Rift Valley Fever Virus Exposure amongst Farmers, Farm Workers, and Veterinary Professionals in Central South Africa

Abstract

Rift Valley fever (RVF) is a re-emerging arboviral disease of public health and veterinary importance in Africa and the Arabian Peninsula. Major RVF epidemics were documented in South Africa in 1950⁻1951, 1974⁻1975, and 2010⁻2011. The number of individuals infected during these outbreaks has, however, not been accurately estimated. A total of 823 people in close occupational contact with livestock were interviewed and sampled over a six-month period in 2015⁻2016 within a 40,000 km² study area encompassing parts of the Free State and Northern Cape provinces that were affected during the 2010⁻2011 outbreak. Seroprevalence of RVF virus (RVFV) was 9.1% (95% Confidence Interval (CI95%): 7.2⁻11.5%) in people working or residing on livestock or game farms and 8.0% in veterinary professionals. The highest seroprevalence (SP = 15.4%; CI95%: 11.4⁻20.3%) was detected in older age groups (≥40 years old) that had experienced more than one known large epidemic compared to the younger participants (SP = 4.3%; CI95%: 2.6⁻7.3%). The highest seroprevalence was in addition found in people who injected animals, collected blood samples (Odds ratio (OR) = 2.3; CI95%: 1.0⁻5.3), slaughtered animals (OR = 3.9; CI95%: 1.2⁻12.9) and consumed meat from an animal found dead (OR = 3.1; CI95%: 1.5⁻6.6), or worked on farms with dams for water storage (OR = 2.7; CI95%: 1.0⁻6.9). We estimated the number of historical RVFV infections of farm staff in the study area to be most likely 3849 and 95% credible interval between 2635 and 5374 based on seroprevalence of 9.1% and national census data. We conclude that human RVF cases were highly underdiagnosed and heterogeneously distributed. Improving precautions during injection, sample collection, slaughtering, and meat processing for consumption, and using personal protective equipment during outbreaks, could lower the risk of RVFV infection.

Figure 1

Distribution of anti–Rift Valley fever virus antibodies in 462 humans on 189 ruminant livestock and game farms in the study area in central South Africa, sampled during 2015–2016.

Figure 2

Hotspot map of anti–Rift Valley fever virus antibody prevalence, adjusted for age, ¹ in 462 humans on 189 ruminant livestock and game farms in the study area in central South Africa, sampled during 2015–2016. ¹ Difference of RVFV observed seroprevalence and predicted seroprevalence (by logistic regression model including age) by farm.