A precocious line of Eimeria media was obtained by selection for early development of oocysts in rabbits. The prepatent period was reduced from 108 to 72 h. The precocious line was less pathogenic than the original strain, and its multiplication rate was lower. Rabbits given oocytes of the precocious line were totally immune to challenge with the original strain as assessed by change in weight gain but were partially protected as assessed by oocyst output. Selection for precocious development was accompanied by morphological changes in the sporulated oocytes; each sporocyst contained only one large refractile body instead of the two smaller bodies seen in the original strain.

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A precocious line of Eimeria intestinalis was obtained by selection for early development of oocysts in rabbits and after six consecutive passages in animals. This line (EiP) was derived from a wild strain (EiO) isolated in 1975 from the caecal content of a rabbit with coccidiosis. The prepatent period of the EiP strain was reduced from 215 h to less than 144 h, the result being that the oocyst sporulation time was the same for both lines. The excreted and unsporulated oocysts had exactly the same shape, but microscopical examination of the sporulated oocysts showed a marked difference between EiP and EiO strains. A huge refractile globule was located in each of two sporocysts of the precocious line, whereas no refractile globule was seen in the other two. The EiP line had a reproductive potential much lower (1000 times) than that of its parent strain EiO and, as judged by the weight gain, mortality and lesions that also occurred in the jejunum and above all in the ileum, its pathogenicity was substantially reduced.

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The pathogenicity and immunogenicity of Eimeria intestinalis was evaluated in SPF rabbits. The animals were given immunizing doses of 6, 6 x 10(2), 6 x 10(3), and 6 x 10(4) sporulated oocysts and were challenged with 3 x 10(3) oocysts. The criteria analysed were the daily weight gain and the oocyst output. This study showed that E. intestinalis had strong immunogenicity, as the inoculation of 6 oocysts was sufficient to minimize the clinical expression of the disease following the challenge and to reduce the oocyst output by about 60%. The immunity towards the excretion of oocysts and the illness was absolute in animals inoculated with 600 or more oocysts. Moreover, this protection seemed to be efficient at least 8 weeks after the challenge. The present results also confirm the pathogenicity of E. intestinalis, although the occurrence of diarrhoea may be irregular, and emphasize the fact that the capacity of this Eimeria for multiplication is not a criterion for clinical diagnosis of the disease.