Introduction

West Nile virus (WNV) was first reported in Africa in 1937, followed by reports in the Middle East, Eastern Europe, and West Asia in the 1950s and 1960s. WNV is a mosquito transmitted arbovirus (arthropod-borne virus) found in birds, humans, and other vertebrate animals. In people, most infections of WNV produce mild symptoms that include fever, headache, body aches, sometimes a skin rash, and swollen lymph nodes. Severe infections may be characterized by high fever, headache, stiff neck, stupor, disorientation, coma, convulsions, tremors, muscle weakness, paralysis, and, occasionally, death. Patients exhibiting the more severe symptoms are usually elderly or immunologically compromised.

West Nile virus was apparently introduced into the United States in New York City in 1999. The mode of entry has not been firmly established but birds imported for zoo exhibits are possible WN virus sources. Since 1999, the WNV has spread northward, southward, and westward at a relatively rapid rate. As of 2005 incidents of WNV infected birds, horses, and/or people have been reported from Canada, Mexico and every state except Hawaii and Alaska.

Since the appearance of the WNV on the east coast in 1999, it has been noted that the virus reappears each year at or near its original foci and, of course, at foci in its recently expanded range. This strong, periodic re-emergence of the WNV enzootic cycle in conjunction with a rapid concentric expansion pattern sharply contrasts with endemic arboviral cycles (e.g., St. Louis encephalitis, eastern equine encephalitis) that sporadically flare-up and then disappear for indeterminate time periods.

How Do People and Animals Get West Nile Virus?

West Nile virus is transmitted to people and animals by infected mosquitoes. Only certain species of mosquitoes carry the virus and very few mosquitoes are actually infective. A mosquito first acquires the infection by feeding on a bird with the virus in its blood. The virus lives in the mosquito and is transmitted to a new host in the mosquito’s saliva when the insect bites a person or animal.  It has been discovered that some species of mosquitoes are able to pass the virus on transovarially, which means that the virus is passed on through the mother to her eggs.  This fact has added another dimension to West Nile virus transmission because some species do not need to find a positive bird in which to become infected by; these mosquitoes are able to infect others from the beginning of their adult life cycle.

Humans and horses are accidental hosts for WNV. Human-to-human or horse-to-human transmission does not occur. Virus transmission is most frequent from May to October when mosquitoes are most abundant (see transmission cycle below).

Transmission Cycle of West Nile Virus

What are the Symptoms of West Nile Virus in People?

Most people who are infected with WNV show no symptoms. However, of those who become ill, symptoms can include fever, headache, nausea, body aches, mild skin rash, or swollen lymph nodes. In a few cases, the disease will progress to encephalitis (inflammation of the brain). The time between the mosquito bite and the onset of illness, known as the incubation period, ranges from 3-15 days in humans. It is estimated that 1 in 150 people who are infected with WNV will require hospitalization. The elderly are particularly susceptible to clinical illness caused by WNV. There is no specific treatment for infection with WNV. Currently no vaccine for humans is available and supportive care is important.

Which Animals Get West Nile Virus?

An infected mosquito can bite a variety of animals, and many animals will become infected, but only a few species exhibit disease. Disease is seen in a several species of birds, in horses, and man. Cats, dogs, bats, chipmunks, skunks, squirrels, and domestic rabbits have been shown to be infected without exhibiting disease symptoms.

Birds

Certain wild birds are the animals from which the mosquito vector primarily acquires the WNV. Infection has been reported in almost 300 bird species, most of which do not appear ill. WNV infection can cause serious illness and death in many avian species including corvids, e.g., crows, jays, ravens, and magpies. Raptors (birds of prey) are also very susceptible.

Horses

Horses are also susceptible to WNV. Clinical signs consist of central nervous system abnormalities similar to those caused by infection with eastern equine encephalitis (EEE) and western equine encephalitis (WEE) viruses. Both EEE and WEE vaccines are available for horses and are recommended for use in the spring. An equine WNV vaccine is available. For more information, see http://www.cdfa.ca.gov/ahfss/ah/wnv_info.htm.