Evaluating the significance of Toxoplasma gondii sporozoite antibodies in cats: a pilot study.

People can acquire Toxoplasma gondii infection by ingestion of sporulated oocysts passed in cat feces; whether this route is common in cats is unknown. The primary objectives of this study were to (a) adapt a commercially available enzyme-linked immunosorbent assay (ELISA) for the detection of T. gondii tachyzoite IgG antibodies in feline sera to detect T. gondii sporozoite IgG antibodies, (b) utilize the ELISA to confirm that exposed cats can mount an antibody response to sporozoites, (c) estimate the prevalence of sporozoite antibodies in naturally exposed cats, and (d) evaluate associations between the serologic status of naturally exposed cats and clinical signs that could be caused by toxoplasmosis. To generate positive control sera, three male cats were orally inoculated with approximately 100,000 sporulated oocysts of the ME49 strain of T. gondii. A human antisporozoite antibody ELISA was then adapted for use with cat sera. Detectable levels of antisporozoite IgG were found in two of the three experimentally inoculated cats. The sera of 100 healthy cats and 295 clinically ill cats were assessed in the prototype sporozoite ELISA and a commercially available tachyzoite ELISA. The ELISA estimated that prevalence of antisporozoite IgG was 2% in healthy cats and 3.1% in clinically ill cats; in contrast, the overall estimated prevalence of antitachyzoite IgG was 15%. Only two of 395 cats (0.5%) had both antisporozoite and antitachyzoite IgG. While experimentally infected and naturally exposed cats developed antisporozoite antibodies, the low prevalence did not allow for the evaluation of associations among clinical signs.