Disease found to be on the rise in highlands of Uganda and Kenya, which are becoming hotter and wetter
A recent study published in the BMJ Global Health journal has revealed that the geographical range of Rift Valley fever disease clusters is expanding across eastern Africa.
Rift Valley fever (RVF), a mosquito-borne viral disease caused by Rift Valley fever virus (RVFV), is known for extreme weather-associated large epidemics characterised by abortions and perinatal mortalities in livestock.
It also causes fever, jaundice, encephalitis, retinitis and haemorrhagic syndrome in humans.
Since its first recorded identification in 1930 in Kenya, RVF outbreaks have been reported throughout Africa with subsequent epidemic deaths in humans and livestock losses.
The BMJ Global Health study challenges previous beliefs about RVF, revealing that it predominantly occurs in small clusters rather than large outbreaks and its association with El Nino is not as pronounced as previously thought.
The study, titled Widening geographic range of Rift Valley fever disease clusters associated with climate change in East Africa, was conducted in the East Africa region – Kenya, Uganda and Tanzania. The region is characterised by a diverse ecosystem ranging from agriculturally productive highlands to semiarid and arid lands inhabited by nomadic pastoralists.
Regional climatic conditions in eastern Africa are not uniform. Kenya, Uganda and northeastern parts of Tanzania witness bimodal rainfall (two wet and two dry seasons annually), with long rains occurring from March to May and short rains from October to December.
In contrast, central and southern Tanzania experience unimodal rainfall (one wet and one dry season) from November to May.
Most of the region experiences substantial levels of rainfall in at least one season, except the northern and eastern parts of Kenya.
Temperatures in most areas range from a moderate 15 degree C (°C) to 25°C, except in coastal belts characterised by humidity and higher temperatures.
The researchers collected data on 100 RVF disease events in eastern Africa from 2008 through 2022.
Of the 100 events, 44 were reported in Uganda, 41 in Kenya and 15 in Tanzania.
Among the 100 events, 91 were classified as ‘small clusters’ with a median of one human and three livestock cases, with minimal human mortality occurring primarily in highlands (67 per cent). Thirty-five per cent of the clusters occurred in regions that had never reported RVF cases before.
The study modelled against possible geo-ecological risk factors of occurrence including altitude, soil type, rainfall, temperature, normalised difference vegetation index (NDVI), livestock production system, land-use change and long-term climatic variations.
A positive NDVI change one month prior to RVF occurrence was observed in 51 per cent of the small clusters.
The small RVF clusters were situated in areas with notable land use changes, particularly those leading to significant water accumulation such as rice farming, mining and irrigation.
A °C increase in temperature and a 1-unit increase in NDVI, one month prior, were associated with increased RVF incidence rate ratios of 1.20 and 1.93 respectively.
The study findings on climatic trends analysis revealed a significant decadal increase in annual mean temperature, associated with decreasing rainfall in arid and semi-arid lowlands but increasing rainfall trends in highlands. These hotter and wetter highlands showed increasing frequency of RVF clusters, accounting for 76 per cent and 43 per cent in Uganda and Kenya, respectively.
Climate change to blame
With ongoing heating of the planet, eastern Africa is projected to undergo changes in temperature and precipitation patterns with continued increases in seasonal and annual mean temperatures, especially in highland areas.
The study highlighted the increasing frequency of small RVF clusters in previously unaffected areas, associated with a combination of higher temperature and rainfall.
The widening geographic range of disease is associated with climatic variations, with the likely impact of wider dispersal of the virus leading to the creation of new regions with virus endemicity.
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