Studies in humans and experimental animals indicate that
infection with schistosomes results in impaired immune response to a variety of
antigens. Since artificial immunization against a number of
infections is frequently attempted in populations in which
schistosomiasis is endemic, we have attempted to determine whether this impairment could be demonstrated in response to a commonly used immunogen. We have compared the serum
antitoxin response to
diphtheria toxoid (DTd) in normal, uninfected mice with that in mice bearing schistosome
infections of different duration. Animals were infected with 100 Schistosoma mansoni cercariae 16, 12, 8, 4 and 2 weeks prior to, on the same day as, and 4 weeks after the first of three 1 microgram doses of DTd given at 3 week intervals. The
antitoxin level of each mouse was taken as the mean of two sera obtained 1 and 2 weeks after the 3rd dose of DTd. The mean
antitoxin level in uninfected-immunized control mice was 0.0457
Antitoxin Units (AU) ml-1. 71% of mice in this group developed
antitoxin levels > or = 0.01 AU ml-1. Only 28% of the infected-immunized mice achieved
antitoxin levels > or = 0.01 AU ml-1. In infected-immunized mice with
infections of all durations, both the percentage of mice with > or = 0.01 AU ml-1 (0 to 50%) and mean
antitoxin levels (0.003 to 0.016 AU ml-1) were lower than in uninfected-immunized mice.
Antitoxin levels in animals infected 16, 12, 8 and 2 weeks prior to, simultaneously with, and 4 weeks after the first dose of DTd were significantly lower (P = < 0.05) than those of controls. Mice infected 4 weeks prior to the first dose of DTd had
antitoxin levels 64% below controls but this difference was not significant. If similar immunosuppression occurs in human
schistosomiasis, these findings may have implications for childhood immunization programs in areas where
schistosomiasis is endemic.