Endogenous stages of a 'chick embryo-adapted' strain of Eimeria tenella.

Abstract

A strain ofE. tenellawhich had been passaged serially in 42 batches of chick embryos appeared to have lost its ability to produce the characteristically large second-generation schizonts in both the embryonic chorioallantois and in the chicken caeca. Numerous small schizonts (10–16 µm in diameter), which contained 8–30 small merozoites, replaced the large second-generation schizonts characteristic of this species. The small schizonts were numerous 3–5 days after inoculation and were restricted to epithelial cells of the embryo allantois when sporozoites were inoculated via the allantoic cavity. These schizonts also occurred in epithelial cells of the caeca of chickens given oocysts via the crop. Cell reactions in the chicken caeca in response to infection were minimal. Numerous minute lesions were present in the chorioallantois 4–6 days after inoculation; these were associated with the small schizonts and later with gamonts and oocysts. The strain reproduced well in the embryo but failed to induce gross pathogenic effects and no mortality resulted from the infection.Only two passages of this strain in chickens were needed to restore a high degree of pathogenicity. This change was accompanied by the reappearance of the characteristically large second-generation schizonts in the tunica propria of the caecal wall.