# Enzootic abortion of ewes
**Ovine chlamydiosis** is an infection of domestic sheep and goats by the bacterium *Chlamydia abortus*, best known as a cause of late-pregnancy abortion called enzootic abortion of ewes (EAE) or ovine enzootic abortion (OEA).[1](https://www.woah.org/en/disease/ovine-chlamydiosis/) Abortion typically occurs in the last 2 to 3 weeks of pregnancy and produces stillborn lambs and inflamed placentas, and the organism can also infect humans, with pregnant women at particular risk.[1](https://www.woah.org/en/disease/ovine-chlamydiosis/) The World Organisation for Animal Health (WOAH) lists the infection among its Listed diseases for sheep and goats, with a dedicated chapter in the Terrestrial Animal Health Code.[2](https://www.woah.org/fileadmin/Home/eng/Health_standards/tahc/2024/en_chapitre_chlamydophila_abortus.htm)
| Key fact | Detail |
|---|---|
| Cause | *Chlamydia abortus*, an obligate intracellular bacterium, formerly classified as *Chlamydia psittaci* immunotype 1 and then *Chlamydophila abortus*[1](https://www.woah.org/en/disease/ovine-chlamydiosis/) • [3](https://www.msdvetmanual.com/reproductive-system/abortion-in-large-animals/abortion-in-sheep?content=summary) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC11545194/) |
| Other names | Enzootic abortion of ewes (EAE); ovine enzootic abortion (OEA)[1](https://www.woah.org/en/disease/ovine-chlamydiosis/) |
| Typical presentation | Abortion in the last 2–3 weeks of pregnancy, stillborn lambs, and inflamed placentas; weak lambs may die within 48 hours of birth[1](https://www.woah.org/en/disease/ovine-chlamydiosis/) |
| Latent course | Infection acquired by ingestion stays latent until conception; ewes exposed late in pregnancy may not show signs until the following pregnancy[2](https://www.woah.org/fileadmin/Home/eng/Health_standards/tahc/2024/en_chapitre_chlamydophila_abortus.htm) |
| Diagnosis | History of late abortion, purulent to necrotising placentitis with vasculitis, and demonstration of large numbers of the organism by quantitative PCR, antigen tests, or stained smears[5](https://www.woah.org/app/uploads/2024/10/202409-bsc-report-sept-public-md.pdf) |
| Zoonotic risk | Humans are infected from products of abortion or parturition; in pregnant women the disease can be severe and life-threatening[1](https://www.woah.org/en/disease/ovine-chlamydiosis/) • [6](https://www.gov.uk/guidance/chlamydophila-abortus) |
| Vaccines | Inactivated and live attenuated vaccines prevent abortion and reduce excretion but assist control without eradicating infection[1](https://www.woah.org/en/disease/ovine-chlamydiosis/) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC11545194/) |
## Clinical disease
The hallmark of ovine chlamydiosis is abortion in late pregnancy. Infected ewes usually show no clinical illness beforehand; behavioral changes and a vulval discharge can be observed in the last 48 hours of pregnancy, but there are rarely any predictive signs that abortion is coming.[1](https://www.woah.org/en/disease/ovine-chlamydiosis/) [Infection](https://www.edgepedia.org/infection) can also result in full-term stillborn lambs or weak lambs that do not survive longer than 48 hours, and in multiple births an infected ewe may produce a mix of dead, weak, and healthy lambs.[1](https://www.woah.org/en/disease/ovine-chlamydiosis/)
The placenta shows the characteristic damage. Gross lesions include placentitis with necrotic, reddish-brown cotyledons and thickened brown intercotyledonary areas covered by exudate; aborted fetuses are typically fresh, with minimal autolysis, and occasionally show ascites, lymphadenopathy, or liver congestion.[3](https://www.msdvetmanual.com/reproductive-system/abortion-in-large-animals/abortion-in-sheep?content=summary) Infection of ewes in early or mid-gestation results in abortion in the final 2 to 3 weeks of gestation or the birth of stillborn or weak lambs that frequently die within the first few days.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC7150237/)
A ewe that has aborted seldom aborts again, but she remains persistently infected and can still transmit the organism.[3](https://www.msdvetmanual.com/reproductive-system/abortion-in-large-animals/abortion-in-sheep?content=summary) Chlamydial abortion occurs to a similar extent in goats, and less frequently affects cattle, pigs, horses, and wild ruminants.[1](https://www.woah.org/en/disease/ovine-chlamydiosis/)
