Brucella

ORGANISM:

GENERAL CONCEPTS:

The Brucellae are generally associated with animal infections but most are also pathogenic for humans.
All human infections come from animals; there is no human to human transmission. Such diseases are called "zoonoses".
B. melitensis is associated with a specific human disease called Malta fever.
Brucellae are intracellular parasites.

DISTINCTIVE PROPERTIES:

The genus Brucella is composed of Gram negative coccobacilli. Most are aerobic but grow best in a 5-10% CO₂-enriched environment. Their metabolism is oxidative.
Brucellae possess a typical Gram-negative LPS endotoxin, as well as two major serological determinants; A and M.

PATHOGENESIS:

Of the four species that cause disease in humans, B. melitensis and B. suis are more transmissible to humans, particularly via the oral route.
These bacteria are intracellular parasites of the reticuloendothelial (RE) system (e.g. spleen, liver, bone marrow, lymph nodes and kidneys).
Following exposure, the organisms may produce a localized abscess, which is followed by bacteremia. Phagocytosis by macrophages and intracellular multiplication leads to localization in the RE tissues. Disease may remain subacute or become chronic with initial symptoms of malaise, chills, weakness and intermittent fever. Granulomas in various RE tissues may occur as a result of a hypersensitivity reaction.
In animals, multiplication occurs in the uterus because of the presence of erythritol, which the bacteria prefer to glucose. This localization can lead to abortion or excretion in milk (human source for infection).

HOST DEFENSES:

Because of the intracellular life-style of the Brucellae, humoral defenses play a minor role. Cell mediated defenses (T-lymphocytes, activated macrophages) are required.

EPIDEMIOLOGY:

Brucella infections have a worldwide distribution but have been mostly eradicated in the United States.
B. abortus affects primarily cows; B. melitensis affects goats and sheep; B. suis affects pigs; B. canus affects dogs. Humans generally acquire disease through occupational exposure, Thus, veterinarians, meat workers and animal handlers are those most likely to be afflicted.

DIAGNOSIS:

Clinical: Symptoms of brucellosis are variable and diagnosis is, therefore, very difficult. Flu-like symptoms with limb and back pain, an intermittent fever with malaise may last up to 3 months for acute disease (a year or more for subacute or chronic disease).
Laboratory: Isolation of Brucella from the blood is possible. Cultures must be incubated 3-4 weeks with added CO₂.

CONTROL:

Sanitary: Pasteurization of dairy products and use of protective clothing prevent human infection. More importantly, systematic identification and elimination of infected animals and vaccination of animals reduces the reservoir.
Immunological: Vaccination for persons at high risk is possible, but they must first be tested to ensure that no hypersensitivity already exists.
Chemotherapeutic: Tetracycline or a tetracycline/streptomycin combination is generally curative.