Seroprevalence and risk factors of brucellosis in pastoralists and their livestock in South Sudan

Background

Brucellosis poses serious public health implications and substantial economic losses in pastoral rural settings in South Sudan. In humans, brucellosis is almost always originating from animals. Current literature provides scant data regarding the seroprevalence of brucellosis in South Sudan. This cross-sectional study investigates the seroprevalence of brucellosis among the pastoral community and livestock and identifies risk factors for the disease from two counties, Terekeka and Juba in Central Equatoria State (CES), South Sudan.

Methodology

A total of 986 serum samples from humans (n=143), cattle (n=478), sheep (n=86), and goats (n=279) were randomly collected from 17 livestock camps in CES. Serum samples for the humans, cattle and goats were screened for Brucella- specific antibodies using rose Bengal plate test (RBPT) and further confirmed by competitive enzyme linked immune sorbent assay (c-ELISA) in series. All the sera from sheep were tested in parallel using RBPT and c-ELISA. A camp was considered positive when at least one animal of either species tested positive on the c-ELISA. Univariate analysis using binary logistic regression with a confidence interval of 95% at a p-value of ≤ 0.05 was used to identify the association between the potential individual risk factors and Brucella seropositivity. The investigated risk factors for livestock included age, sex, species, prior abortion history, retained placenta, parity, and reproductive status. Variables found to have associations in univariate analysis ( p =0.25) with Brucella seropositivity were further included in multivariable logistic regression. The risk factors investigated for humans included, gender, age, educational level, occupation, marital status, drinking of raw milk, aiding female animals during delivery, eating of undercooked meat and blowing of air into the cow’s uterus through the vagina.

Results

The study revealed seroprevalence of 21.7%, 11.8%, and 4.8% in cattle, goats, and humans, respectively. Our results indicated that all sheep serum samples were negative on both RBPT and c-ELISA. The seroprevalence in the 13 camps from TerekekaCounty was 100% (13/13) compared to 50% (2/4) seropositive from 4 camps in Juba County. All the variables investigated in the univariate analysis of risk factors in cattle were significantly associated with Brucella seropositivity: sex (OR:4.5, 95% CI: 2.2 – 8.9, p:<0.001), age (OR:6.6, 95% CI: 2.3 –19.1, p:<0.001), abortion history(OR:3.1, 95% CI: 1.8 – 5.2, p:<0.001), retained placenta (OR:2.5, 95% CI: 1.4 – 4.4, p: 0.001),parity (OR:2.3, 95% CI: 1.1 – 4.7, p:0.020). However, in small ruminants, none of the potential risk factors were associated with Brucella seropositivity. In humans, blowing air through cow’s vagina (OR: 1.4, 95%CI: 0.782 – 2.434, p: 0.035) was the only variable found to be significantly associated with Brucella seropositivity at the univariate analysis. The forceful blowing of air into cow’s vagina to induce milk is a common practice among the pastoral communities in South Sudan.

The multivariable logistic regression model identified sex, age, and abortion history as statistically significant factors for Brucella seropositivity in cattle. The odds of seropositivity were nearly threefold (OR: 2.8; 95% CI: 1.3 – 5.8, p: 0.006) higher in cows compared to bulls (male cattle). Cattle over two years old had higher odds of Brucella seropositivity than young animals (OR: 3.5, 95% CI: 1.2 – 10.3-, p: 0.025). Cows with a history of abortion had higher odds of Brucella seropositivity (OR: 2.8, 95% CI: 1.6 – 4.7, p: <0.001).

Conclusion

This is the first study to report occurrence of brucellosis in goats and its absence in sheep in South Sudan. Altogether, our results suggest that Brucella abortus infecting primarily cattle has spilled over to goats but not (yet) to sheep. The present study also shows occurrence of brucellosis in cattle, goats and people in the pastoral community and recommends implementation of One Health approach for effective mitigation of this disease.

Author summary

Brucellosis is a neglected, bacterial zoonotic disease that is caused by several species of the genus Brucella. Cross-species transmission of Brucella can occur in mixed or integrated farming systems. The disease poses serious public health implications and substantial economic losses particularly in low-income countries including South Sudan.

This study was conducted to estimate the seroprevalence of brucellosis in pastoralists, their livestock as well identifying potential risk factors associated with Brucella infection. Knowledge of the seroprevalence of brucellosis and risk factors is a prerequisite towards planning an effective mitigation strategy of the disease.

The study revealed high seroprevalence of brucellosis in cattle compared to goats, and the following risk factors were identified; prior history of abortion, age (old) and sex (female) significantly associated with Brucella infection. Surprisingly, sheep were found to be seronegative.