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General Information about Brucellosis

Brucellosis is a highly transmissible zoonosis (human infection of animal origin) caused by bacteria of the genus Brucella. In humans brucellosis is also known as undulant fever or Mediterranean fever. A wide variety of mammals are susceptible to Brucella species; disease may be asymptomatic or cause abortion and epididymitis. In endemic countries, infection in livestock may result in significant economic losses through reduced milk yield, decreased fertility, lameness, and loss of young.

B. melitensis - goats, sheep, camels
B. abortus - cattle, camels
B. suis - pigs
B. canis - dogs
B. ovis - goats, sheep
B. neotomae - desert wood rat
Two further presumptive species have been described in marine mammals.

Brucella species are widespread throughout the world; endemic areas include the Mediterranean Basin, South and Central America, Eastern Europe, Asia, Africa, the Caribbean, and the Middle East. However, brucellosis is rare in the UK, and almost all of the human cases are acquired abroad.

B. melitensis, B. abortus and B. suis regularly cause human infection, but only rare infections with canine and marine species have been reported.

Epidemiology of human infections in the UK

With the exception of some cattle-associated B. abortus cases in Northern Ireland, all human cases in the UK are acquired abroad. In England and Wales, an average of ten cases are diagnosed each year.

Transmission

Human infections usually arise from direct contact with infected animals, their milk or milk products. Infection is through ingestion, inhalation or direct inoculation.
 

Ingestion:

  • unpasteurised milk or milk products, including cream and soft cheeses
  • acidified milk products (yoghurts, buttermilk, laban, lassi etc) are less hazardous but still pose a risk

Inhalation:

  • aerosols from infected products of conception in animals
  • aerosols generated in laboratory accidents, or contaminated dust and droplets in abattoirs

Direct contact or inoculation:

  • entry via the conjunctiva (usually in veterinarians or farmers)
  • veterinarians using the live attenuated vaccine
  • blood transfusion, bone marrow/organ transplantation

Infectious Dose

This is dependent on organism type, virulence and host resistance. Based on data from laboratory-acquired infections, the dose is probably < 500 organisms, and may be as low as 10-100 organisms.

Incubation period and communicability

The incubation period is typically 5-30 days, but may be up to 6 months. The infection may persist for several months without causing any symptoms. Person-to-person spread is rare, although mother-to-child, and sexual transmission have been documented.

Infection in humans

Clinical features are diverse with an acute or insidious onset. The major features are prolonged fever and debilitation, weight loss, sweating, headache, lethargy, anorexia and joint pains. Arthritis affecting the large joints and spinal osteomyelitis are common. Patients may have a dry cough or enlarged liver and spleen; neurological involvement and endocarditis are rare but important clinical presentations. Acute disease lasts for weeks, and although a substantial minority will have self-limiting disease, some people develop long-lasting chronic or relapsing disease. This may last for years, with malaise, depression, and destructive arthritis/ osteomyelitis.
Although the illness can be protracted and debilitating, brucellosis is rarely fatal in humans. However, if left untreated, up to 2% of patients will develop endocarditis which can be fatal.

B. melitensis and B. suis infections give rise to more severe acute disease than B. abortus. B. suis tends to induce abscess formation.

Serology

Laboratory diagnosis is usually based on serology using agglutination tests, although positive cultures may occasionally be obtained. Conventional serological testing uses antigens of B. melitensis and B. abortus, but serological responses do not completely distinguish between the two species. The tests will cross-react with B. suis infections but not with B. canis infection.

Consultation with the Reference Laboratory is essential for interpretation of results for both acute diagnosis and for clinical follow-up.

Environmental survival

Brucella species can survive for a long time in both hot and cold environments, particularly in moist conditions. Pastures and animal accommodation on farms may remain contaminated for prolonged periods, with survival for up to two years being reported. However, the organisms are very sensitive to direct sunlight, and can be destroyed by pasteurisation or cooking.

Occupational exposure

Brucella species represent a significant biohazard to laboratory workers, (advice is available for laboratory exposure Brucella Reference Unit Procedures for managing laboratory exposure.pdf (PDF, 93 KB)) and others at occupational risk, such as veterinarians, abattoir workers and farmers. The risk associated with the newly described marine species is unclear; suitable precautions should be taken to avoid any risk of infection by people who handle or work with marine mammals (particularly seals, porpoises and dolphins).

Prevention and control

In most developed countries B. abortus has been eradicated from cattle (through vaccination, test and slaughter of infected herds, and by pasteurisation of milk products) and is now a rare cause of disease.

Visitors to countries where brucellosis in endemic (the Mediterranean Basin [Portugal, Spain, Southern France, Italy, Greece, Turkey, North Africa], South and Central America, Eastern Europe, Asia, Africa, the Caribbean, and the Middle East) should not consume unpasteurised milk or milk products while travelling.
 


Last reviewed: 10 April 2012