Equine lentivirus, comparative studies on four serological tests for the diagnosis of equine infectious anaemia

Serological diagnosis of equine infectious anemia is of necessity group-reactive, i.e. based on viral core protein p26, because viral envelope components as well as the host's immune response to them undergo rapid antigenic change. Since 1970 the agar gel-immunodiffusion test ("Coggins-test") has been the diagnostic method of choice. Recently, ELISA tests have been introduced for faster and theoretically more sensitive serodiagnosis, while Western blots have been used to clarify doubtful results obtained in Coggins-tests. A commercial competitive ELISA was found to give practically equivalent results to the Coggins-test. The sensitivity of this market product is intentionally kept marginal in order to avoid false-positive "reactor horses". Another commercial ELISA, non-competitive, gave inconsistent results, creating great turmoil among horse owners when falsely positive. Caution is also indicated when interpreting Western blots. Sera of strongly positive horses gave as many as eleven bands, of medium positives fewer bands, and of the weakest reactors solely the p26 band. Single p26 banding was, however, also encountered in 5% healthy horses, in two of them consistently over time, which are accordingly considered non-specific. In order to be interpreted as positive, a Western blot for this equine lentivirus must band with its core protein plus at least one glycoprotein, similar to the recommended criterion for a positive reading of serum samples from AIDS patients.