Responses of chickens to a recombinant refractile body antigen of Eimeria tenella administered using various immunizing strategies.

A refractile body antigen (designated SO7') found in sporozoites of Eimeria tenella was administered to chickens in various immunizing forms to assess its ability to protect against virulent challenge. These included native antigen from the parasite (viable oocysts; per os), recombinant antigen (CheY-SO7'; s.c.), and naked DNA immunization (pcDNA3-SO7'; i.m.). Male White Leghorn chicks were inoculated with oocysts or injected with one of the forms of SO7' antigen at 1 wk of age, boosted at 3 wk of age with the same treatment, and subsequently challenged at 5 wk of age with oocysts of Eimeria tenella USDA Strain 80. Seven days postchallenge, chicks were killed, and weight gains and lesion scores were determined and compared with those of the controls. Significant protection against cecal lesions and weight loss was noted in birds inoculated with oocysts or injected with pcDNA3-SO7' (25 microg). Birds injected with other doses of pcDNA3-SO7' (12.5, 50, 60, and 100 microg) showed no reduction in cecal lesions and maintained poor rates of growth similar to controls. The recombinant antigen (CheY-SO7') was also nonprotective. These results offer promise to the use of DNA immunization strategies for controlling avian coccidiosis and show for the first time naked DNA vaccination with a single coccidial antigen that protects chickens (as measured by reduced lesion scores and high rate of growth) against cecal coccidiosis.