Humoral immune response and antigenemia in sheep experimentally infected with Schistosoma bovis. Cross-reactivity with Fasciola hepatica antigens.

Circulating antigen level, IgG antibody response to worm antigens and to excretory/secretory products (ES), and specificity to Fasciola hepatica antigens were determined in 6 Schistosoma bovis-infected sheep at weekly intervals for 15 wk. A noninfected control group was included. An enzyme-linked immunosorbent assay (ELISA) sandwich and a double-antibody ELISA test was used for antibody and antigen detection, respectively. The infection induced an early and relatively low IgG response to adult worm extract. This response was significantly elevated by 3 wk postinfection (PI), reached its maximum level at 9 wk PI, and was followed by a subsequent decrease. The response to ES antigens was slightly higher than that to adult worms, although the response started later, at 8 wk PI, and remained at its maximum level until 15 wk. A remarkable level of cross-reactivity was observed when adult F. hepatica extract was used. However, a low degree of cross-reactivity was found with ES antigen. The ELISA for circulating antigens was performed at weekly intervals for 8 wk. Antigens were detected as early as the first week of infection, although differences were statistically significant from week 5 onward. The highest values were observed at 7 week PI.