In 1930, a new disease was reported in sheep on a farm near the town of Naivasha, in the central Rift Valley region of Kenya. This disease caused mass spontaneous abortions among pregnant ewes and the death of newborn lambs. This disease, which came to be known as ‘Rift Valley fever’, also infected people who’d come into contact with the animals, causing muscle pains, fever and headaches. The causative agent was identified as a member of the Bunyaviridae family of viruses, known as Rift Valley fever virus.
Outbreaks of Rift Valley fever have subsequently happened sporadically in East Africa and other regions of sub-Saharan Africa and Madagascar where the virus is now considered endemic. There have also been large outbreaks in Egypt, and along the Red Sea in Yemen and Saudi Arabia. An outbreak in five East African countries in 1997–1998 caused the death of an estimated 100,000 animals and around 90,000 human infections. In the Garissa District of north-eastern Kenya, the virus was implicated in 170 deaths. Other recent outbreaks have occurred in Eastern Africa (2006–2007) and Southern Africa (2008–2011), as shown in the map below.
In addition to causing serious health problems in humans and animals, Rift Valley fever carries a high financial burden to affected countries. In Somalia, a trading ban on live animals to the Middle East following an outbreak caused the estimated losses of tens of millions of dollars.
While outbreaks are sporadic, they do have one thing in common: rainfall. In parts of East Africa and other endemic regions, mosquitoes lay their eggs in shallow depressions in grassland areas, which are known as dambos in East Africa and pans or vleis in Southern Africa. These areas flood during periods of prolonged and heavy rain (as shown in the picture below), causing the mosquito eggs to hatch, many of which are infected with the virus. Bites from infected adult mosquitoes are the main route of virus transmission between animals. Humans can also become infected through mosquito bites, but many cases come from contact with infected animal blood, perhaps through butchery or veterinary work.
During El Niño years, when the global temperature cycles of the planet lead to wetter conditions in East Africa, the rainy season can be extended, giving an additional three- to five-month period of above normal rainfall. During these periods, millions of mosquitoes can hatch from the dambos, representing a significant increase in risk for virus transmission.
Although El Niño events happen at irregular intervals of up to seven years, they have a recognised beginning, identified by a positive shift in water temperature patterns in the Pacific Ocean, and are often accompanied by a similar warming in the western Equatorial Indian Ocean. There is a one- to two-month gap between this shift and the extended rainfall in Africa, which gives people like Dr Assaf Anyamba time to act. Assaf is a Research Scientist with Universities Space Research Association at NASA’s Goddard Space Flight Center, and he is working on ways to provide early warning of potential Rift Valley fever outbreaks.
He and his colleagues do this by looking at data from polar orbiting satellites flying above the African continent. These satellites carry sensors that can be used to measure environmental variables, such as surface temperatures and the growth of vegetation at an astonishingly high resolution, in some cases looking at areas only 30 metres in diameter. Cooler, wetter areas promote the growth of plants, but also provide appropriate conditions for mosquito vectors to emerge and thrive.
The team used satellite data dating back to the early 1980s to hone their model, showing the correlation between increasing amounts of vegetation and Rift Valley fever cases. In 2006, they successfully predicted areas of East Africa that were at risk of an outbreak, with 64% of the reported human cases happening within predicted areas, and the remaining happening no more than 30 miles away.
Crucially, Assaf was able to provide governments and international agencies – such as the WHO and FAO – up to six weeks warning prior to the 2006 outbreak, allowing increased disease surveillance, animal quarantine and public health messages to be deployed.
The early warning system was used again last year, providing several months of preparation time. In some at-risk areas of Kenya, it appears that there were no reported cases of Rift Valley fever at all, thanks to successful animal vaccination.
There are several animal vaccines available to prevent infection, and this work is helping to target their use, which is much more cost-effective than a mass vaccination programme, particularly in an area as vast as East Africa. This approach to controlling Rift Valley fever is also undoubtedly more economical and ecologically viable than intensive insecticide use.
Assaf and his colleagues have also applied their surveillance model successfully in South Africa, showing the effects that the rain patterns of La Niña (essentially the opposite of El Niño) have on Rift Valley fever outbreaks. Assaf is also involved in a project in the country to learn more about the length of time between rainfall and the first case in an outbreak, and to investigate whether there is another, as yet undiscovered, reservoir for the disease.
“I’m originally from Kenya,” he explains, “and now I’m sitting in Goddard Space Flight Center where I’m not just able to help people from Kenya, but people all over the world.”
Investigations are also underway to see whether the same satellite monitoring technique could be used to provide early warning for other mosquito-borne viral diseases like dengue or chikungunya, although there remains a lot of missing data in the transmission models for these diseases.
More severe rainfall events and increased animal trade suggest that the forthcoming years may see more reported cases of Rift Valley fever. However, unlike many other emerging diseases, this one has vaccine options. Vaccinating livestock will stop the spread of this disease, but in order to be cost-effective, vaccines need to be delivered to the right place, at the right time. As such, the work of Assaf and his colleagues will be vital in stopping this disease.
Assaf’s work is supported by funding from the Department of Defense Health Agency (DHA)–Armed Forces Health Surveillance Branch in support of the Global Emerging Infections Surveillance and Response System (GEIS).
You can read more posts from our On the Horizon series here.