B.S. Hursey and J. Slingenbergh
The authors are Senior Officer (Insect-borne diseases) and Animal Health Officer (Trypanosomiasis), respectively, Animal Health Service, Animal Production and Health Division, FAO, Rome, Italy.
Tsetse flies, through the cyclical transmission of trypanosomiasis to both humans and their animals, greatly influence food production, natural-resource utilization and the pattern of human settlement throughout much of sub-Saharan Africa. It is estimated that the annual direct production losses in cattle alone amount to between US$6 000 million and $12 000 million, while animal deaths may reach 3 million.
The FAO Programme for the Control of African Animal Trypanosomiasis and Related Development was inaugurated during the World Food Conference, in 1974. Initially based on the concept of tsetse eradication from large tracts of sub-Saharan Africa, it has been subjected to significant revision and redirection. These changes have been influenced by two main factors: the development and introduction of new, more refined and environmentally acceptable control techniques (FAO, 1992, 1993) and an increasing awareness of the need to relate disease management to demography, population dynamics, natural-resource potential and agricultural systems. In this way, the basis necessary for strategic planning, at both the national and regional levels, is provided.
In order to carry out the required analysis, and to help clarify the tsetse problem at the subcontinental level, FAO has initiated the development of a geographic information system (GIS) on tsetse and agriculture.
The underlying principle is, in itself, quite simple. There are 37 tsetse-infested countries in sub-Saharan Africa. The disease manifests itself when and where humans and their livestock are placed at risk of infection. Indirect problems also result from efforts to avoid tsetse contact, however. These may be readily demonstrated by the subcontinental cattle distribution. Of the 165 million cattle in sub-Saharan Africa, only 10 million are located in tsetse-infested areas, while the remainder are distributed on the periphery. The difficulty lies in the interpretation of the consequences of this distorted distribution, which, in turn, depends on a knowledge of the factors determining cattle distribution patterns in the absence of tsetse.
In this regard, significant progress has been made by the Environmental Research Group, Oxford (ERGO), who examined the anthropogenic and environmental correlates of livestock distribution in the West African semi-arid and subhumid ecological zones (Wins and Bourn, 1994). This study demonstrated that human habitation patterns and the associated intensity of crop production, as well as rainfall, are all key factors influencing the distribution of ruminant livestock. In general, there is a tendency towards the aggregation of people, crops and livestock. This holds true for both pastoral and village cattle when they are kept on a year-round basis. Cattle densities are highest in the moister subhumid areas, except where tsetse flies exist. This conclusion is of major relevance to the understanding of pastoral and agropastoral production systems as it contradicts the conventional belief that livestock are generally kept in drylands, away from the moister cropping zones.
Screw worm at what price?
The eradication of New World Screwworm-induced myiasis from the Libyan Arab Jamahiriya in 1992, at a cost of US$75 million, yielded an estimated cost-benefit ratio of 50:1 when calculated against the value of the livestock and wildlife in the region overall. Similarly, the ongoing 30-year campaign to eradicate Screw worm from the United States and Mexico, which is to eventually extend south to the Darien gap, so far has cost some $700 million, but it has given an average cost-benefit return to the livestock industry of 6:1.
In Australia, it has been estimated that the invasion of the Old World Screw worm could incur annual livestock losses of up to A$430 million.
Various myiasis-causing insects inhabit vast areas throughout the world, and at what cost to the subsistence farmers and rural poor who struggle to survive in the face of these devastating parasites? Following the success of the Libyan campaign, FAO intends to undertake a global assessment of the impact of myiases in order to set priorities for intervention and to better advise Member Nations.
Recognition of the fact that the ultimate goal of sustainable rural development depends on healthy and productive livestock is the driving force behind this initiative. And the availability of these livestock demands that major constraints, such as the Screw worm be identified and systematically addressed.
