Mosquito-Borne Encephalitis

with special emphasis on equines

There are four mosquito-borne viruses that can cause encephalitis in equines in the U.S.: Eastern equine encephalitis (EEE), Western equine encephalitis (WEE), Venezuelan equine encephalitis (VEE), and West Nile Fever virus (WN). These four viruses have also caused illness and death in humans. Identifying the specific type of virus causing encephalitis in equines is especially important, because there are different medical, veterinary and public health concerns with different viruses.

Eastern equine encephalitis virus is the most pathogenic mosquito-borne virus in the United States. It occurs principally east of the Mississippi River and is transmitted by a variety of mosquitoes, depending on the region where it occurs. Equine cases have been diagnosed in Ohio. More detailed information on EEE is described below.

Western equine encephalitis (WEE) virus is less pathogenic than EEE, but may have fatality rates (proportion of sick horses that die) ranging from 20 to 40%. It occurs principally west of the Mississippi River. The enzootic cycle involves a variety of bird species and the primary vector is Culex tarsalis. Only two WEE cases have been diagnosed in Ohio over the past 30 years, so it is questionable that WEE occurs in Ohio.

Venezuelan equine encephalitis (VEE) may have fatality rate 50 to 80%. There are several strains of this virus, principally in Central and South America, but there is an apparently non-pathogenic strain called Everglades virus that occurs in Florida. The pathogenic Type-I strain has not been seen in the U.S. since 1971, when an epizootic originating in South America swept north through Mexico and entered south Texas. A massive vaccination, quarantine, and mosquito spraying program stopped it there. During an epizootic, many mosquito species can serve as vectors. The enzootic cycle involves rodents and Culex mosquitoes of the subgenus Melanoconion.

West Nile fever has captured the attention of medical, public health and veterinary communities since it appeared in the New York City area in 1999. Of particular interest to veterinarians and horse owners is the fact that this virus can cause mortality in equines. West Nile (WN) Fever virus has been described as only producing a low-grade fever in experimentally infected horses. However, the experience in New York showed, to the contrary, that this particular strain could kill horses. Published reports have related that 25 horses on Long Island showed clinical signs, while 20 others were seropositive, but asymptomatic. Of those with clinical signs, nine died or were euthanized. The enzootic cycle involves principally Culex mosquitoes and a variety of bird species. It is especially fatal to crows, and about 15 other species of birds have died from the infection including exotic species at the Bronx Zoo. In general epidemiology, it is very similar to the closely related St. Louis encephalitis (SLE) virus, except that SLE is not pathogenic to birds, horses, or other animals.

The following is a more detailed description of the epidemiology, signs, diagnosis and prevention of EEE, with special emphasis on the clinical samples and procedures needed to make a confirming diagnosis. These procedures apply to any horse exhibiting signs compatible with a case of mosquito-borne viral encephalitis.