## Transmission and latency
Susceptible animals become infected through ingestion of infectious materials.[2](https://www.woah.org/fileadmin/Home/eng/Health_standards/tahc/2024/en_chapitre_chlamydophila_abortus.htm) The primary infection is thought to become established first in the tonsil, from which it is disseminated by blood or lymph to other organs, although the site of latent infection is not definitely known.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC7150237/) • [8](https://pmc.ncbi.nlm.nih.gov/articles/PMC1281521/) In lambs and non-pregnant ewes the infection remains latent until conception, and ewes exposed to infection late in pregnancy may not exhibit signs until the subsequent pregnancy, a pattern that shapes how the disease moves through a flock.[2](https://www.woah.org/fileadmin/Home/eng/Health_standards/tahc/2024/en_chapitre_chlamydophila_abortus.htm)
The infected placenta and uterine discharges are the most potent sources of the organism.[6](https://www.gov.uk/guidance/chlamydophila-abortus) The risk of transmission through vaginal fluids falls 2 to 3 weeks after an abortion, when the discharges begin to dry and stop, although biosecurity measures remain necessary.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC9152823/) Ruminants can also be infected by ingesting grass or pasture contaminated with abortion products, and *C. abortus* has been identified in conjunctival swabs from ruminants, indicating that other organs may carry the bacteria and possibly contribute to transmission between species.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC9152823/) Persistently infected ewes shed the organism from the reproductive tract for 2 to 3 days before and after ovulation, and rams can be infected and transmit it venereally.[3](https://www.msdvetmanual.com/reproductive-system/abortion-in-large-animals/abortion-in-sheep?content=summary)
## Diagnosis
Confirming that an animal is infected with *C. abortus* requires three things: that abortions, typically during late gestation, have occurred in the flock or herd of sheep or goats; that the placenta shows necrotising to purulent inflammation of its vessels and tissues; and that large quantities of the bacterium are detected in diseased placentas, whether through quantitative polymerase chain reaction (PCR), antigen-based assays, or stained smear preparations.[5](https://www.woah.org/app/uploads/2024/10/202409-bsc-report-sept-public-md.pdf) The still moist fleece of fetuses, abomasal contents, or vaginal swabs from females that have freshly aborted are also useful samples.[5](https://www.woah.org/app/uploads/2024/10/202409-bsc-report-sept-public-md.pdf) Chlamydial elementary bodies can be seen in Gimenez- or Giemsa-stained smears of placenta or vaginal discharge, and a humoral antibody response can be detected after abortion.[3](https://www.msdvetmanual.com/reproductive-system/abortion-in-large-animals/abortion-in-sheep?content=summary) • [5](https://www.woah.org/app/uploads/2024/10/202409-bsc-report-sept-public-md.pdf)
Two differential diagnoses matter in practice. Cotyledonary damage caused by *Toxoplasma gondii* must be distinguished from chlamydial lesions, and in stained smears *C. abortus* is morphologically similar to *Coxiella burnetii*, the agent of [Q fever](https://www.edgepedia.org/q-fever), which also occasionally causes abortion in sheep.[3](https://www.msdvetmanual.com/reproductive-system/abortion-in-large-animals/abortion-in-sheep?content=summary) • [5](https://www.woah.org/app/uploads/2024/10/202409-bsc-report-sept-public-md.pdf) Definitive diagnosis is based on identification of the bacterium by ELISA, fluorescent antibody staining, PCR assay, or isolation.[3](https://www.msdvetmanual.com/reproductive-system/abortion-in-large-animals/abortion-in-sheep?content=summary) As a flock-level rule of thumb, an infectious cause should be expected when more than 2% of the flock aborts.[10](https://www.vet.cornell.edu/animal-health-diagnostic-center/programs/nyschap/modules-documents/infectious-causes-abortions-sheep)
