Effects of the Immune Chicken on the Endogenous Stages of Eimeria tenella

The cecal mucosa of chickens immunized by multiple doses of Eimeria tenella oocysts was susceptible to invasion by sporozoites when the birds were challenged 14 and 21 days after the last im- munizing dose. Fewer parasites were observed in immune than nonimmune birds. Development of the parasite appeared to be suppressed at every stage in the life cycle rather than at one particular stage. Sporozoites removed from the cecal mucosa of immune chickens 48 hr after inoculation and injected into the ceca of susceptible birds failed to initiate infections in the recipient hosts. Isolates from nonimmune birds, however, readily induced an infection. The life cycle of Eimeria tenella (Railliet and Lucet, 1891) Fantham, 1909 in immune chickens has been studied by several workers. Their observations have, however, led to con- flicting views regarding the fate of the para- site. Edgar (1944) concluded that sporozoites failed to invade the cecal mucosa. Pierce, Long, and Horton-Smith (1962) did not find parasites in the tissue but did observe oocysts in the feces of immune birds following chal- lenge inoculation. They later observed that sporozoites invaded the cecal mucosa and were present in glandular epithelial cells up to 72 hr after challenge, but that no first-generation schizonts developed (Horton-Smith, Long, and Pierce, 1963a). Our early examinations of challenged immune birds revealed numerous schizonts in cecal tissue. The present study was undertaken in an at-