DETECTION OF SERUM ANTIBODY-RESPONSES IN CATTLE WITH NATURAL OR EXPERIMENTAL NEOSPORA INFECTIONS

Parasite-specific antibody responses were detected using an indirect f luorescent antibody (IFA) test in cattle that were naturally or experi mentally infected with Neospora parasites. The test was developed usin g Neospora tachyzoites isolated from an aborted bovine fetus and grown in bovine cell cultures (isolate BPA1). In all cases, infections were confirmed by the identification of Neospora tachyzoites and/or bradyz oite cysts in fetal or calf tissues using an immunoperoxidase test pro cedure. Fifty-five naturally infected cows that aborted Neospora-infec ted fetuses had titers of 320-5,120 at the time of abortion. The titer of 6 cows that were serologically monitored over a prolonged period d ecreased to 160-640 within 150 days after they aborted infected fetuse s. Two of the cows showed an increase in their Neospora titers during their subsequent pregnancy, and they gave birth to congenitally infect ed calves that had precolostral titers of 10,240-20,480. Postcolostral titers of these calves and of 4 other calves with congenital Neospara infections were all greater than or equal to 5,120, whereas calves wi th no detectable parasites had titers less than or equal to 160. Two p regnant heifers that were experimentally infected with the BPA1 isolat e at approximately 120 days gestation seroconverted to Neospora antige ns within 9 days and developed peak titers of 5,120 and 20,480 within 32 days of infection. The fetus taken by caesarian section 32 days pos tinfection from 1 heifer and the full-term calf born to the other had Neospora titers of 640 and 10,240, respectively. Nine cows that aborte d uninfected fetuses and 61 adult cattle maintained under pasture or f eedlot conditions, where risk of exposure to Neospora was considered t o be low, had titers less than or equal to 320. Some of the feedlot ca ttle tested had serologic reactivity that was restricted to antigens a t the apical end of both Neospora and Toxoplasma gondii tachyzoites. T his type of reactivity, which may result from serologic cross-reactivi ty between conserved apical complex antigens of closely related sporoz oan parasites, differed from the whole parasite fluorescence that was observed with sera from Neospora-infected animals. The significance of these results and the potential application of the IFA test for the d iagnosis of Neospora infections in cattle are discussed.