Prevention strategies for Rift Valley fever

INTRODUCTION

RVF is a very serious and important zoonosis. Countries at risk should take whatever steps they can to prevent the entry and/or occurrence of the disease. As with all serious livestock diseases, a comprehensive quarantine programme should be regarded as the first line of defence. However, no records show to date that RVF has been transported by animal movement from one country or area to another. The movement of animals has not been associated with new foci of disease in Africa, as has been the case with lumpy skin disease and many other animal diseases. It has been suggested that RVF entered Egypt via camels from the Sudan; while this cannot be disproved, the very brief and low level of viraemia in camels, with the 12-14 day journey, makes it extremely unlikely. Vector movement in air currents is a well-documented and proven means for the dissemination of plant insect pests and malaria. Animal movement should be closely monitored if animals are being imported from known epizootic areas, and this should only take place in demonstrably validated interepizootic periods. Insect vector movement in low-level air currents is uncontrollable and vigilance is necessary to monitor for possible RVF introductions in the receptive areas deemed to be of high risk.

The potential for countries in different regions of the world to implement effective strategies for RVF varies enormously.

PREVENTION STRATEGIES FOR COUNTRIES IN AFRICA

It is virtually impossible to prevent RVF outbreaks in regions of Africa where outbreaks have occurred in the past. Livestock movement controls are unlikely to play any role in propagating RVF in the enzootic/epizootic areas of Africa. This is probably even the case where there are no endemic cycles of infection (such as in Egypt). Continuing mass livestock vaccination programmes during interepidemic periods are unlikely to be an economically viable proposition. However, consideration should be given to routine vaccination of high-grade animals.

This does not mean that nothing can be done. On the contrary, the emphasis must change to early warning programmes (Chapter 5) to detect the first evidence of a likely epidemic and early reaction programmes (Chapter 6) so that disease outbreaks in livestock and humans can be aborted or at least their effects ameliorated.

Many countries in sub-Saharan Africa are embarking upon agricultural and livestock development programmes, whereby they also create conditions for the generation of huge mosquito populations. This happens when countries initiate improved water conservation/utilization schemes or make changes to river systems to allow more irrigation. Countries should be aware of the increased risk of RVF outbreaks following the creation of such extensive new mosquito habitats. This has been the case of the Gezira scheme in the Sudan and the Senegal River barrage in Senegal. The likelihood is that low-level cryptic RVF activity has been taking place for generations in such areas and will be amplified to epizootic proportions if susceptible livestock populations are introduced.

PREVENTION STRATEGIES FOR COUNTRIES IN THE MIDDLE EAST

A large traditional livestock trade exists between countries in the Horn of Africa and countries in the Middle East. A major challenge is to manage the risk of spreading RVF with such livestock shipments. Importing countries must be given adequate safety assurances with respect to RVF, while the livestock trade, which is vital for the livelihoods of agropastoralists in both regions, is maintained as far as possible.

Joint planning and the implementation of programmes by animal health authorities in both exporting and importing countries could achieve this. An FAO/UNDP Expert Consultation on Risk Assessment and Risk Reduction of RVF Transmission in Trade Exchanges between the Horn of Africa and the Arabian Peninsula was held from 15 to 16 May 2001. The meeting proposed a framework of action, which includes activities to be carried out by both exporting and importing countries during periods of epizootic RVF, pre-epizootic RVF conditions and between epizootics. The proposals are summarized in the table on p. 27, prepared by the Expert Consultation.

PREVENTION STRATEGIES FOR COUNTRIES IN OTHER REGIONS

RVF could be introduced to countries in other regions by aerial movement of infected vectors or through the importation of domestic or wild ruminants from infected countries, although this could only happen if importation took place within the short incubation period for the disease. Adoption of the recommended guidelines of the OIE International Animal Health Code for such importations would prevent this (see Box on p. 27).

Another possible mechanism is to transport RVF-infected mosquitoes or people through international flights. They can be moved from RVF endemic countries within a matter of hours. Although there are numerous examples of the establishment of vector-borne diseases (e.g. malaria) through the escape of infected insects in areas near international airports, international standards exist for aircraft disinfection and airport insect abatement programmes, which are designed to prevent the entry of RVF and other insect vector-borne diseases. People infected with the RVF virus may have a high enough viraemia to reinfect biting mosquitoes so that it is theoretically possible for an incoming airline passenger to introduce the disease to a new country. Consequently, cooperation is required with health ministries to ensure that the correct human quarantine procedures are implemented for incoming passengers at international airports. These procedures include identification, isolation and hospitalization of any person who displays symptoms suggestive of RVF or other serious haemorrhagic fever. People arriving from endemic countries should also be warned to seek medical assistance promptly if they become ill within a week of arrival.