There are over 3700 types of mosquitoes worldwide and over 200 types in the continental United States, of which only 12 are associated with transmitting diseases to humans. The majority are just a nuisance. Since they cannot readily be distinguished, strategies to prevent any bites are recommended.
West Nile Virus
In the US, West Nile virus (WNV) is the leading cause of neuroinvasive arboviral disease. Just hearing the name took me back to New York in 1999 when sightings of dead birds around the city and boroughs were reported daily. The virus was isolated that same year. The enzootic circle occurs between mosquitoes and birds, which are the primary vertebrate host via the bite of Culex mosquitoes. After a bite from an infected mosquito, humans are usually a dead-end host since the level and duration of viremia needed to infect another mosquito is insufficient.
Human-to-human transmission is documented through blood transfusion and solid organ transplantation. Vertical transmission is rarely described. Initially isolated in New York, WNV quickly spread across North America and has been isolated in every continent except Antarctica. Most cases occur in the summer and autumn.
Most infected individuals are asymptomatic. Those who do develop symptoms have fever, headache, myalgia, arthralgia, nausea, vomiting, and a transient rash. Less than 1% develop meningitis/encephalitis symptoms similar to other causes of aseptic meningitis. Those with encephalitis in addition to fever and headache may have altered mental status and focal neurologic deficits including flaccid paralysis or movement disorders.
Detection of anti-WNV IgM antibodies (AB) in serum or CSF is the most common way to make the diagnosis. IgM AB usually is present within 3-8 days after onset of symptoms and persists up to 90 days. Data from ArboNET, the national arboviral surveillance system managed by Centers for Disease Control and Prevention and state health departments, reveal that from 1999 to 2022 there were 56,575 cases of WNV including 28,684 cases of neuroinvasive disease. In 2023 there were 2,406 and 1,599 cases, respectively. Those historic totals for WNV are 10 times greater than the totals for all the other etiologies of neuroinvasive arboviral diseases in the US combined (Jamestown Canyon, LaCrosse, St. Louis, and Eastern Equine encephalitis n = 1813).
Remember to include WNV in your differential of a febrile patient with neurologic symptoms, mosquito bites, blood transfusions, and organ transplantation. Treatment is supportive care.
The US began screening all blood donations for WNV in 2003. Organ donor screening is not universal.
Dengue
Dengue, another arbovirus, is transmitted by bites of infected Aedes aegypti and Aedes albopictus mosquitoes, which prefer to feed during the daytime. There are four dengue virus serotypes: DENV-1 DENV-2, DENV-3 and DENV-4. In endemic areas, all four serotypes are usually co-circulating and people can be infected by each one.
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Long-term immunity is type specific. Heterologous protection lasts only a few months. Dengue is endemic throughout the tropics and subtropics of Asia, Africa, and the Americas. Approximately 53% of the world’s population live in an area where dengue transmission can occur. In the US, most cases are reported from Puerto Rico. Dengue is endemic in the following US territories: Puerto Rico, US Virgin Islands, American Samoa, and free associated states. Most cases reported on the mainland are travel related. However, locally acquired dengue has been reported. From 2010 to 2023 Hawaii reported 250 cases, Florida 438, and Texas 40 locally acquired cases. During that same period, Puerto Rico reported more than 32,000 cases. It is the leading cause of febrile illness for travelers returning from the Caribbean, Latin America, and South Asia. Peru is currently experiencing an outbreak with more than 25,000 cases reported since January 2024. Most cases of dengue occur in adolescents and young adults. Severe disease occurs most often in infants, those with underlying chronic disease, pregnant women, and persons infected with dengue for the second time.