Effect of Cryptosporidium baileyi in specific pathogen free chickens vaccinated (CVI988/Rispens) and challenged with HPRS-16 strain of Marek's disease virus

This study was performed to examine the effect of Marek's disease virus (MD V) serotype 1 vaccine (CVI988/Rispens) on the pathogenicity of Cryptosporid ium baileyi, and to determine whether C. baileyi infection could prevent th e development of vaccinal Marek's disease (MD) immunity in specific pathoge n free (SPF) chickens. Sixty-eight SPF homozygous B13 White Leghorn chicken s were divided into seven groups. C. baileyi was orally administered at 5 d ays of age (day 4) in chickens infected with Rispens vaccine at day 0 or at day 8 and challenged with HPRS-16 strain of oncogenic MDV at day 15. Relev ant control groups were constituted. The chickens were kept in isolators un til the end of the experiment at day 62. The parameters evaluated were clin ical signs, kinetics of oocyst shedding, mortality, macroscopic and microsc opic lesions, cryptosporidia location in various organs and serum anti-C. b aileyi antibodies at days 42 and 62. Our results show that C. baileyi, whic h is considered to be non-pathogenic when inoculated orally, may become hig hly pathogenic. It induced severe mortality and developed in organs other t han classical target sites when chickens were vaccinated with Rispens vacci ne and challenged with the HPRS-16 strain of MDV. However, parasite infecti on does not prevent the induction of vaccinal immunity for MD. Our results also show that vaccination of B13 chickens at hatching induces higher prote ction against challenge with HPRS-16 MDV at day 15 than vaccination at day 8.