AGE-DEPENDENT RESISTANCE TO CRYPTOSPORIDIUM-BAILEYI INFECTION IN CHICKENS

The age-dependent aspects of resistance to Cryptosporidium baileyi wer e studied in chickens orally inoculated with C. baileyi oocysts at 1 a nd 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 thos e 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. T hese results indicate that both innate and acquired resistance to C. b aileyi are age-related. The numbers of animals needed per experimental group to estimate the length of the prepatent period, the patency, an d the total oocyst output were assessed. Relatively small numbers of a nimals are required for the estimation of the mean length of the prepa tent period, in contrast with the large numbers of birds needed for th e estimation of the mean length of the patent period or that of the to tal oocyst excretion.