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Summary Equine infectious anemia (EIA) was diagnosed in a high percentage of horses gathered from public lands administered by the Bureau of Land Management in the area of the confluence of the White River and the Green River in Northeastern Utah in 1998. Twenty-three mares with foals by their sides were gathered in this area. The twelve mares that were positive on serologic tests for EIA were euthanatized. Their foals were moved to an isolation facility where they became subjects of a prospective study. The antibodies in the foals' serum against EIAV antigens were determined to be passive in origin, as levels declined and eventually became undetectable in official tests for EIA in all twelve foals. No evidence for the genetic material of EIA virus (EIAV) was found in sensitive reverse transcriptase-polymerase chain reaction (RT-PCR) assays of plasma samples from the foals collected at the time of separation from all test-positive adult horses, and during their 8 months in isolation. The passively acquired antibodies against EIAV antigens were detected with highest sensitivity in the immunoblot test. Of the official ELISA-based test kits, the competitive ELISA (CELISA) was highest in sensitivity, followed by the other ELISA assay (Vira-CHEK) for detection of antibodies against the major core protein (p26) of EIAV. These results correlated well with those obtained in the agar gel immunodiffusion (AGID or Coggins) test, the internationally recognized gold-standard serologic test for EIA, which also detects antibody against the p26 antigen. The least sensitive indicator for passive antibodies in these foals against EIAV was the SA-ELISA test, which detects activity against the transmembrane protein, gp45. From another perspective, the SA-ELISA test was the initial serologic indicator of lack of infection in these foals, with negative reactions a mean of ≥101 days earlier than in AGID tests. The results confirm that foals of test-positive mares can be raised free of infection with EIAV. The data indicate that the lack of detectable EIAV-RNA in plasma of foals with declining serum antibody titers against EIAV are excellent predictors for lack of infection in foals of test-positive dams. |
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