EIA Testing Of Wild Free-Roaming Horses

Every year, the majority of new cases of equine infectious anemia (EIA) is found in the same states where EIA has been diagnosed with the highest frequency since testing was initiated in the early 1970s. Thus, an untested reservoir of infection appears to be present and serves as a source for transmission to our mobile and tested population.

In areas of the United States where infection with EIA virus (EIAV) has been diagnosed infrequently, there has been hesitation to embrace wide-scale testing because of the perceived low benefit/cost ratio. The result of not testing is difficult to judge. Given sufficient time, EIAV has the potential to spread within equid populations, but the rate cannot be predicted accurately.

This study documents testing for EIA in an area where testing has traditionally occurred at a low rate annually. Wild free-roaming and domesticated horses populate the more remote rural areas of the Uinta Basin of northeastern Utah. The wild free-roaming bands owned by the people of the United States and of the Ute Nation own domesticated or other free-roaming equids.

As the areas has few fences and is rugged sagebrush desert country, the equids are relatively free to commingle on public and private lands. Testing was initiated in response to the gathering of 593 equids on Ute Nation lands in April 1998; about 15% of those sampled were test-positive for EIA. The exact origin of each individual was not recorded, but most test-positive ones were gathered in the White River area.

The areas considered at-risk and tested were within designated BLM Horse Management Areas (HMAs) or in areas immediately contiguous to those where test-positive horses had been gathered earlier. Horses were located and gathered by helicopter and hazed into traps. Animals were immediately sorted into foals and adults and transported to holding pens at a distant site. Once samples were collected at the holding pens, mares and foals were reunited; stallions were placed in separate enclosures.

An accredited veterinarian collected paired blood samples in sterile evacuated glass tubes using multisample needles to minimize blood contamination of the sampler and the area. One blood sample was used for immediate horse-side testing in ELISA test formats (CELISA kits from IDEXX; Vira-CHEK EIA kits from Synbiotics; SA-ELISA kits from Centaur), the second was transported to the state laboratory in Salt Lake City for testing in the agar gel immunodiffusion (AGID or Coggins) test. The manufacturers kindly donated ELISA test kits. The rapid ELISA test results were used to effect immediate separation of negative and positive/suspect groups, which were separated by at least 200 yards. Final disposition was made after confirmation of the field data by AGID test results.

The topography of this area of Utah is not conducive to rapid or efficient capture of horses. In most of the units, the land is broken with plateaus and draws that carry water along washes, which appear with the infrequent rains. In areas where more vegetation is present, horses often elude gathering by remaining motionless, camouflaged beneath trees or large brush. Despite these limitations and frequent high velocity spring winds which restrict helicopter flight, as estimated >95% of horses were gathered from three HMAs and from the area immediately contiguous to the index area.

Samples collected from adult horses yielded identical results in field and laboratory assays. Samples with positive ELISA test results were confirmed positive in all three ELISA formats before the horses were segregated, generally within two hours of the time of blood collection. The rate of positive reactors within the other sites ranged from 0-49.5%.

The infection was limited to horses in the BLM Managed Lands contiguous to the index cases, with one exception. The other positive horse (a 2-year-old bachelor stallion) found in the BLM HMA, however, had markings and hoof structure/wear that suggested it originated from the population on the Managed Lands. The only results that did not agree in all official test formats were of foals out of test-positive mares. Those samples were confirmed as positive in the immunoblot test, which has proven to detect antibodies against multiple EIAV proteins with higher sensitivity than the current official tests. Repeated testing will be required to determine if these antibodies are passive in origin or are present as the result of infection.

Control of EIA by traditional test/quarantine/removal/retest methods will be difficult in this type of situation and will require a high degree of individual, inter-nation and in interagency cooperation to succeed. Results generated in multiple ELISA tests offer additional power to the diagnosis of EIA. They can be adapted to horse-side testing and should be adopted for these types of applications.

—Equine Disease Quarterly, Funded By Underwriters At Lloyd's, London, Brokers And Their Kentucky Agents.

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