Effect of Cryptosporidium baileyi on broilers infected at 26 days of age.

Cryptosporidium baileyi causes respiratory disease, pallor, death, and transient increased feed conversion (FC) and decreased body weight gain (BWG) when intratracheally (IT) inoculated into young chicks. Unlike the case for young chicks in previous studies, C. baileyi had no effect on FC, BWG, or mortality in IT-inoculated 26-day-old chicks. Results from two different histologic evaluations indicated that IT inoculation with 10(4) oocysts caused higher epithelium lesion scores (P less than 0.05) and higher mucosal area scores (P less than 0.05) than did IT inoculation with more, fewer, or no oocysts. Reasons for these findings and implications of chicken immune system response to or modulation by C. baileyi are discussed.