Incidence of Brucella spp. in various livestock species raised under the pastoral production system in Isiolo County, Kenya.

Background Brucellosis is an important zoonosis with a worldwide distribution. The disease is caused by multiple species of Brucella that can infect a wide range of mammalian hosts. In the sub-Saharan Africa, many studies have been implemented to determine the prevalence of the disease in livestock, but not much is known about its incidence. We implemented a longitudinal study to determine the incidence of Brucella spp. infection in cattle, camels, sheep and goats that were being raised in a pastoral area in Isiolo County, northern Kenya. Methods An initial cross-sectional survey was implemented to identify unexposed animals for follow up; that survey used 141 camels, 216 cattle, 208 sheep and 161 goats. A subsequent longitudinal study recruited 31 cattle, 22 sheep, 32 goats and 30 camels for follow up. All the samples collected were screened for Brucella spp. using the Rose Bengal Plate test (RBPT), a modified RBPT, and an indirect multispecies Enzyme Linked Immunosorbent Assay (iELISA) kit. Samples that tested positive by any of these serological tests were further tested using real-time PCR-based assays to detect genus Brucella DNA and identify Brucella species. These analyses targeted the alkB and BMEI1162 genes for B. abortus , and B. melitensis, respectively. The longitudinal study took 12 months and data were analysed using Cox proportional hazards model that accounted for clustering of observations within herds. Changes in anti- Brucella IgG optical values between successive sampling periods were determined to confirm primary exposures. Results The mean incidence rate of Brucella spp. was 0.024 (95% confidence interval [CI]: 0.014 – 0.037) cases per animal-months at risk. Brucella spp. incidence in camels, cattle, goats and sheep were 0.053 (0.022 – 0.104), 0.028 (0.010 – 0.061), 0.013 (0.003 – 0.036) and 0.006 (0.0002 – 0.034) cases per animal-month at risk, respectively. A higher incidence rate of Brucella spp. was found among females (0.020, 0.009 – 0.036) than males (0.016, 0.004 – 0.091), while young animals (0.026, 95% CI; 0.003 – 0.097) had a slightly higher incidence rate compared to adults (0.019, 95% CI; 0.009 – 0.034). RT PCR analyses showed that B. abortus was more prevalent than B. melitensis in the area . The results of multivariable Cox regression analysis identified species (camels and cattle) as an important predictor of Brucella spp. exposure in animals. On the diagnostic tests, modified RBPT provided similar findings as the iELISA test.   Conclusions Our findings indicated that camels and cattle have a higher incidence of Brucella spp. exposure than the other livestock species. This could be due to the higher prevalence of B. abortus , which readily infects these species, than B. melitensis . More studies are underway to identify ecological factors that influence the persistence of the key Brucella species in the area. The study further concluded that modified RBPT test can give reliable results as those of a formal iELISA test, and it can therefore be used for routine surveillance in the region.