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 (MDV) serotype 1 vaccine (CVI988/Rispens) on the pathogenicity of Cryptosporidium baileyi, and to determine whether C. baileyi infection could prevent the development of vaccinal Marek's disease (MD) immunity in specific pathogen free (SPF) chickens. Sixty-eight SPF homozygous B13 White Leghorn chickens were divided into seven groups. C. baileyi was orally administered at 5 days 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. Relevant control groups were constituted. The chickens were kept in isolators until the end of the experiment at day 62. The parameters evaluated were clinical signs, kinetics of oocyst shedding, mortality, macroscopic and microscopic lesions, cryptosporidia location in various organs and serum anti-C. baileyi antibodies at days 42 and 62. Our results show that C. baileyi, which is considered to be non-pathogenic when inoculated orally, may become highly pathogenic. It induced severe mortality and developed in organs other than classical target sites when chickens were vaccinated with Rispens vaccine and challenged with the HPRS-16 strain of MDV. However, parasite infection does not prevent the induction of vaccinal immunity for MD. Our results also show that vaccination of B13 chickens at hatching induces higher protection against challenge with HPRS-16 MDV at day 15 than vaccination at day 8.