Long-term humoral antibody responses by various serologic tests in pigs orally inoculated with oocysts of four strains of Toxoplasma gondii

Antibody titers to Toxoplasma gondii were determined in 16 pigs orally inoculated with 1000 or 10000 oocysts of one of the four strains (GT-1, ME-49, TS-2, TC-2) of T. gondii. Pigs were euthanized on postinoculation days 103-875 and their tissues were bioassayed for T. gondii. Antibody titers were measured in the modified agglutination test (MAT) using formalin-preserved (test A) or acetone-preserved (test B) tachyzoites, latex agglutination test (LAT), indirect hemagglutination test (IHA), enzyme-linked immunobsorbant assay (ELISA), and the Sabin-Feldman dye test (DT). Toxoplasma gondii was isolated from all but two (one with GT-1 strain and one with TC-2 strain) of the inoculated pigs. Results of the serologic tests varied by test used, by strain of T. gondii and from pig to pig within groups. One pig inoculated with the TC-2 strain was considered not to be infected because it remained seronegative in all tests and T. gondii was not isolated from its tissues by bioassay. The IHA and LAT did not produce consistently positive results with infected pigs and two pigs remained seronegative (< 1:64) in both tests. At the time of necropsy, IHA titers had declined to < 1:64 in five pigs and LAT titers had declined to < 1:64 in four pigs. The MAT (test A) and ELISA detected antibodies in all infected pigs but ELISA did not detect antibody at the time of necropsy in one pig. Antibody titers peaked earliest in the DT. Antibody titers in the MAT (test B) peaked at 2560 in all seropositive pigs by 6 weeks postinoculation (PI) but declined to < 1:160 by 15 weeks PI; this test may be useful in determining recency of T. gondii infection in pigs. Overall, the MAT (test A) gave most consistent results.