Brucella

Brucellosis is a common zoonotic disease throughout Africa, Asia, the Middle East, Mediterranean Basin, Eastern Europe, the Caribbean, Central America and South America. A review of data from 2010 through 2024, demonstrated that 71 to 165 human 140 cases brucellosis are reported annually in the United States. The primary human pathogens were B. melitensis, B. abortus, B. suis and B. canis.

Brucellae are small, slow-growing Gram-negative coccobacilli that typically infect livestock including goats, sheep, cattle and pigs. Most exposures occur in countries where brucellosis is endemic and unpasteurized milk or milk products are consumed. Less commonly, infections occur in farm settings after inhalation of organisms or direct contact with blood or body fluids during the slaughter of infected animals. Brucella bacteria can be easily aerosolized and inhaled. Certain occupations or activities have higher risk of exposure including slaughterhouses, laboratories, veterinary practice, and hunting. Human-to-human transmission is possible but rarely occurs. 

B. melitensis and B. abortus comprise 70-75% of brucellosis cases and are caused by consumption of unpasteurized dairy products, such as raw milk and cheese. Americans most likely get infected after consuming unpasteurized dairy products while traveling or brought to the U.S. from abroad. 

A strain of B. abortus (RB51) is found in the Brucella livestock vaccine and rarely causes human infection. The few cases that have occurred were most often associated with veterinary needle-stick exposures while vaccinating cattle. However, RB51 can also be transmitted by consumption of raw milk products.

B. suis infections account for 25-30% of brucellosis cases. They usually occur in people who hunt and butcher feral swine that are infected. Dogs can contract brucellosis from feral swine and spread the infection to their owners.

B. canis is the least common species that infects people. It's transmitted by dogs and generally causes mild illness in people.

The Incubation period ranges from weeks to months. Symptoms consist of fever, profuse sweating, fatigue, headache, weight loss, myalgia and arthralgia. Lymphadenopathy, hepatomegaly, splenomegaly, cytopenia, and hepatitis are common. Relapse may occur in spite of lengthy antibiotic therapy.

B. melitensis, B. abortus, B. suis, and B. canis infections are typically treated with a combination of doxycycline and rifampin for at least 6 weeks. Rifampin should not be used for B. abortus RB51 infection, because they are resistant to it. Instead, Trimethoprim-sulfamethoxazole (TMP-SMZ) should be prescribed. 

Most commercial clinical laboratories offer serological tests that can detect antibodies to three Brucella species: B. abortus, B. melitensis, or B. suis.  IgM antibodies are predominant in acute infection but decline within weeks. Sensitivity ranges from 67% to 100%. Relapses are accompanied by transient elevations of IgG and IgA antibodies but not IgM. Brucella IgG tests require two serum samples to confirm brucellosis: the first within 7 days of symptom onset and the second 2-4 weeks later. A four-fold rise in IgG titer between the acute and convalescent phase specimens is diagnostic.  Infection with B. canis and Brucella RB51 cannot be confirmed by serology and must be confirmed by culture.

Culture is the gold standard for diagnosis of Brucellosis. However, Brucellae can be fastidious, slow growers. Culture from primary specimens may require up to 21 days of incubation. Persons with chronic infections are less likely to be culture-positive. Brucellae can be isolated from blood, bone marrow, joint fluid, CSF, abscesses and purulent discharge.Bone marrow culture is more sensitive than blood, but is a more invasive procedure.

Polymerase chain reaction (PCR) testing of isolates can be used to identify species but is not widely available in the United States. PCR has a high false-negative rate due to low bacillary levels. In certain patients with brucellosis, Brucella bacteria might continue to be detectable by PCR testing after treatment because of the persistence of DNA in their blood.

Brucellosis is the most commonly reported laboratory-acquired bacterial infection, possibly because of two reasons. Brucellae can be easily aerosolized in the laboratory, leading to inhalation or ingestion and the infectious dose is very low. CDC recommends that laboratories use procedures that minimize splashes or aerosols of unidentified isolates and that culture plate sniffing should be prohibited. Slow growing Gram-negative and Gram-variable organisms should be handled in a BSL-3 biological safety cabinet. Clinicians need to notify the microbiology laboratory that brucellosis is suspected so that cultures can be incubated for at least 10 days and biosafety precautions are taken to prevent laboratory exposures.

Brucellosis is a reportable and nationally notifiable condition. 

References

https://www.cdc.gov/brucellosis/site.html#hcp

Centers for Disease Control and Prevention, Brucellosis Reference Guide: Exposures, Testing, and Prevention, February 2017, https://www.cdc.gov/brucellosis/media/pdfs/2025/02/brucellosi-reference-guide.pdf

Locke NS, et al. Brucellosis Surveillance — United States, 2010–2024. MMWR Surveill Summ 2026;75(No. SS-2):1–12. 

Di Bonaventura G, et al. Microbiological laboratory diagnosis of human brucellosis: an overview. Pathogens 2021;10:1623.

Yagupsky P, Morata P, Colmenero JD. Laboratory diagnosis of human brucellosis. Clin Microbiol Rev 2019;33:e00073–19.