Chlamydiosis is a highly contagious infection that commonly occurs in sheep flocks of the western U.S. However, because laboratory confirmation of chlamydial abortion can be difficult, the true incidence is not easy to quantify and many infections are not reported. The organism that causes chlamydiosis is Chlamydia psittaci. Among flocks where the disease has been present for several years, most of the older ewes are immune and the annual abortion rate is around 1 to 5 percent (primarily due to ewe lambs and yearling ewes aborting during their first pregnancy). However, in recently infected flocks, or among nonimmune ewes recently introduced to an infected flock, the abortion rate can be as high as 30 percent. Lambs born weak at birth also are common.

The infective organism is excreted in high numbers in the aborted fetal membranes and fluids. Transmission often occurs when susceptible ewes lick the aborted fetus or consume feed or water that is contaminated by the aborted fluids or tissues. Once ingested, the infective agent incubates and can cause abortion within about 60 to 90 days. Ewes that become infected late in gestation may have weak lambs. Nonpregnant ewes and lambs and ewes in the final stages of gestation may become infected and harbor the organism until the next pregnancy, when they abort. Aborting ewes generally do not get sick unless secondary infection occurs.

Treatment of chlamydiosis usually is not an issue because ewes normally show no signs prior to abortion. However, prevention depends on interrupting the infective cycle. Removing afflicted ewes from the flock for several days after abortion and discarding the aborted fetus and fluids will lower the level of contamination. Additionally, feeding farm flock sheep in feeders may prevent them ingesting feed that has been contaminated on the ground. Vaccinating ewes will increase their disease resistance, but the resistance is neither absolute nor long-lasting. In flocks infected with Chlamydia psittaci, adding tetracyclines to feed for daily consumption throughout the lambing period may reduce the incidence of abortion. In flocks that experience some chlamydial abortions annually, prophylactic administration of tetracyclines in the feed during gestation may be beneficial.