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 werereported, according to the U.S. Department of Agriculture (USDA) NationalVeterinary Services Laboratories. Cases have reached as far as Colorado,Montana, New Mexico and Wyoming.

As a result, WNV-affected western state and federal veterinarians, imploredUSDA Animal and Plant Health Inspection Service (APHIS) and the Bureau ofLand Management (BLM) to probe the potential impact of WNV on wild horseand 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 Nilevirus in Free-roaming Horses in the Western United States."

"(Western states) were looking ahead and realized this could becomean issue of concern to them, and that was the incentive for the report,"says Chris Kopral, USDA-APHIS, Veterinary Services (VS), CEAH statisticianand 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 LisaHatcher, federal veterinarian and coordinator of a partnership of USDA-APHISand 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 majorityinhabiting Nevada. Herd management areas, administered by BLM, are locatedin 10 western states. The National Park Service, individual states and privategroups handle wild horse herds elsewhere.

If WNV settles in the West, it would require local avian reservoir hostpopulations to become endemic, the report states. Bird species found positivein the East are common out West. Although American crows and blue jays arenot 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 wildhorse habitats, in comparison to the East, where 27 species of mosquitowere found positive for WNV. Study authors say this may indicate reducedwild horse exposure. Yet it may be that as WNV extends west, additionalmosquito 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'salso no mosquito abatement being done, because these are very remote areas,"says Kane.

Risk factor

Beyond mosquitoes, what may leave wild horses more exposed to risk ofWNV than domestic horses is the content of its diet. The report notes, "Clinicaldisease in free-roaming horses may be more common than in domestic horsesdue to deficiencies in the diet."

"If a wild horse were to become infected, the disease may have adifferent course than in domestic horses, because many of these horses areunder environmental stress, and sometimes nutrition on the range is notoptimal," says Kane. "Whether the risk of them showing clinicalsigns is different from what (is seen) in domestic horses ... we reallydon't know."

Risk of mortality

In 2001, in 470 horses for which an outcome was reported, case fatalityrate was 33 percent. One potential scenario uses the observed clinical attackrate in Florida in 2001 (42 per 10,000) and maximum observed case fatalityrate (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 resultof WNV infection.

WNV vaccination

Only wild horses designated for BLM's Adopt-A-Horse or Adopt-A-Burroprograms are vaccinated for WNV and other diseases. That includes 8,000horses annually. Other wild horses on the range are managed with minimalhuman intervention, says Hatcher.

"We'd like to reassure people that, yes, the horses in the Adopt-A-Horseprogram are being protected to the standard of care that is out there forthe domestic horse population," adds Kane.

Conclusions

The report notes, "Although it appears likely that free-roaminghorses will be exposed to WNV, the frequency of such exposure may be lowerthan in the eastern U.S."