Enzootic Abortion of Ewes (Ovine Enzootic Abortion) - Chlamydia abortus Infection

Enzootic abortion of ewes (EAE), also known as ovine enzootic abortion (OEA) or chlamydial abortion, is a highly contagious, infectious cause of late-term abortion, stillbirth, and the birth of weak lambs in sheep and goats. The disease is caused by the obligate intracellular bacterium Chlamydia abortus (formerly Chlamydophila abortus). It is one of the most important infectious causes of abortion in sheep and goats worldwide, leading to significant economic losses due to abortion storms, reduced lamb/kid crop, increased culling, and zoonotic risk to pregnant women. The disease primarily affects the placenta, causing placentitis and subsequent fetal death or premature birth. In naive flocks, abortion rates can reach 20-30%, with up to 60% of ewes aborting in severe outbreaks. The disease is endemic in many sheep-rearing regions, and control relies on vaccination, biosecurity, and management of infected animals. In goats, the disease is similar but may be less prevalent; however, it is also zoonotic, causing abortion in pregnant women. The disease is not typically associated with respiratory signs, but it is classified under infectious epidemic diseases due to its rapid spread and high impact on flock health.

The primary causative agent is Chlamydia abortus, a Gram-negative, obligate intracellular bacterium belonging to the family Chlamydiaceae. It has a unique biphasic life cycle with infectious elementary bodies (EBs) and metabolically active reticulate bodies (RBs). The organism primarily infects the placenta, where it multiplies in trophoblast cells, causing severe placentitis and necrosis. Other chlamydial species, such as Chlamydia pecorum, may occasionally cause abortion but are less common. The bacterium is shed in large numbers in vaginal discharges, aborted fetuses, placentas, and uterine fluids. It can survive in the environment for several weeks, especially in cool, moist conditions. The organism is resistant to freezing but is inactivated by heat (60°C for 30 minutes) and common disinfectants. Infection typically occurs via the oral route through ingestion of contaminated feed or water, or by inhalation of contaminated dust. After initial infection, the organism establishes a latent infection in the non-pregnant ewe, which reactivates during a subsequent pregnancy, leading to abortion. The incubation period is variable, but abortion typically occurs in the last 2-3 weeks of gestation (from 120 days to term).

Enzootic abortion of ewes is distributed worldwide, with higher prevalence in Europe, North America, and Australia. It is a major cause of abortion in sheep, and to a lesser extent, goats. The disease is more common in intensive and semi-intensive production systems where sheep are housed or closely confined, facilitating transmission. In sheep, the disease primarily affects ewes that have been infected for the first time during a previous pregnancy; these ewes develop immunity and do not abort in subsequent pregnancies, but they may shed the organism. However, in naive flocks, abortion storms can occur, affecting up to 30% of pregnant ewes. The disease is more prevalent in the last trimester of pregnancy, with most abortions occurring between 120 and 150 days of gestation. Breed susceptibility varies, with some breeds showing higher abortion rates. Age is a risk factor, with young ewes (2-3 years old) being more commonly affected, likely due to first exposure. In goats, the disease is similar but may be less severe. The economic impact includes loss of lambs/kids, reduced milk production, increased veterinary costs, and culling of affected animals. Zoonotic transmission to pregnant women can cause severe abortion and maternal illness, making the disease a public health concern.

The pathogenesis of Chlamydia abortus infection involves a complex interaction between the bacterium and the host's immune system. After oral or nasal exposure, the bacterium disseminates via the bloodstream to various organs, including the spleen, liver, and lymph nodes, where it establishes a latent infection. During subsequent pregnancy, the organism reactivates and spreads to the placenta via the bloodstream, likely within macrophages. The bacterium invades the trophoblast cells of the placenta, where it multiplies and causes severe necrotizing placentitis. The infection leads to inflammation, edema, and necrosis of the cotyledons and intercotyledonary areas, resulting in placental dysfunction. This impairs nutrient and oxygen transfer to the fetus, leading to fetal hypoxia, growth retardation, and eventually abortion or premature birth. The fetus is often expelled before or at the time of death, and the placenta is typically retained. The immune response, particularly cell-mediated immunity, plays a role in controlling the infection, but the bacterium has evolved mechanisms to evade the immune system, including modulation of apoptosis and cytokine production. The release of pro-inflammatory cytokines, such as tumor necrosis factor-alpha and interleukins, contributes to the inflammatory damage. In addition, the bacterium can cause systemic infection in pregnant women, leading to severe illness and abortion.

