Hemagglutinin (HA) titers to sRBC were chronologically observed in chickens orally inoculated at 2 days of age with 5 x 10(5) oocysts of Cryptosporidium baileyi. All the infected chickens exhibited negligible HA titers by 44 days postinoculation (PI). The titers were elevated as time progressed, and peaked on day 52 PI, declined gradually thereafter, and eventually reached to normal titers on day 92 PI. On the contrary, the titers in uninfected chickens were higher in comparison with infected chickens during the experiment. Chickens infected with the protozoa showed normal oocyst shedding profiles during this period. These data suggest that C. baileyi infection suppress development of humoral immunity to sRBC in chickens. It is possible that impairment of the bursa of Fabricius by cryptosporidiosis rendered chickens vulnerable to other pathogens.
