Chlamydia abortus and Ovine Enzootic Abortion (EAE): Diagnosis and Control
Etiology and Taxonomy
Chlamydia abortus is an obligate intracellular Gram-negative bacterium belonging to the family Chlamydiaceae, order Chlamydiales. The organism was formerly classified as Chlamydia psittaci serovar 1 before being reclassified as a distinct species based on genomic and phenotypic differences [1]. C. abortus is the primary etiological agent of ovine enzootic abortion (EAE), also known as enzootic abortion of ewes (EAE) or chlamydial abortion. The bacterium exhibits a biphasic developmental cycle alternating between infectious elementary bodies (EBs) and metabolically active reticulate bodies (RBs). EBs are the extracellular, spore-like form resistant to environmental degradation, while RBs are the intracellular replicative form found within membrane-bound inclusions in host cells. The genome of C. abortus is approximately 1.1 Mb and encodes a type III secretion system critical for host cell invasion and immune evasion.
Epidemiology and Host Range
C. abortus is a major cause of reproductive failure in sheep and goats worldwide. The pathogen is endemic in many regions including Europe, North America, North Africa, the Middle East, and Asia [2, 3, 4, 5, 6, 7]. Seroprevalence studies have demonstrated significant variation by breed, flock management, and geographic location. In Montana domestic rangeland sheep, seroprevalence varied by breed and herding practices [2]. In the Layyah district of Punjab, Pakistan, a prevalence of 18.5% was reported in sheep [3]. In North-Central Algeria, seroprevalence in small ruminants ranged from 12% to 28% depending on the region [4]. Tibetan sheep in Qinghai Province, China, showed a seroprevalence of 14.3% [6]. A six-year epidemiological study in Turkiye identified C. abortus in 8.2% of ovine and caprine fetuses submitted for abortion diagnosis [7]. In Spanish Mediterranean ecosystems, seroprevalence was detected in both domestic and wild ruminants, indicating potential wildlife reservoirs [5]. Zoo ungulates in Spain also demonstrated seropositivity, suggesting a broad host range [8]. A large-scale serosurvey in Ethiopian small ruminants identified chlamydiosis as one of several neglected zoonoses [9]. In Northern Cyprus, a preliminary study in abortion-inexperienced sheep populations reported a seroprevalence of 6.7% [10]. In Southern Benin, enzootic ovine abortion was confirmed in small ruminants with a prevalence of 11.3% [11]. Risk factors associated with seropositivity include flock size, introduction of new animals, contact with other ruminant species, and poor biosecurity practices [3, 4, 5].
Transmission occurs primarily via the oral route through ingestion of contaminated feed or water with aborted fetuses, fetal membranes, or vaginal discharges. Latent infections can reactivate during pregnancy, leading to placental colonization and abortion. The bacterium can also be shed in milk and feces. Venereal transmission from infected rams is possible but less common. The role of avian strains of C. abortus in livestock infections has been increasingly recognized, with evidence of spillover from birds to mammals [1, 12, 13].
Clinical Signs and Pathogenesis
The hallmark of C. abortus infection in sheep and goats is late-term abortion, typically occurring in the last two to three weeks of gestation. Abortion storms can affect up to 30% of naive ewes in a flock. Infected ewes may show no prodromal signs, but some exhibit vaginal discharge, placentitis, and retained fetal membranes. Live-born lambs from infected ewes are often weak and may die shortly after birth. In non-pregnant animals, infection is usually subclinical. The pathogenesis involves bacterial invasion of the placenta, specifically the trophoblast cells of the chorionic epithelium. The organism replicates within these cells, causing necrosis, inflammation, and disruption of fetal-maternal nutrient exchange. The immune response, particularly interferon-gamma and cell-mediated immunity, is critical for controlling infection. However, C. abortus can downregulate major histocompatibility complex (MHC) class I expression on infected cells, facilitating immune evasion.
