Pregnancy failure following vaginal infection of sheep with Chlamydia psittaci prior to breeding.

Enzootic abortion in sheep, caused by Chlamydia psittaci, has been associated with pregnancy failure in most sheep-producing countries. Late-term abortions or the birth of weak low-birth-weight lambs occurred following primary C. psittaci infection in pregnant ewes. However, the mode by which C. psittaci can be transmitted among sheep has not been established. The present study was designed to determine whether the vaginal tracts of nonpregnant ewes were susceptible to C. psittaci infection and whether such infections had an impact during the next pregnancy. At day 0 of the estrus cycle, the vaginal tracts of 10 nonpregnant ewes were inoculated with C. psittaci and 10 ewes were exposed by subcutaneous injection. The ewes were bred 6 weeks postinfection. Five ewes from the vaginally infected group and four from the subcutaneously infected group were reinfected by subcutaneous injection at day 60 of gestation. Pregnancy outcomes and antibody responses to infection were compared with that of ewes that were infected with C. psittaci, either subcutaneously or intravaginally, for the first time during pregnancy and with that of noninfected control ewes. Subcutaneous infection of nonpregnant ewes did not cause subsequent pregnancy failure; rather, this provided protection against abortion following reinfection during pregnancy. As expected, abortions or the birth of weak lambs was observed in those ewes that received primary C. psittaci infection by either route during pregnancy. Similarly, abortion or the birth of weak lambs was a consequence of vaginal inoculation prior to breeding, thereby confirming the susceptibility of the vaginal mucosa to infection and demonstrating the potential for venereal transmission.