WNV may veer toward free-roaming horses
Equine cases of West Nile Virus (WNV), not unlike human and bird cases, are spreading faster than the wildfires that scourged the West earlier this year.
As of September, 2,361 positive equine cases of WNV in 28 states were reported, according to the U.S. Department of Agriculture (USDA) National Veterinary Services Laboratories. Cases have reached as far as Colorado, Montana, New Mexico and Wyoming.
As a result, WNV-affected western state and federal veterinarians, implored USDA Animal and Plant Health Inspection Service (APHIS) and the Bureau of Land Management (BLM) to probe the potential impact of WNV on wild horse and burro populations, in which no cases of WNV have been found.
In mid-August USDA-APHIS' Centers for Epidemiology and Animal Health (CEAH) answered with an 11-page report, "Potential Impact of West Nile virus in Free-roaming Horses in the Western United States."
"(Western states) were looking ahead and realized this could become an issue of concern to them, and that was the incentive for the report," says Chris Kopral, USDA-APHIS, Veterinary Services (VS), CEAH statistician and lead author. "There was a lot of input from (state and federal) veterinarians from the origin of the project to gathering of the data ... and reviewing the document."
Drs. Al Kane, USDA-APHIS and BLM Wild Horse and Burro Program advisor, Josie Traub-Dargatz of Colorado State University, Fort Collins, and Lisa Hatcher, federal veterinarian and coordinator of a partnership of USDA-APHIS and BLM's Wild Horse and Burro Program, contributed to the report.
Wild horse dispersal
The wild horse and burro population represents 45,500 mammals, the majority inhabiting Nevada. Herd management areas, administered by BLM, are located in 10 western states. The National Park Service, individual states and private groups handle wild horse herds elsewhere.
If WNV settles in the West, it would require local avian reservoir host populations to become endemic, the report states. Bird species found positive in the East are common out West. Although American crows and blue jays are not familiar in the West, similar corvids, such as the western scrub jay, which have been found WNV-positive, are. Western edge
The mosquito volume is likely to be low in the arid climate of many wild horse habitats, in comparison to the East, where 27 species of mosquito were found positive for WNV. Study authors say this may indicate reduced wild horse exposure. Yet it may be that as WNV extends west, additional mosquito species will serve as vectors.
How mosquitoes behave in wild horse habitats is another factor to consider.
"The areas in which wild horses and burros live are usually dry, desert type climates. It isn't a great environment for mosquitoes, but there's also no mosquito abatement being done, because these are very remote areas," says Kane.
Beyond mosquitoes, what may leave wild horses more exposed to risk of WNV than domestic horses is the content of its diet. The report notes, "Clinical disease in free-roaming horses may be more common than in domestic horses due to deficiencies in the diet."
"If a wild horse were to become infected, the disease may have a different course than in domestic horses, because many of these horses are under environmental stress, and sometimes nutrition on the range is not optimal," says Kane. "Whether the risk of them showing clinical signs is different from what (is seen) in domestic horses ... we really don't know."
Risk of mortality
In 2001, in 470 horses for which an outcome was reported, case fatality rate was 33 percent. One potential scenario uses the observed clinical attack rate in Florida in 2001 (42 per 10,000) and maximum observed case fatality rate (38 percent), according to the report. In this scenario, 16 per 10,000, or 73 free-roaming horses, would be expected to die annually as a result of WNV infection.
Only wild horses designated for BLM's Adopt-A-Horse or Adopt-A-Burro programs are vaccinated for WNV and other diseases. That includes 8,000 horses annually. Other wild horses on the range are managed with minimal human intervention, says Hatcher.
"We'd like to reassure people that, yes, the horses in the Adopt-A-Horse program are being protected to the standard of care that is out there for the domestic horse population," adds Kane.
The report notes, "Although it appears likely that free-roaming horses will be exposed to WNV, the frequency of such exposure may be lower than in the eastern U.S."