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Immunity to coccidiosis: protective effects of transferred serum and cells investigated in chick embryos infected with Eimeria tenella

SUMMARY The chick embryo proved to be an eminently suitable medium for immunological studies on E. tenella infection, some of the difficulties encountered in the normal host, the hatched chicken, being circumvented. Adoptive transfer of immunity to E. tenella infections in the chick embryo was obtained with antibodies and with cells from caecal tonsils. Protective factors (antibodies) were demonstrated in the serum of fowls which had been infected with E. tenella and in their progeny. Suspensions of caecal tosnil cells inoculated on to the CAM of developing chick embryos reduced the extent of subsequent infections with E. tenella and this was particularly marked when the cell donors had been immunized with E. tenella. Serum globulin protected only when the donors were becoming immune, i.e. after the third inoculum of E. tenella oocysts. The experiment was not continued to the point at which the serum donors were ‘solidly immune’, when previous attempts to protect passively with serum have failed. Maternally transmitted protection was demonstrated in embryos only at the time at which peak protection was afforded by serum globulin, but this is probably an unsatisfactory method of testing since antibodies are not present in allantoic fluid until late in incubation, when coccidial infections are already well established.Some reduction in the oocyst output of the progeny of immunized hens was found when these were challenged at 4 days of age. The protection given by the inoculation of suspensions of cells from the caecal tonsils may have resulted from the production of antibodies by these cells or from their interaction with the parasite in some form of cell-mediated immune mechanism. We are grateful to Mrs Patricia Hesketh, Mr Martin Shirley and Mr Ron Waters for technical assistance and to Mr Peter Townsend for care of the animals.