Chapter 3. OVERVIEW OF METHODS FOR TSETSE-TRYPANOSOMIASIS CONTROL OR ERADICATION

CONCEPTS AND DEFINITIONS

In order to determine how much progress has been achieved in the past or can be achieved in minimizing the impact of tsetse-trypanosomiasis on humans, livestock, wildlife and rural economies in general, the concepts of “control” and “eradication” must be elaborated and applied appropriately to the vector (tsetse fly), the disease-causing organism (trypanosomes), the disease (trypanosomiasis), reservoirs (wild and domestic animals) and to the hosts (humans and livestock). Budd (1999) adopted the term “trypanosomiasis control” for those drug-based strategies that eliminate parasite organisms in the host or for the use of innate resistance in host animals to combat the effect of the parasite. The indirect control of the trypanosome through attacking the vector (tsetse fly) is referred to as “tsetse control”. Where a complete removal of the vector from a given or defined area is the objective, the strategy is referred to as “eradication”.

Tsetse control

The earliest methods of tsetse control included widespread bush clearing to destroy tsetse breeding habitats, supported by shooting of game (wildlife) or installation of game fences to prevent the game hosts from carrying flies into the tsetse-free area, and clearing wide forest corridors to prevent re-invasion of tsetse. Insecticide-based control techniques, i.e. ground and aerial spraying, were introduced after the Second World War to complement the methods employed to destroy the vector habitat (Budd, 1999). In view of environmental concerns and public outcry regarding the use of chemicals and the destruction of forests in the 1980s, these methods gave way to other less-intrusive techniques including trapping, the use of odour-baited targets (Jordan, 1986) and the treatment of animals with insecticide (Bauer et al., 1992). The application of the sterile insect technique (SIT) has also had a limited impact.

The major shortcomings of these methods lie in the limited size of area for which they can be economically deployed relative to the total size of the tsetse-affected area and the continual costs associated with preventing re-invasion. There are also concerns about non-tsetse targets, for example ticks, becoming resistant to pyrethroids used as insecticides on livestock. Yet in theory, tsetse control appears to offer the greatest likelihood of success in ridding the African continent of the tsetse-trypanosomiasis problem.

Drug-based trypanosomiasis control

The use of trypanocidal drugs to control trypanosomiasis is well established and represents the most widely used approach (d’Ieteren et al., 1998). As a control measure, drug therapy strategies are currently protecting more cattle (approximately 25 percent of all affected cattle) against the disease than any other method (Budd, 1999). However, the most serious setback in the use of drugs to control trypanosomiasis is the increasing trend in drug resistance, especially to isometamidium, diminazene and homidium bromide - the three most commonly used trypanocidal drugs (d’Ieteren et al., 1998). Not only are individual cases recognized (Peregrine, 1994), but regional distribution is increasingly being reported in East and West Africa. A recent study by the International Livestock Research Institute (ILRI) and partner institutions discovered serious drug resistance in Kenedougou, a southern region of Burkina Faso (McDermott et al., 2003).

Furthermore, the much anticipated breakthrough in the development of vaccines to control trypanosomiasis appears unattainable in the near future owing to the antigenic variation of trypanosomes and the complexity of their antigenic repertoire. New strategies that integrate several options in controlling trypanosomiasis are being developed to extend the period during which the currently used drugs remain effective (FAO, 1998; Holmes, 1997).

Exploitation of trypanotolerant livestock as an option for trypanosomiasis control

Trypanotolerant livestock play a significant role in moderating the problem of tsetse-trypanosomiasis in West and Central Africa, primarily through their use for food, traction and as a source of cash income in areas where livestock agriculture would otherwise not be possible. The wide geographical distribution of these animals (especially small ruminants), scattered from the southern limits of the semi-arid zone to the coastal humid zones and found in almost all the countries in the region, suggests that livelihoods are being supported in significant ways by the presence of these stock in the affected areas.

In addition to the use of trypanotolerant livestock in directly minimizing the tsetse-trypanosomiasis problem, there is some suggestion that because of their capacity to rid themselves of trypanosome parasites and maintain low parasitaemia, once infected, they indirectly reduce the trypanosome parasite load associated with any given location. While the mechanisms that lead to the maintenance of lower parasite loads in trypanotolerant breeds compared with susceptible breeds are not clearly understood, results from laboratory experiments indicate that the killing of trypanosomes in host animals results from inhibition of the trypanosome glycolytic pathway and of adenosine triphosphate (ATP) production (Muranjan et al., 1997). There is also the suggestion that animals infected with trypanosomes tend to attract more tsetse flies than uninfected animals and that the feeding success on animals is substantially greater in infected than in uninfected animals (Baylis and Mbwabi, 1995). Thus, it can be argued that, given two areas with similar initial trypanosome prevalence and the same number of either trypanotolerant or trypanosusceptible livestock, the rate of infection will be lower among the trypanotolerant stock. A logical extension of this hypothesis is that where the population of trypanotolerant livestock is high relative to trypanosusceptible livestock (maintained under chemotherapy or chemoprophylaxis), or where there are only trypanotolerant livestock, the rate of transmission of the disease will be much lower. Thus, the investment made to control the tsetse-trypanosomiasis problem by drug-based methods (chemoprophylaxis, chemotherapy, etc.) may be higher. These postulations are supported by the observations of Leak et al. (1990) who demonstrated that for a given tsetse challenge it is possible to estimate the prevalence in trypanotolerant and trypanosusceptible cattle and that, in general, the rate at which trypanotolerant cattle appear to acquire infections is lower compared with trypanosusceptible cattle.

