Preliminary Studies on the Relevance of Antibodies and Bioassay to Experimental Infection with Toxoplasma gondii In Ducks

Little is known about the relationship between antibody dynamics and positive bioassay with Toxoplasma gondii (T. gondii) infection in ducks. In this study, 42 ducks were allocated randomly into groups A, B and C with 14 birds of 14 days of age in each group. Three groups were infected intravenously with 10 5 , 10 6 and 10 8 of T. gondii tachyzoites, respectively. Blood samples were collected on day 7, 14, 21, 28, 42, 56, 70 postinfection (p.i.). Sera was prepared from each group to evaluate the anti-T. gondii antibodies by the modified agglutination test (MAT); from each group 2 duck samples were used to bioassay T. gondii in mice. The T. gondii were identified by PCR and nucleic acids sequencing. In group A, the parasite was successfully bioassayed from day 7, 14 and 21 in post-infected ducks, with the MAT titers of 1:2, 1:5 and 1:80, respectively. However, in ducks with MAT titers 1:80, 1:40, 1:20 and 1:4 on the following days 28, 42, 56 and 70 p.i., the bioassay was negative. In group B, T. gondii was successfully biossayed at all the time points, regardless of the seropositivity or seronegativity for T. gondii antibodies. All birds of group C died before day 4 p.i, and therefore they were not bioassayed. PCR and sequencing results confirmed that all the isolates belong to T. gondii. This study demonstrated that successful bioassay does not completely correlate with positive MAT diagnosis. The existence of T. gondii and the concentration of the parasite may be the key parameters during the bioassay studies.