Ovine and Caprine Brucellosis (Brucella melitensis & Brucella ovis)

Ovine and caprine brucellosis is a chronic, contagious, and zoonotic bacterial disease of sheep and goats caused primarily by Brucella melitensis (biotypes 1-3) and, in rams, by Brucella ovis. The disease is characterized by abortion storms in late gestation (usually the last trimester), stillbirths, weak lambs/kids, retained placenta, and reduced fertility. In rams, B. ovis causes epididymitis, orchitis, and seminal vesiculitis, leading to decreased libido and infertility. The disease has significant economic impact due to reproductive losses, culling of infected animals, trade restrictions, and public health concerns, as B. melitensis is highly zoonotic, causing undulant fever in humans. Brucellosis is a notifiable disease in many countries, and control programs focus on vaccination, test-and-slaughter, and biosecurity. The disease is endemic in Mediterranean countries, the Middle East, Central Asia, and parts of Africa and Latin America, while B. ovis is more widespread in sheep-raising areas worldwide, including Australia, New Zealand, North America, and Europe.

The primary causative agents are Brucella melitensis (a Gram-negative, facultative intracellular coccobacillus) and Brucella ovis (a rough colony variant that is non-zoonotic). B. melitensis has three biotypes (1, 2, 3) and is the most pathogenic for goats and sheep, causing severe abortion storms. B. ovis is a rough strain that primarily affects sheep, causing epididymitis in rams and occasional abortion in ewes. Both species are facultative intracellular pathogens that survive and replicate within macrophages, evading the host immune response. They are shed in high numbers in aborted fetuses, fetal fluids, placentas, and vaginal discharges, as well as in milk, urine, and semen. The bacteria are highly resistant to environmental conditions, surviving for months in damp soil, manure, and contaminated feed, but are susceptible to common disinfectants (e.g., 1% sodium hypochlorite, 70% ethanol, 2% glutaraldehyde) and heat (60°C for 10 minutes).

Brucellosis affects both sheep and goats, but goats are more susceptible to B. melitensis and are considered the primary reservoir. The disease is more prevalent in intensive dairy goat operations and in flocks with high animal density, poor biosecurity, and uncontrolled animal movement. Age is a significant factor: sexually mature animals are more susceptible, with abortion occurring in the first pregnancy after infection. Breed differences exist, with some Mediterranean breeds showing higher susceptibility. The disease is transmitted horizontally through ingestion of contaminated feed, water, or licking of aborted fetuses and placentas, and venereally in the case of B. ovis. Vertical transmission occurs in utero and through milk. Morbidity can be high, with up to 30-50% of susceptible pregnant females aborting in an outbreak, while mortality is low except for secondary complications. Economic losses include abortion, reduced milk production, infertility, culling, and trade restrictions. The disease is endemic in many regions, and the prevalence varies from 1-20% in sheep and goats in endemic areas.

After ingestion or inhalation, Brucella organisms penetrate the mucosal epithelium and are phagocytosed by macrophages. They survive intracellularly by inhibiting phagolysosomal fusion and replicating within the endoplasmic reticulum. The bacteria then disseminate via the lymphatic system and bloodstream to the reproductive tract, particularly the pregnant uterus, where they colonize the chorionic trophoblasts. The trophoblast cells provide erythritol, a sugar alcohol that stimulates Brucella growth, leading to massive bacterial proliferation. This results in placentitis, necrosis of the cotyledons, and disruption of the maternal-fetal interface, causing abortion in the last trimester. The release of endotoxin (lipopolysaccharide) and the inflammatory response contribute to fetal death and expulsion. In rams, B. ovis colonizes the epididymis, causing inflammation, fibrosis, and obstruction of the epididymal ducts, leading to sperm granulomas and reduced semen quality. Chronic infection can lead to orchitis and testicular atrophy.

Intrinsic factors include age (sexually mature animals), pregnancy (especially first pregnancy after infection), and breed susceptibility. Extrinsic factors include high stocking density, poor hygiene, introduction of infected animals without quarantine, shared grazing with infected flocks, and lack of vaccination. Management practices such as mixing sheep and goats, especially dairy goats, increase the risk of B. melitensis transmission. In rams, B. ovis is more common in mature rams used for breeding, and the disease is transmitted venereally, so the use of infected rams is a major risk factor. Environmental factors such as contaminated pastures and water sources can maintain the infection. Stress factors like malnutrition, concurrent diseases, and poor biosecurity also predispose to outbreaks.

