Environmental Dermatology

What’s Eating You? Culex Mosquitoes and West Nile Virus


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West Nile virus (WNV) commonly presents cutaneously as a maculopapular rash on the trunk and extremities that most often appears around the time of defervescence and may serve as a positive prognostic indicator. Several laboratory tests can aid in diagnosis of WNV, including an IgM enzyme-linked immunosorbent assay (ELISA), but an antibody response may not be detectable for up to 8 days after symptom onset. Taking a comprehensive history in any patient presenting with a generalized maculopapular rash, fever, nonspecific symptoms, or neurologic changes can aid the astute dermatologist in promptly recognizing the possibility of WNV.

Practice Points

  • Dermatologists should be aware of the most common rash associated with West Nile virus (WNV), which is a nonspecific maculopapular rash appearing on the trunk and extremities around 5 days after the onset of fever, fatigue, and other nonspecific symptoms.
  • Rash may serve as a prognostic indicator for improved outcomes in WNV due to its association with decreased risk of encephalitis and death.
  • An IgM enzyme-linked immunosorbent assay for WNV initially may yield false-negative results, as the development of detectable antibodies against the virus may take up to 8 days after symptom onset.



What is West Nile virus? How is it contracted, and who can become infected?

West Nile virus (WNV) is a single-stranded RNA virus of the Flaviviridae family and Flavivirus genus, a lineage that also includes the yellow fever, dengue, Zika, Japanese encephalitis, and Saint Louis encephalitis viruses.1 Birds serve as the reservoir hosts of WNV, and mosquitoes acquire the virus during feeding.2 West Nile virus then is transmitted to humans primarily by bites from Culex mosquitoes, which are especially prevalent in wooded areas during peak mosquito season (summer through early fall in North America).1 Mosquitoes also can infect horses; however, humans and horses are dead-end hosts, meaning they do not pass the virus on to other biting mosquitoes.3 There also have been rare reports of transmission of WNV through blood and donation as well as mother-to-baby transmission.2

What is the epidemiology of WNV in the United States?

Since the introduction of WNV to the United States in 1999, it has become an important public health concern, with 48,183 cases and 2163 deaths reported since 1999.2,3 In 2018, Nebraska had the highest number of cases of WNV (n=251), followed by California (n=217), North Dakota (n=204), Illinois (n=176), and South Dakota (n=169).3 West Nile virus is endemic to all 48 contiguous states and Canada, though the Great Plains region is especially affected by WNV due to several factors, such as a greater percentage of rural land, forests, and irrigated areas.4 The Great Plains region also has been thought to be an ecological niche for a more virulent species (Culex tarsalis) compared to other regions in the United States.5

The annual incidence of WNV in the United States peaked in 2003 at 9862 cases (up from 62 cases in 1999), then declined gradually until 2008 to 2011, during which the incidence was stable at 700 to 1100 new cases per year. However, there was a resurgence of cases (n=5674) in 2012 that steadied at around 2200 cases annually in subsequent years.6 Although there likely are several factors affecting WNV incidence trends in the United States, interannual changes in temperature and precipitation have been described. An increased mean annual temperature (from September through October, the end of peak mosquito season) and an increased temperature in winter months (from January through March, prior to peak mosquito season) have both been associated with an increased incidence of WNV.7 An increased temperature is thought to increase population numbers of mosquitoes both by increasing reproductive rates and creating ideal breeding environments via pooled water areas.8 Depending on the region, both above average and below average precipitation levels in the United States can increase WNV incidence the following year.7,9

What are the signs and symptoms of WNV infection?

Up to 80% of those infected with WNV are asymptomatic.3 After an incubation period of roughly 2 to 14 days, the remaining 20% may develop symptoms of West Nile fever (WNF), typically a self-limited illness that consists of 3 to 10 days of nonspecific symptoms such as fever, headache, fatigue, muscle pain and/or weakness, eye pain, gastrointestinal tract upset, and a macular rash that usually presents on the trunk or extremities.1,3 Less than 1% of patients affected by WNV develop neuroinvasive disease, including meningitis, encephalitis, and/or acute flaccid paralysis.10 West Nile virus neuroinvasive disease can cause permanent neurologic sequelae such as muscle weakness, confusion, memory loss, and fatigue; it carries a mortality rate of 10% to 30%, which is mainly dependent on older age and immunosuppression status.1,10

What is the reported spectrum of cutaneous findings in WNV?

Of the roughly 20% of patients infected with WNV that develop WNF, approximately 25% to 50% will develop an associated rash.1 It most commonly is described as a morbilliform or maculopapular rash located on the chest, back, and arms, usually sparing the palms and soles, though 1 case report noted involvement with these areas (Figure).11,12 It typically appears 5 days after symptom onset, can be associated with defervescence, and lasts less than a week.1,13 Pruritus and dysesthesia are sometimes present.13 Other rare presentations that have been reported include an ill-defined pseudovesicular rash with erythematous papules on the palms and pink, scaly, psoriasiform papules on the feet and thighs, as well as neuroinvasive WNV leading to purpura fulminans.14,15 A diffuse, erythematous, petechial rash on the face, neck, trunk, and extremities was reported in a pediatric patient, but there have been no reports of a petechial rash associated with WNV in adult patients.16 These findings suggest some potential variability in the presentation of the WNV rash.

Maculopapular rash in a patient with West Nile virus distributed over the upper back and posterior arm. Reproduced with permission from Sejvar,12Viruses; published by MDPI, 2014.

What role does the presence of rash play diagnostically and prognostically?

The rash of WNV has been implicated as a potential prognostic factor in predicting more favorable outcomes.17 Using 2002 data from the Illinois Department of Public Health and 2003 data from the Colorado Department of Public Health, Huhn and Dworkin17 found the age-adjusted risk of encephalitis and death to be decreased in WNV patients with a rash (relative risk, 0.44; 95% CI, 0.21-0.92). The reasons for this are not definitively known, but we hypothesize that the rash may prompt patients to seek earlier medical attention or indicate a more robust immune response. Additionally, a rash in WNV more commonly is seen in younger patients, whereas WNV neuroinvasive disease is more common in older patients, who also tend to have worse outcomes.10 One study found rash to be the only symptom that demonstrated a significant association with seropositivity (overall risk=6.35; P<.05; 95% CI, 3.75-10.80) by multivariate analysis.18

How is WNV diagnosed? What are the downsides to WNV testing?

Given that the presenting symptoms of WNV and WNF are nonspecific, it becomes challenging to arrive at the diagnosis based solely on physical examination. As such, the patient’s clinical and epidemiologic history, such as timing, pattern, and appearance of the rash or recent history of mosquito bites, is key to arriving at the correct diagnosis. With clinical suspicion, possible diagnostic tests include an IgM enzyme-linked immunosorbent assay (ELISA) for WNV, a plaque reduction neutralization test (PNRT), and blood polymerase chain reaction (PCR).


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