## Prevention and control
To manage the disease within an infected flock, the veterinarian-directed response involves separating sick ewes and their lambs from the rest of the group and giving oxytetracycline of the long-acting form, or tetracycline by mouth, to ewes that have been exposed.[3](https://www.msdvetmanual.com/reproductive-system/abortion-in-large-animals/abortion-in-sheep?content=summary) Commercial vaccines are widely used across many countries: inactivated products include Mydiavac, Chlamysure, and INMEVA, and live vaccines contain the attenuated temperature-sensitive *C. abortus* 1B strain, marketed as Enzovax and CEVA Chlamydia.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC11545194/) Vaccines have been reported to prevent abortion and to reduce excretion; they assist control of the disease but will not eradicate it.[1](https://www.woah.org/en/disease/ovine-chlamydiosis/) One caution from the field is that an abortion storm mimicking natural wild-type infection has been induced by the live 1B vaccine strain in a vaccinated flock.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC11545194/)
The WOAH Terrestrial Code frames trade measures around the disease's latent course. Importing countries are advised to require certification that breeding sheep and goats have remained in establishments free of EAE, shown no clinical signs on the day of shipment, and tested negative within 30 days before shipment; flock freedom requires official veterinary surveillance, two years without clinical evidence, negative diagnostic tests in a statistically valid sample of animals over six months of age, permanent identification, and controlled introduction of new animals.[2](https://www.woah.org/fileadmin/Home/eng/Health_standards/tahc/2024/en_chapitre_chlamydophila_abortus.htm)
## Public health significance
Because the *C. abortus* infection that produces chlamydiosis in sheep and goats can pass to humans, the bacterium should only be handled while observing suitable biosafety measures.[5](https://www.woah.org/app/uploads/2024/10/202409-bsc-report-sept-public-md.pdf) Human infection may be acquired from infected products of abortion or parturition or from carelessly handled laboratory cultures, with manifestations ranging from subclinical infection to acute influenza-like illness.[1](https://www.woah.org/en/disease/ovine-chlamydiosis/) The route of transmission to humans is uncertain, but inhaling aerosols and dusts heavily contaminated with the organism is the likely route; the risk is limited mainly to people actively working with sheep, including veterinary surgeons, and their immediate families, and indoor lambing may carry greater exposure than open pastures.[6](https://www.gov.uk/guidance/chlamydophila-abortus)
Pregnant women are at special risk and should not be exposed to sources of infection. Authenticated cases of human placentitis and abortion caused by *C. abortus* of ovine or caprine origin have been documented.[1](https://www.woah.org/en/disease/ovine-chlamydiosis/) During pregnancy, the chief consequences are serious maternal illness that can at times endanger the mother's life, together with miscarriage or stillbirth; when a pregnancy makes it through the acute infection, no long-term difficulties seem to follow, and nothing indicates the infection produces congenital malformation.[6](https://www.gov.uk/guidance/chlamydophila-abortus) Veterinary or husbandry care around abortion, stillbirth, or foaling in horses, in which *C. abortus* is present in equine fetal membranes, should also be considered a potential zoonotic risk.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC9152823/)
## Distribution and surveillance
EAE occurs worldwide except in Australia and New Zealand, and it is most important in intensively managed sheep.[3](https://www.msdvetmanual.com/reproductive-system/abortion-in-large-animals/abortion-in-sheep?content=summary) Australia reports the infection as free, with no occurrence on record.[11](https://animalhealthaustralia.com.au/wp-content/uploads/dlm_uploads/2026/05/AHiA-Annual-Report-2025.pdf) A systematic review of seroprevalence in small ruminants found the average prevalence of *C. abortus* infection highest in Asia at 48.5% and lowest in North America at 3.0%, and emphasized the global paucity of prevalence data for sheep and goats.[12](https://revistas.ufpr.br/veterinary/article/view/90853)
