Early warning enables rapid detection of the appearance of or sudden increase in the incidence of serious livestock diseases before they develop to epidemic proportions and provoke serious socio-economic consequences. It embraces all the initiatives, mainly based on disease surveillance, reporting and epidemiological analyses that lead to improved awareness and knowledge of the distribution and behaviour of disease outbreaks (and of infection). Early warning enables forecasting of the probable occurrence, source and evolution of disease outbreaks and monitoring of the effectiveness of disease control campaigns.
Early warning programmes are critical for RVF. They provide the cornerstone for contingency planning for the disease. In regions where RVF is present, forecasting of the high probability of epidemics at least three months, and possibly up to six months, before they start will enable an effective response to be mounted against the disease. Conversely, if a RVF outbreak does not come to official attention until it is under way, the capacity of animal and human health authorities to mount an effective disease control campaign against it will be severely limited.
As is the case with all aspects of contingency planning for RVF, it is essential that there be close cooperation with counterpart officials in the Ministry of Health and that joint national and local early warning systems be developed.
The success of a country's capability for forecasting and rapid detection of outbreaks of RVF depends on the following:
It is beyond the scope of this manual to discuss the above issues in detail. For more information, reference should be made to the Manual on the preparation of national animal disease emergency preparedness plans (FAO Animal Health Manual No. 6) and the Manual on livestock disease surveillance and information systems (FAO Animal Health Manual No. 8). However, some of the most important issues for RVF early warning preparedness will be described below.
In many countries, even those in which RVF outbreaks have occurred in the past, few veterinarians and animal health workers have had any direct, first-hand experience with the disease. If RVF is considered to be a major threat, this deficiency needs to be rectified. A systematic training programme should be established for all those who, in their professional capacity, may possibly be the first to come into contact with an incursion or outbreak of the disease. Training programmes should be comprehensive and regular, both because the disease may strike in any part of the country and also because of staff turnover. Training must extend to staff in the remotest parts of the country, as well as to selected officials (agri-cultural extension officers, local authorities) and livestock owners.
It will obviously be neither practicable nor necessary to train personnel to a high level of expertise in the disease. It is sufficient for trainees to be familiar with the basic clinical, pathological and epidemiological features of RVF and know what they need to do if they suspect its presence. Perhaps the most important thing to inculcate in people is a “mind-set”; if they are confronted by an unusual disease outbreak involving abortion storms and deaths in young ruminant animals, either in the field or in the diagnostic laboratory (through submitted diagnostic specimens), they should include RVF in the range of their differential diagnostic possibilities and act accordingly. People should be trained in the steps they need to take to secure a confirmatory diagnosis, including collection and transport of diagnostic specimens, and in the immediate disease control actions that need to be instituted at a disease outbreak site. More specialized training will be needed for personnel who are nominated as members of specialist diagnostic teams (see below).
A number of training possibilities may be selected, as appropriate.
These programmes are one of the most critical, but sometimes neglected, aspects of preparedness planning for emergency diseases. They are necessary for fostering “ownership” and support for emergency disease control/eradication campaigns from livestock farmers and other key stakeholders. They also engender a bottom-up approach to planning and implementation of disease control programmes, to complement the more traditional top-down approach adopted by governments.
Communication strategies should aim to make stakeholders aware of the nature and potential consequences of RVF and other important livestock diseases and of the benefits to be derived from prevention and control/eradication. Furthermore, these strategies should always have an element of rallying the community to the common cause of preventing and fighting a disease epidemic, ideally resulting in farmer sanitary defence groups and farmer organizations.
One of the important messages to get across is that it is essential to notify and seek help from the nearest government animal health official as soon as an unusual disease outbreak is observed. The way to do this needs to be carefully explained. Publicity campaigns should not be directed only towards farmers but also to local authorities and livestock traders.
Livestock traders are an important target group for public awareness campaigns and are often overlooked. The movement of animals through livestock traders is often the key epidemiological factor in the spread of contagious epidemic livestock diseases. The need to build up a climate of trust and confidence between animal health officials and livestock traders is as important as that discussed for farmers and the general themes for emergency disease awareness should be similar. More emphasis should be placed on the importance of doing the "right thing", about sourcing animals from disease-free areas where possible; not buying any sick stock; and following any rules about quarantine, vaccination, testing or identification of animals. The potential consequences of the occurrence of a disease for internal and international trade should be emphasized.
