Epidemiologic and environmental risk factors of rift valley fever in southern Africa from 2008 to 2011

Authors: GLANCEY, MARGARET - Goddard Space Flight Center; ANYAMBA, ASSAF - Goddard Space Flight Center; Linthicum, Kenneth - Ken

Submitted to: Vector-Borne and Zoonotic Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/28/2015
Publication Date: 8/1/2015
Citation: Glancey, M.M., Anyamba, A., Linthicum, K. 2015. Epidemiologic and environmental risk factors of rift valley fever in southern Africa from 2008 to 2011. Vector-Borne and Zoonotic Diseases. 15(8):502-511.

Interpretive Summary: Southern Africa experienced the worst series of Rift Valley fever outbreaks in nearly 40 years between 2008 and 2011. Rift Valley fever is a serious disease of cattle, sheep, goats and humans, and it is transmitted by the bite of infected mosquitoes. The outbreaks in South Africa initially impacted cattle in the eastern provinces and later sheep in the interior and southern provinces. The outbreaks occurred after increased rainfall; however, their correlation with rainfall and vegetation development was less than has been observed in Eastern Africa. Outbreaks of Rift Valley fever in South Africa are also related to geographical factors such as land use, drainage and topography.

Technical Abstract: BACKGROUND: Rift Valley fever outbreaks have been associated with periods of widespread and above average rainfall over several months which allows for the virus infected mosquito vector populations to emerge and propagate. This has provided basis to develop complex models based on environmental factors to predict Rift Valley fever outbreaks in Eastern Africa. From 2008 to 2011, South Africa experienced the worst multi-year wave of Rift Valley fever outbreaks in almost 40 years. We investigated similar environmental factors in Southern Africa to be used in future prediction models for this region. METHODS: We tabulated and summarized the Rift fever epidemic records obtained from the World Animal Health Information Database (WAHID) in terms of the livestock species affected, location, and time. Environmental variables including rainfall and satellite-derived normalized difference vegetation index (NDVI) data were assessed in outbreak regions in order to understand the underlying drivers of the outbreaks. RESULTS: The predominant species affected depended on the location of the outbreak. In 2008 and 2009, outbreaks were concentrated in the eastern provinces of South Africa, mainly affecting cattle. In 2010 and 2011 outbreaks occurred in the interior and southern provinces of South Africa affecting over 16,000 sheep. Epidemics occurred in different regions every year and moved from the northeast of South Africa toward the southwest with each progressing year. The highest intensity of cases occurred in mid-summer to fall between January and April each year. The South African outbreaks generally showed a pattern of increased rainfall before epizootics began; however, NDVI and rainfall were less correlated than has been observed in Eastern Africa. CONCLUSIONS: The multi-year Rift Valley outbreak from 2008 to 2011 in South Africa indicates that besides rainfall and NDVI, other important environmental and geographical factors such as land use, drainage, and topography play a role with respect to Rift Valley fever emergence. Future investigations into these factors as well as the current findings will be able to contribute to updating forthcoming models to increase spatial accuracy of mapping risk areas, allowing for adequate preparation and prevention time before an outbreak occurs.