Pathology
Gross pathological findings in aborted fetuses are often non-specific. The placenta is the primary target organ. Lesions include necrotic placentitis with thickened, edematous, and discolored cotyledons. Intercotyledonary areas may appear thickened and opaque. Fetal lesions may include subcutaneous edema, serosanguinous fluid in body cavities, and hepatomegaly. Histopathological examination reveals necrotizing and suppurative placentitis with infiltration of neutrophils and macrophages. Intracytoplasmic chlamydial inclusions can be visualized in trophoblast cells using modified Ziehl-Neelsen (MZN) staining or immunohistochemistry.
Diagnosis of Chlamydia abortus Ovine Enzootic Abortion Chlamydiosis EAE
Accurate and timely diagnosis is essential for implementing control measures. Diagnostic approaches include direct detection of the organism, serological testing, and molecular methods.
Sample Collection
Appropriate samples for diagnosis include fetal membranes (placenta), fetal tissues (liver, lung, spleen, abomasal contents), vaginal swabs from aborting ewes, and blood samples for serology. Samples should be collected aseptically and transported under cold conditions.
Direct Detection Methods
Modified Ziehl-Neelsen (MZN) Staining: Smears from placental cotyledons or vaginal swabs are stained with MZN. C. abortus EBs appear as small, red-stained coccoid bodies against a blue background. This method is rapid and inexpensive but has limited sensitivity and specificity.
Immunohistochemistry (IHC): IHC using monoclonal or polyclonal antibodies against C. abortus antigens can detect the organism in formalin-fixed, paraffin-embedded tissues. This method provides high specificity and allows visualization of the pathogen within lesions.
Antigen Detection ELISA: Commercial ELISA kits for direct detection of chlamydial lipopolysaccharide (LPS) or specific proteins in placental or swab samples are available. These assays offer moderate sensitivity and are useful for screening.
Molecular Diagnostics
Molecular methods, particularly real-time PCR (qPCR), have become the gold standard for diagnosis due to their high sensitivity, specificity, and rapid turnaround time.
Conventional PCR and Real-Time PCR: Several PCR assays targeting the ompA gene (encoding the major outer membrane protein, MOMP), the 16S rRNA gene, or the incA gene have been developed. A cross-European laboratory evaluation of commercial and in-house real-time PCR assays demonstrated high inter-laboratory agreement and diagnostic accuracy for detecting C. abortus in small ruminants [14]. These assays can differentiate C. abortus from other Chlamydiaceae species.
Metagenomic Next-Generation Sequencing (mNGS): mNGS has been increasingly used for the diagnosis of infectious diseases, including C. abortus infections. This unbiased approach can detect the pathogen directly from clinical samples without prior knowledge of the infectious agent. Several case reports have described the use of mNGS to diagnose C. abortus pneumonia in humans, highlighting its utility in complex or mixed infections [15, 16]. In veterinary settings, mNGS can be applied to abortion cases where conventional testing is negative or when co-infections are suspected.
Genotyping and Molecular Epidemiology: Molecular typing methods, such as multilocus sequence typing (MLST) and ompA genotyping, are used to characterize C. abortus strains for epidemiological investigations. These methods can identify sources of infection and track transmission patterns within and between flocks.
Serological Diagnosis
Serological tests detect antibodies against C. abortus and are used for flock-level screening and prevalence studies.
Complement Fixation Test (CFT): The CFT has been the traditional serological test for chlamydial infections. It detects antibodies against the genus-specific LPS antigen. However, CFT has limitations, including low sensitivity in early infection and cross-reactivity with other Chlamydiaceae.
Enzyme-Linked Immunosorbent Assay (ELISA): Commercial ELISA kits using recombinant antigens (e.g., MOMP, TARP, or IncA) offer improved sensitivity and specificity compared to CFT. These assays can differentiate between infected and vaccinated animals (DIVA capability) when subunit vaccines are used. The seroprevalence data from numerous studies have been generated using ELISA [2, 3, 4, 5, 10, 8, 6].
