Brucellosis: Patient History Is the Key

Maria Negron, DVM, PhD, MS


December 07, 2017

Editorial Collaboration

Medscape &

It's not every day that you see a case of brucellosis in your practice. The disease is rare in the United States, with only about 120 cases annually. But when you do encounter brucellosis, it's vitally important to order the right tests so that you can diagnose properly and treat the disease promptly with appropriate antibiotics. Your patient's health could depend on it; those who don't receive treatment for brucellosis can develop serious disease that can affect their lives and persist for years.

Although brucellosis is mainly a bacterial disease of animals, several species of Brucella bacteria are known to cause disease in humans. Because the diagnostic test and treatment vary, it is important to identify whether a patient is infected with Brucella, and if so, which [species of] Brucella is causing disease.

Each species of Brucella has distinct risk factors. When you suspect brucellosis, getting a thorough medical history is an important first step because symptoms can develop anywhere from 5 days to 6 months after exposure.

Diagnosing brucellosis cannot be done by symptom presentation alone because initial symptoms are nonspecific and resemble those of other febrile illnesses. A person who is infected with Brucella typically presents with a history of fever; sweats; malaise; anorexia; headache; fatigue; and muscle, joint, or back pain.

Approximately 70%-75% of US brucellosis cases reported annually to CDC are due to the bacterial species B melitensis and B abortus. Most of these cases are associated with consumption of unpasteurized dairy products (such as raw milk and cheese) while travelling internationally to places where brucellosis is endemic in animals. Although acquired outside the United States, these cases are not diagnosed until after the patient has returned home, due to the long incubation period for Brucella.

In the United States, 25%-30% of brucellosis cases are due to B suis and almost all are diagnosed in people who hunt and slaughter feral swine. People usually become infected through contact with blood and fluids from infected swine while dressing the carcass. However, keep in mind that dogs can contract brucellosis from feral swine and they can then spread the infection to their owners.

Although less common in humans, B canis is found in dogs all over the world, including the United States, and generally causes mild illness in people. Canine brucellosis can spread to humans by close contact to vaginal secretions, aborted puppies, or urine from infected dogs.

Finally, people can become infected with Brucella RB51, a strain of B abortus that is used to vaccinate cattle in the United States and other parts of the world. Although RB51 was developed to be less pathogenic, it can cause disease in humans. Human cases of RB51 are most often associated with needle-stick exposures while vaccinating cattle, and cases are usually veterinarians or veterinary technicians. And although very rare, the CDC has received notification of two human cases of RB51 associated with consumption of raw milk in the United States this year.

The clinical history will help you determine which test you should order to confirm brucellosis. In particular, ask patients if they have traveled internationally, if they hunt (especially feral swine), if they have consumed unpasteurized milk or milk products recently, if they work in a laboratory, if they vaccinate livestock, or if they have had close contact with animals or animal tissues.

Most clinical diagnostic laboratories are able to run serologic tests, which detect an antibody response to most of the Brucella species. However, B canis and Brucella RB51 won't show up on serology, so culture is needed to confirm the infection. If your local lab cannot perform culture, contact your state health department for assistance. They can also guide you and assist you to determine the appropriate route of sample submission for a proper diagnosis. Always remember to inform the lab that you suspect brucellosis when submitting a sample. This will ensure that the lab staff will take appropriate precautions when performing tests and prevent additional exposures to the bacteria.

Once brucellosis has been confirmed, start treatment immediately to prevent chronic infection. Treatment for brucellosis is typically a combination of doxycycline and rifampin for at least 6 weeks. Please be aware that when dealing with an RB51 infection, rifampin should not be used because this particular strain is resistant to it. Other antibiotic options can be considered when patients have contraindications (eg, drug allergies or pregnancy) or if they develop side effects. Remember: If not treated appropriately, the infection can lead to long-term disease associated with arthritis, endocarditis, chronic fatigue, depression, or swelling of the liver or spleen.

Visit the CDC's brucellosis website or consult the CDC's brucellosis reference guide for more information on the different types of diagnostic tests available, treatment, and prevention of brucellosis.

Web Resources


Zoonoses and Select Agent Laboratory (ZSAL)

Brucellosis reference guide

Brucellosis: Risks from Unpasteurized Dairy Products