Several factors increase the risk of enzootic abortion of ewes. Intrinsic factors include age (young ewes are more susceptible), breed (some breeds may be more resistant), and parity (first pregnancy after infection is most likely to abort). Extrinsic factors include management practices such as introducing new animals into a flock without quarantine, mixing pregnant and non-pregnant animals, and poor hygiene in lambing pens. Overcrowding and poor ventilation in housing facilities facilitate the spread of the organism. Nutritional stress, such as inadequate energy or protein, may exacerbate the severity of infection. Concurrent infections, such as toxoplasmosis or border disease, can increase the risk of abortion. The presence of infected placentas and vaginal discharges in the environment is a major source of infection. In addition, the use of contaminated equipment or personnel can transmit the organism. Zoonotic transmission to pregnant women is a significant risk, especially for those handling aborted material or lambing ewes.

The most characteristic clinical sign is abortion in the last 2-3 weeks of gestation, typically between 120 and 150 days. Aborted fetuses are often fresh and may show no gross lesions, but the placenta is typically thickened, edematous, and has necrotic cotyledons with a reddish-brown or grayish appearance. Some ewes may give birth to weak, premature lambs that fail to thrive. In some cases, ewes may show a slight vaginal discharge before abortion, but systemic signs are usually absent. After abortion, ewes may have a persistent vaginal discharge for several days, which is highly infectious. In naive flocks, abortion storms can occur, with many ewes aborting within a short period. In goats, the clinical signs are similar, but abortion may occur earlier in gestation. Some animals may develop metritis or retained placenta, leading to secondary bacterial infections. In rare cases, pneumonia or polyarthritis may occur, but these are not typical. The disease is not associated with respiratory signs, but the classification in the respiratory and infectious diseases category is due to its epidemic nature.

Differential diagnoses for enzootic abortion of ewes include other causes of late-term abortion in sheep and goats. These include: 1) Toxoplasmosis (Toxoplasma gondii) - causes abortion at similar stages, but placental lesions are more focal and white spots (foci of necrosis) are seen on cotyledons; histopathology shows necrotic foci and protozoal cysts. 2) Border disease (pestivirus) - causes abortion, stillbirth, and hairy shaker lambs; diagnosis via virus isolation or PCR. 3) Q fever (Coxiella burnetii) - causes abortion and is zoonotic; diagnosis via serology or PCR. 4) Brucellosis (Brucella melitensis) - causes abortion in goats and sheep; diagnosis via serology and culture. 5) Campylobacteriosis (Campylobacter fetus) - causes abortion storms; diagnosis via culture of abomasal contents. 6) Listeriosis (Listeria monocytogenes) - causes abortion and encephalitis; diagnosis via culture. 7) Leptospirosis - causes abortion and systemic signs; diagnosis via serology. 8) Nutritional causes such as pregnancy toxemia - typically occurs in late gestation with metabolic signs, not infectious. 9) Other infectious agents like Salmonella abortusovis, Mycoplasma spp., and fungal infections. Definitive diagnosis requires laboratory testing, including PCR, ELISA, or culture of the organism from placenta or fetal tissues.

The diagnostic approach for enzootic abortion of ewes involves a stepwise process. First, obtain a thorough flock history, including abortion rate, stage of gestation, vaccination status, and recent introductions. Perform a physical examination of affected ewes, noting any vaginal discharge or retained placenta. Collect samples from aborted fetuses and placentas for laboratory analysis. The placenta should be examined grossly for characteristic lesions, and samples should be taken from cotyledons and intercotyledonary areas. Laboratory tests include: 1) Modified Ziehl-Neelsen (MZN) staining of placental smears to detect chlamydial elementary bodies (red-stained organisms). 2) PCR on placental tissue or vaginal swabs to detect Chlamydia abortus DNA. 3) ELISA for antibodies in serum, but serology is less useful in individual cases due to latent infections. 4) Culture of the organism in embryonated eggs or cell lines, but this is less commonly used. 5) Histopathology of the placenta to show necrotizing placentitis with intracytoplasmic inclusions. 6) Differential diagnosis to rule out other causes of abortion. In a flock outbreak, it is important to submit multiple samples to increase the chance of diagnosis. The diagnostic algorithm should also include a biosecurity assessment and review of management practices.

