Dr Bernard Bett is a veterinary epidemiologist at the International Livestock Research Institute (ILRI), where he co-leads research on emerging zoonotic diseases in developing countries. He is a partner in the Dynamic Drivers of Disease in Africa Consortium, an ESPA-funded research programme exploring the linkages between ecosystems, disease and poverty. He works with various actors in the public and private sectors to develop decision support tools for managing disease risks based on information generated from research.
Recent climate predictions suggest East Africa may be in line for an epidemic of Rift Valley fever – an infectious disease which can hit people, their livestock and livelihoods, and national economies hard.
Data from the Climate Prediction Centre and International Research Institute for Climate and Society suggest there is a 99.9% chance there will be an El Niño occurrence this year, with a 90% chance it will last until March/April 2016.
At least two of the most recent Rift Valley fever epidemics in East Africa – those in 1997/98 and 2006/2007 – were associated with El Niño weather patterns, with Kenya suffering losses amounting to US$32 million in the most recent.
Given the strong predictions of an El Niño occurrence, and the established association between El Niño and Rift Valley fever risk, countries in the Horn of Africa need to start laying out measures to manage the developing risk. In particular, public education on the linkages between the expected weather patterns and disease risk is vital to minimise human exposure to the disease should an epidemic occur.
Disease and climate change
El Niño – the warm phase of El Niño Southern Oscillation (ENSO) – is a warming of sea surface temperatures in central and eastern equatorial Pacific Ocean which occurs every three to seven years, triggering a chain of atmospheric changes throughout the world. In the Horn of Africa it results in heavy rainfall, particularly when there is a simultaneous increase in the sea surface temperatures in the western Indian Ocean. The current predictions give a strong indication that sea surface temperatures in the Indian Ocean are indeed also on an upward trend.
Questions are often raised on whether there are linkages between ENSO and global warming. ENSO is not considered a driver of climate change, but it is the strongest naturally occurring source of climate variability globally. As such, global warming is likely to double the frequency of ‘super’ El Niños.
Epidemics of malaria, Rift Valley fever and foot rot in animals have all been reported in eastern Africa during past El Niño events.
Rift Valley fever is a zoonotic disease, i.e. one that is passed to humans from a vertebrate animal ‘host’. It is caused by the Rift Valley fever virus which is carried by mosquitoes. Intense and persistent rain provides perfect breeding conditions for the mosquito vectors which initiate and intensify the virus’s transmission.
Cattle, sheep and goats play a critical role in the amplification of the virus’s transmission, especially during initial stages of an epidemic. Animals that recover acquire lifetime immunity, but high livestock turnover rates among the pastoralists of East Africa mean herd immunity is rarely sustained for long. This could be one of the factors that make arid and semi-arid areas more prone to epidemics than other areas.
The disease causes sudden abortion in sheep, goats, cattle and camels, as well as high mortality rates in their young. People get exposed to the disease either by coming into direct contact with the tissues of infected animals or via mosquito bites. Most human cases present as acute but mild febrile illness, though a few (less than 8%) result in severe illnesses, characterised by haemorrhagic fever, retinitis or encephalitis.
The disease has substantial impacts on livestock productivity and trade. At the farm level, it results in extensive productivity and food security losses. At industry level, market closures, reduced demand for livestock products and embargoes on livestock exports are highly damaging.
Recovery from an epidemic can be slow as the extensive abortions and perinatal mortality associated with the disease decimate the replacement stock. Emergency interventions such as surveillance and vaccination campaigns also result in heavy financial losses for countries that are already among the world’s poorest.
Vaccination, education and surveillance
Several risk management tools have been developed since the last Rift Valley fever epidemic. These include Contingency Plans, Standard Operating Procedures and Decision Support Framework (DSF) . More can be done though and all efforts now need to be geared towards managing an impending epidemic.
Surveillance, starting with known disease hotspots, but extending to other areas with time, is essential. Risk maps can be used to design risk-based surveillance measures.
Livestock vaccination should also be implemented, especially in disease hot spots. Recent model-based analyses carried out as part of research for the Dynamic Drivers of Disease in Africa Consortium suggest that where there has been no prior or periodic vaccination, at least 70% vaccination coverage needs to be implemented at least two months before an epidemic (or when the areas are still accessible) to prevent an epidemic. This level of coverage, though, is quite high and may not be feasible for most production systems, especially where livestock producers have a low perception of the risk they face.
These model-based analyses also demonstrate that vaccinating strategically over a five-year period could reduce the thresholds that a reactive vaccination needs to attain to stop an epidemic. Though clearly this recommendation is not applicable at the moment, it is an approach which should be considered in the future.
For now, there is an urgent need to make the most of existing climate predictions. The heads of public health and veterinary departments in most countries in the region have alerted their field staff to the current risk and some actions such as mass vaccination of livestock have been initiated in various areas. However, in the past, the deployment of response measures has been hampered by the slow processes required to mobilise resources. This should not be allowed to happen again.
Ebola and lessons from other recent epidemics
Lessons that have been learnt from the recent epidemics of Ebola, bird flu and MERS should be used to inform response to this disease. The recent Ebola epidemic in West Africa, for example, demonstrated that a delay in response to such an epidemic would result in a severe health crisis and extensive socio-economic impacts. A few countries such as Nigeria were able to manage the epidemic by responding swiftly and isolating the initial cases. Given the high frequency at which emerging infectious diseases are now occurring, there is need for early warning systems, preparedness and response plans that identify ways of involving the local communities to effectively manage these risks.
This blog is a shorter version of a longer article by Dr Bernard Bett on the STEPS Centre website.
Dr Bernard Bett is a veterinary epidemiologist at the International Livestock Research Institute (ILRI), where he co-leads research on emerging zoonotic diseases in developing countries. He is a partner in the Dynamic Drivers of Disease in Africa Consortium, an ESPA-funded research programme exploring the linkages between ecosystems, disease and poverty.