West nile encephalitis
Overview
West Nile encephalitis (WNE) is distinguished from other arthropod-borne causes of viral encephalitis (eg, western equine encephalitis, eastern equine encephalitis [EEE], Japanese encephalitis, Venezuelan encephalitis) based on its geographic distribution, clinical features, and laboratory findings. WNE is endemic in the Middle East, Africa, and Asia. In North America, WNE first occurred in the northeast United States along the eastern seaboard and now extends nationwide. WNE may have existed in antiquity in the Middle East. Some have suggested that Alexander the Great may have died from WNE.1, 2 Seventeen species of wild birds transmit WNE to humans via the Culex, Aedes, and Anopheles mosquitoes. WNE first causes symptomatic or asymptomatic illness in wild migratory birds that act as viral replication factories. Wild birds infected with WNE contain high titers of the virus and remain viremic for 1-2 weeks, making them ideal hosts to perpetuate the disease. Mosquitoes transmit WNE from birds to humans. Horses, dogs, and other small animals may harbor WNE after being bitten; however, they are inefficient transmitters because viral titers are relatively low, and WNE viremia is short-lived in these animals.
Symptoms
Most people who are infected (at least most children) are asymptomatic or have a flulike illness. A minority of people develop neurologic disease. Many patients describe prominent GI symptoms, especially vomiting and diarrhea. Nonspecific symptoms may include sore throat, backache, myalgias, and arthralgias. Most patients with WNE present with features of encephalitis, aseptic meningitis (meningoencephalitis), or both. Symptoms include a mild febrile illness accompanied by headache, mental confusion, tremors, or flaccid paralysis. Symptoms are most prominent in very young and very old people.
Causes
WNE usually occurs in the summer, when mosquitoes, wild migratory birds, and humans are in close proximity outdoors. Mosquito bites, which are particularly likely during feeding times (dawn and dusk) in the summer months, transmit West Nile virus. Prolonged contact or multiple mosquito bites enhances the risk. West Nile virus may be transmitted in organ transplants. West Nile virus has been found in breast milk.
Resources
The following recommendations can help reduce the risk of becoming infected with the virus: Stay indoors at dawn, dusk, and in the early evening. Wear long-sleeved shirts and long pants whenever you are outdoors. Apply insect repellent sparingly to exposed skin. An effective repellent contains 2%-3% DEET (N,N-diethyl-meta-toluamide). DEET in high concentrations (greater than 3%) may cause side effects, particularly in children. Avoid products containing more than 3% DEET. Repellents may irritate the eyes and mouth, so avoid applying repellent to the hands of children. Insect repellents should not be applied to very young children (under 3 years of age). Spray clothing with repellents containing permethrin or DEET since mosquitoes may bite through thin clothing. Whenever you use an insecticide or insect repellent, be sure to read and follow the manufacturer's directions for use, as printed on the product. Note: Vitamin B and "ultrasonic" devices are not effective in preventing mosquito bites.