In pregnant ewes and does, the most common sign is abortion in the last trimester (usually 4-6 weeks before term), often with retained placenta. Aborted fetuses may be autolyzed, and there may be a bloody vaginal discharge. Some animals may give birth to weak, non-viable lambs/kids. After abortion, there may be a transient fever, decreased milk production, and metritis. In non-pregnant animals, infection is often subclinical. In rams infected with B. ovis, clinical signs include scrotal swelling, epididymitis (often unilateral), orchitis, and reduced libido. Palpation of the scrotum may reveal firm, enlarged epididymides. Semen quality is poor, with reduced sperm count and motility. In goats, B. melitensis can also cause mastitis, with reduced milk yield and abnormal milk. Systemic signs are rare but may include arthritis, hygromas, and in chronic cases, weight loss.

Differential diagnoses for abortion in sheep and goats include: 1) Chlamydial abortion (Chlamydia abortus) - causes late-term abortion, but with necrotic placentitis and no epididymitis in rams; 2) Toxoplasmosis (Toxoplasma gondii) - causes abortion, but with characteristic white spots on the cotyledons and fetal brain lesions; 3) Campylobacteriosis (Campylobacter fetus subsp. fetus) - causes abortion storms, but with hepatitis and no epididymitis; 4) Q fever (Coxiella burnetii) - causes abortion, but with placentitis and is highly zoonotic; 5) Listeriosis (Listeria monocytogenes) - causes abortion and encephalitis, but with microabscesses in the brainstem; 6) Border disease (pestivirus) - causes abortion and congenital defects, but with hairy lambs and no epididymitis; 7) Salmonellosis (Salmonella abortus ovis) - causes abortion, but with enteritis and septicemia; 8) Mycotic abortion (Aspergillus spp.) - causes sporadic abortion with characteristic placental lesions. For epididymitis in rams, differentials include Actinobacillus seminis, Histophilus ovis, and trauma.

The diagnostic approach begins with a thorough flock history, including abortion history, vaccination status, and introduction of new animals. Physical examination of aborting females and rams is essential. For abortion cases, collect fetal tissues (lung, abomasal contents, spleen), placenta, and maternal blood for serology. For rams with epididymitis, collect semen and serum. Laboratory tests include: 1) Serology: Rose Bengal Test (RBT) as a screening test, followed by complement fixation test (CFT) or ELISA for confirmation. For B. ovis, ELISA and gel diffusion tests are used. 2) Bacteriology: Culture of Brucella from fetal stomach contents, placenta, or semen on selective media (e.g., Farrell's medium) under biosafety level 3 conditions. 3) PCR: Real-time PCR on clinical samples for rapid detection and species identification. 4) Histopathology: Examination of placenta and fetal tissues for characteristic lesions. 5) Milk ring test for bulk tank milk surveillance. The algorithm should follow a stepwise approach: initial screening with RBT, confirmation with CFT or ELISA, and isolation for definitive diagnosis.

In infected animals, hematology may show mild leukocytosis or leukopenia, but is non-specific. Serum biochemistry may reveal elevated acute-phase proteins (haptoglobin, serum amyloid A). Serological tests: RBT is positive in infected animals, but false positives can occur; CFT is more specific and is the confirmatory test. ELISA (both indirect and competitive) is highly sensitive and specific. For B. ovis, ELISA and gel immunodiffusion are used. Bacteriological culture: Brucella melitensis grows slowly (3-7 days) on selective media, producing small, smooth colonies; B. ovis produces rough colonies. PCR: Real-time PCR targeting the IS711 gene or bcsp31 gene is highly sensitive and specific. In rams with B. ovis, semen analysis shows reduced sperm motility and increased numbers of abnormal spermatozoa. Histopathology of the placenta shows necrotic cotyledons with inflammatory infiltrates.

Ultrasonography can be used to assess fetal viability and detect placental abnormalities in pregnant animals. In rams with epididymitis, ultrasonography of the scrotum can reveal an enlarged, hypoechoic epididymis with fluid accumulation. Radiography is not commonly used, but may show calcification in chronic epididymitis. Computed tomography (CT) is rarely used in clinical practice but can provide detailed images of the reproductive tract in research settings.

Histopathology of the placenta in B. melitensis infection shows severe necrotizing placentitis with infiltration of neutrophils and macrophages, and the presence of numerous intracellular bacteria in trophoblasts. Fetal tissues may show bronchopneumonia, hepatitis, and splenitis. In rams with B. ovis epididymitis, histopathology reveals interstitial inflammation, fibrosis, and sperm granulomas in the epididymis. Cytology of semen may show increased numbers of neutrophils and macrophages. In chronic cases, testicular atrophy and fibrosis are evident.

