Recent media reports have prompted many questions regarding West Nile virus (WNV) infection in species other than birds, horses, and humans. As an "emerging disease" in the United States, WNV has clinical and health effects on various animal species and populations that have yet to be described or fully reported. The following information is offered on the current knowledge of WNV infection in other species.
It is currently believed that any type of bird or mammal may be susceptible to WNV infection, but very few species appear to develop clinical illness due to infection. Since entering North America in 1999, WNV has been reported in thousands of birds, horses, and humans (over 2768 people and 146 deaths, as of Wed 9 Oct 2002). Before this year, WNV infection had been reported in several bat species, chipmunks, gray squirrels, striped skunks, a rabbit, and 3 cats. Laboratory trials had indicated that cats would become viremic and ill, but that dogs were relatively resistant to infection with WNV.
Serosurveys in New York following the 1999 outbreak revealed that 8-11 percent of dogs had antibody titers to WNV, but had not become ill. As WNV has moved across the country this summer, infection and illness have now been reported in a domestic sheep, a mountain goat, a dog, a 7-month-old wolf pup, a llama, and an alpaca. Serologically positive black bears and white-tailed deer have also been detected, but these animals did not develop clinical illness.
Some of the animals that became ill had another underlying health condition and/or a compromised immune system predisposing them to development of clinical disease. For instance, the one 8-year-old dog in Illinois was immune-compromised. From what is currently known, WNV does not appear to pose a significant health risk for species other than birds, horses, and humans. The extremely small number of cases in other species, the active surveillance conducted for the last 4 years, and reports from 42 states that have detected WNV suggest that most of these species are extremely resistant to developing clinical illness from infection. There is also no current evidence that any of these species are capable of serving as a reservoir for the virus.
It is likely more cases in other hosts will be found as WNV enters and becomes established in new areas. As additional information becomes available, the relative health significance of WNV infection in these populations will become more clearly established.