Serological evidence of human infection with the protozoan Neospora caninum

Neospora caninum is a protozoan parasite that is closely related to Toxopla sma gondii. Dogs are a definitive host. Prior,to its discovery in 1988, N. caninum infection in animals was often mistakenly diagnosed as toxoplasmosi s. Neosporosis in animals is characterized by encephalitis, abortion, and o ther conditions that clinically and pathologically resemble toxoplasmosis. The potential of N. caninum to infect humans is unknown. Therefore, evidenc e of human exposure to this parasite aas sought by screening for antibodies in blood donors by indirect fluorescent antibody (IFA) tests and immunoblo tting. Of 1,029 samples screened, 69 (6.7%) had titers of 1:100 by IFA test ing. Fifty of the 69 (72%) sera that were positive for N. caninum were also negative for a closely related protozoan pathogen of humans, T. gondii. Im munoblot analysis confirmed the specificity of the positive sera for N. can inum antigens, with several sera recognizing multiple Neospora antigens,vit h molecular masses similar to those of antigens recognized by monkey anti-N . caninum serum. An immunodominant antigen of approximately 35 kDa was obse rved with 12 sera. These data provide evidence of human exposure to N. cani num, although the antibody titers in healthy donors were low. The significa nce of human exposure to, and possible infection with, this parasite is unk nown and warrants further study.