Abstract

West Nile virus (WNV) has been known for many years as a pathogen endemic to certain areas of Africa and eastern Asia, and has occasionally been responsible for epizootics in Europe. It was first recognized in the western hemisphere in 1999 as the cause of morbidity and mortality in birds, horses and humans living in New York state. Since that time, WNV has spread to many eastern and midwestern states, as well as into Canada, and will almost certainly continue to spread southward and westward.

WNV is a mosquito-borne flavivirus, and numerous species of mosquitos have been implicated as vectors. Birds serve as amplifying hosts, with some species developing extremely high-titer viremia following infection. Natural or experimental infection has also been demonstrated in a variety of mammals, including horses, donkeys, pigs, rabbits and rodents.

Pioneering studies conducted in France several decades ago delineated the response of equids to WNV infection, and work done using isolates of virus from New York have confirmed and extended our understanding of equine WNV infection. Most infected horses develop a viremia 2–4 days after infection that persists for up to several days. The magnitude of viremia in horses is low, and likely not sufficient to allow efficient infection mosquitos. Based on field data and experimental studies, it appears that roughly one in ten infected horses develop severe clinical disease. Initial signs appear 6 to 10 days after infection and include listlessness, obtundation, ataxia, paresis and occasional mania. The disease progresses over 1 to 3 days, reflecting severe and widespread non-suppurative encephalomyelitis. Horses with less severe diseases may recover.

The response of dogs and cats to WNV infection has recently been reported. Both species develop low-titer viremia associated with no or mild clinical disease. Thus, like horses, these animals are unlikely to serve as amplifying hosts. Cats are readily infected by feeding on infected mice, thus defining a oral transmission cycle for this virus.

Several companies are developing and testing vaccines for control of WNV infection in horses and cats.