ABSTRACT
Coccidian parasites belonging to the genus Cryptosporidium are transmitted by an environmentally robust stage known as the oocyst. The slides were examined by bright-field microscopy and scored as either positive or negative based on the presence or absence of Cryptosporidium developmental stages in the brush borders of the intestinal villi. The proportion of animals infected at each oocyst dose was determined from the plus or minus scores of the tissue slide examinations. The oocysts voided in the feces of experimentally infected calves were clarified by using cesium chloride density gradient and stored in phosphate-buffered saline containing antibiotics. Examination of process control viability also confirmed that there was no reduction in the ability of the oocysts to cause infection. The oocyst inocula selected for the process control infectivity experiments were designed such that they would enable measurement of up to 3-log inactivation of oocysts.
