West Nile virus encephalomyelitis in eight horses

Special ReportWest Nile virus encephalomyelitis in eight horses W est Nile virus was first isolated in the United States in 1999 1 and caused human, avian, and equine illness and death.The virus was first isolated from a febrile woman in Uganda in 1937 2,3 and West Nile virus encephalomyelitis has been reported in humans in Africa, Europe, the Middle East, west and central Asia, and Oceania. 3 Equine encephalomyelitis caused by the virus has been reported in Egypt, 4 Morocco, 5 Portugal, 6 France, 7 and Italy.8 The most recent epizootics occurred in Morocco in 1996, Italy in 1998, the northeastern United States in 1999, and France in 2000.3 Identification of West Nile virus in the United States in 1999 represented the first occurrence of this virus in the western hemisphere. 1 West Nile virus is an arbovirus in the Japanese encephalitis antigenic complex of the genus Flavivirus, family Flaviridae.3 The virus has been isolated from 43 mosquito species, primarily in the genus Culex.Virus isolation from other hematophagous arthropods such as ticks has also been reported, but the role of ticks in transmission has not been documented.The virus has been isolated from a large number of avian species (56 in the United States alone 9 ) and mammalian species including lemurs, chimpanzees, rodents, bats, camels, mules, donkeys, horses, goats, cattle, water buffalo, sheep, pigs, rabbits, and dogs, as well as amphibians.3 Results of most research studies suggest that birds are the only animals in which a viremia develops that is sufficient to infect mosquitoes and propagate the infective cycle.3 A transient, low-level viremia has been documented in naturally and experimentally infected horses and donkeys.4,10 The level of viremia is thought to be insufficient to infect mosquitoes.2,[10][11][12] Madagascan lemurs may be one of the few mammals that develop a viremia sufficient to infect feeding mosquitoes.13 Because of these findings, it is highly unlikely that a horse infected with West Nile virus would transmit the virus to humans or other horses in typical circumstances.West Nile virus encephalomyelitis was diagnosed in 8 horses with neurologic signs hospitalized at the George D. Widener Large Animal Hospital at New Bolton Center, University of Pennsylvania in the fall of 1999 and 2000.The diagnosis was made via serologie testing in 5 horses and via serologie testing and reverse transcriptase polymerase chain reaction (RT-PCR) testing performed on brain tissue in 3 horses.The purpose of this report was to describe findings in the 8 affected horses.