ENVIRONMENTAL CHANGE AND RIFT VALLEY FEVER

Climate change in Africa is likely to create the necessary conditions for more outbreaks of disease. RVF is a virus transmitted by blood feeding flies and mosquitos. It affects both humans and valuable livestock such as sheep, goats, buffalos and camels. The virus was first identified in 1931 during an epidemic among cattle (Greenhalgh 2015). RVF occurs following periods of extreme precipitation. Extreme precipitation events flood fly and mosquito breeding habitats, creating an ideal environment for RVF infected insect eggs to hatch and spread the disease. The RVF virus is extremely difficult to control because the infected fly and mosquito eggs can survive for years in wet soil. Following RVF infection, animals develop a high level of virus in their bloodstream. This infects previously uninfected flies and mosquitos, which in turn infect other animals and humans. Humans can catch the disease either through direct contact with the blood, from the bite of a blood-feeding fly or mosquito, or through consuming the meat of infected animals, which for many people are a primary source of milk and food (Greenhalgh 2015).

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 In 2005-2007, RVF outbreaks in Madagascar, Tanzania, Kenya, Somalia and Sudan caused more than 200,000 human infections and was responsible for approximately 500 deaths (WHO 2021). In Kenya alone, international export bans and livestock losses resulting from the disease cost the country $32 million (WHO 2021) The RVF outbreak of 1997 caused 170 RVF associated deaths in Kenya and caused approximately 28,000 infections (Greenhalgh 2015). The most recent outbreak occurred in Kenya in February 2021, causing approximately 40 human infections and 11 deaths. The disease also affected livestock including sheep, goats and camels. The 2021 outbreak is thought to be linked with the flooding of the Ewaso and Dawa rivers in Kenya (WHO 2021).

Climate Change and Precipitation

It is expected that the warming in Africa will continue and is likely to exceed 2°C by the end of this century (Chou et al 2007). It is also projected that the African land temperature will increase faster than the global land average, especially in more arid regions such as Northern Africa (Niang et al 2014). Increased temperatures will cause a significant reduction in precipitation in North Africa and Southern Africa by the end of this century. This is likely to cause longer periods of drought and increased irregularity in river discharge (Chou et al 2007). However, climate change will also increase the variability of precipitation events in Africa, causing more extreme precipitation events and fewer lower and moderate intensity precipitation events.

The increase in extreme rainfall events in the African continent is largely due to global warming and the associated effects of the ‘Clausius Clapeyron relation’ (CCR). The CCR is based on atmospheric thermodynamics and expresses the relationship between the temperature, pressure and the moisture-holding capacity of the atmosphere (Murphy et al, 2017). This relation projects that the moisture-holding capacity of the atmosphere increases by approximately 7% per degree temperature increase (Niang et al 2014). A warmer atmosphere therefore holds more moisture, resulting in more frequent extreme precipitation events.

Future RVF projections

East Africa is projected to experience a significant increase in extreme precipitation events in comparison to the rest of the African continent (Murphy et al, 2017). These precipitation events will form an ideal environment for more RVF outbreaks in East Africa. They also create the potential for RVF to expand its geographic range (Niang et al 2014).

Figure 1: 2015 projections of extreme precipitation and RVF outbreaks (Source)

Studies have found that there is a high risk of future RVF outbreaks in parts of East Africa that are currently unaffected by the disease. The greatest projected changes in RVF outbreaks are in Tanzania and Kenya. RVF changes remain stable until the year 2050 when compared with baseline results, before increasing significantly by the end of the century (Greenhalgh 2015). The projected increases are most evident in Western Uganda, Rwanda, Berundi and Western Kenya (Greenhalgh 2015). The study also highlighted the link between socio-economic factors and risk of disease. Risk of RVF is highest at the border between South Sudan and North-Western Kenya due to the high densities of livestock, high levels of poverty and poor infrastructure (including poor health services) (Greenhalgh2015). The studies show that with climate change and increasing events of extreme rainfall, there is a clear need to be cautious and address both the environmental and socio-economic factors that influence disease outbreaks.