Serological response to in vitro shed antigen(s) of Tritrichomonas foetus in cattle

We developed a serological assay for detection of (i) an erythrocyte-adhering molecule(s) shed by the bovine venereal pathogen Tritrichomonas foetus and (ii) serum antibodies to this antigen(s) in exposed cattle. Sera from exposed and unexposed cattle were tested for their ability to induce complement-mediated lysis of bovine erythrocytes that had been previously incubated overnight at room temperature in pH-adjusted supernatants of T, foetus culture media. Eight of 180 serum specimens from six groups of presumably unexposed cows or heifers showed a positive (greater than or equal to 1:2) hemolytic titer (specificity = 95.6%). Thirteen of 14 females in two experimentally infected groups showed a positive hemolytic titer following infection (sensitivity = 94%). In experimentally infected heifers, there was little correlation (r(2) = 0.33) between serum hemolytic titers with respect to shed antigen and titers obtained in serum enzyme-linked immunosorbent assays in which whole T. foetus served as the antigen. Serum hemolytic titers rose 3 to 4 weeks sooner than did previously described vaginal mucus immunoglobulin G1 or immunoglobulin A titers with respect to whole cell antigen or TF1.17 subunit antigen, respectively. Among 14 chronically infected bulls, only 6 (43%) showed a positive hemolytic titer. This study is the first, to our knowledge, to show a specific serological response in the host to an in vitro-shed antigen(s) of T, foetus and suggests a useful diagnostic test for potentially exposed herds.