Treatment of brucellosis in sheep and goats is not recommended due to the zoonotic risk and the poor response to antibiotics. Antibiotic therapy (e.g., oxytetracycline, enrofloxacin) may reduce shedding but does not eliminate the infection. The mainstay of control is vaccination and test-and-slaughter. In endemic areas, vaccination with B. melitensis Rev.1 vaccine (live attenuated) is recommended for young animals (3-6 months of age) via conjunctival or subcutaneous administration. For B. ovis, a killed vaccine is available in some countries. Infected animals should be culled to prevent spread. In a flock outbreak, quarantine, hygiene measures, and disposal of aborted fetuses and placentas are critical. Antibiotic treatment of humans exposed to Brucella is essential, with doxycycline and rifampin for 6 weeks.

The prognosis for individual animals is poor, as they remain infected for life and can shed the bacteria. The prognosis for the flock is guarded, as the disease can persist and cause ongoing reproductive losses. With vaccination and test-and-slaughter, the disease can be eradicated from a flock, but this requires strict biosecurity and monitoring. In endemic areas, vaccination reduces the incidence of abortion and human infection. The zoonotic risk makes the prognosis for public health significant, and prompt treatment of human cases is essential.

After an outbreak, implement a flock monitoring program: 1) Serological testing of all animals at 30-day intervals until two consecutive negative tests are obtained. 2) Vaccination of all replacement animals with Rev.1 vaccine. 3) Strict quarantine of new animals for 30 days with testing. 4) Proper disposal of aborted fetuses and placentas (incineration or deep burial with lime). 5) Disinfection of contaminated areas with 1% sodium hypochlorite. 6) In rams, test for B. ovis before breeding season and cull positive rams. 7) Educate farm workers about zoonotic risks and personal hygiene. 8) Monitor bulk tank milk with milk ring test for dairy flocks.

Pearls: 1) Brucellosis should be suspected in any abortion storm in sheep and goats, especially in endemic areas. 2) The Rose Bengal Test is a rapid, inexpensive screening test, but positive results must be confirmed with CFT or ELISA. 3) Vaccination with Rev.1 is highly effective in reducing abortion and shedding. 4) Always wear gloves when handling aborted fetuses and placentas due to zoonotic risk. 5) In rams, palpate the epididymides during breeding soundness exams to detect B. ovis infection. Pitfalls: 1) Do not treat infected animals with antibiotics, as this can lead to chronic carriers and antibiotic resistance. 2) Avoid using live vaccines in pregnant animals, as they can cause abortion. 3) Do not ignore the zoonotic risk; human cases can occur in farm workers and veterinarians. 4) False-positive serological tests can occur due to cross-reactions with other Gram-negative bacteria (e.g., Yersinia enterocolitica O:9). 5) Failure to implement biosecurity can lead to re-infection.

There is no effective treatment for brucellosis in sheep and goats. Antibiotics such as oxytetracycline (10 mg/kg IV or IM, q24h for 7-10 days) or enrofloxacin (2.5-5 mg/kg IM, q24h for 5-7 days) may be used in valuable animals, but they do not eliminate the infection and are not recommended. Vaccination protocols: B. melitensis Rev.1 vaccine: administer 1-2 x 10^9 CFU subcutaneously or conjunctivally to lambs/kids aged 3-6 months. For adult flocks in outbreak situations, a reduced dose (10^5 CFU) can be given conjunctivally. For B. ovis, a killed vaccine is available in some countries; administer 1 mL subcutaneously, followed by a booster in 4-6 weeks. Withdrawal times: For antibiotics, meat withdrawal is 28 days and milk withdrawal is 7 days (varies by country). No withdrawal times are required for vaccines.

Landmark studies have demonstrated the efficacy of Rev.1 vaccine in controlling B. melitensis infection in sheep and goats. A meta-analysis by Blasco (1997) showed that Rev.1 vaccination reduces abortion rates by over 90% in endemic areas. Studies on B. ovis have shown that vaccination with killed vaccines reduces the incidence of epididymitis in rams. The World Organisation for Animal Health (WOAH) provides guidelines for surveillance and control. The AASRP and ECSRHM recommend test-and-slaughter programs in low-prevalence areas and vaccination in high-prevalence areas. Recent research focuses on improved diagnostic tests, such as real-time PCR, and on the development of new vaccines that do not interfere with serological surveillance. The zoonotic impact of B. melitensis is well-documented, with human cases occurring in Mediterranean and Middle Eastern countries, emphasizing the need for public health collaboration.