Laboratory findings in enzootic abortion of ewes include: 1) Placental smears stained with MZN show red-stained elementary bodies within trophoblast cells. 2) PCR of placental tissue or vaginal swabs is highly sensitive and specific for Chlamydia abortus. 3) Serology (ELISA) may show rising antibody titers in paired samples, but a single sample is not diagnostic due to latent infections. 4) Histopathology of the placenta reveals severe necrotizing placentitis with infiltration of neutrophils and macrophages, and intracytoplasmic chlamydial inclusions in trophoblasts. 5) Fetal tissues may show non-specific lesions, but the liver and spleen may have focal necrosis. 6) Complete blood count and serum biochemistry are usually unremarkable in the ewe. 7) In goats, similar findings are observed. 8) Other laboratory tests may be performed to rule out other causes of abortion, such as serology for toxoplasmosis, Q fever, and border disease. 9) Bacterial culture of the placenta may be attempted, but Chlamydia abortus is difficult to culture and requires specialized media. 10) In cases of zoonotic infection, PCR of human samples may be performed.

Imaging is not commonly used in the diagnosis of enzootic abortion of ewes, but ultrasonography can be used to assess fetal viability and detect signs of placentitis. In pregnant ewes, transabdominal ultrasonography may reveal fetal death, placental thickening, or separation of the placenta. However, these findings are non-specific and not diagnostic for chlamydial infection. In cases of retained placenta, ultrasonography may be used to evaluate the uterus for metritis. Radiography is rarely used, but it may be helpful in detecting fetal skeletal abnormalities in cases of abortion. Computed tomography (CT) is not used in routine diagnosis. In research settings, advanced imaging may be used to study placental pathology, but this is not practical in clinical practice. Therefore, imaging has a limited role in the diagnosis of enzootic abortion of ewes, and laboratory testing is the primary diagnostic tool.

Cytology and histopathology are important for confirming the diagnosis of enzootic abortion of ewes. Cytological examination of placental smears stained with MZN can reveal chlamydial elementary bodies as small red-stained coccoid organisms within trophoblast cells. Histopathology of the placenta is the gold standard for diagnosis. The characteristic lesion is a severe necrotizing placentitis, with extensive necrosis of the cotyledons and intercotyledonary areas. The chorionic epithelium is destroyed, and there is infiltration of neutrophils, macrophages, and lymphocytes. Intracytoplasmic chlamydial inclusions are often visible in trophoblast cells, especially in the early stages of infection. The fetal membranes are thickened and edematous. In the fetus, histopathology may show non-specific changes, such as congestion and focal necrosis in the liver and spleen. In cases of systemic infection, the lungs may show interstitial pneumonia. Immunohistochemistry using specific antibodies against Chlamydia abortus can be used to confirm the presence of the organism in tissue sections. Electron microscopy can also be used to visualize the elementary bodies and reticulate bodies, but this is not routinely performed.

Treatment of enzootic abortion of ewes is primarily aimed at controlling the outbreak and preventing further spread. Antibiotics such as oxytetracycline (long-acting) can be administered to pregnant ewes to reduce the severity of abortion and shedding, but they are not always effective in clearing the infection. The recommended dose for sheep is 20 mg/kg of long-acting oxytetracycline administered subcutaneously, repeated after 10-14 days. However, treatment is most effective when given early in the outbreak, before abortion occurs. In goats, similar protocols are used. In addition, supportive care for affected ewes includes ensuring adequate nutrition and hygiene. Aborted ewes should be isolated from the rest of the flock, and contaminated areas should be cleaned and disinfected. Vaccination is the most effective control measure, with inactivated vaccines available for sheep and goats. Vaccination should be administered to replacement ewes before breeding, and annual boosters are recommended. In an outbreak, vaccination of all pregnant ewes may be considered, but it is less effective in already infected animals. There is no specific treatment for the fetus, and aborted lambs are usually non-viable. Zoonotic precautions should be taken to protect pregnant women from exposure.

