A. A. LatifVeterinary Research Laboratory, Harare, Zimbabwe
Introduction
Problems of present control methods
Sustainable control methods
Host-resistance to ticks
Tick-resistant cattle in Africa
Conclusions
References
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Summary The two major livestock diseases of economic significance in Africa are trypanosomiasis and tick-borne diseases (TBDs). Both affect subsistence and commercial farmers and limit the exploitation of productive land. Present methods of vector and disease control remain inadequate, costly and pose environmental problems. Control methods for tick-borne diseases that are safe and effective are now available. Immunisation with reduced dipping practices make it possible to tolerate moderate tick burdens on cattle With the advent of immunisation, integrated tick and TBD management (ITM) becomes desirable. Tick-resistant breeds of cattle, vector population studies and concurrent reduced use of chemicals are the crucial elements in the control strategy. The ITM package is economically and technically attractive and practically desirable. However, the social and political feasibility of tick-borne- disease control needs to be addressed. The host's resistance to tick infestation manifests itself against attacking ticks of different species and at varying stages. Highly resistant cattle keep tick populations low Quantitative studies on host-resistance to ticks in zebu cattle in Africa are available. Several indigenous breeds have been found to have high levels of resistance, including Africander, Bonsmara, Nkoni and Brahman (S. Africa), Nkoni (Zimbabwe), Boran and East African Zebu (Kenya), Butana and Kenana (Sudan) and Boran and Horro (Ethiopia). The practice of opposing resistant cattle to tick infestations for tick and TBD control needs to be given support and priority in research. There is am urgent need for an integrated network approach to tick-control research similar to the trypanotolerance network, to integrate epidemiological studies on ticks and TBDs, animal science, socio-economic studies and dissemination of information. |
Tick-borne diseases are a major constraint to livestock improvement throughout the tropics. World-wide, an estimated 600 million cattle are exposed to anaplasmosis and babesiosis and 200 million cattle are exposed to theileriosis. In sub-Saharan Africa as many as 175 million cattle may be exposed to cowdriosis. In eastern, central and southern Africa theileriosis (East Coast fever), caused by Theileria parva, is considered to be the most significant tick-borne disease of cattle. The parasite infects cattle and buffalo (Synerus caffer) in 12 countries of the region and 25 million cattle are at risk. One million cattle are estimated to have died of East Coast fever (ECF) in 1989 alone (Mukhebi et al, 1992).
Most indigenous cattle in areas where tick-borne diseases occur possess a natural resistance to these diseases. The association between the parasites and cattle has produced states of endemic stability in which cattle are immune and parasites are transmitted constantly between cattle and ticks. Cattle that are exposed to Anaplasma, Babesia or Cowdria organisms early in life do not usually develop the clinical disease and are subsequently immune. This form of age-related immunity does not occur with T. parva.
During the last 80 years the control of ticks and the diseases they transmit has been largely through the application of acaricides in dips or sprays. The strict short-interval application of acaricides prevents transmission of the parasites and this method has been used very successfully throughout the region, in conjunction with control on animal movement, quarantine and slaughter. Introduced in 1902, the use of acaricides was used to eradicate ECF from South Africa by 1954 and Swaziland by 1960 (Lawrence, 1991). However, the development of resistance in ticks to successive acaricide compounds has been a major problem. This has been compounded by the increasing cost of acaricides, illegal cattle movement, civil unrest, poor management and inadequate maintenance. The devastating extent of the drought in Africa has made many dip-tanks non-operational due to lack of water. Another complication associated with the use of acaricides is that they are environmental pollutants and may also contaminate milk and meat.
Strict acaricide application results in highly susceptible cattle populations because the cattle are not exposed to the parasites; when tick control breaks down, enormous losses can occur. An extreme example was the breakdown of dipping infrastructure during the war of independence in Zimbabwe, where a compulsory dipping policy had been in force since 1914. Between 1974 and 1979 an estimated one million cattle died, mainly of tick-borne diseases (Lawrence et al, 1980). Research has shown that game animals of various species maintain a tick population and tick-borne diseases (Young et al, 1988).
Strict tick control is difficult to maintain in many countries and more rigorous methods for controlling ticks and tick-borne diseases are being investigated. The broad approach has been to use integrated control measures which include the natural exposure to Anaplasma, Babesia or Cowdria organisms while animals are very young. Other means include immunising older animals with live vaccines; immunisation against ECF by infection and treatment; chemotherapy; and strategic acaricide application to control overwhelming tick infestation or disease challenge.
However, the most important element in this control package is the use of breeds of cattle that are genetically resistant to tick infestations. Resistant animals have a tendency towards light tick burdens and require less dipping, making control relatively easy and cheaper. In addition, other biological control methods could be used in support of control methods, e.g. anti-tick grasses (Thompson et al, 1978; Sutherst et al, 1982; Zimmerman et al, 1984) and application of aggregation-attachment pheromone-baited acaricide to single sites on bovine hosts (Rechav et al, 1977; Sonenshine et al, 1979). These approaches require an understanding of the parasites that exist in a particular environment and their epidemiology. It is also necessary to have access to acaricides, vaccines and drugs and to be able to monitor tick infestations and disease outbreaks.
