A phytosociological analysis and description of wetland vegetation and ecological factors associated with locations of high mortality for the 2010-11 Rift Valley fever outbreak in South Africa

<div><p>Rift Valley fever (RVF) is endemic in Africa and parts of the Middle East. It is an emerging zoonotic disease threat to veterinary and public health. Outbreaks of the disease have severe socio-economic impacts. RVF virus emergence is closely associated with specific endorheic wetlands that are utilized by the virus’ mosquito vectors. Limited botanical vegetation surveys had been published with regard to RVF virus (RVFV) ecology. We report on a phytosociological classification, analysis and description of wetland vegetation and related abiotic parameters to elucidate factors possibly associated with the 2010–2011 RVFV disease outbreak in South Africa. The study sites were located in the western Free State and adjacent Northern Cape covering an area of ~40,000 km² with wetlands associated with high RVF mortality rates in livestock. Other study sites included areas where no RVF activity was reported during the 2010–11 RVF outbreak. A total of 129 plots (30 m²) were selected where a visible difference could be seen in the wetland and upland vegetation. The Braun-Blanquet method was used for plant sampling. Classification was done using modified Two-Way Indicator Species Analysis. The vegetation analysis resulted in the identification of eight plant communities, seven sub-communities and two variants. Indirect ordination was carried out using CANOCO to investigate the relationship between species and wetland ecology. The study also identified 5 categories of wetlands including anthropogenic wetlands. Locations of reported RVF cases overlapped sites characterized by high clay-content soils and specific wetland vegetation. These findings indicate ecological and environmental parameters that represent preferred breeding habitat for RVFV competent mosquito vectors.</p></div

A phytosociological analysis and description of wetland vegetation and ecological factors associated with locations of high mortality for the 2010-11 Rift Valley fever outbreak in South Africa - Fig 5

<p><b>Categorization of five, freshwater wetland depressions-types with descriptions of vegetation and ecology.</b> A. Deelpan, a typical saline endorheic pan, with narrow, the dense vegetated pan-margin, providing ideal breeding habitat for <i>Aedes</i>. Aa. Holpan, a non-saline pan, covered with <i>Eragrostis bicolor</i> the low, caespitose, specialist arid-region grass. B. De Dam, shallow depression wetland with clay soil and emerging sedge <i>Fuirena coerulescens</i>, grass <i>Echinochloa colona</i> and fern, <i>Marisela capensis</i>. Bb. Petrusburg wetland with large-tufted <i>Scirpoides dioecious</i>, emerging <i>Cyperus laevigatus</i> sedges, and the spreading, prostrate forb <i>Hypertelis salsoloides</i>. C. Mature ox-bow cut-off, 100cm deep, with wetland vegetation, sedges, grasses, on the margins. Cc. Inundated ox-bow wetland-type at Bougainvillea, a site of high sheep mortality during the 2010 outbreak. D. Riet River in flood, near Mokala National Park, with dense, monotypic stands of <i>Phragmites australis</i>. Dd. Seven Dams had no RVF mortalities. The most species-rich wetlands with extensive stands of <i>Phragmites australis</i> (foreground) and <i>Typha capensis</i>. E. Sedge and Juncus dominated wetland. The deep grove is created by the wheel of the pivot irrigator. Ee, Extensive, spill-over wetland created at Rooibokpan near Jacobsdal, dominated sedges, <i>Juncus</i> and OBL forbs in the <50 cm deep water.</p

Selected wetland soil properties correlate to Rift Valley fever livestock mortalities reported in 2009-10 in central South Africa.

Outbreaks of Rift Valley fever have devastating impacts on ruminants, humans, as well as on regional and national economies. Although numerous studies on the impact and outbreak of Rift Valley fever exist, relatively little is known about the role of environmental factors, especially soil, on the aestivation of the virus. This study thus selected 22 sites for study in central South Africa, known to be the recurrent epicenter of widespread Rift Valley fever outbreaks in Southern Africa. Soils were described, sampled and analyzed in detail at each site. Of all the soil variables analyzed for, only eight (cation exchange capacity, exchangeable Ca2+, exchangeable K+, exchangeable Mg2+, soluble Ca2+, medium sand, As, and Br) were statistically identified to be potential indicators of sites with reported Rift Valley fever mortalities, as reported for the 2009-2010 Rift Valley fever outbreak. Four soil characteristics (exchangeable K+, exchangeable Mg2+, medium sand, and Br) were subsequently included in a discriminant function that could potentially be used to predict sites that had reported Rift Valley fever-associated mortalities in livestock. This study therefore constitutes an initial attempt to predict sites prone to Rift Valley fever livestock mortality from soil properties and thus serves as a basis for broader research on the interaction between soil, mosquitoes and Rift Valley fever virus. Future research should include other environmental components such as vegetation, climate, and water properties as well as correlating soil properties with floodwater Aedes spp. abundance and Rift Valley fever virus prevalence

Annual comparison over 3 years of numbers of <i>Culex</i> and <i>Aedes</i> collected.

<p>The columns show the clear relationship between rainfall and mosquito numbers. The annual comparison of mosquito samples from 2014 to 2017 is derived from the Rift Valley fever virus vector surveillance work package.</p

Rainfall data for the study sites.

<p>Fig 3a. Annual total rainfall map for the study region showing a gradient on decreasing rainfall from East to West/Southwest. Also shown are locations of weather stations (that are coincident with vector sampling sites) and other vector sampling sites at farm locations with high mortality during the 2010/2011 epizootic outbreak. Fig 3b. Cumulative daily rainfall profiles for Graspan/Holpan and Brakput monitoring locations. Graspan and Brakput locations showing rainfall trajectories for different years including the RVF epizootic year 2010/2011 (above normal rainfall shown in green) and the record drier-than-normal year 2015/2016.</p

Ordination: The ordination diagram illustrates the gradients of ecological and microclimatic conditions.

<p>Community 8 to Community 1, horizontal axis represents an east to west gradient of geology, solid, species diversity. Vertical axis shows plant height, wetland status and degree of wetness.</p

Reported deaths of livestock due to Rift Valley Fever, during the 2010 outbreak, centred in the Free State, South Africa.

<p>The diagonally lined box indicates the 200km x 200km study area. Rift Valley Fever deaths of livestock were reported in eight of the nine South African provinces. The study area was centred in the region of highest mortality in the western Free State. Mortality data derived from the RSA, OIE Report 17.</p

Location of study sites and nearest towns, with endorheic pans, upland depressions and palustrine wetlands shown in blue.

<p>Study sites are situated in the areas of highest Rift Valley Fever mortalities coinciding with the most dense concentration of wetlands in the western Free State.</p

Relationship between named plant communities, vegetation characteristics and reported RVF mortalities.

<p>Relationship between named plant communities, vegetation characteristics and reported RVF mortalities.</p

Animal cases and deaths during the 2010 Rift Valley fever outbreak.

<p>Animal cases and deaths during the 2010 Rift Valley fever outbreak.</p