Indirect Immunofluorescence Assay (IFA): IFA is a sensitive method for detecting antibodies, but it is more labor-intensive and subjective than ELISA. It is primarily used as a confirmatory test.
Diagnostic Algorithm
The following Mermaid diagram illustrates a recommended diagnostic workflow for investigating ovine abortion suspected to be caused by C. abortus.
flowchart TD
A[Abortion Storm in Sheep/Goats], > B{Collect Samples}
B, > C[Placenta, Fetal Tissues, Vaginal Swab]
B, > D[Maternal Blood for Serology]
C, > E{Direct Detection}
E, > F[MZN Stain of Placental Smear]
F, > G{Positive?}
G, >|Yes| H[Presumptive Diagnosis]
G, >|No| I[Real-Time PCR for C. abortus]
I, > J{Positive?}
J, >|Yes| K[Confirmed Diagnosis]
J, >|No| L[Consider Other Pathogens]
D, > M[ELISA for Anti-C. abortus Antibodies]
M, > N{Positive?}
N, >|Yes| O[Flock-Level Exposure Confirmed]
N, >|No| P[Recent Infection or Naive Flock]
K, > Q[Implement Control Measures]
O, > Q
Treatment
Treatment of infected animals is generally not effective once abortion has occurred. Antibiotic therapy can be used to reduce shedding and prevent further abortions in at-risk pregnant ewes.
Tetracyclines: Long-acting oxytetracycline is the most commonly used antibiotic. A single intramuscular injection (20 mg/kg) can reduce the incidence of abortion if administered before the onset of clinical signs. However, treatment does not eliminate the carrier state.
Macrolides: Tylosin and tilmicosin have shown efficacy against C. abortus in vitro and in vivo. Their use may be considered in cases of tetracycline resistance or intolerance.
Fluoroquinolones: Enrofloxacin and danofloxacin have good intracellular penetration and activity against C. abortus. However, their use in pregnant animals should be carefully evaluated due to potential effects on fetal cartilage development.
Antimicrobial susceptibility testing is not routinely performed for C. abortus due to the difficulty of culturing the organism. However, resistance to tetracyclines has been reported in some strains, necessitating the use of alternative antibiotics.
Control and Prevention
Control of EAE relies on a combination of biosecurity, management practices, and vaccination.
Biosecurity and Management
Quarantine and Testing: New animals should be quarantined and tested for C. abortus before introduction to the flock. Serological testing (ELISA) and PCR on vaginal swabs can identify carriers.
Segregation of Aborting Ewes: Aborting ewes and their lambs should be isolated from the rest of the flock. Contaminated bedding, fetal membranes, and aborted fetuses should be removed and disposed of by incineration or deep burial.
Hygiene and Disinfection: C. abortus is susceptible to common disinfectants, including 1% bleach, 70% ethanol, and quaternary ammonium compounds. Thorough cleaning and disinfection of pens and equipment are essential.
Flock Management: Maintaining a closed flock or sourcing animals from known negative flocks reduces the risk of introduction. Avoiding contact with other ruminant species and wildlife can also reduce transmission.
Vaccination
Vaccination is the most effective long-term strategy for controlling EAE.
Live Attenuated Vaccines: A live attenuated vaccine (strain 1B) has been used for decades. It provides good protection against abortion but has several drawbacks. It can cause abortion if administered to pregnant ewes, and it can interfere with serological diagnosis. The vaccine strain can also be shed and potentially revert to virulence.
Inactivated Vaccines: Inactivated whole-cell vaccines are safer than live vaccines but generally induce a weaker immune response. They require adjuvants and booster doses.
