It may be impossible to eradicate RVF from a country in which it has become established. Nevertheless, countries that are free of the disease and are threatened by it should prepare contingency plans that have eradication as their goal. This goal should be pursued for as long as epidemiological monitoring of the disease control campaign indicates that eradication remains a viable option.
Countries where RVF virus persists should prepare contingency plans based on the premise that epidemics of the disease are forecast and/or detected as early as possible and immediate action taken to prevent outbreaks or at least limit their geographic range and size. The aim is thus to minimize the socio-economic, trade loss and public health consequences of the disease.
The early warning that epidemiological and environmental conditions suggest pre-epizootic conditions for RVF and that outbreaks are likely to occur provides a window of opportunity for national animal and human health authorities to mount an effective response. This opportunity will be progressively lost as RVF virus activity in the field becomes apparent. It is always a difficult decision for national animal health authorities to commit funds and resources to combat a possible disease threat in advance of actual outbreaks. In the future, this decision-making process will become easier as long-range forecasting methodology for RVF becomes more precise.
When there is a forecast that RVF activity may be imminent, the first thing is to define the areas that are likely to become infected. This definition should be based on:
This information can be compiled and evaluated using the Geographical Information System (GIS).
Public awareness and education programmes are critical factors in the success of control and eradication campaigns against major diseases. They are far more relevant when one of the deadly haemorrhagic fevers of human beings is being considered. Once the high-risk areas for RVF outbreaks have been identified, a public awareness programme, targeted at livestock-raising communities and others at risk, should be mounted. This should be delivered by medical and animal health personnel by whatever means are most effective in reaching the target audience - meetings, posters, radio, television or newspapers.
Information that needs to be communicated includes:
RVF public awareness poster produced in Senegal
Available vaccines. Both attenuated (live) and inactivated vaccines have been used to immunize animals against RVF. The modified live Smithburn vaccine has been widely used in eastern and southern Africa to protect the exotic and exotic cross-breeds of sheep, goats and cattle imported into Africa to improve livestock production. The vaccine is easily produced and is relatively inexpensive. A single vaccination protects sheep and cattle within five to seven days and produces a long-lasting immunity. It is quite safe when used in non-pregnant adult animals and in relatively resistant genotypes. Its main disadvantage is that it is not safe for use in pregnant ewes or young lambs. Foetal abnormalities, including microencephaly, hydrancephaly and arthrogryposis, may occur, particularly when pregnant sheep are vaccinated in the first semester of pregnancy. Up to 30 percent of ewes may abort and others develop hydrops amnii towards the end of the term. Young lambs may suffer neurological effects. The vaccine is safe for use in pregnant cattle of Bos taurus and indicus breeds.
There is at least a theoretical risk of reversion to virulence, although this has not occurred to date. For this reason it is not recommended that the live vaccine be used in non-endemic areas.
A live mutagen-attenuated vaccine, MP-12, has also shown considerable promise experimentally as a safe and effective vaccine.
The inactivated vaccine is quite safe for all types of animal (including pregnant animals) but is poorly immunogenic. It is more difficult to produce and is very expensive. At least two to three doses are required to produce an adequate level of immunity and even then breakdowns may occur during epizootics.
Vaccination campaigns. A mass vaccination campaign in the RVF epizootic areas should be seriously considered when and where climatic and epidemiological evaluations suggest that there is a high probability of outbreaks of RVF. The earlier this is done, the greater the chances of success in preventing the more serious consequences of the disease. An early warning of at least two to four months would be required to mount an effective campaign and ensure that adequate supplies of vaccine are available.
The modified live Smithburn vaccine is the most appropriate vaccine to use in such a mass campaign. A value judgement would need to be made as to whether to vaccinate all animals. There may be less overall loss by including pregnant sheep even though some abortions and foetal abnormalities may occur. Many of the indigenous breeds of sheep and goats in Africa do not abort when vaccinated during pregnancy. Those in the Sahelian zones appear to be more susceptible than elsewhere.
Vaccination should cease in areas where evidence of RVF virus transmission has been detected. However, vaccination may be continued in other risk areas but combined with a high level of disease surveillance.
At present, the options for insect vector control programmes as components of an RVF campaign are very limited. Mass insecticide spraying may be impractical and prohibitively expensive, as well as environmentally unacceptable. However, ultra-low volume insecticide spraying of well-defined mosquito breeding and resting sites may be a valuable intervention in some circumstances. Moving susceptible animals away from areas of high mosquito activity or stabling animals to protect them from mosquito bites are unlikely to be viable options.
The best vector control strategy is through larvicidal treatment of potential mosquito breeding sites. At this stage, this must be regarded as experimental for RVF. The most practical way of application is through burying larvicides in the mud of pans before flooding occurs. Toxins derived from Bacillus thurigiensis and sphericus and larval growth inhibitors, such as methoprene, have been used experimentally and given excellent results. Larvicide treatment is applicable where well-defined, discrete areas are expected to flood and where the likely floodwater area can be estimated. The Sahelian areas of West Africa are the best candidate areas for this approach, where the walo and dieri or seasonal water pans of alluvial clays are quite discrete.
When RVF virus activity has been confirmed in a country where the disease is enzootic and which is exporting livestock, the veterinary authorities should:
Vaccination in outbreak areas is not recommended at this time, when there is evidence of high levels of RVF transmission by mosquitoes. Needle transmission will exacerbate the situation. It is not an option when RVF is confirmed in floodplain zones, when multifocal simultaneous emergence of RVF infection occurs.
Peri-focal vaccination could however be considered in areas where past epidemiological experience has shown that there is only likely to be gradual extension of RVF from outbreak foci; this may include areas that are marginal for mosquito activity (e.g. higher altitude plateaus).
Livestock should not be moved into/out of the high-risk epizootic areas during periods of greatest virus activity, unless they can be moved to an area where no potential vector species exist (such as at high altitudes).
All trade should cease once pre-epizootic conditions have been recognized and until at least six months after the last evidence of RVF virus activity has been obtained.
Local people may traditionally move their animals away from flood zones to higher drier areas during the danger periods when there is flooding. Herders have been known to move their animals from the floor of the Rift Valley to the top of the scarp at its edge. This generally takes animals away from the infected mosquito populations and disease ceases within three to five days. There is no reason to prevent such animal movements.
There is real danger that sheep and goats may be moved from an epizootic area where no clinical disease may be apparent because of the genetic resistance of the livestock. Any such movement for trade could result in viraemic animals arriving in a distant country within the incubation for the disease. Viraemic animals constitute a real hazard if introduced into an RVF-free country that has large mosquito populations. Foci of RVF virus propagation could develop. These animals also present a danger to any human beings who slaughter them.
Local animal health authorities may wish to impose movement controls within their own country during RVF epizootics and even to prevent slaughter activities during the periods of greatest risk.
RVF should be viewed in regional terms on the basis of shared ecological zones and climatic conditions. The history of past RVF epidemics demonstrates that these natural ecological zones frequently traverse territory in two or more neighbouring countries, for example, Kenya, the United Republic of Tanzania and Somalia; South Africa and Namibia; Mauritania and Senegal; and with Saudi Arabia. It is therefore very important for countries to cooperate with each other on a regional basis in their efforts against this serious disease.
This regional international collaboration should extend inter alia to:
