Enzyme-linked immunosorbent assays for the measurement of specific antibodies in experimentally induced ovine toxoplasmosis

Tachyzoites of the RH strain of Toxoplasma gondii were inoculated intravenously into sheep following which serum samples were collected at approximately weekly intervals for 9 months. The sera were examined by the toxoplasma dye test and two enzyme-linked immunosorbent assays (ELISA) specifically developed for investigations of ovine toxoplasmosis. One was an antibody class capture assay for the detection of anti-toxoplasma specific IgM, the other an indirect assay which detected anti-toxoplasma IgG. Some of the sheep had antibodies to toxoplasma prior to inoculation but none had specific IgM. Sera collected 17 days after inoculation showed that all had raised specific antibody levels but the only sheep that produced specific antitoxoplasma IgM were those that were initially without any antibody. Specific IgM could be detected in all these particular sheep for at least 1 month after infection and up to 3 months in some. Specific IgG persisted at high levels for at least 3 months and could still be detected at moderate levels for at least 9 months. The ELISA methods described are simple to perform and could clearly distinguish between previous infection and this experimental infection with Toxoplasma gondii.