Subcellular and Recombinant Vaccines: Recent research has focused on developing safer and more effective vaccines. Subcellular vaccines containing recombinant proteins (e.g., MOMP, TARP, or IncA) have shown promise in experimental challenge models. Studies have evaluated the protective efficacy of different antigen doses and adjuvant formulations [17, 18, 19]. A subcellular vaccine with decreasing antigen doses demonstrated dose-dependent protection in a pregnant sheep challenge model [18]. Another study evaluated the effects of different adjuvants on the protective efficacy of a subcellular vaccine [17]. Bacterial ghosts engineered with lipidated antigens have been developed as an adjuvant-free vaccine platform [20]. Surface display of cholera toxin B subunit on recombinant Escherichia coli ghosts further enhanced resistance to C. abortus infection in mice [21]. The role of the delivery route on vaccine-induced immune responses has been investigated in a murine model, showing that mucosal delivery can enhance genital tract immunity [22]. Overexpression of IL-10 has been shown to reduce the protective response of an experimental vaccine, highlighting the importance of the cytokine milieu in vaccine efficacy [23].
DIVA Capability: Subunit vaccines allow for differentiation between infected and vaccinated animals (DIVA), which is critical for serological surveillance and eradication programs.
Zoonotic Considerations
C. abortus is a zoonotic pathogen. Pregnant women are at particular risk, as infection can cause severe abortion or stillbirth. Veterinarians, farmers, and abattoir workers should take appropriate precautions when handling aborting ewes, fetal tissues, and placentas. The use of personal protective equipment (gloves, masks, and eye protection) is recommended. Several case reports have documented severe pneumonia caused by C. abortus in humans, including cases complicated by hemophagocytic syndrome, pneumomediastinum, and acute respiratory distress syndrome (ARDS) [24, 25, 12, 13, 26, 16, 27]. The emergence of avian-associated C. abortus strains as a cause of community-acquired pneumonia underscores the need for enhanced surveillance and awareness [1, 12, 13].
Conclusion
Chlamydia abortus remains a significant cause of ovine enzootic abortion (EAE) worldwide, with substantial economic and animal welfare impacts. Accurate diagnosis using molecular methods such as real-time PCR and mNGS is essential for timely intervention. Control strategies should integrate biosecurity, management, and vaccination. The development of safe and effective subunit vaccines with DIVA capability represents a major advance in the control of this disease. Continued surveillance and research are needed to address emerging challenges, including antimicrobial resistance and the zoonotic potential of this pathogen.
References
[1] Borel N, Longbottom D, Greub G, et al. Zoonotic infections due to avian Chlamydia abortus: what are we missing? Lancet Microbe. 2025. https://pubmed.ncbi.nlm.nih.gov/40848734/
[2] Simmons AJ, Borgogna JC, Roeder BL, et al. The Seroprevalence of Chlamydia abortus in Montana Domestic Rangeland Sheep Varies by Breed, Flock and Herding Practices. Vet Med Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/41930915/
[3] Baqir M, Lodhi LA, Aslam MA, et al. Prevalence and risk factors of ovine enzootic abortion caused by Chlamydia abortus in the Layyah district of Punjab, Pakistan. BMC Vet Res. 2025. https://pubmed.ncbi.nlm.nih.gov/41068903/
