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.
