Ovine enzootic abortion: experimental studies of immune responses

Eight pregnant and four non-pregnant ewes were exposed to experimental oral infection with the chlamydial agent of ovine enzootic abortion. Four non-pregnant ewes received a sham inoculum. Immune responses were investigated using a complement fixation test (CFT), a delayed-type hypersensitivity (DTH) skin test and a lymphocyte transformation assay (LTA). When these responses were correlated with clinical observations it was concluded that fetopathy occurred in those ewes which failed to mount or mounted only weak DTH responses. High antibody titres were not an indication of protective immunity; in pregnant animals there was evidence of an inverse correlation between DTH and humoral responses as detected by the CFT. In the LTAs, chlamydia specific blastogenesis was recorded in all infected sheep but there was little correlation with clinical findings.