Response of ewes to temperature-sensitive mutants of Chlamydia psittaci (var ovis) obtained by NTG mutagenesis.

Two temperature-sensitive strains, 1B and 1H, of ovine Chlamydia psittaci obtained by mutagenesis were used as live vaccine: eleven weeks before breeding, 20 ewes received 3.9 X 10(6) PFU of 1B strain, and 20 others 3.5 X 10(6) PFU of 1H strain. The consequences of the vaccination were studied during pregnancy by recording CF antibody titer, chlamydial vaginal excretion and lambing performance. The vaccination did not disturb pregnancy and none of the ewes excreted chlamydiae at lambing. The immunity of the ewes was challenged one year after vaccination, by intradermic inoculation of 2.5 X 10(6) PFU of the virulent parental strain at 78 days of pregnancy, while nine out of ten control ewes excreted chlamydiae at lambing, none of the 16 pregnant ewes vaccinated with 1B strain, and only one the 18 pregnant ewes vaccinated with 1H strain did so. After further controls of safety and efficacy, 1B strain could provide a live vaccine against abortive chlamydiosis.