Trade of livestock between the Horn of Africa and the Arabian Peninsula (Port of Berbera, northern Somalia)
By Marc Bleich
It is recommended that a specialist RVF diagnostic or emergency disease investigation team be nominated within a country so that it can be mobilized when there is a report of a suspect outbreak of the disease from the field. These arrangements should be made well in advance of any emergency. Members should be available and equipped to travel to a disease outbreak site at short notice. They should carry all the equipment needed for the preliminary investigation of a disease and for collection and transport of diagnostic specimens.
The composition of the diagnostic team will vary according to circumstances, but could include:
Ideally, a joint medical and veterinary specialist diagnostic team should be mobilized in any outbreak situation. The team would travel to a disease outbreak site with local veterinary staff if so directed by the CVO (and would be provided with the transport to do so). They would be expected to make clinical examinations, collect histories and make preliminary epidemiological and entomological investigations. They should collect a range of diagnostic specimens specifically for RVF and for any endemic or exotic diseases that might be included in the differential diagnosis and transport them back to the laboratory.
The team should also be able to take any necessary immediate disease control actions at the outbreak site and have the appropriate authority to do this; they should be empowered to provide any immediate instructions to local animal health officials.
The team would be expected to report back immediately to the state/provincial/regional veterinary officer and the CVO on their assessment of the disease outbreak. They should report on the steps taken to secure a confirmatory diagnosis and give advice on further disease control strategies, including declaration of infected and surveillance zones. They may also advise on any necessary measures to improve disease reporting from the outbreak area and on the desirability of setting up a local disease control centre.
The rapid and certain diagnosis of diseases can only be assured in fully equipped laboratories, with a range of standardized diagnostic reagents, with experienced staff and a sufficient throughput of diagnostic specimens to maintain expertise. The relatively simple facilities required for testing sera by ELISA are a realistic possibility for most countries with P-2 facilities. Additionally, the development of diagnostic expertise for exotic diseases for tests, which require handling live RVF virus (e.g. SN tests), should only be attempted in laboratories with appropriate biosafety facilities (P-3/P-4).
It would therefore be impractical and excessively costly for most countries to maintain a national veterinary diagnostic laboratory with full capacity for confirmatory diagnosis of all transboundary and other emergency diseases, many of which will be exotic. However, it is to be expected that all countries with significant livestock populations will have a veterinary diagnostic laboratory. This must be equipped and competent to undertake a broad range of standard techniques in pathology, virology, bacteriology and serology to the level where preliminary identification of aetiological agents for most, if not all, emergency livestock diseases can be attempted. If RVF is deemed to be a high-threat disease, consideration should be given to developing capabilities for some primary key diagnostic tests for the RVF antigen (formolized tissue and immuno-histochemistry or PCR) and antibody detection (ELISA tests).
Specimen transport containers should be kept at both central and state or provincial veterinary laboratories and be made readily available for FVOs and specialist diagnostic teams. Containers should ideally consist of primary leakproof glass universal bottles with a metal screw top and rubber washer or good-quality plastic screw-top jars. These are then packed into a leakproof secondary container (e.g. a steel paint tin or a plastic or styrofoam™ cool box) with absorbent material and an ice pack, and finally put into a well-labelled robust outer container. Specimen advice notes should also be provided.
A network of FAO and OIE Reference Laboratories and Collaborating Centres for RVF exist around the world and are available for providing advice and assistance to countries. Their names, full contact details, subjects and geographic areas of responsibility are given in Appendix 1.
As part of their RVF contingency planning, countries should establish contact and a dialogue with the appropriate reference laboratories and collaborating centres. The countries should determine the nature and range of diagnostic specimens or isolated agents to be sent for confirmatory diagnosis or further characterization. The specific means of transport, method of packaging and refrigeration, labelling of package (including correct address and any necessary customs or International Air Transport Association [IATA] declarations) should all be determined. This information should be documented in country plans.
Potential or confirmed aetiological agents from emergency disease outbreaks must be sent to the appropriate International Reference Laboratory for further characterization. It is recommended that several isolates from different geographic locations and at different phases of the outbreak be forwarded. Submission of samples to any laboratory outside the country of origin should always be subject to prior agreement with the recipient and transportation in containers meeting IATA regulation standards.
Reference laboratories and collaborating centres can provide, for example, opportunities for training, provision of specialized advice in planning and standardized diagnostic reagents.
The three essential prerequisites for an epidemic to occur are a susceptible livestock population, a massive build-up in the populations of vector mosquitoes and the presence of a virus. Assuming the continuing presence or at least the close proximity of the virus in regions where the disease has occurred previously, the first two factors become the key to early forecasting of likely RVF activity.
