Patterns of Rift Valley fever virus seropositivity in domestic ruminants in central South Africa four years after a large outbreak

Yusuf B Ngoshe,Melinda K Rostal,Janusz T Paweska,William B Karesh,Alan Kemp,Claudia Cordel,Peter N Thompson,Whitney Bagge,Petrus Jansen Van Vuren,Veerle Msimang,Alida Avenant

doi:10.1038/s41598-020-62453-6

Abstract

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.

Highlights

Rift Valley fever (RVF) is a mosquito-borne viral zoonosis showing complex epidemiological patterns that are poorly understood in South Africa
Rift Valley fever (RVF) is an arthropod-borne viral zoonosis caused by RVF virus (RVFV), a member of the Phlebovirus genus, family Phenuiviridae of the recently established order Bunyavirales[1]
After adjustment for survey design, including sampling fraction and clustering within herd, overall RVFV seroprevalence in domestic ruminants was estimated to be 29.7%

Summary

Introduction

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; the level of herd immunity of ruminant livestock, a key determinant of outbreaks, is unknown. Outbreaks tend to occur following above average rainfall and localized flooding[9] These climatic conditions favour breeding of floodwater mosquitoes that are the proposed maintenance vectors of this virus via transovarial transmission[10]. Studies elsewhere in Africa have reported varying RVFV seroprevalences in livestock during interepidemic periods. Despite the fact that the herd immunity of domestic ruminant populations is a key determinant of outbreak occurrence and extent[17] there are no published studies on the seroprevalence of antibodies to RVFV in the central interior of SA. Effective vaccines are available, they are generally infrequently used in SA except in the face of an outbreak[6]

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