The ERGO data were kindly made available to FAO for more extensive analysis in the context of defining the impact of tsetse on livestock and agriculture. These studies revealed that, in the tsetse-infested areas of moist subhumid regions of Nigeria, village cattle are virtually absent, despite high population density, land pressure, intensive cereal cropping and a favourable length of growing period. Therefore, in otherwise identical agro-ecological sets of circumstances, it is the presence of tsetse alone that prohibits the keeping of cattle on a more permanent basis. This was corroborated by relating cattle densities to the amount of woody vegetation as measured by satellite imagery. The results confirmed that the discrepancy between livestock densities in tsetse-infested and non-infested areas is explained by the presence of the fly, as well as by the disease risk it poses to humans and animals, rather than by the mere fact that woody vegetation tends to coincide with tsetse distribution (Rogers, Hendrickx and Slingenbergh, 1994).
A computer simulation model was designed to forecast the patterns of human and cattle population densities and those of arable land use in the moist subhumid zone of Nigeria. The data have since been transferred to GIS for mapping purposes. Some preliminary outputs of this model are shown in Figures 1 to 4.
The model predicts the results of interactions between such variables as human and cattle populations, their growth rates and the intensity of cultivation. It is also able to take into account the influence of tsetse/trypanosomiasis on these interactions. At this stage, the environmental impacts of arable land expansion and increased human and livestock populations, for example, a reduction in woody vegetation, have not yet been included. Their importance is recognized, however, and basic interrelationships between the main variables affecting the environment can be readily incorporated into the model by using baseline normalized differential vegetation index (NDVI) data obtained through satellite imagery (Hendrickx et al., 1995).
More important, the model depicts the areas where agricultural expansion is the most significant and the need for ruminant livestock integration is the greatest. It is the persistence of tsetse, particularly the riverine species, around these areas that causes major damage. Fortunately, the technical capacity to suppress the vector as well as the incentives to activate the local communities, in terms of more rewarding farming practices, are greatest in this particular set of circumstances.
A further indication of the significant influence of trypanosomiasis on livestock distributions has been obtained from an FAO field project in Togo. In recent years, this project has systematically collected all relevant data required for a trypanosomiasis impact analysis. The findings shed new light on the influence of tsetse on farming systems. As for Nigeria, while the number of rural household cattle increases proportionally with the amount of land brought into the cultivation cycle, the positive association between cropping and cattle becomes weaker as exposure to trypanosomiasis increases. In areas with a trypanosomiasis prevalence of more than 30 percent, it becomes virtually impossible to establish and maintain a mixed-farming system.
1 Human habitation pattern in subhumid Nigeria at present and predicted - Structure des habitations humaines, situation présente et prévisions, dans le Nigéria subhumide - Distribución actual y prevista de la población humane en Nigeria subhúmeda (Village rooftops per km2 in 1995)
1 Human habitation pattern in subhumid Nigeria at present and predicted - Structure des habitations humaines, situation présente et prévisions, dans le Nigéria subhumide - Distribución actual y prevista de la población humane en Nigeria subhúmeda (Village rooftops per km2 in 2010)
2 Cattle distribution pattern in subhumid Nigeria at present and predicted - Répartition du bétail, situation actuelle et prévisions, dans le Nigéria subhumide - Distribución actual y prevista del ganado vacuno en Nigeria subhúmeda (Total cattle in 1995)
2 Cattle distribution pattern in subhumid Nigeria at present and predicted - Répartition du bétail, situation actuelle et prévisions, dans le Nigéria subhumide - Distribución actual y prevista del ganado vacuno en Nigeria subhúmeda (Total cattle in 2010)
3 Land utilization pattern in subhumid Nigeria at present and predicted - Modes d'utilisation des terres, situation actuelle et prévisions, dans le Nigéria subhumide - Distribución actual y prevista de la utilización de la sierra en Nigeria subhúmeda (Arable use in 1995)
3 Land utilization pattern in subhumid Nigeria at present and predicted - Modes d'utilisation des terres, situation actuelle et prévisions, dans le Nigéria subhumide - Distribución actual y prevista de la utilización de la sierra en Nigeria subhúmeda (Arable use in 2010)
Projection of mixed-farming areas in Nigeria for the year 2010 - Zones d'agriculture mixte, projection pour l'an 2010, au Nigéria - Proyección hasta en año 2010 de las zonas agropecuarias mixtas en Nigeria
The influence of trypanosomiasis risk on the distribution of various cattle breeds was also examined by the Togo project, which reaffirmed that the trypanotolerant Baoulé are significantly more resistant to the disease than zebu. It was also established that the degree of disease resistance, measured on a herd basis, strongly correlated with the number of Baoulé genes as estimated from the phenotypes. Moreover, the study suggested that zebu introgression rapidly reduced the breed's capacity to survive under tsetse challenge. It is noteworthy that livestock owners have appreciated this, and, as a result, the distribution of Togolese cattle breeds is very much a function of the level of disease risk to which they are exposed: 50 percent of the herds consist entirely of purebred Baoulé animals. The implication is that the integration of crop and livestock production develops very gradually because of the risk involved in introducing zebu animals, while the Baoulé cattle lack the size and strength required to provide adequate draught power.
