Hemagglutination-inhibition titers (log2) to Newcastle disease (ND) virus were chronologically observed in chicks, which were orally inoculated with 5 x 10(5) oocysts of Cryptosporidium baileyi at 2 days of age and subsequently vaccinated with inactivated ND virus at 4 and 21 days postinoculation. In general, the titers were considerably lower in the infected chicks than those in the uninfected control throughout the experimental period (p < 0.01), and rapid negative seroconversions were observed in the infected chicks. The titers reached a peak on weeks 2 and 4 post-booster-vaccination in the control and infected chicks, respectively. Thus, C. baileyi infection was shown to have an immunosuppressive effect on ND vaccination when the agent was given to 2-day-old chicks. It is suggested that C. baileyi infection in chicks may increase the host susceptibility to ND virus.
