LONG-TERM ANTIBODY-RESPONSES OF CATS FED TOXOPLASMA-GONDII TISSUE CYSTS

As part of a long-term study on immunity to oocyst shedding, 12 4-6-mo -old cats were inoculated orally with tissue cysts of the ME-49 strain (6 cats) or the TS-2 strain (6 cats) of Toxoplasma gondii. Two cats f ed the ME-49 strain died or were killed because of acute toxoplasmosis 12 and 13 days after inoculation (DAI), respectively. On day 39 after primary infection, 5 cats (2 infected with the ME-49 strain and 3 inf ected with the TS-2 strain) were challenged orally with tissue cysts o f the ME49 strain. One cat died following rechallenge infection due to causes unrelated to toxoplasmosis. Seventy-seven months after primary infection, the remaining 9 cats were challenged orally with tissue cy sts of the P89 strain of T. gondii. Blood samples were obtained weekly or monthly and sera were analyzed for antibodies to T. gondii using t he modified agglutination test (MAT), the Sabin-Feldman dye test (DT), and the enzyme-linked immunosorbent assay (ELISA) for IgM (IgM-ELISA) or Ige (IgG-ELISA). The MAT was performed using both formalin-fixed ( FF) and acetone-fixed (AF) tachyzoites. The MAT (FF) was the most sens itive test; cats seroconverted within 14 DAI and high titers (greater than or equal to 10,000) persisted > 6 yr, although cats had no clinic al signs. The MAT titers using the AF detected recent exposure and tit ers declined sharply after 2 mo postinoculation. DT and ELISA titers w ere lower and developed slower than MAT titers. Fluctuations in antibo dy titers were limited to 8-fold during the 6-yr observation period. A namnestic serum antibody responses were seen in 2 cats after the final challenge, but not after first challenge.