The genus Brucella encompasses a group of bacteria that cause disease in both animals and humans. These small, Gram-negative coccobacilli are facultative intracellular pathogens, surviving and multiplying inside host cells. Brucella species are responsible for brucellosis, a globally prevalent zoonotic infection and a significant public health concern, particularly where animal health programs are less developed.
Key Brucella Species and Their Animal Hosts
Brucella species exhibit a preference for specific animal hosts, though cross-species infection can occur. These bacteria localize in the reproductive organs of infected animals, frequently leading to reproductive issues.
B. melitensis
Brucella melitensis is primarily associated with sheep and goats. It is the most virulent species and the most common cause of human brucellosis worldwide. It has also been reported in camels and cattle in some regions.
B. abortus
Cattle are the usual hosts for Brucella abortus, which is also known to infect bison, water buffalo, and elk. The name abortus reflects its tendency to cause abortions in these animals. While less pathogenic for humans than B. melitensis, B. abortus has historically been a frequent cause of brucellosis in North America.
B. suis
Domesticated and wild pigs are the typical hosts for Brucella suis. Human infections with B. suis are well-documented, often leading to a prolonged illness with suppurative lesions. Other biovars have been found in caribou, reindeer, and wild rodents.
B. canis
Brucella canis primarily infects dogs and other canids. It can cause reproductive failure in dogs, including abortions, stillbirths, epididymitis, or orchitis. Although human infections are less common, they can occur, often presenting with mild symptoms.
Other Brucella species include B. ovis in sheep, B. ceti in cetaceans, and B. pinnipedialis in seals. These demonstrate the genus’s broad host range, though B. ovis is not known to cause human disease. Newly identified Brucella species in wildlife, such as rodents and baboons, are still being investigated for human infectivity.
Transmission from Animals to Humans
Brucellosis is a zoonosis, naturally transmitted from animals to humans rather than typically spreading between people. Humans are considered accidental hosts, acquiring the infection primarily through specific pathways involving contact with infected animals or their products.
Consumption of contaminated food products is the most common route of human infection. This frequently involves unpasteurized milk, cheese, and other dairy items from infected animals like cows, goats, or sheep. Eating undercooked meat from infected animals can also lead to transmission.
Direct contact with infected animals or their tissues is another significant pathway, especially for individuals in certain occupations. Farmers, veterinarians, slaughterhouse workers, and hunters face increased risk through contact with blood, urine, placentas, aborted fetuses, or other bodily fluids. The bacteria can enter the human body through breaks in the skin or via mucous membranes in the eyes, nose, or mouth.
Inhalation of aerosolized bacteria is a less common but recognized route of transmission. This risk is heightened in environments like barns, stables, or laboratories where Brucella organisms can become airborne. While rare, human-to-human transmission has been documented in isolated cases, primarily through routes such as blood transfusions, organ transplantation, breastfeeding, or sexual contact.
The Disease of Brucellosis
Once Brucella bacteria enter the human body, they can cause a systemic disease known as brucellosis. This condition is also referred to by historical names like Undulant Fever, Malta Fever, or Mediterranean Fever, reflecting its characteristic symptoms and geographical prevalence. The incubation period for brucellosis can vary widely, ranging from about 5 days to several months, with an average onset of two to four weeks after exposure.
Symptoms often begin acutely, resembling a flu-like illness. Patients may experience:
Fever
Chills
Profuse sweating
Headache
Malaise
Generalized muscle and joint pain
A distinguishing feature of brucellosis is the “undulating” fever pattern, where body temperature rises in the evening and gradually subsides in the morning, sometimes accompanied by drenching sweats.
If left untreated, the disease can progress to a chronic phase, potentially lasting for months or even years. Chronic manifestations may include recurrent fevers, persistent fatigue, arthritis, and localized infections in various organ systems. Complications can involve the musculoskeletal system (spondylitis, sacroiliitis), genitourinary tract (epididymo-orchitis), or, less commonly, the central nervous system (meningitis, encephalitis) and the heart (endocarditis), with endocarditis being a severe complication.
Diagnosis of brucellosis typically involves a combination of methods. Blood tests to detect antibodies against Brucella are commonly used for screening, though their specificity can be limited due to cross-reactivity with other bacteria. Confirmation often relies on isolating the bacteria through cultures of blood, bone marrow, or other affected tissues, which can be slow-growing. Treatment for brucellosis involves a prolonged course of antibiotics, usually a combination of two drugs, administered for a minimum of six weeks to prevent relapse and complications.
Veterinary Impact and Control Measures
Brucellosis has a substantial economic impact on livestock industries worldwide, particularly in countries with intensive farming practices. The disease in animals primarily manifests as reproductive losses, leading to significant financial burdens for farmers.
Infected female animals frequently experience spontaneous abortions, stillbirths, and the birth of weak offspring. Reduced milk production and impaired fertility, including longer calving intervals, are also common consequences in dairy herds. In male animals, symptoms can include epididymitis and orchitis. These reproductive failures directly translate into lost productivity and revenue, affecting both meat and dairy sectors.
Control and eradication strategies for brucellosis in animal populations are fundamental to protecting both animal health and reducing human exposure. Vaccination programs are a primary measure, with specific live attenuated vaccines available for different livestock species. For instance, Brucella abortus strain 19 (S19) and RB51 vaccines are used for cattle, while Brucella melitensis REV-1 vaccine is effective for sheep and goats.
Alongside vaccination, “test-and-slaughter” policies are implemented in many regions. This involves serological testing of animals to identify infected individuals or herds, followed by the culling of positive reactors to prevent further spread. Surveillance programs, including milk ring tests for dairy cattle, are also employed to monitor disease prevalence and ensure the effectiveness of control efforts. Preventing animal-to-human transmission is achieved through a combination of animal health interventions and public health awareness campaigns, emphasizing food safety practices like pasteurization.