Brucellosis is a zoonotic disease - a disease that can be transmitted from animals to humans. Humans can become infected through contact with an infected animal, infected food, or inhalation. Four species of bacteria from the genus Brucella infect humans: B. abortus, B. canis, B. melitensis, and B. suis.

Brucella bacteria, 3D illustration. Gram-negative pleomorphic bacteria which cause brucellosis in cattle and humans and are transmitted to man by direct contact with ill animal or by contaminated milk - Image Copyright: Kateryna Kon / Shutterstock

History of Brucellosis

British army surgeon Cleghorn first identified brucellosis in 1751, noting similarities to a disease first described by Hippocrates more than 2000 years earlier. In 1887, David Bruce isolated the causative organisms of the disease. The genus Brucella was later named after him. Scientists further characterized the disease by connecting it with abortions in cattle. Brucella bacteria infect the reproductive organs of the host, resulting in abortions and sterility. Animals shed the bacteria in urine, milk, and other bodily fluids.

Brucellosis also infects wild animals, but animal-to-human transmission primarily occurs with domestic animals. Worldwide, there are about 500,000 new cases of brucellosis each year. Animal vaccination programs have reduced the incidence of brucellosis in the US.

Brucellosis Evades the Immune System

Once inside the body, Brucella bacteria evade the immune system through a number of mechanisms.

That makes Brucella difficult for the immune system to eliminate. Cell-mediated immunity is the primary mode of defense for the host. This host response to B. abortus produces tissue granulomas that resemble sarcoidosis. Infection with the more virulent B. melitensisand B. suis can cause visceral microabscesses.

Brucella infects the human body through breaks in the skin, mucous membranes, respiratory and gastrointestinal tracts, and conjunctivae. Inside the body, Brucella bacteria enter the lymphatic system and replicate. They can then spread to the kidney, liver, spleen, breast, or joints. The infection can involve any organ system.

Symptoms of brucellosis include:

• fever
• sweating
• headache
• anorexia
• fatigue
• muscle pain
• joint pain
• back pain
• arthritis
• swelling of scrotum and testicles
• endocarditis
• chronic fatigue
• depression
• swelling of liver
• swelling of spleen
• neurological symptoms

The total number of reported cases in the US is about 100 per year. Most cases of brucellosis in the US result from the use of illegal unpasteurized dairy product from Mexico. About 60 percent of US cases are found in Texas and California.

Early Treatment is Key

Brucellosis responds well to treatment. If treated within a few months of onset, a complete cure with low risk of relapse or chronic disease is possible. However, if treatment is delayed, mortality approaches 85 percent. Illness can become chronic and debilitating. Overall mortality from brucellosis is less than 2 percent.

Brucellosis is typically diagnosed from blood cultures and testing for antibodies specific to the disease. The standard treatment for brucellosis is antibiotics. Bed rest is often recommended.

Biological Warfare

Brucella bacteria have been weaponized in experimental US biological warfare programs. Qualities such as airborne transmission, ability to cause chronic disease, and vague clinical characteristics (leading to difficulty in diagnosing) made it appealing for a weapons program. The program was terminated by 1967, but the organisms are still considered a potential weapon and potential threat to US troops.