BACKGROUND: Brucellosis is a worldwide zoonosis. Non-specific symptoms and long incubation periods complicate brucellosis diagnosis and control. We reviewed two surveillance systems to understand human brucellosis characteristics in the United States.
METHODS: Incidence rates (IR) were calculated using confirmed and probable brucellosis cases reported to the National Notifiable Disease Surveillance System (NNDSS) from 2000 to 2014. Population estimates were obtained from the U.S. Census Bureau. Clinical symptoms and risk factors were abstracted from the CDC brucellosis surveillance program (BSP) database for 2010-2014. Poisson regression, the Cochran-Armitage test, and t-test were used for statistics calculation. We excluded unknown or missing values when calculating clinical symptoms and risk factor variables.
RESULTS: One thousand three hundred twelve cases were reported through NNDSS from 2000 to 2014. The average reported IR was 0.29 cases per million people per year (PMY) (range from 0.19 to 0.33, p=.2889). IRs in Arkansas, Hawaii, Montana, North Dakota and Texas were higher than the average IR (p<.05). IRs increased with age (p=.0393); people aged 25+ had higher IR than those aged below 25 (IR=0.34, 0.20 PMY respectively, p=.0170). IRs did not significantly differ between males and females (IR=0.34, 0.24 PMY respectively, p=.0611). Three hundred eighty-five cases were obtained from the BSP database from 2010 to 2014. Fever (244/262, 93%), night sweats (136/177, 77%), headache (122/171, 71%), fatigue (150/211, 71%), anorexia (106/151, 70%), myalgia (112/157, 71%), arthralgia (81/119, 68%), and weight loss (69/106, 65%) were commonly reported symptoms. Seventy-six percent (180/236) of cases were hospitalized. The case fatality ratio was 2.50% (4/151). Sixty-seven percent (82/122) of cases reported contact with animals, of which up to 80% had contacted swine. Fifty-three percent (116/220) of cases had travelled to brucellosis endemic countries before illness onset and consumed unpasteurized dairy or undercooked meat. Fifteen percent (22/146) of cases were epidemiologically linked to other confirmed brucellosis cases.
CONCLUSIONS: Human brucellosis IR has remained stable in the United States for the past 15 years. Animal contact (particularly swine contact), travel to brucellosis endemic countries, and consumption of unpasteurized dairy or undercooked meat account for most brucellosis cases. Physicians should inquire about exposure history when diagnosing patients who presented symptoms compatible with brucellosis.