Brucellosis: Infectious Disease Caused by the Brucella Bacteria

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  • 0:05 Biological Weapons
  • 1:43 Brucella
  • 3:32 Brucellosis
  • 5:10 Diagnosis and Treatment
  • 5:50 High-Risk Groups and…
  • 6:48 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

In this lesson we will learn about the bacterial pathogen Brucella. Brucella is a human and livestock pathogen that was also the first biological agent weaponized by the United States.

Biological Weapons

In the 1950s, the memory of World War II was still fresh in the minds of the American people. Not only that, but the Soviet Union was gaining in global influence and prominence. American scientists and government officials began working together to identify a potential weapon that could be used to incapacitate enemy troops while causing minimal deaths and at the same time inflict major economic damage on the enemy. In the context of our current global views, it might surprise you to learn that the United States began developing biological weapons to accomplish these tasks.

The organism they chose to weaponize was the bacterium Brucella. Brucella and the illnesses it can cause had many of the major characteristics required for a successful biological weapon: an ability to aerosolize for dispersal, prolonged illness with low mortality in humans, and abortions and more severe illness in livestock, impacting the economy. The Brucella biological weapon was developed but fortunately never used.

In the 1970s, the United States, the Soviet Union, and the United Kingdom signed an agreement that they would no longer develop or stockpile biological weapons, so Brucella is no longer part of the American arsenal. Today, about 170 countries have signed this agreement. Brucella holds the title as the first biological agent to be weaponized by the United States. In this lesson, we'll examine Brucella and the foodborne illness brucellosis.


Brucella is a genus of Gram-negative, nonmotile, coccobacillus bacteria. The term coccobacillus might be a new one to you. Basically, a bacterium with a coccobacillus shape is a short rod, or bacillus, that can easily be confused with the spherical, or cocci, shape. Gram-negative means they have a cell wall with a thin peptidoglycan layer and an outer membrane.

There are four species of Brucella that can cause disease in humans, each normally associated with a different host animal. B. abortus, which infects cattle, and B. canis, which infects dogs, are species that cause only very mild diseases in humans. B. suis, which infects pigs, and B. melitensis, which infects sheep and goats, can cause fatal disease in humans.

People acquire Brucella through direct contact with infected animals, specifically contact with animal secretions or through consumption of animal products. Any secretions, like saliva, blood, or wastes, that get into cuts and broken skin, in the eyes, or inhaled, can cause active infections. But the majority of cases result from people drinking or eating milk products and undercooked meat from infected livestock. The Brucella bacteria concentrate in the mammary glands of infected cattle and goats and can be found in high concentrations in the milk. Pasteurization will kill 100% of the Brucella cells, so it is unpasteurized milk and cheese that commonly spread the disease.

It is also possible to pass Brucella from person to person, but this is very, very rare. There have been a couple of confirmed cases in the United States acquired from breastfeeding and sexual contact, but most infections are from unpasteurized milk or cheese.


Brucellosis is a disease caused by the bacteria Brucella often acquired by ingesting the bacteria in contaminated food. Once consumed, the bacteria will be engulfed by immune cells and transported to the lymph nodes. Unfortunately, Brucella can survive this attack, pass out of the lymph nodes to the blood stream, and invade the bone marrow, spleen, and liver. For many healthy people, it is possible that no noticeable symptoms will develop. A minority of people will have very mild symptoms that can be mistaken for flu symptoms. The symptoms can be vague and include fatigue, loss of appetite, weakness, and fever.

It is the fever that makes brucellosis unique. The fever rises very high in the day, causing drenching sweats. In the evening, the fever goes down, causing chills. For this reason, brucellosis is often called undulant fever for the cyclic rise and fall of body temperature unique to the disease. Most people will completely recover from brucellosis in a couple weeks without treatment. In an unlucky few, the disease can become chronic and life-threatening. The fever and fatigue can last for months or longer.

Brucella cells can localize in the brain, lungs, heart, and bones and be very difficult to eliminate. Eventually, some patients die due to chronic inflammation in the heart muscle. It is important to note that the symptoms of brucellosis are caused by the actively growing bacteria. This makes brucellosis an example of food infection, not food poisoning, which involves ingesting only microbial toxins.

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