Multidisease integrated approach

In terms of cost and simplicity, it can be considered desirable to be able to control several diseases simultaneously with one or a few packaged treatments or control options. Such an approach to controlling several diseases with one treatment or option in an integrated manner may be referred to as a multidisease integrated approach.

Thus, the application of one drug or chemical compound to control agents or vectors of one disease and that might simultaneously control vectors of other diseases should be considered as a superior approach to alternatives that involve attacking the vectors with several chemicals. For example, the use of trypanocidal pour-ons on livestock to control tsetse flies and nuisance flies may lead to the control of ticks, and hence tick-transmitted diseases. Another example of such an approach may be the theory that genes conferring trypanotolerance in cattle breeds might be linked with those controlling dermatophilosis. The use of trypanotolerant livestock in areas endemic for tsetse-transmitted trypanosomiasis and tick-borne diseases associated with dermatophilosis might provide an integrated approach to dealing with multiple diseases of trypanosomiasis and various tick-borne diseases. Given the range of diseases that trypanotolerant livestock are credited to be resistant, tolerant or resilient to, it can be argued that their utilization in several ecosystems is feasible and could thus play a significant role in multidisease integrated control approaches and methods.

However, a combination of two or more control measures may be more effective in overcoming a particular disease that is difficult to treat with one control measure. For example, Gibson (2002) noted that, in most instances, utilization of genetic resistance will be one component of an integrated approach to disease control and argued that, with the use of purebred indigenous livestock, resistance is not always complete and that a proportion of animals often suffer some degree of production loss as a result of the disease. Furthermore, in cross-breeding and other systems that exploit resistance, the level of resistance will usually be incomplete. Consequently, Gibson concluded that the design of cost-effective, disease-control strategies requires knowledge of the degree and nature of the resistance of the breed(s) used in the production system. Based on this reasoning, Gibson hypothesized that sheep that are partially resistant to helminthosis can exhibit lower susceptibility to infection and have lower faecal egg counts when infected. Where stringent management is difficult to ensure, this may mean that rotational grazing will be more effective with the use of partially resistant animals than with the use of susceptible breeds.

Integrated approaches to combating tsetse-trypanosomiasis

The relationship between tsetse challenge and the options available for combating tsetse-trypanosomiasis has been studied or modelled and some “rule-based models” have been advocated (Snow and Rawlings, 1999). Jahnke et al. (1988) suggest that if the challenge is not permanent or if tsetse eradication campaigns are not possible, chemical treatments are the relevant choice; when dealing with trypanosusceptible breeds, the eradication of tsetse flies may be a solution if the area is small and isolated.

As pointed out by d’Ieteren et al. (1997), there is no single solution that will be valid for all production systems, ecological zones, or regional or national markets. However, the increasing drug resistance in trypanosusceptible populations and the difficulties of sustaining tsetse fly control increase the need for enhancing trypanotolerance through selective breeding either within breeds or through crossbreeding. These observations support the analysis by Holmes (1997) and Geerts and Holmes (FAO, 1998) that more integrated strategies need to be developed.

TABLE 5
Summary of suggested control and management responses to different rank intensities of African animal trypanosomiasis (AAT) problems in the Gambia

Note: NA = not considered appropriate at this problem level.
Source: Snow and Rawlings (1999).

The deployment of any integrated control of tsetse-trypanosomiasis must be based on quality information, not only on the abundance of tsetse flies and the infection rates in the flies and the host livestock, but also on how the livestock owners view the extent of the problem. Cost has been a major factor in the collection of such data because longitudinal surveys are usually required in establishing seasonal prevalence of the disease in the host. Long-term tsetse surveys are also usually required for a proper assessment of challenge. Long-term research carried out at the International Trypanotolerance Centre in the Gambia has demonstrated that, using information gathered from rapid-appraisal questionnaires addressed to local informants (in a participatory rural appraisal format), one-off tsetse surveys and assessments of the prevalence in village livestock, it is possible to develop rule-based guidelines for making management decisions about which of the control options or combination of options are appropriate in a given situation (Snow and Rawlings, 1999). Table 5 summarizes suggested control and management responses developed for the agropastoral mixed farming system in the Gambia.

The information in Table 5 suggests that, in situations where tsetse challenge is considered to be medium, high or very severe, the use of pure trypanotolerant breeds is indispensable, while trypanosusceptible breeds may be used only with heavy investment in chemotherapy, chemoprophylaxis and/or tsetse control. In zero to low tsetse challenge situations, trypanosusceptible breeds and cross-breeds with some level of tolerance may be used. However, in order to make this model applicable to several situations, the dynamic nature of production systems in which livestock are managed should be taken into account. It may also be considered that, if the prevailing markets highly favour special attributes of, and products from, trypanotolerant livestock, they may be competitive even in the zero to low tsetse challenge situations. Furthermore, as more drug-based control options, for example vaccines, become available, the relative advantage of trypanotolerant breeds may change in some production systems. Thus, other analytical frameworks may be required to capture the changes in production systems, market values and technological innovations.