Brucella abortus and Bovine Brucellosis

Brucella abortus belongs to the phylum Proteobacteria, class Alphaproteobacteria, order Rhizobiales, family Brucellaceae, and genus Brucella. It is one of the most widespread species within the genus.

Microscopically, the bacterium appears as a Gram-negative coccobacillus or short rod. It does not form spores and lacks flagella, making it non-motile. Colonies grown on solid culture media are typically small, smooth, and translucent.

A key biological feature of B. abortus is its ability to survive and replicate within host cells, particularly macrophages. This facultative intracellular lifestyle enables the pathogen to evade immune responses and establish chronic infections.

The primary natural host of Brucella abortus is cattle, including dairy cattle, beef cattle, and water buffalo. The bacterium may also infect other animals such as camels, deer, and horses.

Animal-to-animal transmission: Infection in cattle typically occurs through contact with aborted fetuses, placental tissues, vaginal secretions, or contaminated environments. The organism may also be present in semen, allowing transmission during breeding.

Animal-to-human transmission: Human infection generally occurs through occupational exposure or ingestion of contaminated food products.

• Direct contact with infected animals or tissues
• Consumption of unpasteurized milk or dairy products
• Inhalation of infectious aerosols in laboratory or farm environments

In animals: The most characteristic sign in cattle is abortion during late pregnancy, typically between the fifth and eighth months of gestation. Other manifestations may include retained placenta, endometritis, infertility, orchitis in males, and chronic joint inflammation.

Infected animals may become chronic carriers and continue shedding bacteria into the environment, acting as a long-term source of infection within herds.

In humans: Human infection leads to brucellosis, often referred to as “undulant fever” because of its recurring fever pattern.

The incubation period is typically 1–4 weeks. Common symptoms include:

• Intermittent fever
• Night sweats
• Fatigue and weakness
• Joint and muscle pain
• Headache
• Enlargement of liver, spleen, or lymph nodes

If untreated, the infection may progress to chronic disease affecting multiple organs, including bones, joints, heart valves, and the nervous system.

Because clinical symptoms are often nonspecific, laboratory testing is essential for definitive diagnosis.

Bacterial isolation: Culturing the pathogen from blood, bone marrow, joint fluid, or animal tissues remains the diagnostic gold standard. However, the bacterium grows slowly and poses laboratory biosafety risks, requiring specialized containment facilities.

Serological testing: Widely used for screening and diagnosis.

• Rose Bengal plate test (rapid screening)
• Standard tube agglutination test
• Complement fixation test
• ELISA for detection of IgM and IgG antibodies

Molecular detection: PCR-based methods provide rapid and highly specific detection of Brucella DNA, particularly useful in chronic infections or culture-negative cases.

Human brucellosis requires prolonged antibiotic therapy to prevent relapse. Standard treatment often involves a combination of doxycycline and rifampicin for at least six weeks. In severe cases, additional antibiotics such as streptomycin may be included.

Control of brucellosis primarily depends on livestock management and disease prevention at the animal level.

Brucella abortus represents a classic example of a zoonotic pathogen linking animal health, food safety, and human public health. Effective surveillance, improved diagnostic technologies, and coordinated veterinary-medical collaboration are essential for controlling brucellosis and reducing its global impact.