Risk factors associated with Rift Valley fever epidemics in South Africa in 2008–11

Abstract

Rift Valley fever (RVF) is a zoonotic and vector-borne disease, mainly present in Africa, which represents a threat to human health, animal health and production. South Africa has experienced three major RVF epidemics (1950–51, 1973–75 and 2008–11). Due to data scarcity, no previous study has quantified risk factors associated with RVF epidemics in animals in South Africa. Using the 2008–11 epidemic datasets, a retrospective longitudinal study was conducted to identify and quantify spatial and temporal environmental factors associated with RVF incidence. Cox regressions with a Besag model to account for the spatial effects were fitted to the data. Coefficients were estimated by Bayesian inference using integrated nested Laplace approximation. An increase in vegetation density was the most important risk factor until 2010. In 2010, increased temperature was the major risk factor. In 2011, after the large 2010 epidemic wave, these associations were reversed, potentially confounded by immunity in animals, probably resulting from earlier infection and vaccination. Both vegetation density and temperature should be considered together in the development of risk management strategies. However, the crucial need for improved access to data on population at risk, animal movements and vaccine use is highlighted to improve model predictions.

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

“…For a vector-borne disease, R 0 is determined by several factors which may vary in different settings, including the vector to host ratio, the biting rate and the vector survival rate 54 . The HIT for RVF near the start of the 2010 outbreak in our study area was previously estimated to be between 50% and 85%, based on an estimated effective reproduction number that peaked at 4.3 in February 2010 17 . Estimated seroprevalence in our study was well below this range in all three species, indicating that, should conditions similar to those in early 2010 recur, the likelihood of another large outbreak may be high.…”

“…Outbreaks may then be amplified by epidemic vectors, of which Culex theileri is considered the most important on the interior plateau 12 . Risk factor studies conducted during and after outbreaks in both humans and animals have identified several other environmental, human, and animal factors that may be associated with RVF outbreaks and RVFV seropositivity [13][14][15][16][17] . The presence of large water bodies was found to be associated with seropositivity in Somalia 15 and southwest Saudi Arabia 16 .…”

Prevalence and identification of arthropod-transmitted viruses in Kassala state, Eastern Sudan

“…Dengue, yellow fever, chikungunya, Rift Valley fever, alkhurma haemorrhagic fever, and Sindbis viruses have been reported in more than 34 countries in sub-Saharan Africa and Middle East [10][11][12][13][14]. YFV originated in Africa and spread with the slave trade dating back to at least 1650 [15,16].…”

Anomalous High Rainfall and Soil Saturation as Combined Risk Indicator of Rift Valley Fever Outbreaks, South Africa, 2008–2011

“…In this regard, irrigation is of particular importance in outbreaks in the Orange River region during October–December 2009 and in outbreaks that occurred in low-risk areas of Western Cape Province in 2010 and 2011. Findings from previous studies strongly suggest that irrigation could create suitable breeding habitats for mosquito vectors and lead to subsequent outbreaks of RVF ( 9 , 24 ). …”