Seroincidence of Brucella spp. infection among humans and livestock in a pastoralist setting in Northern Kenya.

Brucellosis is one of the most widespread zoonoses globally. The disease is associated with significant human illnesses and livestock production losses, particularly among pastoralist communities. Despite its recognition as a significant zoonotic disease, reliable estimates of the health burden of brucellosis in humans and animals are limited. This study aimed to determine the annual seroincidence of Brucella spp. in humans and animals in a pastoralist community and to explore risk factors associated with exposure. We conducted a one-year, household-based longitudinal study in Isiolo County, northern Kenya. Households within a 20 km radius of Kinna town were eligible for enrollment if they owned at least one livestock species (cattle, goats, or sheep). All consenting humans and five animals per species were included. Quarterly blood sampling was performed, and Brucella immunoglobulin G (IgG) antibodies were detected using enzyme-linked immunosorbent assays (ELISA). A structured questionnaire was administered to collect data on potential risk factors for human and animal exposures. An incident case was defined as any human or animal that seroconverted during follow-up. Counts of incident cases were used to estimate exact incidence rates, with human- and animal-years at risk serving as the denominators. The data were analyzed using the Cox proportional hazards model, which allowed for type I right censoring. Crude and adjusted hazard rate ratios were estimated using univariable and multivariable models. The study enrolled 814 individuals (median age, 21.5 years; range, 2-90 years). A total of 1938 animals were recruited, comprising 687 cattle (35.5%), 622 sheep (32.1%), and 629 goats (32.4%). We identified 25 human incident cases over 435 person-years, resulting in an incidence rate of 0.057 (95% CI 0.035-0.080). The standard multivariable Cox regression analysis (with right censoring) revealed significant associations with age (p = 0.005), placenta removal (p = 0.031), and the occurrence of an incident case in livestock (p = 0.026). We identified 22 animal incidents across 1143 animal years, yielding an annual incidence of 0.019 cases per year (95% CI 0.011-0.027). Incidence was higher among younger animals and goats. The population-based incidence of Brucella spp. in humans and livestock indicates active transmission in this pastoralist community. Age, placenta removal, and household livestock infection were independently associated with human seroconversion, while goats and younger animals had significantly higher exposure rates than other livestock species and older animals, respectively. This highlights the human-livestock interface as a key pathway for transmission. Future studies should investigate the link between seroconversion and active infection in humans and livestock.