History

Mosquito bites may or may not be present in an infected person. A history of travel to or from an area that is known to harbor the virus is common.

The incubation period for the West Nile virus is postulated to be approximately 5-15 days. Symptoms of mild infection may last 3-6 days and include fever in 20% of cases. Other symptoms include nausea, anorexia, malaise, myalgia, headache, backache, rash, eye pain, and vomiting.^[2]

Symptoms of more severe illness include severe muscle weakness, flaccid paralysis or increased muscle spasticity, photophobia, seizures, mental status changes, respiratory symptoms, and an erythematous, maculopapular, or morbilliform rash involving the neck, trunk, arms, or legs.^{[6, 11]}The severity of the illness is related to the degree of central nervous system invasion by the virus.

Physical

Signs of encephalitis and meningoencephalitis may be seen. These include mental status changes, such as confusion, stupor, or coma. Other findings include positive Brudzinski and Kernig signs, papilledema, cranial nerve involvement (eg, facial weakness, double vision, visual loss, decreased taste sensation), motor strength weakness, decreased sensation, hyperreflexia, and positive pathologic reflexes (eg, Babinski sign, Hoffman sign).

Causes

The West Nile virus is transmitted to humans by the bite of an infected mosquito. Typically, warm climates and the summer months provide an ideal environment for mosquitoes to breed.^{[2, 25]}Multiple mosquito bites and greater exposure to environments with a large mosquito population increase the risk of infection. However, some cases have been linked to organ transplantation, breastfeeding, and (possibly) blood transfusions.^[26]

A study by Paull et al indicated that drought is the main climate component that increases West Nile virus epidemics and that by the late 2040s, climate change–driven increases in drought severity could cause cases of the virus to triple in regions where human immunity to it is low. The investigators found some evidence that drought increases West Nile virus epidemics by altering the prevalence of mosquito infection, as opposed to increasing the number of mosquitoes.^[27]

Similarly, a study by Uelmen et al that looked at West Nile virus rates in Wisconsin indicated that environmental factors that favor West Nile virus transmission include increases in temperature and decreases in precipitation, as well as urbanization and human population growth, owing to their effect on mosquito biting rates.^[28]

Differential Diagnoses

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