It has long been recognised that some animals, or whole breeds, consistently carry fewer ticks than others kept in the same environment (Roberts, 1968a; Wagland, 1975). Such differences are caused by variation in the animals' abilities to respond immunologically to tick infestation (Roberts, 1968b). The ability to develop resistance is heritable (Hewetson, 1972; Seifert, 1984) and the actual manifestation is acquired (Riek, 1962; Roberts, 1968a). It is stable over longer periods, although stresses such as lactation or sickness cause a drop in resistance (Wharton et al, 1970; Seifert, 1971; Utech et al, 1978). Different levels of resistance occur in all breeds, but this is manifested more strongly in zebu cattle and their crosses (Rick, 1962; Wilkinson, 1962; Wharton et al, 1969; Seifert, 1971; Hewetson, 1979).
The improved tick control following the use of tick-resistant cattle has been demonstrated in various breeds of cattle and crossbreds (Rick, 1962). Kelly (1943) suggested that the tick resistance of zebu cattle should be utilised for tick control. Moreover, a cross between zebu and taurine cattle was shown to carry fewer ticks and required less dipping than temperate breeds of cattle on similar pastures (Wharton et al, 1969).
The criteria for the assessment and quantification of resistance to ticks in cattle are given in many reviews (Willadsen, 1980; Tatchell, 1986; Latif and Pegram, 1992). Resistant animals consistently carry fewer ticks than susceptible animals. Female ticks completing engorgement are fewer and smaller on resistant animals than on susceptible animals.
The earlier observations on cattle mortality due to heartwater and its relation to the number of ticks on animals of different breeds made by Bonsma in 1940s (Bonsma, 1981) in South Africa formed the basis of the studies on host-resistance to tick infestations. The study showed that Africander cattle carried far fewer ticks than British beef cattle and had a far lower mortality rate than the British cattle (6% vs 60%). This pioneer work by Bonsma was resurrected by researchers in Africa 40 years later. Assessment and quantification of host-resistance were carried out on several African indigenous breeds of cattle (Table 1). The majority of these breeds are tick-resistant and of high productivity (Trail and Gregory, 1982; Saeed et al, 1987).
The work shown in Table 1 comprises short-term studies (up to two years) carried out to assess the levels of host-resistance in different breeds of cattle. There has been only one long-term programme aimed at raising herd resistance by selection, breeding or gene alteration (de Castro, 1991); this was based at the International Centre of Insect Physiology and Ecology, Kenya. Although the progress and results obtained were meaningful and in the right direction, the programme was terminated after three years due to lack of funding and expertise. Over 100 bulls and 450 heifers at the Boran National Stud were assessed for tick resistance by repeated monthly exposure to ticks. The proportion of bulls ranked in different classes for total tick counts (Table 2) showed that the majority (55%) of these animals had a high level of resistance (rank 1) while a smaller proportion (6%) were ranked as of low resistance (rank 3). The repeatability of the observations suggests a high degree of heritability of resistance.
Table 1. Tick-resistant zebu came in Africa 8.
|
Breed |
Country |
Reference |
|
Afrikander |
South Africa |
Bonsma (1940) in Bonsma (1981) |
|
Brahman |
South Africa |
Rechav (1987), Rechav et al (1990) |
|
Bonsmara |
South Africa |
Scholtz et al (1991) |
|
Ngunis-Nkoni (Sanga) |
South Africa |
Scholtz et al (1991) |
|
Boran |
Kenya |
de Castro (1991) |
|
East African Zebu |
Kenya |
Latif et al (1991) |
|
Kenana |
Sudan |
Latif (1984) |
|
Butana |
Sudan |
Latif (1984) |
|
Boran |
Ethiopia |
All (1989) in: de Castro (1991) |
|
Horro |
Ethiopia |
Ali (1989) in: de Castro (1991) |
|
Nkoni (Sanga) |
Zimbabwe |
Norval et al (1988) |
Table 2. Frequency of bulls (100) ranked by total tick counts for six months, Boran National Stud, Kenya.
|
Rank |
% |
Resistance |
|
1 |
55 |
High |
|
2 |
39 |
Middle |
|
3 |
8 |
Low |
Source: Adapted from de Castro (1991).
Current control of ticks and tick-borne diseases in Africa is unsatisfactory, costly and requires excessive effort. Therefore a package for integrated control of ticks and tick-borne diseases is presented. The features of the strategy are: maintenance of enzootic stability to tick-borne diseases, adoption of resistant breeds of cattle; and reduction of chemical treatments to low levels. Similar methods of integrated control have also been suggested by Young et al (1988). The future of breeding programmes for tick resistance is two steps ahead of breeding for trypanotolerance because, firstly, vaccination against tick-borne diseases is possible and, secondly, resistant breeds of different types are present in most regions of Africa.
Research into trypanotolerant cattle has received long-term support from donor agencies and research institutes. The use of cattle resistant to tick infestations (zebus and their crosses) and its effective role in tick and disease control needs support and priority in research.
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