The prognosis for individual ewes with enzootic abortion is generally good, as most ewes recover and develop immunity. However, the prognosis for the fetus is poor, as abortion is usually fatal. In a flock, the prognosis depends on the control measures implemented. With appropriate vaccination and management, the abortion rate can be reduced in subsequent years. However, the disease can become endemic in a flock, leading to ongoing losses. The economic impact can be significant, especially in naive flocks with high abortion rates. In goats, the prognosis is similar. Zoonotic infection in pregnant women can be severe, with a high risk of abortion and maternal illness, so prompt medical attention is essential. Overall, the long-term prognosis for the flock is favorable with proper control measures, but the disease can be challenging to eradicate once established.

Follow-up for enzootic abortion of ewes involves monitoring the flock for subsequent abortions and implementing control measures. After an outbreak, all aborted ewes should be identified and isolated. The flock should be vaccinated annually, and replacement animals should be vaccinated before breeding. Biosecurity measures should be enhanced, including quarantine of new animals and strict hygiene in lambing areas. Regular monitoring of abortion rates is important to detect any recurrence. In addition, serological monitoring can be used to assess the level of exposure in the flock. In endemic flocks, vaccination is the mainstay of control. It is also important to educate farm personnel about the zoonotic risk and the importance of personal hygiene. Follow-up should include a review of management practices and a plan for future prevention. In cases of zoonotic infection, medical follow-up is necessary.

Clinical pearls: 1) Enzootic abortion of ewes should be suspected in any flock with late-term abortions, especially if the placenta is thickened and necrotic. 2) Modified Ziehl-Neelsen staining of placental smears is a rapid and inexpensive diagnostic test. 3) Vaccination is the most effective control measure, and it should be given to replacement ewes before breeding. 4) Pregnant women should avoid contact with aborting ewes and contaminated materials. 5) Long-acting oxytetracycline can reduce shedding if given early in an outbreak. Pitfalls: 1) Failure to submit placental samples for laboratory testing can lead to misdiagnosis. 2) Serology is not reliable for individual diagnosis due to latent infections. 3) Antibiotic treatment is not a substitute for vaccination. 4) Inadequate biosecurity can lead to recurrence. 5) Zoonotic risk is often underestimated, leading to human infections.

Current drug protocols for enzootic abortion of ewes include: 1) Long-acting oxytetracycline: 20 mg/kg subcutaneously, repeated after 10-14 days. Withdrawal times: meat 28 days, milk 7 days (check label). 2) Inactivated Chlamydia abortus vaccine: 1 mL subcutaneously, given 4 weeks before breeding, annual booster. No withdrawal time. 3) Supportive care: fluids and electrolytes if needed. 4) In goats, similar protocols. 5) For zoonotic exposure, doxycycline 100 mg twice daily for 7-14 days in non-pregnant women; in pregnant women, erythromycin or azithromycin may be used under medical supervision. 6) Always follow label instructions and consult a veterinarian for specific dosages and withdrawal times.

Evidence-based literature supports the use of vaccination as the most effective control measure for enzootic abortion of ewes. A systematic review by Longbottom and Coulter (2003) highlighted the efficacy of inactivated vaccines in reducing abortion rates. Studies have shown that vaccination can reduce abortion rates by up to 90% in affected flocks. Antibiotic treatment with oxytetracycline has been shown to reduce shedding but not eliminate infection. A study by Gutierrez et al. (2011) demonstrated that long-acting oxytetracycline given at 20 mg/kg reduced abortion rates when administered early in an outbreak. The use of PCR for diagnosis is well-established, with high sensitivity and specificity. The zoonotic risk is well-documented, and guidelines from public health agencies recommend precautions for pregnant women. Overall, the literature emphasizes the importance of vaccination, biosecurity, and early diagnosis in controlling the disease.