Enzootic Abortion of Small Ruminants

Enzootic Abortion of Small Ruminants

Dr Rick Last – Specialist Veterinary Pathologist

Vetdiagnostix – Veterinary Pathology Services


Enzootic abortion caused by Chlamydophila abortus is a major cause of bacterial abortion in sheep and goats worldwide.  Previously, it was believed that all the chlamydial diseases in sheep and goats, including abortion, polyarthritis, and conjunctivitis, were caused by Chlamydia psittaci and that the organism could also be found in the intestines of sheep with no clinical signs of disease. Chlamydophila abortus, previously Chlamydia psittaci immunotype / serovar 1, has now been shown to be a different and separate organism to Chlamydia psittaci. 

Therefore, Enzootic Abortion frequently occurs as a reproductive flock / herd disorder in the absence of other so-called Chlamydia “syndromes” such as conjunctivitis, polyarthritis, gastroenteritis, mastitis, meningoencephalitis.  It should be appreciated that ovine chlamydiosis is not a spectrum of diseases caused by a single bacterial agent but rather a group of different disease syndromes caused by different chlamydial species.  The new revised classification system has unravelled the mystery and confusion and allowed for clear classification of the disease syndromes caused by these Chlamydiales bacteria.


  • Chlamydophila abortus – Enzootic Abortion (Ovine Chlamydial Abortion, Ovine Chlamydiosis).
  • Chlamydophila pecorum – polyarthritis, conjunctivitis, pneumonia, metritis, encephalomyelitis and subclinical enteric infections.  Experimental studies with oral infection of pregnant ewes with C. pecorum failed to induce placentitis, fetal infection or abortion.
  • Chlamydophila psittaci – rarely associated with pneumonia, but usually in conjunction with other respiratory pathogens.  Not considered a primary lung pathogen of sheep or goats


Enzootic abortion (C.abortus) is characterised by rare fever, late abortions, stillbirths and weak lambs.  Initially abortion rates run at 25-60% for the first year or 2, but drops to 1-5% once the disease becomes endemic.  In an infected flock first and second lamb ewes are most susceptible.  Many authors claim that abortion always appears in the last weeks of gestation regardless of the moment of infection.  However, these assumptions are based on studies where ewes were infected after 60 days of gestation.  In a 1993 study by Papp et al – Can J Vet Res where ewes were infected at 50 days gestation 26% of these infected ewes experienced in apparent fetal loss before day 105 of gestation.  Possibly related to fetal immunity?  Therefore, C.abortus needs to be considered when investigating infertility in sheep flocks and goat herds.



Oral infection through contaminated feces or abortion products (fetus, placenta, birth fluids) is the primary route of infection with inhalation considered less common.  Following infection C.abortus colonize the intestine but are rarely associated with gastrointestinal signs.  These bacterial may then become established within the intestinal mucosa (1st carrier state).  In pregnant animals there is systemic spread from this intestinal mucosal site to the uterus and placenta.  Placental lesions usually take about 2 weeks to develop.  Initially there is a localized placentitis without fetal involvement and many abortions may occur at this stage, prior to fetal involvement.  In other instances this initial placentitis is followed by spread to the fetus causing generalized disease.

Animals which become infected and abort often maintain protective immunity explaining the significant drop off in abortion rate in endemically infected flocks / herds.  Rarely immune animals may shed bacteria in uterine discharges and placental tissues in subsequent pregnancies and this is a 2nd type of carrier state.


Gross pathology provides extremely good indicators for Chlamydophila abortus infection.  Placental pathology is pale characterised by soft thickening of the periphery of cotyledons (necrosis) with adherent cheese-like debris and in some instances hemorrhage.  Marked edema and opacity of the adjacent inter-cotyledonary areas accompany the cotyledonary changes.  Frequently only placental pathology is evident in the abcscence of any fetal pathology, highlighting the vital importance of always submitting placenta to the laboratory for abortion investigations.

Aborted foetuses are usually fresh with minimal post mortal autolysis.  Often there are no gross lesions but when they are present they include haemorrhages of the subcutis, thymus and lymphnodes.  There is usually an ascites with hepatic congestion and rarely military necrosis of the liver.  Mesenteric lymphadenopathy is a common and consistent foetal lesion when foetal infection has occurred.

Confirmatory Tests

  • Impression smears of placenta and abomasal fluid smears for Gimenez stains.
  • Placenta, lung, liver impression smears for Chlamydophila FAT stains or fresh on ice for C.abortus PCR.
  • Placenta (cotyledonary and inter-cotyledonary), liver, spleen, lymphnodes, lung and brain in
  • 10% buffered formalin for histopathology.
  • IHC staining for Chlamydophila antigen on formalin fixed tissues.
  • As C.abortus is ubiquitous in sheep and goats environment, antibody testing of dam sera is of little diagnostic use.  Foetal infections are often chronic and late term and so antibodies may be detected in fetal fluids.  However, this foetal serology has largely been superceeded by PCR and IHC.


  1. Buxton D et al. 2002.  Ovine chlamydial abortion: Characterization of the inflammatory immune response in placental tissues.  Journal of Comparative Pathology 127:133-141.
  2. Entrician G et al. 2001. Chlamydial infection in sheep: immune control versus fetal pathology.  Journal of the Royal Society of Medicine. 94:273-277.
  3. Njaa B L. 2012.  Diagnosis of abortion and neonatal loss in animals. 4th edn.  Wiley-Blackwell.
  4. OIE Terrestrial Manual 2012. Enzootic Abortion of Ewes.
  5. Papp et al. 1993. Chlamydia psittaci infection and associated infertility in sheep.  Canadian Journal of Veterinary Research. 57:185-189.


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