Public concern has grown in New England after findings of the West Nile virus in more dead birds and mosquitoes. (For updated WNV coverage, visit https://thehorse.com/TopicSearch/Default.aspx?n=West+Nile+Virus+(WNV)&nID=6&ID=79.) West Nile virus (WNV) is a type of encephalitis spread by the bite of an infected mosquito, and made its first appearance in the Western Hemisphere last fall. The illness was found first in dead birds, but also caused both illness and death in horses and humans. The virus has not been found in any horses or humans this year. A man who died July 19 in Staten Island, N.Y., of symptoms confused with those of WNV was found negative after testing.

New developments on West Nile’s appearance in the United States include additional bird findings in New Jersey and New York, and the detection of West Nile in a new species of mosquito.

The virus has most commonly been found in Culex pipiens mosquitoes, but discovery of another type of mosquito carrying the virus has made livestock owners intensify mosquito eradication. This is the first time that WNV has been isolated from the Aedes japonicus mosquito. Aedes japonicus originated in Asia (Japan or Korea) and is suspected to have arrived in the Northeast in the past few years in shipments of used tires. The species has spread rapidly throughout Connecticut, New Jersey, and New York.

It is unknown whether this type of mosquito can transmit the virus. However, the reason positive Aedes japonicus findings have raised concern is because unlike the Culex species, these mosquitoes are active both in the day and nightime, and are found in wooded areas. Larvae are found in places where water is contained, such as in discarded tires or rock holes