[4] Boukhalfa N, Douifi M. Prevalence and risk factors of Chlamydia abortus infection in small ruminants from North-Central Algeria. Comp Immunol Microbiol Infect Dis. 2025. https://pubmed.ncbi.nlm.nih.gov/41066919/
[5] Jiménez-Martín D, García-Bocanegra I, Jiménez-Ruiz S, et al. Seroepidemiological study of Chlamydia abortus in domestic and wild ruminants in Spanish Mediterranean ecosystems. Prev Vet Med. 2025. https://pubmed.ncbi.nlm.nih.gov/40554956/
[6] Lei F, Yu J, Niu Y, et al. Seroprevalence of Chlamydia abortus infection in Tibetan sheep in Qinghai Province, China. Acta Trop. 2025. https://pubmed.ncbi.nlm.nih.gov/40158645/
[7] Şevik M. A six-year epidemiological study of selected zoonotic abortifacient agents in ovine and caprine foetuses in Türkiye. Epidemiol Infect. 2024. https://pubmed.ncbi.nlm.nih.gov/39696935/
[8] Beato-Benítez A, García-Bocanegra I, Gonzálvez M, et al. Large-scale serosurvey of Chlamydia abortus in zoo ungulates in Spain. Res Vet Sci. 2025. https://pubmed.ncbi.nlm.nih.gov/40480044/
[9] Alemayehu G, Mamo G, Alemu B, et al. Prevalence and risk factors of neglected zoonoses in Ethiopian small ruminants: a focus on Q fever, brucellosis, chlamydiosis, and toxoplasmosis. One Health Outlook. 2026. https://pubmed.ncbi.nlm.nih.gov/41715195/
[10] Baloğlu H, Çolakoğlu HE, Cam NS. Preliminary seroprevalence study of zoonotic abortigenic agents in the abortion inexperienced sheep population in the Northern Cyprus. Vet Res Commun. 2025. https://pubmed.ncbi.nlm.nih.gov/40493315/
[11] Kifouli AH, Alowanou G, Challaton P, et al. Enzootic Ovine Abortion among small ruminants in Southern Benin. Vet Ital. 2025. https://pubmed.ncbi.nlm.nih.gov/39812231/
[12] Gooskens J, van Essen EHR, Kraakman MEM, et al. Community-Acquired Pneumonia Caused by Avian Chlamydia abortus, the Netherlands. Emerg Infect Dis. 2025. https://pubmed.ncbi.nlm.nih.gov/40023822/
[13] Raven S, Heijne M, Koomen J, et al. Circulation of avian Chlamydia abortus in the Netherlands and community-acquired pneumonia: an outbreak investigation and retrospective cohort study. Lancet Infect Dis. 2025. https://pubmed.ncbi.nlm.nih.gov/39426392/
[14] Schnee C, Laroucau K, Livingstone M, et al. Cross-European laboratory evaluation of commercial and in-house real-time PCR assays for the detection of Chlamydia abortus in small ruminants. Res Vet Sci. 2025. https://pubmed.ncbi.nlm.nih.gov/40120389/
[15] Liu L, Su P, Gong F, et al. Diagnosis and management of mixed Chlamydia abortus and psittaci pneumonia guided by metagenomic next-generation sequencing: a case report. BMC Infect Dis. 2026. https://pubmed.ncbi.nlm.nih.gov/42192344/
[16] Chen Y, Lan P, Liu L, et al. Prone positioning ventilation treatment rescuing a patient with chlamydia abortus-induced ARDS diagnosed by next generation sequencing: a case report. Front Med (Lausanne). 2024. https://pubmed.ncbi.nlm.nih.gov/39328319/
[17] Livingstone M, Aitchison K, Palarea-Albaladejo J, et al. Effects of Different Adjuvants on the Protective Efficacy of a Subcellular Vaccine Against Chlamydia abortus Infection in Sheep. Vaccines (Basel). 2025. https://pubmed.ncbi.nlm.nih.gov/40573940/
[18] Livingstone M, Aitchison K, Palarea-Albaladejo J, et al. Protective Efficacy of Decreasing Antigen Doses of a Chlamydia abortus Subcellular Vaccine Against Ovine Enzootic Abortion in a Pregnant Sheep Challenge Model. Vaccines (Basel). 2025. https://pubmed.ncbi.nlm.nih.gov/39852869/
