Rift Valley fever (RVF) is a mosquito-borne viral zoonosis showing complex epidemiological patterns
that are poorly understood in South Africa. Large outbreaks occur in the central interior at long,
irregular intervals, most recently in 2010–2011; however, the level of herd immunity of ruminant
livestock, a key determinant of outbreaks, is unknown. During 2015–2016 a cross-sectional study
on 234 randomly-selected farms investigated the prevalence, patterns of, and factors associated
with, antibodies to RVF virus (RVFV) in livestock in an area heavily affected by that outbreak. A RVFV
inhibition ELISA was used to screen 977 cattle, 1,549 sheep and 523 goats and information on potential
risk factors was collected using a comprehensive questionnaire. The estimated RVFV seroprevalence,
adjusted for survey design, was 42.9% in cattle, 28.0% in sheep and 9.3% in goats, showing a high
degree of farm-level clustering. Seroprevalence increased with age and was higher on private vs.
communal land, on farms with seasonal pans (temporary, shallow wetlands) and perennial rivers and in
recently vaccinated animals. Seropositivity amongst unvaccinated animals born after the last outbreak
indicates likely viral circulation during the post-epidemic period. The current level of herd immunity in
livestock may be insufficient to prevent another large outbreak, should suitable conditions recur.