Annual Change in Immunoglobulin G and M Antibody Levels to Toxoplasma gondii in Human Sera

Sera collected from a Japanese population over 4 years at yearly intervals were examined for antibodies to Toxoplasma gondii by enzyme-linked immunosorbent assay (ELISA). Distributions of immunoglobulin (Ig) G antibody levels for 6 age groups consistently showed two antibody groups with constant mean ELISA values. Quantitative analysis of annual changes in IgG antibody levels for 1,366 pairs indicated increase of ≥0.1 in 28 samples and decrease of ≥0.1 in 58 samples, with the remaining 1,280 samples showing changes of <0.1. Mean ELISA values of samples with increase and decrease were 0.479 and 0.833, respectively, and the difference of 0.354 was similar to 0.412, that was the sum total of mean annual changes in samples with increase (0.228) and decrease (0.184). Increase of ≥0.1 from the initial IgG antibody level of <0.1, which was considered as a serological evidence for newly acquired infection, was observed in 3 samples, but no increase was demonstrated in IgM antibody level. The changing pattern of IgG and IgM antibody levels in people during the initial and chronic phases of asymptomatic infections is discussed.