Epidemiology of Rift Valley Fever Virus in South Africa: assessing herd immunity, viral persistence and drivers of epizootics in livestock

Rostal, Melinda K. (2020) Epidemiology of Rift Valley Fever Virus in South Africa: assessing herd immunity, viral persistence and drivers of epizootics in livestock. PhD thesis, University of Glasgow.

Abstract

Rift Valley fever virus (RVFV) is a zoonotic arbovirus that can cause devastating outbreaks in ruminant livestock. These Rift Valley fever (RVF) outbreaks may be multinational or regional and occur every 5-36 years. While a significant amount of research has been conducted, there remain many questions about the epidemiology RVFV. This study aims to examine drivers of interepizootic and epizootic RVF in livestock in central South Africa. We hypothesized that interepizootic transmission of RVFV was occurring in central South Africa; that antibodies induced by vaccination will wane over time; that RVFV can persist in a single pan setting with two vector species and a single host species; and that factors associated with endemic RVFV circulation on the farm and importation of RVFV through trade would drive the occurrence of RVF on a farm during the 2010 RVF outbreak, while factors associated with vector amplification and host susceptibility would drive the number of abortions and mortalities during the outbreak.

We collected serological and questionnaire data from longitudinal and cross-sectional field studies. We analysed these data using a variety of statistical methods, including Generalized Linear Models, mathematical modelling and Bayesian statistical analysis. We determined that low-level interepizootic RVFV transmission is occurring in central South Africa. Despite this, herd immunity declined in unvaccinated cattle, while remaining relatively constant in sheep and goats. Our simulation of the underlying biological mechanisms, suggested that RVFV persistence in a single pan is possible. However, persistence was very sensitive to the vector bite rate, host-vector ratio and transovarial transmission. We also determined that vaccination with Smithburn is effective in inducing long-lasting antibodies. Despite this, only a small portion of reportedly vaccinated sheep were seropositive. This was likely due to suboptimal vaccine handling. If the vaccine were mishandled along the supply chain, prior to administration it could significantly reduce the impact of vaccination on herd immunity.

Our results also suggested that drivers of interepizootic RVF may be different than those previously identified for widespread epizootics. Likewise, drivers of widespread epizootics were insufficient to predict the probability of RVF occurring though we were able to identify drivers of the severity of RVF in livestock measured through the number of RVFassociated abortions and mortalities on a farm during the 2010 outbreak. Further research is needed to identify different drivers of interepizootic transmission and the number of RVF-associated abortion and mortalities during and epizootic.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: RVFV, RVF, sheep, transovarial transmission, antibody.
• Subjects: Q Science > QR Microbiology > QR355 Virology
• Colleges/Schools: College of Medical Veterinary and Life Sciences > Institute of Biodiversity Animal Health and Comparative Medicine
• Funder's Name: U.S. Defense Threat Reduction Agency
• Supervisor's Name: Cleaveland, Professor Sarah, Haydon, Professor Daniel, Matthews, Professor Louise and Noam, Dr. Ross
• Date of Award: 2020
• Depositing User: Dr Melinda Rostal
• Unique ID: glathesis:2020-81395
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Oct 2026 13:06
• Last Modified: 19 Oct 2026 11:32
• Thesis DOI: 10.5525/gla.thesis.81395
• URI: https://theses.gla.ac.uk/id/eprint/81395