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.
WNV Progress Report
• Only one in 10 horses testing positive for WNV antibodies ever show symptoms of disease.
• In extreme challenge studies, modified-live virus vaccines have demonstrated a high degree of efficacy.
• There have been no reports of horses coming down with WNV disease more than once.
• In 2006, fewer than 1,000 veterinary cases of WNV were reported in the United States (although it may be underreported).
• Vaccination is still key, since an estimated 35%-50% of WNV encephalitis cases are fatal.
I've been able to see this happening literally in my own backyard. I live in Lancaster County, Pennsylvania. West Nile virus was first detected in my home state in 2001, with three human and four equine cases. In 2002, human cases reached 57, equine 73. The year 2003 was our worst, with 235 cases in people and 540 reported in horses, most of them from my county.
That equine number is way below the actual count. Every vet I talked to in the county stopped sending blood for diagnosis fairly early in the epidemic. They were all on overload and knew what they were dealing with. Cases were occurring both in vaccinated and unvaccinated horses (only the killed vaccine was available at that time).
In 2004, the storm was over. There were only 15 human cases statewide and nine in horses. Lancaster County has continued to test among the highest for virus activity in mosquitoes, but there hasn't been a single equine case this year, despite the continued presence of a large unvaccinated equine population among the Amish and Mennonites, and only one human case.
Challenge Studies & New Vaccines
Whenever we see a drop in disease occurrence, vaccines tend to get the credit. However, many other factors are at work, making it nearly impossible to tell what effect vaccination is having. This is likely the case regarding West Nile virus statistics. Multiple influences have likely helped to avert what once was feared would turn into an epidemic in the horse population.
The gold standard for establishing the effectiveness of a vaccine is what's known in science as a "challenge study." During a challenge, test animals that are known to be free of exposure to the disease are separated into two groups. One group remains unvaccinated, while the other is vaccinated. Approximately two weeks after the vaccination protocol has been completed, both groups are exposed to the disease. Then they are observed to see how many of each group show symptoms.
Attempts to prove effectiveness of the equine West Nile virus vaccines by exposing the test animals to mosquitoes infected with the virus have failed because neither the vaccinated nor the unvaccinated horses came down with West Nile encephalitis.
Merial, the manufacturer of the Recombitek West Nile vaccine, decided to go one step further. Merial repeated its challenge trial by directly injecting the virus into the horse's nervous system. This is a much more severe challenge than a horse is likely to receive by natural exposure, because the immune system doesn't have a chance to participate in stopping the virus before it reaches the nervous system. Nevertheless, the Recombitek-vaccinated horses came through with flying colors, with only one of 10 showing fever and transient muscle trembling, while nine out of 10 unvaccinated horses exposed this way developed encephalitis.
There is also a newly approved, modified-live virus vaccine from Intervet, PreveNile, which replaces part of the DNA from a yellow fever virus with West Nile DNA. This vaccine has also proven to be highly protective in the same type of challenge trial. PreveNile uses the same technology as is in the human WNV vaccine currently in clinical trials. The original Fort Dodge killed-virus vaccine has not been put to the test in a similar way.
DNA-based vaccines, like Recombitek and PreveNile, are well known to provide strong and long-lasting disease protection, but the Recombitek vaccine has only been available since January of 2004. PreveNile was just released in September 2006. When killed vaccines are tested in species where the disease is easier to induce (such as birds), they typically perform poorly when it comes to the bottom line of actually providing protection from the disease. Partial protection, at best, is also found with other killed vaccines such as those for Rhinopneumonitis, EPM and Potomac Horse Fever.
With the availability of high-tech, proven-to-be-effective West Nile vaccines, it just doesn't make sense not to vaccinate a horse that has never been exposed to West Nile virus. The fatality rate for this disease runs between 35% to as high as 50%. Once the current epidemic runs its natural course, case numbers will likely settle down to a pattern more like that seen for other types of viral encephalitis, such as EEE. But, while the magnitude of the threat may drop, WNV is here to stay and will continue to be a threat to the health, even the lives, of our horses.