticosteroid injection or even surgical excision.

Most hard ticks have a 3-host life cycle, meaning they attach for one long blood meal during each phase of the life cycle. Because they search for a new host for each blood meal, they are efficient disease vectors. The larval ticks, so-called seed ticks, have 6 legs and feed on small animals. Nymphs and adults feed on larger animals. Nymphs resemble small adult ticks with 8 legs but are sexually immature.

Distribution

Amblyomma americanum has a wide distribution in the United States from Texas to Iowa and as far north as Maine (Figure 2).² Tick attachments often are seen in individuals who work outdoors, especially in areas where new commercial or residential development disrupts the environment and the tick’s usual hosts move out of the area. Hungry ticks are left behind in search of a host.

Figure 2. Distribution of Amblyomma americanum in 2014. Red states represent areas with established populations, while brown states represent areas with isolated reports of the tick. Data from Springer et al.2

Disease Transmission

Lone star ticks have been implicated as vectors of Ehrlichia chaffeensis, the agent of human monocytic ehrlichiosis (HME),³ which has been documented from the mid-Atlantic to south-central United States. It may present as a somewhat milder Rocky Mountain spotted fever–like illness with fever and headache or as a life-threatening systemic illness with organ failure. Prompt diagnosis and treatment with a tetracycline has been correlated with a better prognosis.⁴ Immunofluorescent antibody testing and polymerase chain reaction can be used to establish the diagnosis.⁵ Two tick species—A americanum and Dermacentor variabilis—have been implicated as vectors, but A americanum appears to be the major vector.^6,7

The lone star tick also is a vector for Erlichia ewingii, the cause of human ehrlichiosis ewingii. Human ehrlichiosis ewingii is a rare disease that presents similar to HME, with most reported cases occurring in immunocompromised hosts.⁸

A novel member of the Phlebovirus genus, the Heartland virus, was first described in 2 Missouri farmers who presented with symptoms similar to HME but did not respond to doxycycline treatment.⁹ The virus has since been isolated from A americanum adult ticks, implicating them as the major vectors of the disease.¹⁰

Rickettsia parkeri, a cause of spotted fever rickettsiosis, is responsible for an eschar-associated illness in affected individuals.¹¹ The organism has been detected in A americanum ticks collected from the wild. Experiments show the tick is capable of transmitting R parkeri to animals in the laboratory. It is unclear, however, what role A americanum plays in the natural transmission of the disease.¹²

In Missouri, strains of Borrelia have been isolated from A americanum ticks that feed on cottontail rabbits, but it seems unlikely that the tick plays any role in transmission of true Lyme disease^13,14; Borrelia has been shown to have poor survival in the saliva of A americanum beyond 24 hours.¹⁵ Southern tick–associated rash illness is a Lyme disease–like illness with several reported cases due to A americanum.¹⁶ Patients generally present with an erythema migrans–like rash and may have headache, fever, arthralgia, or myalgia.¹⁶ The causative organism remains unclear, though Borrelia lonestari has been implicated.¹⁷ Lone star ticks also transmit tularemia and may transmit Rocky Mountain spotted fever and Q fever.¹³

Bullis fever (first reported at Camp Bullis near San Antonio, Texas) affected huge numbers of military personnel from 1942 to 1943.¹⁸ The causative organism appears to be rickettsial. During one outbreak of Bullis fever, it was noted that A americanum was so numerous that more than 4000 adult ticks were collected under a single juniper tree and more than 1000 ticks were removed from a single soldier who sat in a thicket for 2 hours.¹² No cases of Bullis fever have been reported in recent years,¹² which probably relates to the introduction of fire ants.