Age-dependent resistance to Cryptosporidium baileyi infection in chickens.

The age-dependent aspects of resistance to Cryptosporidium baileyi were studied in chickens orally inoculated with C. baileyi oocysts at 1 and 9 wk of age. The prepatent period was significantly shorter and the patency was significantly longer in younger birds. Chickens infected at 1 wk of age excreted 3 times the number of oocysts excreted by those inoculated at 9 wk of age. There was a good correlation between the length of the patent period and the total oocyst output of chickens. These results indicate that both innate and acquired resistance to C. baileyi are age-related. The numbers of animals needed per experimental group to estimate the length of the prepatent period, the patency, and the total oocyst output were assessed. Relatively small numbers of animals are required for the estimation of the mean length of the prepatent period, in contrast with the large numbers of birds needed for the estimation of the mean length of the patent period or that of the total oocyst excretion.