Brucella melitensis is a Gram-negative coccobacillus bacterium that causes brucellosis, a serious zoonotic disease. Historically, this illness has been known as Malta fever, Mediterranean fever, or undulating fever. The organism is classified as a facultative intracellular pathogen, meaning it can survive and multiply within the host’s cells. This intracellular nature complicates both the body’s immune response and antibiotic treatment.
Reservoirs and Transmission Pathways
The primary natural reservoirs for Brucella melitensis are small ruminants, specifically domestic sheep and goats, though it is also found in camels in some regions. The bacteria localize in the reproductive organs of these animals, leading to high concentrations in birth products like the placenta, aborted fetuses, and vaginal discharges. Infected animals can also shed the bacteria continuously or intermittently in their milk and urine, often without showing outward signs of illness.
The most frequent way humans become infected is through ingestion, particularly by consuming unpasteurized milk and dairy products made from infected animals. Unpasteurized products, such as soft cheese and fresh milk, present a significant risk because the heating process of pasteurization effectively kills the bacteria. This route accounts for the majority of human cases globally, especially in regions where the disease is common in livestock.
The second major pathway is direct contact, which primarily affects those working closely with infected animals, such as farmers, veterinarians, and slaughterhouse workers. Infection occurs when the bacteria enter the body through breaks in the skin or mucous membranes, often when handling birth products or carcasses. A third, less common route is inhalation, which can happen when aerosols containing the bacteria are generated in highly contaminated environments or during laboratory procedures.
Clinical Manifestations of Brucellosis
The symptoms of brucellosis in humans often begin insidiously, typically appearing two to four weeks after exposure but sometimes taking up to six months to manifest. Initial signs are non-specific and resemble a severe flu, including generalized malaise, muscle aches (myalgia), headache, and profound fatigue. Patients frequently experience drenching sweats, especially at night.
A characteristic feature of the disease is a fever pattern that rises and falls over a period of days or weeks, giving it the name “undulating fever.” This cyclical pattern of fever, chills, and sweats can lead to delays in diagnosis because the symptoms can temporarily resolve. The infection often results in prolonged illness if not treated correctly.
If the infection progresses, the bacteria can spread from the lymph nodes to nearly any organ system, leading to localized complications. Osteoarticular involvement is the most commonly reported complication, affecting up to 60% of patients and presenting as arthritis. This often targets the joints of the spine, such as sacroiliitis.
Neurological symptoms, collectively termed neurobrucellosis, occur in a small percentage of cases and can include meningitis, encephalitis, or nerve root involvement. Although rare, endocarditis, which is the infection of the heart’s inner lining, is the most life-threatening complication of brucellosis. Endocarditis is the leading cause of death associated with the disease.
Diagnostic Procedures and Treatment Regimens
Diagnosing brucellosis requires a high index of suspicion due to the non-specific, flu-like presentation and the necessity of confirming a history of exposure. Laboratory confirmation can be achieved through blood cultures, where the bacteria can be grown from a patient’s blood sample. However, Brucella is a slow-growing organism, and cultures may take several weeks to become positive, requiring special notification to the laboratory to prevent premature disposal.
Serological testing is generally the most common and practical diagnostic method, particularly the Standard Tube Agglutination Test (STAT). This test detects specific antibodies against the bacterium in the patient’s serum. A positive test result or a four-fold rise in antibody titer between acute and convalescent phase samples helps to confirm an active infection. Molecular methods, such as Polymerase Chain Reaction (PCR), are increasingly used to detect the bacterial DNA directly in clinical samples.
Treatment of B. melitensis infection necessitates a prolonged course of combination antibiotic therapy to prevent relapse. The standard regimen for adults typically involves a combination of two antibiotics. This includes doxycycline taken for a minimum of six weeks, paired with either rifampin or an injectable aminoglycoside like streptomycin or gentamicin for a shorter period. Adherence to this extended, dual-drug protocol is required because monotherapy has a high failure rate.
Strategies for Prevention and Control
Effective prevention of Brucella melitensis infection hinges on controlling the disease in the animal reservoir and implementing rigorous public health measures. For the general public, the single most effective preventive step is ensuring that all milk and dairy products consumed are pasteurized. Avoiding the consumption of raw or unpasteurized dairy items eliminates the primary transmission route.
Occupational safety is important for individuals who handle livestock, animal tissues, or laboratory samples. This includes consistently wearing protective equipment, such as gloves, masks, and face shields, when assisting with animal births or processing animal carcasses. Strict laboratory safety protocols must also be followed to avoid accidental infection from aerosols or inoculation.
At the public health level, controlling the disease in the animal population is the most sustainable long-term strategy for protecting humans. This involves organized surveillance, testing, and vaccination programs for sheep and goats. The use of live attenuated vaccines, such as Brucella melitensis strain Rev.1, helps to reduce the prevalence of infection in flocks. Furthermore, the identification and subsequent culling of infected animals are often implemented to eliminate the source of the bacteria from herds.