Use of IgG- and IgM-specific ELISAs for the assessment of exposure status of chickens to Eimeria species.
Simple and reliable methods for the determination of the exposure status of chickens to Eimeria species are required. For this purpose an enzyme-linked immunosorbent assay (ELISA) detecting specific IgG and IgM antibodies in serum samples was evaluated. Sera from chickens hyperimmunized by intramuscular injection of a saline extract of Eimeria tenella sporozoites were used to determine optimal reaction conditions in the ELISA which were found to be at a serum dilution of 1:100 and an antigen concentration of 0.2 microgram per reaction well. Saline extracts of sporulated oocysts and purified sporozoites of E. tenella were also potent antigens but most studies were carried out with sporozoite antigen. In a trial with 80 chickens, concentrations of serum IgM directed against sporozoite antigen increased significantly 9 days after primary infection with 10,000 oocysts of E. tenella per animal. IgM levels subsequently decreased rapidly reaching a plateau level only slightly higher than uninfected controls by about 15 days post-infection. In chickens challenged with 10,000 oocysts 21 days after primary infection significant increases of IgM levels were observed 2, 6 and 12 days later. In contrast IgG levels increased only slightly after primary infection but significant increases occurred after challenge infection so that by Day 12 after challenge sporozoite-specific IgG levels were much higher than in control chickens. Thus, it may be possible to discriminate between chickens actually infected with Eimeria (as indicated by high levels of antiparasite IgM), chickens which have been repeatedly exposed to Eimeria (as indicated by high levels of antiparasite IgG) and unexposed birds. The applicability of this ELISA, using sporozoite antigen of E. tenella to practical situations was substantially confirmed, since sampling of over 1000 sera from commercially reared broilers and laying hens indicated that broilers, maintained on medicated food, had low levels of IgM and IgG whereas 84-97% of the laying hens, receiving drug-free feed, had relatively high IgG concentrations. These results reflect low and rare exposure to Eimeria infections in broilers and repeated exposure of the hens.