A comprehensive epidemiological study of this nature facilitates the identification of effective strategies for disease intervention. In the Togo example, such strategies were established in accordance with the diverse disease-risk situations and the degree of innate resistance found in the herds. In areas where animals are mostly tolerant and the disease risk is low, the problem may be contained by treating only those individual animals affected with drugs. Whereas, at the other extreme, where trypanosusceptible zebu are exposed to high risk, the intervention of choice would be risk reduction through vector control. The intermediate scenarios, which more closely approximate the practical situation, would demand the appropriate mix of control methods.
In West Africa, this constraint to the development of mixed farming is most severe in the areas recently freed from onchocerciasis. The significant reoccupation of fertile valleys has triggered a major expansion of agricultural production. It is also evident that the presence of tsetse prohibits effective and productive use of natural resources, however, by constraining the integration of livestock with crop production. The alleviation of this constraint is a matter 1of some urgency because of the extremely high and increasing demand for land resulting from the demographic explosion, a situation that is particularly acute in this subregion. These high population pressures also influence the movement of traditional pastoral societies away from the Sudano-Sahelian zone. For some decades, this increasing pressure has been forcing pastoralists to move from the growing intensity of cultivation in the moister regions of the arid and semi-arid zones towards the underutilized tsetse-infested areas. However, as the majority of pastoralist cattle are trypanosusceptible zebu, which cannot produce or survive under the constant risk of trypanosomiasis, the results are often disastrous.
An analysis made to identify priority areas for intervention indicates that, in the medium term, more sustainable socio-economic benefits are to be gained by focusing on the wetter mixed-farming areas of the subhumid zone, especially where human populations are at their greatest. A major consideration when implementing vector control in such situations is the readiness of rural societies to play an active role in sustaining the tsetse campaign long enough to achieve autonomous disease control through the gradual and progressive transformation of the landscape.
Tsetse-transmitted trypanosomiasis is unique in that it has considerable impact over a vast area of some 9 million km2, where it so profoundly influences and distorts the patterns and density of agriculture that they are often contrary to the demand and the resource potential. Realizing the magnitude of this problem, the programme is in the process of changing towards a more realistic and practical approach. Most notably, this includes strengthening the coordinating role of FAO within a global programme to clarify and solve the problem of African trypanosomiasis, both human and animal, and to ensure the active participation of all players, from the subsistence farmer to the international research institutes and donor community (FAO, 1995; FAO/IBAR, 1995). It is now generally appreciated that strategic planning for tsetse and trypanosomiasis cannot be undertaken within the narrow framework of animal health, nor, indeed, confined to the livestock sector alone. It requires an understanding of resource potential, environmental implications, farming systems and the constraints thereon, as well as consideration of the dynamics of population growth and food demand over time. It has rural development dimensions.
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