Chlamydial or enzootic abortion is a zoonotic disease caused by the bacterium Chlamydia abortus (C abortus). This is one of the most common infectious causes of abortion and the birth of weak lambs. C abortus strains are widespread among ruminants and have been found to be associated with abortion in horses, rabbits, guinea pigs, mice, pigs and humans.

Causative agent

C abortus is an obligate intracellular gram-negative bacterium belonging to the family Chlamydiaceae. 


The disease has been reported in herds of sheep and goats in Europe, North America, Africa, and Iran. 


Ovine enzootic abortion leads to important economic losses worldwide. Abortion rates can reach 40% of pregnant animals. This high percentage of abortions in ewes is accompanied by a high mortality rate of lambs and a reduction in milk production. In addition, C abortus can also be a cause of infertility and affect early lamb development.


Infection usually occurs via the oral-nasal route through the ingestion of chlamydia present in contaminated water or food, or through the licking and ingestion of placental residues. After the abortion, ewes continue to excrete the organism occasionally during births and oestrus in the following three years. This favours the maintenance and spread of the disease in the affected flock and making its eradication difficult.

Clinical signs

Abortion is the most evident clinical sign of infection by C abortus. It occurs in the last 2 to 3 weeks of pregnancy. Animals that have been infected before pregnancy show no clinical signs of infection. Infected ewes remain latent carriers of the infection, until the microorganism reactivates in the next pregnancy.


Vaccination can help to control the disease, but variable efficacy values have been described, possibly associated with factors related to both the host and the vaccine.

Currently, two types of vaccine (inactivated and attenuated live vaccines) are available commercially.

A single administration of live vaccine to the females prior to breeding has been proven to reduce abortion and shedding for up to 3 years. However, live vaccines cannot be used in pregnant animals and it also poses safety concerns.

By contrast, the inactivated vaccines are safe for administration during pregnancy. However, they do not completely prevent bacterial shedding and reproductive failures may still occur. In addition, their efficacy is highly variable, with reports of occasional enzootic abortion outbreaks in vaccinated flocks.

 For further information, view our Small Ruminant Reproductive Multivalent Vaccine programme.