Chlamydial (enzootic) abortion is one of the two most commonly diagnosed causes of ovine abortion in the UK, which costs the sheep industry about £15 million per year. 

Ewes become infected by ingestion or inhalation of the bacterium Chlamydophila abortus from an environment contaminated by infected placentae or vaginal discharge. 

The outcome following infection is dependant on the ewe's immune status and stage of pregnancy. 

• ewes which have previously aborted due to chlamydial abortion are immune to further chlamydial abortions, but may continue to shed C. abortus in vaginal discharges following subsequent lambings

• infection of susceptible ewes during the first half of pregnancy sometimes results in abortion during the final three weeks of the same pregnancy  (in practice this is uncommon, other than in flocks with both early and late lambing ewes)

• infection of susceptible ewes during late pregnancy or when not pregnant usually results in abortion during the final three weeks of the subsequent pregnancy

• surviving ewe lambs born to aborted ewes, or normal ewe lambs born to ewes which aborted during a previous season, become infected through licking/inhaling placental fluids/vaginal discharges on their coats and abort during their first pregnancy 

• rams and wethers are not involved in transmission of infection

Chlamydial abortion typically occurs in the final 3 weeks of pregnancy.