High seroconversion rate to Rift Valley fever virus in cattle and goats in far northern KwaZulu-Natal, South Africa, in the absence of reported outbreaks

PLoS Negl Trop Dis. 2019 May 3;13(5):e0007296. doi: 10.1371/journal.pntd.0007296. eCollection 2019 May.

Abstract

Background: Rift Valley fever (RVF) is a mosquito-borne zoonotic disease characterized in South Africa by large epidemics amongst ruminant livestock at very long, irregular intervals, mainly in the central interior. However, the presence and patterns of occurrence of the virus in the eastern parts of the country are poorly known. This study aimed to detect the presence of RVF virus (RVFV) in cattle and goats in far northern KwaZulu-Natal province and to estimate the prevalence of antibodies to the virus and the incidence rate of seroconversion.

Methodology: Cross-sectional studies were performed in communally farmed cattle (n = 423) and goats (n = 104), followed by longitudinal follow-up of seronegative livestock (n = 253) 14 times over 24 months, representing 160.3 animal-years at risk. Exposure to RVFV was assessed using an IgG sandwich ELISA and a serum neutralization test (SNT) and seroconversion was assessed using SNT. Incidence density was estimated and compared using multivariable Poisson models and hazard of seroconversion was estimated over time.

Principal findings: Initial overall seroprevalence was 34.0% (95%CI: 29.5-38.8%) in cattle and 31.7% (95%CI: 22.9-41.6%) in goats, varying by locality from 18-54%. Seroconversions to RVFV based on SNT were detected throughout the year, with the incidence rate peaking during the high rainfall months of January to March, and differed considerably between years. Overall seroconversion rate in cattle was 0.59 per animal-year (95% CI: 0.46-0.75) and in goats it was 0.41 per animal-year (95% CI: 0.25-0.64), varying significantly over short distances.

Conclusions/significance: The high seroprevalence in all age groups and evidence of year-round viral circulation provide evidence for a hyperendemic situation in the study area. This is the first study to directly estimate infection rate of RVFV in livestock in an endemic area in the absence of reported outbreaks and provides the basis for further investigation of factors affecting viral circulation and mechanisms for virus survival during interepidemic periods.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Antibodies, Viral / blood*
  • Cattle
  • Cattle Diseases / blood
  • Cattle Diseases / epidemiology
  • Cattle Diseases / virology*
  • Cross-Sectional Studies
  • Disease Outbreaks
  • Female
  • Goat Diseases / blood
  • Goat Diseases / epidemiology
  • Goat Diseases / virology*
  • Goats
  • Humans
  • Immunoglobulin G / blood
  • Male
  • Rift Valley Fever / blood
  • Rift Valley Fever / epidemiology
  • Rift Valley Fever / virology*
  • Rift Valley fever virus / genetics
  • Rift Valley fever virus / immunology
  • Rift Valley fever virus / isolation & purification*
  • Seroconversion
  • Seroepidemiologic Studies
  • South Africa / epidemiology

Substances

  • Antibodies, Viral
  • Immunoglobulin G

Grants and funding

This project was partially supported by the Cooperative Agreement Number 5 NU2GGH001874-02-00, funded by the Centers for Disease Control and Prevention. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the Centers for Disease Control and Prevention or the Department of Health and Human Services. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.