The disease's international standing has been actively updated. The Code chapter was first adopted in 1992 and most recently updated in 2019, while the corresponding Terrestrial Manual chapter was last adopted in 2018; a WOAH case definition for infection with *C. abortus* was developed between April and July 2025.[13](https://www.woah.org/app/uploads/2025/10/a-scad-report-sep-2025-final2.pdf)
## History and nomenclature
An abortifacient chlamydial disease of sheep was first reported by Greig in 1936 and was later described by Stamp and colleagues as enzootic abortion of ewes, caused by an organism of the psittacosis–lymphogranuloma venereum–trachoma group.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC11545194/) The accepted nomenclature has changed repeatedly, from *Chlamydia psittaci* immunotype 1 to *Chlamydophila abortus* and finally back to *Chlamydia abortus*, and recent studies have proposed two subtypes under this designation, a mammalian and an avian *C. abortus*.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC11545194/) A 1998 review of ovine chlamydial abortion traced how ribosomal gene analysis identified nine chlamydial genotypic groups in two lineages, and described the role of latent infection in transmission and the kinetics of placental colonization that lead to abortion in ruminants.[14](https://pubmed.ncbi.nlm.nih.gov/9689742/)
## References
1. [Ovine chlamydiosis, World Organisation for Animal Health (WOAH)](https://www.woah.org/en/disease/ovine-chlamydiosis/)
2. [Terrestrial Animal Health Code, Chapter 14.4: Infection with Chlamydia abortus (Enzootic abortion of ewes, ovine chlamydiosis), WOAH](https://www.woah.org/fileadmin/Home/eng/Health_standards/tahc/2024/en_chapitre_chlamydophila_abortus.htm)
3. [Abortion in Sheep: Chlamydiosis (Enzootic Abortion of Ewes), MSD Veterinary Manual](https://www.msdvetmanual.com/reproductive-system/abortion-in-large-animals/abortion-in-sheep?content=summary)
4. [A Review on Chlamydial Diseases in Animals (Chlamydiosis in Animals), PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC11545194/)
5. [Annex 29, Chapter 3.8.5 Enzootic Abortion of Ewes (Ovine Chlamydiosis), WOAH Biological Standards Commission report, September 2024](https://www.woah.org/app/uploads/2024/10/202409-bsc-report-sept-public-md.pdf)
6. [Chlamydia abortus: epidemiology, transmission and prevention, GOV.UK](https://www.gov.uk/guidance/chlamydophila-abortus)
7. [Diseases Primarily Affecting the Reproductive System, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7150237/)
8. [Chlamydial infection in sheep: immune control versus fetal pathology, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC1281521/)
9. [Recent advances and public health implications for environmental exposure to Chlamydia abortus: from enzootic to zoonotic disease, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC9152823/)
10. [Infectious Causes of Abortions in Sheep, Cornell University College of Veterinary Medicine](https://www.vet.cornell.edu/animal-health-diagnostic-center/programs/nyschap/modules-documents/infectious-causes-abortions-sheep)
11. [Animal Health in Australia 2025 Annual Report, Animal Health Australia](https://animalhealthaustralia.com.au/wp-content/uploads/dlm_uploads/2026/05/AHiA-Annual-Report-2025.pdf)
12. [Ovine enzootic abortion disease seroprevalence in small ruminants around the world: a systematic review, Archives of Veterinary Science](https://revistas.ufpr.br/veterinary/article/view/90853)
13. [Report of the WOAH Scientific Commission for Animal Diseases, September 2025, including the case definition for infection with Chlamydia abortus](https://www.woah.org/app/uploads/2025/10/a-scad-report-sep-2025-final2.pdf)
14. [Rodolakis A, Salinas J, Papp J (1998). Recent advances on ovine chlamydial abortion. Veterinary Research 29(3-4):275-88](https://pubmed.ncbi.nlm.nih.gov/9689742/)
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*Topic: Encyclopedia › Life and health › Applied biology and nonhuman health › Veterinary medicine and animal health › Animal disease and health › Zoonoses and veterinary public health › Occupational zoonotic risk in animal work*
*Initially written Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license