Brucellosis Surveillance — United States, 2010–2024

A review published in 2008 that summarized NNDSS human brucellosis case reports during 1951–2001 highlighted a significant decline in the 1960s from more than 6,000 cases per year to 100–150 cases per year, followed by decades of stability in case prevalence (4). This decline was attributed to the implementation of effective control measures such as the Cooperative State-Federal Brucellosis Eradication Program (17) and the widespread pasteurization of dairy products (4). The findings in this report align with that 2008 review, demonstrating that annual human brucellosis cases reported through NNDSS remained relatively stable but persistent at 71–165 cases annually during 2010–2024. To explain this continued level of brucellosis transmission, an analysis of supplemental CRF data identified two exposures which could put persons in the United States at risk for brucellosis: 1) consumption of unpasteurized dairy products sourced from outside the United States and 2) direct contact with wild animals in the United States, particularly among hunters and persons involved in the slaughtering or skinning of animals.

Among analyzed cases with supplemental CRFs, reported consumption of unpasteurized dairy products or undercooked meat was common, particularly among those with international travel. Unpasteurized dairy products imported from outside the United States have been linked to multiple U.S. brucellosis outbreaks (18). Although rare, U.S.-produced, unpasteurized products have also been associated with brucellosis cases, often as a result of infection with the cattle vaccine strain B. abortus RB51 (19–21). This vaccine is a modified live vaccine that is used in the United States. The vaccine is typically cleared from the animal’s bloodstream within 3 days of vaccination, although it might occasionally be persistently shed by the animal in milk after vaccination (22). Thus, continuing to highlight the risks for consuming unpasteurized dairy products, both from within and outside the United States, is essential for prevention of brucellosis infection.

A second pattern in the analyzed data was reported animal contact among persons without international travel, particularly during hunting-related activities and skinning or slaughtering of wild animals; feral swine was the most frequently reported animal exposure in this subgroup. A small number of interactions were reported with deer, elk, and caribou. Although brucellosis was eradicated from the U.S. domestic swine population in 2011, the disease remains prevalent among feral swine (23). As the geographic range of feral swine expands in the United States, the exposure risk for hunters increases (24). To reduce this risk, hunters should practice safe field dressing techniques, such as wearing latex or rubber disposable gloves, using eye protection, and avoiding contact with animal fluids or organs (25).

One third of the persons with brucellosis cases in this analysis with reported animal contact indicated contact with dogs; however, this observation is difficult to interpret for multiple reasons. First, because approximately 45% of U.S. households own a dog (26), determining the significance of dog contact is difficult. Second, because of limitations in the data, whether these dogs were infected with Brucella bacteria is unknown. Third, human cases of B. canis in the United States are believed to be underreported because of 1) a lack of awareness about the disease and 2) challenges with the availability of reliable diagnostic tests (27); no serologic tests are available in the United States for detecting human antibodies against B. canis. Finally, humans appear to be relatively resistant to infection with B. canis and might experience milder disease than they do with infection by other Brucella species (28).

Brucellosis symptoms often are nonspecific, leading to underdiagnosis, underreporting, and delays in treatment. Among the confirmed and probable cases analyzed, approximately 40% were among persons who reported arthralgia, and 13% reported arthritis or spondylitis, which are lower than previously reported rates (65% and 26%, respectively) (29). The surveillance system recorded six deaths (1%), consistent with the 1%–2% mortality range previously reported (8,9); however, because death status was missing for a substantial proportion of CRFs, this finding should not be interpreted as a definitive case-fatality estimate.

Most patients received multiple antimicrobials, with doxycycline and rifampin being the most frequently reported combination. This combination often is used for treating uncomplicated brucellosis because of its ease of administration (i.e., oral) (30). In contrast, only six persons received doxycycline and streptomycin, likely because of the logistic challenges of administering streptomycin intramuscularly (30). In addition, 14% of persons were treated with a single antimicrobial. Monotherapy for brucellosis is associated with high relapse rates (approximately 50%) and is generally considered inadequate (30).

The 2010 CSTE brucellosis case definition required fever as part of the clinical criteria for a case, which not only complicates the interpretation of clinical symptom data but also reduces the sensitivity of brucellosis surveillance; approximately 37% of persons with culture-confirmed cases of brucellosis were excluded from this analysis because of the absence of reported fever. Certain excluded persons likely had unrecorded fevers, and others might not have developed a fever despite positive Brucella bacterial cultures. Because culture is considered the gold standard for brucellosis diagnosis, in 2024, CSTE removed fever as a clinical criterion among persons with culture-positive results in its updated case definition position statement, which went into effect in January 2025 (31).

All but two confirmed cases were among patients with Brucella-positive cultures, which aligns with the gold standard for brucellosis diagnosis (8,32). Approximately 25% of patients with culture results yielding Brucella bacteria also had a serology test performed, and 5% of persons with probable cases diagnosed after a single agglutination test result also had a positive PCR test result, indicating that clinicians are requesting multiple testing modalities for patient diagnosis. CDC conducts serologic testing using BMATs, and several commercial laboratories in the United States offer Brucella species ELISA and reflex agglutination testing services; however, commercial laboratories use different test kits with varying performance characteristics, making data comparison challenging.

Overall, agglutination testing was reported twice as often as ELISA testing among the culture-confirmed cases, and 67% of patients with probable cases were diagnosed on the basis of a single positive agglutination test result. The 2010 CSTE brucellosis case definition did not include ELISA testing among the laboratory criteria, likely influencing the higher frequency and reporting of agglutination testing. Brucella species agglutination testing is recommended most often for persons with acute, noncomplicated cases (8,33).

Of the 35 patients with positive cultures and ELISA testing, 31 (89%) received a positive ELISA result (24 positive for either IgM or IgG and seven positive for both IgM and IgG), suggesting that ELISA antibody detection was common among this small number of culture-confirmed cases (34). Thirty-four percent of patients with probable cases who had a single positive agglutination test result also reported Brucella species antibodies detected by ELISA IgM or IgG testing, which cannot be easily interpreted without additional clinical or laboratory test data. Reports have described cross-reactivity for ELISA IgM tests in suspected brucellosis cases, leading to false-positive results (35,36). The general recommendation is to interpret the combined results for ELISA IgM and IgG testing, rely only on the ELISA IgG result, or conduct other laboratory testing for patient diagnosis. Because of the broad accessibility of ELISA testing and the frequency with which ELISA testing was reported among brucellosis cases in this study, CSTE incorporated ELISA IgG testing as a criterion for suspected case classification in the updated 2025 brucellosis case definition.