By Laurie Fio
In the summer of 1999, West Nile fever appeared in New York and caused great concern among health officials as it claimed the lives of birds, horses and humans. The University of California, Davis Center for Equine Health and the Bernard and Gloria Salick Equine Viral Disease Laboratory responded by hosting a meeting with regulatory officials in various state and government health agencies, viral disease experts and representatives of the equine industry. The seminar provided a forum to discuss the implications of West Nile fever on the California equine industry and to help prepare for another outbreak. As a follow-up to that seminar, this is an update on the current status of West Nile fever.
West Nile fever is a mosquito transmitted viral disease of humans and animals whose natural life cycle involves viral transmission between mosquitoes and birds. West Nile virus (WNV) occurs throughout much of Africa, the Middle East, Asia and Europe. WNV first appeared in the Western Hemisphere in 1999 in Queens, New York, where it caused disease in humans, birds and animals. Currently, crows and blue jays are especially susceptible hosts in the United States, but the 1999 U.S. outbreak also infected a substantial number of humans (7 deaths) and horses (9 deaths). In addition to causing death and neurologic disease in horses, WNV outbreak resulted in the European community imposing movement restrictions of horses from affected areas.
The 2000-year began with many questions as to how WNV infection would develop in the United States. Optimistically, some scientists predicted that the epidemic would burn itself out. However, others speculated that the virus would rapidly become endemic throughout the entire United States. As the year has progressed, it has become increasingly clear that the latter opinion is likely to be correct.
The isolation of WNV from both insects and birds during the winter of 2000 confirmed that it had indeed overwintered and with the advent of the vector season this past summer, the virus rapidly and dramatically expanded its incursional foothold. Despite widespread spraying of insecticide to kill the mosquito vectors, the virus now has been positively identified in 11 states as well as the District of Columbia.
Many varieties of mosquitoes have been identified as carriers of WNV, several of which are native to the United States. So far this year, WNV infection has been confirmed in nearly 4,000 birds (mainly crows), 18 humans (one fatality), and in other animals including the bat, rabbit, cat, raccoon, squirrel, skunk and chipmunk. In horses, 53 cases of WNV infection have now been confirmed in eight states including New York, Rhode Island, New Jersey, Maryland, Massachusetts, Connecticut, Pennsylvania and Delaware. At least 22 of these horses either died or were euthanized.
In horses, WNV infection can cause severe neurologic disease and associated encephalitis. Other diseases that can be easily confused with West Nile fever include rabies, eastern, western and Venezuelan equine encephalitis, and the California serogroup Bunyaviruses. All of these diseases cause viral encephalitis in horses. Horsemen should also be aware that the clinical manifestations of West Nile fever could be confused with the symptoms of equine protozoal myeloencephalitis (EPM).
Another issue that causes concern is the occurrence of subclinical WNV infection in horses (infection occurs but the horse shows no signs of disease). In test horses, which were experimentally infected with WNV, only a low incidence of disease occurred which, indicates that subclinical infections are common. If true, further study is also necessary to determine if there is a risk of spreading disease from these carrier horses. Vaccination is considered a viable option for the future; however, a vaccine suitable for use in horses still needs to be developed.
Perhaps, the most disturbing aspect of the 2000 WNV epidemic is the rapid southward extension of the virus, which strongly suggests that it could soon establish a year-round cycle of infection in warmer regions of the southeastern United States. Many experts now predict that WNV infection will spread to all 48 contiguous states of the Unites States, as well as extensive areas of the Caribbean, and Central and South America. As a consequence, movement restrictions might be imposed on horses from regions in which WNV infection becomes endemic.
An additional very disturbing aspect of WNV infection is that extremely high levels of the visor occur in the blood of susceptible birds such as crows and the virus can be directly transmitted between birds without a mosquito vector. This direct transmission of WNV certainly raises enormous public health issues, particularly for laboratory personnel handling suspect birds and mammals. Because of this threat, WNV is classified in the United States as a BL-e pathogen, a level that mandates sophisticated facilities and strict protocol for the safe handling of diagnostic specimens.
Despite the fact that events such as the Sydney Olympic Games and race meets on the eastern seaboard were not impacted this year, the adverse impact of WNV on the U.S. equine industry will undoubtedly increase. During an extensive outbreak of WNV infection of horses in southern Europe (40 horses died in France this summer), the European Union again imposed movement restrictions of horses from WNV-endemic areas in the United States. Because of the considerable mortality that has occurred in WNV-infected horses in both the United States and France, it is feared that the ultimate economic hardship imposed on the equine industry by WNV could be devastated if left unchecked.
In response to these uncertainties, scientists from the Bernard and Gloria Salick Equine Viral Disease Laboratory have initiated a collaborative research investigation on WNV infection of horses with scientists at the equine research Center, Faculty of Veterinary Science, University of Pretoria, South Africa. South Africa provides a unique environment in which to study the evolution and epidemiology of WNV in a region where WNV infection is endemic, yet disease rarely is reported in either humans or horses. The Maurice Stans Foundation, The Equine Research Center in South Africa, and the Bernard and Gloria Salick Equine Viral Disease Laboratory Endowment have generously supplied funding support for this research support for this research.
In summary, it appears that what scientists feared from last year’s WNV outbreak has occurred: WNV appears to be establishing itself in the United States and may become yet another infectious viral disease of humans and animals that is endemic to the United States. The Center for Equine Health and the Bernard and Gloria Salick Equine Viral Disease Laboratory are facing the challenge head on by establishing a network of scientific experts and conducting the necessary research to establish effective preventive measures and treatment protocols.
The Center for Equine Health
UC Davis School of Veterinary Medicine,
One Shields Ave.,
Davis, CA 95616-8589