Brucellosis

Brucellosis is a significant zoonotic disease, meaning it spreads from animals to humans. It is caused by a group of small, rod-shaped bacteria known as *Brucella*. These bacteria are classified as Gram-negative, nonmotile, and nonspore-forming coccobacilli. Because they act as facultative intracellular parasites, they can live inside host cells. This characteristic often leads to chronic infections that may persist for a person's entire life.

There are four primary species of *Brucella* that infect humans. *B. abortus* is primarily a disease of cattle and is considered less virulent than other types. *B. canis* is the species that affects dogs. *B. suis* infects pigs and possesses an intermediate level of virulence. The most invasive and virulent species is *B. melitensis*, which typically infects goats and occasionally sheep.

Transmission occurs through several specific pathways. The most common method is the ingestion of unpasteurized milk or soft cheeses made from infected animals. Many people also contract the disease through occupational exposure. This includes veterinarians, slaughterhouse workers, and laboratory personnel. Humans can also be infected by direct handling of infected animals or through aerosol exposure. Hunters are at risk when field-dressing carcasses or consuming undercooked wild game meat.

In the first stage of infection, bacteremia occurs, which is the presence of bacteria in the blood. This often leads to a classic triad of symptoms: undulant fevers, profuse sweating, and migratory arthralgia and myalgia. The sweating can sometimes have a distinct, foul, or moldy smell similar to wet hay. Gastrointestinal symptoms occur in approximately 70% of cases. These include nausea, vomiting, abdominal pain, and unintentional weight loss. The disease can also cause an enlarged liver or an enlarged spleen.

If the infection is left untreated, it can lead to serious complications called focalizations. These often occur in the bones and joints. Specific conditions include osteomyelitis, which is bone inflammation, or spondylodiscitis of the lumbar spine. Other severe consequences include meningitis, endocarditis, and various neurological disorders known as neurobrucellosis. In men, orchitis is also a common complication.

Diagnosing brucellosis requires several specialized medical tests. Doctors may attempt to isolate the bacteria through blood cultures or bone marrow cultures. However, *Brucella* grows extremely slowly and can take up to two months to appear in a culture. Because the bacteria are highly infectious, this poses a risk to laboratory staff. Clinicians also use serological tests to detect antibodies. These include the Rose Bengal plate test, the Huddleson reaction, and the Wright reaction.

Modern diagnostic tools like the ELISA test measure specific antibodies with high sensitivity. Polymerase Chain Reaction, or PCR, is another fast method used to detect the infection. While PCR is highly specific, some centers report persistent positivity even after successful treatment. Prevention relies heavily on controlling the disease in animal populations. This includes the use of livestock vaccines, such as *B. abortus* strain 19 or *B. melitensis* Rev-1. For humans, the most effective protections are pasteurization and the use of personal protective equipment when handling animals.