Infection with Chlamydia abortus (Enzootic abortion of ewes, ovine chlamydiosis)

Ewes show no illness before aborting, though behavioural change and a vulval discharge may appear in the last 48 hours. Abortion typically happens in the last 2-3 weeks of pregnancy, producing stillborn lambs and grossly inflamed, necrotic placentas. Other outcomes are full-term stillbirths, weak lambs that die within 48 hours, or a mix of dead, weak and healthy lambs in one litter. The placenta shows purulent to necrotising placentitis with vasculitis. A newly infected flock typically sees a few abortions in year one, then an abortion storm in year two.

Enzootic abortion of ewes is caused by Chlamydia abortus, a Gram-negative, obligate intracellular bacterium of the family Chlamydiaceae. The disease process starts at about day 90 of gestation, when the fetus is growing rapidly: the organism invades the placentomes and sets off a spreading, diffuse inflammation with thrombotic vasculitis and tissue necrosis. Milder changes appear in the fetal liver and lung, and severe placental damage can cause hypoxic brain injury in the fetus. Abortion is thought to result from a combination of impaired nutrient and gas exchange between dam and fetus, disturbed hormonal control of pregnancy and damaging cytokine responses.

Sheep and goats are mainly affected; cattle, pigs, horses and wild ruminants less often. Infected ewes release huge numbers of organisms in the placenta and uterine discharges at abortion or lambing, and animals become infected by swallowing or inhaling dust, feed or water contaminated with these materials. Losses are greatest where flocks are crowded at lambing. A clean flock is usually infected by bought-in replacements. In Eastern Saudi Arabia, true seroprevalence was about 11% in sheep and goats, every sampled flock had a seropositive animal, introducing new animals raised risk, and the authors recommend chlamydial vaccination alongside management measures. The infection is zoonotic: people catch it from abortion or birth products, and pregnant women are at special risk.

Toxoplasmosis also damages the placenta, but its necrosis is confined to the cotyledons, whereas C. abortus causes diffuse inflammation and necrosis. In stained smears C. abortus can resemble Coxiella burnetii (Q fever). Brucellosis, campylobacteriosis, listeriosis, salmonellosis, and abortion due to Chlamydia pecorum or Chlamydia psittaci are separated with agent-specific laboratory tests.

Placenta (placentomes, cotyledons), uterus, fetus (liver, lung, brain), reproductive tract

Inactivated and live vaccines reduce abortion and shedding but do not eradicate infection; avoid bringing in infected replacements; post-lambing serological flock screening; farm hygiene; pregnant women must avoid lambing flocks.

Ewes are not ill and usually do not abort again from this infection. Lamb losses are the main outcome; an abortion storm can affect around 30% of ewes in the second year after a flock is infected. Weak lambs rarely survive beyond 48 hours.

Wild ruminants