The pattern of oocyst production of 8 inbred lines of chickens was compared for each of the 7 species of Eimeria which infect this host. Both the overall numbers and the pattern of oocyst production differed in the inbred lines, but there was no evidence of prolonged cycling of schizogenic developmental stages. Comparison of the numbers of oocysts produced by the different lines indicates that there may be common genetic factors affecting susceptibility to 6 of the 7 species. Surprisingly there appears to be an inverse relationship between susceptibility to E. tenella and susceptibility to the other species: lines which produced most oocysts of E. tenella produced least oocysts of the other species and vice-versa.

SUMMARYA comparison was made between the early events (a sexual stages) in the life-cycle of Eimeria tenella in specifically immunized and control chickens. Particular attention was paid to the quantitative aspects and to the transport of sporozoites within intra-epithelial lymphocytes (IEL) from the enterocytes of the surface epithelium to the enterocytes of the crypts. There was a moderate decrease in the number of parasites initially seen in the mucosa of the immune birds, suggesting that some of the effects of immunity are exerted before penetration of the surface enterocytes, but the reduction in the numbers of developing parasites was more marked. This latter effect was due, at least partly, to failure of transfer of sporozoites from IELs to crypt enterocytes. These findings are discussed in relation to the efficacy of the immunity induced.

1. Experiments have been carried out to compare weight gain, mortality and oocyst production in 7 inbred and partially inbred lines of chickens after challenge with the coccidial parasites Eimeria tenella and E. maxima. 2. There were large differences between lines in the effects of challenge on weight gain and mortality for both species of parasite. However, the lines suffering the greatest mortality were not those showing the greatest effects on weight gain, indicating that mortality alone is not an adequate criterion in selection for resistance. 3. Although oocyst production differed between lines there was no correlation with mortality or with weight loss, implying that the variation observed in these traits was not due to a restriction of the parasite but to an accommodation of its effects. 4. Mortality and weight loss resulting from challenge with E. tenella in the different lines did not correlate with that caused by E. maxima. 5. There was evidence of an association of genes of the major histocompatibility complex genes with mortality, but not with weight loss or oocyst production: there was no indication of association of resistance to coccidiosis with resistance to Marek's disease.

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A study was made of the incidence of coccidial oocysts in approximately 1,660 samples of litter from 48 broiler houses on 5 broiler farms during a 12 month period during 1974-75. Oocysts of Eimeria acervulina and Eimeria maxima predominated in the samples but oocysts resembling Eimeria brunetti and Eimeria tenella were sometimes present. Oocyst numbers were extremely low and variable at 2 weeks, probably because of the small proportion of faecal material present at this time, reached a peak at 4-6 weeks and dropped to a low level at 8-9 weeks. For the last 3 crops of birds reared in these houses, the anticoccidial drug Monensin was used and extremely small numbers of oocysts were found in the litter. Significantly improved food conversion ratios and body weights were noted in these broilers. The results indicate that the examination of a few litter samples for oocysts, provided that they are taken from broiler houses at regular intervals after the 3rd week, could provide useful information on the effectiveness of anticoccidial drugs in controlling coccidiosis.