A study looking at antibodies for West Nile virus in more than 400 horses on the Yucatan Peninsula in Mexico found 52% were positive, but none had ever shown symptoms. In a similar scenario, all the research herd horses at the USDA Laboratory in Ames, Iowa, eventually tested positive for West Nile virus antibodies, but none ever developed the disease. Large numbers of antibody-positive but asymptomatic horses have also been found in other areas of the world where West Nile virus has been a problem, such as France.
Immunity to West Nile following natural exposure is believed to be lifelong in humans, and there have not been any reports of horses developing the disease more than once. Under conditions of high virus activity, a large number of horses can be expected to be exposed. The first year or two this happens, WNV encephalitis (swelling of the brain) case numbers will be high.
However, for every one of the most susceptible horses that develops the disease, at least nine more (or possibly higher) will be infected without obvious symptoms. These horses, as well as those that recovered, will then have a strong natural immunity and case numbers should drop. This is precisely the pattern that has been shown in West Nile virus cases over the last seven years. Maps are available at the U.S. Geological Survey site at http://diseasemaps.usgs.gov.
Although there are many complicating factors such as weather conditions most favorable to mosquitoes, the existence of "niches" of infection that favor large populations of virus carrying birds and mosquitoes that will feed on both them and mammals, susceptibility of local birds to the virus, insecticide spraying, etc., the general trend has been for cases to peak over one or two years, then drop off sharply despite testing of mosquitoes and birds that shows the virus is still most definitely present. This pattern is seen in both human cases, where there is no vaccine, and equine cases.