[19] Livingstone M, Aitchison K, Palarea-Albaladejo J, et al. Evaluation of the Protective Efficacy of Different Doses of a Chlamydia abortus Subcellular Vaccine in a Pregnant Sheep Challenge Model for Ovine Enzootic Abortion. Animals (Basel). 2024. https://pubmed.ncbi.nlm.nih.gov/39457934/
[20] Zhang H, Li W, Li Y, et al. Bacterial ghosts engineered with lipidated antigens as an adjuvant-free vaccine for Chlamydia abortus. Int J Pharm. 2024. https://pubmed.ncbi.nlm.nih.gov/39368676/
[21] Zhang H, Li Y, Li W, et al. Surface Display of Cholera Toxin B Subunit Recombinant Escherichia coli Ghosts Further Enhances Resistance to Chlamydia abortus Infection in Mice. Microorganisms. 2024. https://pubmed.ncbi.nlm.nih.gov/39203498/
[22] Richardson S, Medhavi FNU, Tanner T, et al. Role of route of delivery on Chlamydia abortus vaccine-induced immune responses and genital tract immunity in mice. Microbes Infect. 2025. https://pubmed.ncbi.nlm.nih.gov/39645188/
[23] Del Río L, Salinas J, Ortega N, et al. IL-10 Overexpression Reduces the Protective Response of an Experimental Chlamydia abortus Vaccine in a Murine Model. Animals (Basel). 2024. https://pubmed.ncbi.nlm.nih.gov/39199857/
[24] Ding B, Xu Z, Li Y, et al. Chlamydia Abortus-Induced Pneumonia with Psychiatric Symptoms and Pneumomediastinum: A Case Report. Infect Drug Resist. 2025. https://pubmed.ncbi.nlm.nih.gov/41487436/
[25] Xu J, Zeng H, Li H, et al. Severe pneumonia caused by Chlamydia abortus complicated by hemophagocytic syndrome: a case report. Front Med (Lausanne). 2025. https://pubmed.ncbi.nlm.nih.gov/40134914/
[26] Wang M, Zhang B, Wan H, et al. Atypical pneumonia caused by Chlamydia abortus in HIV patient: a case report. BMC Pulm Med. 2024. https://pubmed.ncbi.nlm.nih.gov/39334069/
[27] Ye D, Li Y, Yan K, et al. A Case Study of Severe Pneumonia Caused by mixed Infection of Chlamydia Abortus and Influenza a in a Female Patient. Infect Drug Resist. 2024. https://pubmed.ncbi.nlm.nih.gov/39161470/
[28] Li H, Gao X, Jing X. Characterization of the vaginal bacterial microbiota of sheep exhibiting abortion or repeat breeder syndrome. Vet Anim Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/42211226/
[29] Chahota R, Gupta T, Gupta S, et al. Fatal encephalomyelitis outbreak among dairy cows caused by rabies virus phylogroup-1 with co-occurrence of Chlamydia abortus. Braz J Microbiol. 2026. https://pubmed.ncbi.nlm.nih.gov/42113068/
[30] Hossein E, Hasan G, Mona H, et al. Prevalence and Identification of Infectious Abortion Pathogens in Sheep Flocks of North Khorasan, Iran. Arch Razi Inst. 2025. https://pubmed.ncbi.nlm.nih.gov/41179643/
[31] Kifouly AH, Bekindaka NO, Ali KS, et al. First assessment of the health status of pregnant women, detection of prevalence and risk factors associated with enzootic ovine abortion disease among pregnant women in southern Benin. Front Public Health. 2025. https://pubmed.ncbi.nlm.nih.gov/40469588/
[32] Şevik M. Zoonotic Abortifacient Agents in Bovine Abortion: Diagnostic Assessment of 125 Cases (2015-2017). Vet Med Sci. 2025. https://pubmed.ncbi.nlm.nih.gov/40261042/
[33] Esmaeili H, Ghorani M, Hamidiya Z, et al. Causes of abortion in Iranian goat herds and associated risk factors. Prev Vet Med. 2025. https://pubmed.ncbi.nlm.nih.gov/39536431/
[34] Semango GP, Buza J. Review of the Current Status on Ruminant Abortigenic Pathogen Surveillance in Africa and Asia. Vet Sci. 2024. https://pubmed.ncbi.nlm.nih.gov/39330804/