A new method for the experimental production of necrotic enteritis in chickens is described. The main features are the use of a diet high in wheat and fish meal content; oral administration of a non-lethal inoculum of the coccidium Eimeria maxima followed 6 days later by the bacterium Clostridium perfringens type A per cloaca, so that the bacterial inoculum is deposited at the time and place when and where the intestinal coccidial lesions are maximal; grading of coccidial and clostridial lesions in individual birds sampled during the 14 days following the coccidial infection. The new method was used to examine the relationship between clostridial and coccidial infections. Frank coccidiosis, caused by virulent E. maxima, exacerbated the lesions of necrotic enteritis and other clinical effects due to a subsequent challenge with virulent C. perfringens type A. Immunization with a live, pentavalent, attenuated anticoccidial vaccine (Paracox-5) protected against a severe challenge with heterologous E. maxima. Furthermore, vaccination with Paracox-5, by virtue of its protection against clinical coccidiosis due to the E. maxima challenge, indirectly protected birds against a subsequent challenge with virulent C. perfringens. The results are reconciled with previous field observations on concomitant coccidiosis and necrotic enteritis in chicken flocks.

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The behaviour of four species of Eimeria was studied in lambs which were given either monospecific or multispecific infections. In the presence of other species the patency of oocyst production of E. ovina and E. weybridgensis was extended and the total number of oocysts produced by all species except E. ninakohlyakimovae was increased. Clinical symptoms were observed only in lambs which received E. ninakohlyakimovae.

SUMMARYEimeria flavescens and E. irresidua from the domestic rabbit are redescribed. The relatively smaller ovoidal oocysts of E. flavescens which measure on average 31·7 × 21·4 μm, possess a wide micropyle at the broad end. First-generation schizonts of this species develop deep in the glands of the lower small intestine. Merozoites migrate to the caecum and colon where second, third and fourth-generation schizonts develop in the superficial epithelium followed by the fifth-generation schizonts and gametocytes which form in the glands. In young Dutch rabbits E. flavescens is very pathogenic; low doses of oocysts produce a severe enteritis with high mortality and morbidity. The larger, broadly ellipsoidal oocysts of E. irresidua measure on average 38·4 × 23·2 μm and often possess a very small cryptic oocyst residuum. The endogenous stages develop in the small intestine only; first-generation schizonts in the glands and second-generation schizonts in the lamina propria whilst third and fourth-generation schizonts and gametocytes develop in the villous epithelium. E. irresidua is not pathogenic in young Dutch rabbits; even heavy infections produce only a transient pause in weight gain.

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