Investigations on equine infectious anemia in equids from the Uintah Basin in Utah, 1999-2000

摘要

In 1998, equine infectious anemia (EIA) was diagnosed in the Uintah Basin in NE Utah in a substantial number of free roaming horses on Ute Tribal lands and on federal lands administered by the Bureau of Land Management (Issel et al, Proc US An Health Assn 102:376-384, 1998). The horses gathered in the Bonanza herd area were tested for EIA and released in June of 1998; the test-negative horses from the area with the highest rate of EIA were held in quarantine for 45 days, retested, branded and released in July into the Bonanza herd area. In both cases, they were released after being in contact with an EIA test-positive horse, and with the understanding that gathering and testing of each group would occur in 1999. From November 1999 through March 2000 a total of 216 horses were gathered in the Bonanza herd area. In initial tests for EIA, 32 of the 216 horses (15 percent) were positive for EIA and were destroyed. Of the 46 foals of 1999, 2 were test-positive and both were out of test-positive dams.Both proved to have passive immunity, as their antibodies decayed, and they became test-negative during the quarantine period. During the first 60-day quarantine, 1 additional horse, a 2 year-old stallion, became test-positive for EIA and was destroyed.No further transmission was found during additional quarantine periods. In 1999 the rate of EIA was highest in the group of horses with BLM brands (from the index area in 1998): 15 of the 52 (29 percent) were test-positive, compared with 17/164 (10 percent) of those without brands. This suggests the possibility that some of these infections occurred during the quarantine period in 1998 and that the horses were released before serologic evidence of infection was present. An attempt was made to gather allhorses in this area in 1998. In 1999, at least 21 horses were gathered in the Bonanza HA that had not been previously gathered/released there. This represents another possible source for maintenance of EIA in this area. EIA could also be introduced intothe free roaming bands through entry or release of EIA virus-infected domesticated horses onto the open range country. To further document the risk posed by this source, domestic equids were sampled during an intense "testing clinic" for EIA in the Uintah Basin in April of 2000. During the 2-day exercise, 558 equids were sampled and 1 was positive. The horse had been recently purchased from a neighbor without a test for EIA, further documenting the importance of pre-purchase health exams and negative tests for changes of ownership. A surprisingly high rate of infection with EIA virus (19 percent in ~31 year-olds; 0 percent in foals of 1999) was found in horses gathered in the Bonanza herd area in 1999-2000. Although it is not possible to determine whythe infection persisted in this herd, it is probable that one or more horses were incubating EIA at the time of their release in 1998. Another factor that could have contributed to the spread of EIA was the reshuffling of horses that occurred after their releases in June and July of 1998. The increased level of interaction to establish and maintain a social hierarchy and staliion dominance behavior to establish and maintain harems could have contributed to an increased risk of transmission of EIA. Effective control of EIA in open range country and with free-roaming populations will require a high level of cooperation and continuous surveillance.