Since the incidence of cryptosporidiosis appears to be widespread among domestic poultry, it is thought that its effect on the susceptibility to other important poultry diseases should be further examined. Avian infectious bronchitis (IB) is an acute, highly contagious viral disease of the respiratory tract of chicks, causing high mortality in young chicks. It is characterized by a lightning-fast spread and occurs worldwide.

It has been demonstrated that infectious bursal disease virus induced an immuno-suppression against challenge infection with IB virus in chicks at 1 or 5 days of age (Pejkovski et al., 1979). The objective of this study was to determine the effect of cryptosporidiosis on vaccination against IB in chicks in the field because the effect of the former is largely unknown, with all both diseases have been prevailing in Korea as well as other parts of the world.

Two-day-old commercial chicks (Dekalb-Warren, Scx-Sal-Link, male) with maternal antibody to IB virus were each orally inoculated with 2 × 10⁶ oocysts of C. baileyi (infected group), as previously described by Rhee et al. (1997). Oocysts have been maintained in our laboratory since 1990. Age-matched chicks were served as uninfected control group. Each bird in both groups was orally given 10^3.5 EID₅₀/head of a commercially available IB live virus vaccine at 4 and 14 days postinoculation (PI), as recommended by the manufacture. Following inoculation, the feeding of chicks and the examination of fecal samples were referred to the methods previously described by Rhee et al, (1995 & 1996). Furthermore, 10 commercial chicks immediately after incubation were allocated into intrinsic control group (uninfected and unvaccinated) to monitor maternal antibodies to IB virus.

The serum samples were collected weekly from the same 10 chicks per stage in the intrinsic, the infected and the uninfected groups, respectively. The immune response to IB vaccination was estimated by hemagglutination inhibition (HI) levels determined by the method described by King and Hopkins (1983) using Massachusetts 41 IB antigen prepared by 4 hemagglutinating units. The experiment in the present study was repeated at least three times with similar results. HI levels to IB virus between the uninfected and the infected groups were analyzed by Student's t-test.

In the intrinsic control chicks, the mean log₂ based HI levels of maternal antibody to IB virus at one day-old were 5.16 ± 1.39, subsequently decreased, and reached to negative seroconversion at 6 weeks of age. A negative relationship between the ages of chicks and the levels of maternal antibodies was linear, and in agreement with the linear equation Y = -0.90X + 4.67, showing that the age (X) was a parameter (R = 0.9454). As presented in Table 1, the mean levels of HI to IB virus were, in general, significantly lower in the infected group chicks compared with the uninfected group throughout the experimental period (p < 0.05). The mean levels were gradually decreased with day post-booster-vaccination (PBV), dropped at the lowest grade at 2 weeks PBV, and then increasingly raised with somewhat constant profiles thereafter in both groups. Such diminutions of the mean levels in initial stages of both groups were nearly in line with those of maternal antibody of the intrinsic control chicks.

In the infected group, oocyst output in the excrements revealed profiles similar to those of the previous study of Rhee et al (1998b). However, the oocysts did not show up in the excremental samples of the intrinsic and the uninfected groups during this period.

Although IB viruses perform well as vaccines with SPF chickens, it is well known that maternal antibodies to IB block vaccine viruses and reduce the immune responses when vaccination is carried out at few days of age (Klieve and Gumming, 1988; Calnek, 1991). After booster-vaccination such a temporary depression of HI titer is supposed to be due to the interference phenomenon of maternal antibody. This suggests that the vaccines' efficacy is diminished when birds with maternal antibody are vaccinated with the virus of the same serological subtype. Overall, the data presented confirm reports that maternal antibodies reduce the activity of vaccine reaction.

Based on previous findings (Rhee et al., 1997, 1998a & b) and the present result, it is thought that cryptosporidiosis has an immunosuppressive effect on important poultry disease such as Newcastle disease and IB owing to disturbance of the ontogeny of B-cells by hyperplasia and thickening of bursal epithelium and a mild to moderate depletion of lymphocytes in bursal lymphoid follicles. Therfore, the results give rise to suggestion that the infection with C. baileyi tends to increase the susceptibility of chicks to other important poultry diseases.

The mean HI titers of IB virus were kept up high during 20 to 60-week-old chicks ascribed to several vaccination against IB virus in Korea (Seong et al., 1993). Moreover, the disease is characterized by epizootiological character, as previously described and has been prevailing in Korea (Rhee et al., 1986). Hence, chicks with maternal antibody to IB virus were used as an experimental animal model for the present study to document immunosuppressive effect on other important poultry diseases of chicks at field and general poultry farms. Therefore, the outcome obtained from the present investigation can effectively execute disease prevention practices. It indicate that the managers of poultry flocks have to counteract the incidence of crypto-sporidiosis initially by good managements prior to prevent important poultry diseases.