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