Brucellosis is a zoonotic disease caused by bacteria of the Brucella genus. It can be transmitted to humans through the meat, milk, and dairy products of infected animals such as cattle, sheep, pigs, goats, and buffalo, as well as through contact with body fluids like urine or reproductive secretions. The disease is characterized by high fever accompanied by chills, along with severe joint and muscle pain.
Causative Agent
Brucella species are Gram-negative, coccobacillus-shaped, facultative intracellular, non-capsulated, non-motile, and non-spore-forming bacteria. These species are classified based on phage sensitivity and their growth characteristics on culture media. Nine different Brucella species have been identified. Of these, seven (B. abortus, B. microti, B. melitensis, B. ovis, B. suis, B. canis, B. neotomae) affect terrestrial animals, while two species (B. ceti and B. pinnipedialis) affect marine mammals.
Among the Brucella species that cause disease in humans:
- B. melitensis is primarily found in sheep and goats,
- B. suis in pigs, and
- B. abortus mainly in cattle and buffalo.
B. canis, which infects dogs, rarely causes disease in humans. B. melitensis is considered the most pathogenic species for humans.B. abortus is less frequently associated with infection and usually presents subclinically. B. suis shows variable virulence but typically causes moderate and localized infections.
Brucella Infection in Animals
Among domestic mammals such as sheep, goats, cattle, camels, and pigs, the most common clinical sign of Brucella infection is abortion. Abortions typically occur during the second half of gestation and are often accompanied by metritis and retained placenta.
Cattle are most commonly infected with Brucella abortus, but when they share grazing areas with infected pigs, goats, or sheep, they can also become infected with B. melitensis and B. suis. These two species can be transmitted through cow’s milk and pose significant public health risks. Infected animals may experience a reduction in milk production of up to 25%.
In most infected animals, Brucella localizes in the mammary glands and mammary lymph nodes, and the bacteria can be excreted in the milk for the rest of the animal’s life. Once an infected animal aborts, it typically gives birth to healthy offspring in subsequent pregnancies.In goats, the classical Brucella species is B. melitensis. As in cattle, abortion, stillbirths, reduced fertility, and decreased milk production are characteristic symptoms in infected goats.
Distinctions in Sheep Brucellosis
In sheep, brucellosis is differentiated into ram epididymitis and classical brucellosis. Ram epididymitis is caused by B. ovis, which is not zoonotic, whereas classical brucellosis is caused by B. melitensis.
Brucellosis in Pigs
In pigs, brucellosis causes orchitis, lameness, abortion, spondylitis, hind limb paralysis, abscesses, and metritis.
Brucellosis in Camels
Camels can become infected with B. abortus and B. melitensis when grazing in shared pastures with other infected animals. Camel milk is recognized as a significant source of infection in the Middle East, yet necessary preventive measures are often lacking.
Brucellosis in Dogs
Dogs are the only known species naturally infected by B. canis, which is also an important zoonotic species. In dogs, brucellosis can cause abortion, ocular disorders, reduced fertility, skin lesions, and musculoskeletal disorders.
Complications of Brucellosis in Humans
- Musculoskeletal involvement
- Neurological involvement
- Genitourinary system involvement
- Gastrointestinal system involvement
- Cardiovascular system involvement
- Hematopoietic system involvement
- Pulmonary system involvement
- Dermatological manifestations
- Endocrine system manifestations
Zoonotic Importance and Public Health
Out of the nine known Brucella species, five infect humans. The most pathogenic species for humans is B. melitensis. In fact, B. melitensis, B. suis, and B. abortus are considered potential biological warfare agents in the United States.Most human infections occur through direct or indirect contact. Although the symptoms of brucellosis are well known, they can often be confused with those of other diseases. Common symptoms include chills, shivering, unexplained high fever, fatigue, headache, joint pain, weight loss, and lymphadenopathy.
Clinical Course and Transmission in Humans
Brucellosis is a multisystemic disease in humans that can present in both acute and chronic forms. The primary source of infection is usually milk and dairy products. Although human-to-human transmission is rare, it can occur through breastfeeding from mother to infant.
Certain occupational groups are considered high-risk for brucellosis, including veterinarians, animal caretakers and breeders, meat packing workers, and shepherds. Another possible route of transmission is through vaccines administered to animals.
Control of Brucellosis
Besides causing significant public health concerns due to its zoonotic nature, Brucella infections also result in substantial economic losses in developing countries’ livestock industries. A major challenge in controlling the disease is its latent (subclinical) course. The exact prevalence of latent infection among goats and sheep is not well known. Approximately 5% of offspring born to Brucella-infected animals carry latent infections.
In regions with high incidence rates, the most effective and essential control measure is the vaccination of all suspected animals and the culling of infected ones.
Control of brucellosis generally involves three main approaches:
- Hygienic measures,
- Testing and slaughtering,
- Vaccination.
This blog post has been verified by Veterinarian Alp Emre YILDIZ
Sources:
Brucellosis: an Overview, 1st International Conference on Emerging Zoonoses Jerusalem, Israel. Emerging Infectious Disease 1997; 3 (2): 213-21.
Topçu AW, Söyletir G, Doğanay M. Đnfeksiyon Hastalıkları. Ankara: Nobel Tıp Kitabevleri, 1996: 486-91. 2.
Y, K. (2011). Brusella hastalığı. Kocatepe Veterinary Journal, 4(1), 57-64.
Young EJ. Brucella species. In: Mandell GL, Douglas RG, Bennett JE, Ed. Principples and Practise of Infectious Diseases. New York: Churchill Livingstone Company, 2000: 2386-92.3.
photo sources:
https://www.medicana.com.tr/saglik-rehberi-detay/15047brusella-hastaligi
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