Genetic influence on nitric oxide production during Eimeria tenella infections in chickens

Studies were carried out to assess the importance of nitric oxide (NO) production in vivo in resistance to Eimeria tenella infections through treatment of resistant SC and susceptible TK strains of chickens with compounds proven to be effective inhibitors of induced nitric oxide synthase. The SC strain produced higher levels of plasma NO2- + NO3- in response to primary infection than did the TK strain. SC chickens were also more responsive in reduction of NO2- + NO3- levels and increased oocyst output in response to treatments with S-methylisothiourea or NG-methyl-L-arginine (LNMA) during primary infection. However, the SC strain, regardless of drug treatment, consistently yielded higher oocyst output during primary infection, indicating it could be susceptible to a higher parasite burden. On the other hand, the SC strain seemed to develop immunity more rapidly than the TK strain, in that oocyst output was decreased in both untreated and LNMA-treated chicks upon challenge infection compared with that of the TK strain. Oocyst output from both SC and TK strains was further decreased by LNMA treatment during challenge infection rather than increased, suggesting minimal involvement of NO production in the immune response to challenge. These results indicate that NO production during primary E. tenella infection may be only one of several immune responses and may not be the main anticoccidial effector.