Cordel, C. | bioRxiv Science

bioRxiv2025

A Descriptive Study of wetland vegetation, ecology, habitat and Rift Valley Fever virus vectors in South Africa.

Outbreaks of Rift Valley fever (RVF) have been closely linked to environmental conditions that support large vector populations with what is required for breeding of vectors. Fine-scale vegetation surveys were used to assess how wetland plant communities vary across sites with historical RVF outbreaks in Free State Province of South Africa and to detail plant species density that correlates with mosquito habitat. MethodsAt 22 sites, we compiled 201 releves using the modified Braun-Blanquet method in combination with 200 transects using the line-point method developed specifically for this study. A total of 195 plants were identified and vouchered. These data were combined with soil analyses carried out at all 22 sites and with data from bimonthly mosquito trapping at all sites. Results and significanceWe identified an increase of invasive plants and upland species compared to the earlier study and observed that drought causes a change in wetland vegetation community structure and species composition. However, the wetland vegetation was still dominated by grasses and sedges, with a limited presence of Juncus and forbs. Livestock grazing pressure on wetlands to the drought results in reduced vegetation cover, abundance and diversity. Our work describes a changing landscape that may impact RVFV maintenance and transmission, and provides insights into extensive livestock farming and rainfall over shallow endorheic depressions in temperate grasslands and subtropical riparian environments.

ecology↗

bioRxiv2023

Localized Rift Valley Fever Virus Persistence Depends on a High Transovarial Transmission Fraction

Rift Valley fever virus (RVFV) has spread beyond continental Africa and threatens to follow West Nile, chikungunya and Zika viruses into the Americas. Its impact in new localities and the capacity to control future outbreaks, depends on whether and how RVFV persists at small spatial scales. Transovarial transmission (TOT) is hypothesized as an important mechanism for local persistence, yet its role in RVFV ecology remains poorly understood. We examine whether RVFV can persist locally via TOT while maintaining a realistic seroprevalence pattern of interepidemic and epidemic transmission. We developed a mechanistic, compartmental model of RVFV dynamics within a single host (sheep) and two vector (mosquito) populations, driven by temperate climatic factors. Decades-long persistence was possible in our simulations, which generally captured the observed outbreak patterns in central South Africa with a mean annual seroprevalence ([~]23%) within the range reported during interepidemic periods (5-40%). Persistence was only possible with a substantial TOT fraction and over a narrow range of parameters. The basic reproduction number (R0) was close to one at mean vector population sizes, suggesting a relatively limited expansion of the infected vector population during outbreaks. This limited expansion provides the system with the flexibility to support both low-level transmission and large outbreaks and, counterintuitively, large outbreaks resulted in smaller infected Aedes egg populations. This has important consequences for control: low-level vaccination may prevent large outbreaks without eliminating RVFV and local control efforts may be most effective immediately following an outbreak, suggesting elimination may be possible after emergence in temperate regions.

ecology↗