Encephalitis is an inflammation of the brain, and it can be caused by many different bacterial and viral infections. West Nile Virus belongs to the family Flavoviridae. The animals which are best known to become infected with this disease are birds and horses, however, disease has occured in other species such as mules, donkeys, sheep, pigs and wild pigs, bats, reptiles, and amphibians. Typically, birds, sheep, pigs and wild pigs show no clinical signs, but have serological evidence. Unusual in the recent New York cases was a high death rate in crows and zoo birds with over 70 other dead birds virus positive.
Birds are the natural reservoir of the virus.
They circulate the virus within their blood for one to four days and usually do not develop clinical signs. During this time they are bitten by mosquitos which then become infected with the virus. After this infection the bird acquires life long immunity. Birds that are infected with West Nile Virus in the U.S. can become ill and die, as was shown in the New York epidemic. Horse infections vary in severity dependent on the degree of previous exposure, and the possibility of increased virulence within the strain.