Early work on forecasting was centred at a study site in Kenya where ground truth data for RVF virus activity had been generated for many years. Periodic outbreaks of RVF over a 40-year period were found to correlate with the positive value of a statistic based upon the number of rain days and the quantity of rainfall. The three-month rolling mean value formed a positive spike when RVF virus activity occurred and this was a function of cumulative persistent rainfall, rather than heavy precipitation over a short period. Data were based upon longitudinal rainfall data generated and recorded in the old-fashioned manner. The characteristics of the ITCZ were also important as a determinant of prevailing conditions conducive to RVF virus activity. These data allowed forecasting of RVF outbreaks, with a four- to ten-week period during which vaccination could be carried out before cases occurred.
More sophisticated studies were possible when RSSD became available. These data enabled national and regional monitoring of rainfall and climatic patterns and their effects upon the environment. CCD measurements are closely correlated with rainfall and have replaced the laborious daily collection of rainfall data from many stations. Climatic patterns are regional in East Africa and the Horn of Africa and may be studied on this basis. Detailed analysis was made with virus isolation data over a 25-year period and the NDVI for the study areas. NDVI data are derived from probes measuring relative "greenness" and "brownness" of the vegetation. As the water table rises to the point where flooding may occur, the ratio approaches 0.43 to 0.45. This point was reached at each of the epizootic periods in the study period.
More recent retrospective studies using the same ground truth data have included the surface sea temperatures (SST) for the Indian and Pacific Oceans. When these are combined with NDVI data, they approached 100 percent accuracy in predicting periods of RVF virus activity during the study period. This has a pre-epizootic predictive period of two to five months before virus activity occurs.
A new statistic has been derived from satellite data, known as BERMS. These measure rainfall in the catchment areas of river/wadi systems and are based upon digital maps of basin and river networks. They can predict periods when flooding might occur, which is particularly valuable for the floodplain zones in the Horn of Africa countries and the Arabian Peninsula. Early data suggest that BERMS might be able to predict virus activity five months before its occurrence. The advantages of RSSD for RVF predictive epidemiology are in the relatively low costs of the systems used for analysis. These are readily available on a country and regional basis and give time for preventive measures such as the vaccination of susceptible stock and mosquito larval control methods, wherever possible. There are discrete floodwater mosquito breeding sites in the West African Sahelian zones, which lend themselves to control with larvicides. The floodplain zones in the Horn of Africa do not allow such control methods.
International agencies are best placed to analyse satellite and other data and to provide risk countries with early warning about likely weather patterns conducive to increased RVF activity. FAO, through its Global Information and Early Warning System on Food and Agriculture (GIEWS) and EMPRES/Livestock Programme, will take a central role in generating these data on a continuing basis, thus providing an early warning/risk assessment service.
It must be recorded that little work has been done in other parts of Africa to validate the RSSD systems because the ground truth data has not been available and it takes many years of dedicated work to generate such data. Recent outbreaks in Somalia and northeast Kenya in 1997-98 showed retrospectively that the foci of RVF virus activity in these countries could be correlated with high NDVI values.
Suitable area for mosquito breeding in the Afar region of Ethiopia
By Vincent Martin
Sentinel herd monitoring by a field agent in Mauritania
By Fabien Schneegans
Activities should be directed towards active disease surveillance in order to build up baseline information on interepidemic virus transmission patterns, areas at risk and early warning of any increased virus activity or build-up in vector mosquito populations. This surveillance should be carried out by regular field visits and contact with livestock farmers and communities and should include periodic purposefully designed and geographically representative serological surveys and participatory epidemiological techniques. The detection of RVF virus activity by serology is usually too late to be of any relevance to control.
Sentinel herds are an important means of obtaining baseline epidemiological information on RVF. These are small ruminant herds located in geographically representative areas. Locations where mosquito activity is likely to be greatest, e.g. near rivers, swamps and dams, should be selected.
Arrangements should be made with owners to ensure that sentinel animals are available for regular inspection and sampling and that the animals are permanently identified by eartagging. Arrangements should also be made so that any illness in the herds can be reported and investigated quickly. Serum samples should be collected at regular intervals from 20-30 young adult small ruminant animals in the herds and tested for both IgM and IgG antibodies against RVF. As a general guide, sentinel herds should be sampled twice to four times annually, with an emphasis during and immediately after rainy seasons.