New outbreak of fatal Rift Valley fever in the Horn of Africa

Rift Valley fever is a viral disease of people and ruminant animals transmitted by mosquitoes. Epidemics frequently present as extensive abortion storms in small ruminants and cattle combined with heavy mortality in young animals. In people, the disease is most often a febrile illness without serious consequences. In a low percentage of human cases (about 1% or less), hemorrhagic complications can arise. Blindness also occasionally results. 118 deaths have been confirmed since the outbreak in November 2006 in the North-eastern province and coastal region of Kenya.

The disease is transmitted by the bite of infected mosquitoes or heavy exposure to aerosols in situations such as the slaughtering of infected animals. Outbreaks of the disease are associated with changes in local water resource management or periods of heavy rainfall. Examples have been the construction of new dams or El Nino rain events such as the one in East Africa in 1997-98 when there was a major outbreak of Rift Valley fever in Kenya and Somalia. The virus has been shown to over-winter in infected mosquito eggs. At the onset of the rains, infected mosquitoes transmit the disease to suitable amplifying hosts such as small ruminants. If vector densities are sufficiently high due to favourable environmental conditions, this starts a cascade-like recrudescence of the virus in the host and vector populations, leading to an epidemic.

Severe human cases, although an infrequent outcome of infection, are often the event that triggers recognition that an epidemic is under way.  There is need to develop early warning systems and to validate prevention and control strategies that can mitigate the evolution of outbreaks. Rift Valley fever causes serious economic losses in livestock particularly in cattle and sheep, although goats, camels, Asian water buffalo and wild antelopes may be vulnerable.

Key research questions

A number of important research questions related to Rift Valley fever and its impact remain unanswered and worthy of further research.  These include the following:

•  What is the economic impact of an RVF outbreak, particularly in terms of distribution, livelihoods, international trade, public health, and other macro-level factors?  How does the disease affect unrelated sectors (e.g., tourism)?
•  How has the disease broadly affected trade patterns in livestock products from the horn of Africa and what are potential future impacts? How can these be mitigated?
•  How effective are current vaccines in their ability to prevent disease and how frequent are side effects? There are two types of vaccines currently in use, both of which have serious disadvantages.  For human use, a ‘killed vaccine’consists of formalin-inactivated virus for restricted use.   It requires several doses and annual revaccination.  It is not approved for general distribution and is used only for laboratory workers and other specialized groups.  A live, attenuated vaccine is approved for use in livestock.  It induces a solid, life-long immunity but may cause abortions if administered to pregnant animals. 
•  What is the epidemiological impact and cost-effectiveness of alternative types of vaccination and movement control strategies?  How can these tools be best used in the face of outbreaks like the one we are experiencing now?
•  Can diagnostic tests for the disease be improved to make them more ‘user-friendly’ for field workers and remote laboratories?  Is it possible to develop good diagnostic tests to distinguish between active and past infections, and to distinguish previously exposed animals from vaccinated animals?
•  How can we enhance decision-making and promote the application of risk-based standards to ensure safe international trade of livestock products and scientifically sound trade restrictions?

 The Nairobi-based International Livestock Research Institute (ILRI) is actively seeking to become engaged in two areas.

In diagnostics, ILRI recently held discussions with the Kenya’s Department of Veterinary Services and South Africa’s Onderstepoort Veterinary Institute (OVI).  OVI have developed a field-based test to diagnose RVF infection in cattle.  This test requires only the application of a small blood sample to the device with a result obtained in about three minutes.  Such a test has advantages over a laboratory-based test, in terms of speed of diagnosis and no need for electricity or other equipment.  Although the test has profed successful in the laboratory, it has yet to undergo extensive testing in the field to ensure that it is sufficiently accurate.  It is envisaged that ILRI will be involved in this testing, using samples from the current outbreak.

On another front, ILRI is pursuing the possibility of working with a Walter Reed Project (WRP) and the US-based Centers for Disease Control (CDC) to support their ongoing efforts to understand and control this present outbreak of Rift Valley Fever.  Internal discussions within ILRI highlight three key areas in which ILRI could contribute in this process:

• Sensitise key stakeholders, particularly in government of the epidemiological and economic magnitude and impact of the current outbreak in Kenya.
• Initiate a process to identify appropriate veterinary control strategies to reduce both animal   and human incidence of the disease
• Take advantage of the current situation to collect key epidemiological and economic data to guide further research and improve risk mitigation tools

ILRI is in discussions with the WRP-CDC teams to define roles specific for ILRI in the areas of assessing the socio-economic impacts of the disease, participatory epidemiology and surveillance, and the interface between livestock and public health.  ILRI aims to help WRP-CDC in their short-run emergency response efforts as well as to use this current outbreak to help design decision-support tools to better manage future occurrences of Rift Valley Fever.

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