Risk factors for brucellosis in indigenous cattle reared in livestock–wildlife interface areas of Zambia

We conducted this cross-sectional study to investigate risk factors of Brucella seropositivity in cattle herds reared in livestock–wildlife interface areas of Blue Lagoon and Lochinvar National Parks in Zambia between August 2003 and September 2004. Sera were collected from cattle aged ≥2 years from 124 herds. Data on husbandry practices, grazing strategies, and herd structure (sex and age composition) were also collected. Sera were screened for anti-Brucella antibodies using the Rose Bengal test (RBT) as a presumptive test and a competitive-ELISA (c-ELISA) as a confirmatory test. A herd was classified as Brucella seropositive if at least one animal tested positive on both RBT and c-ELISA in series testing. Risk factors for herd-level brucellosis seropositivity were tested using multivariable logistic regression; risk factors for increases in the within-herd counts of seropositive cattle were analyzed using the negative binomial regression model with the number of seropositive animals as outcome and total number of cattle tested in a herd as the population at risk (exposure). Of the 110 herds tested, 68 (62; 95% CI: 53, 71% after adjusting for clustering by area) tested seropositive for exposure to Brucella spp. The final logistic-regression model identified geographical area, with Lochinvar (OR = 3.4; CI: 0.97, 12) and Kazungula (OR = 4.3; CI: 0.91, 20) recording higher odds of Brucella infections compared to Blue Lagoon. Herds coming in contact with wildlife had higher odds compared to those without contact (OR = 3.4; CI: 1, 11). Similarly, the odds of Brucella infection were progressively higher in the larger herd categories (26–40 cattle, OR = 2.6; CI: 0.70, 10; 41–82 cattle, OR = 4.9; CI: 0.93, 26; >82 cattle, OR = 9.4; CI: 1.7–51) compared to the smallest herd category (10–25). The negative binomial regression model identified geographical area, contact with wildlife, and herd size as having significant effect on counts of seropositive cattle in a herd.