Tick control policy and practice in Africa with particular reference to the use of acaricide mixture and rotation
Division of Tropical
Animal Production, Long Pocket Laboratories,
Private Bag No 3 PO, Indooroopilly, Queensland 4068, Australia
A workshop organised by the Food and Agriculture Organization of the United Nations (FAO) to review acaricide use in Africa was held in Malawi on 25 April 1994. The workshop reviewed tick control policies and practices with emphasis on the use of acaricide mixtures and the philosophy of alternating or rotating acaricides. The use of mixtures or alternating chemicals has been adopted in other bio-industries because it is believed that it may delay the development of resistance to insecticide compounds and because it is thought to be more effective. Others believe it may worsen the problem of resistance. A selection of representative country reports on tick control policies and practices were presented as case studies.
The need for acaricides in tick control
Acaricides are needed to control tick infestations and tick-borne diseases (TBD). In sub-Saharan Africa, the ticks of importance are Rhipicephalus appendiculatus transmitting East Coast fever (ECF), Amblyomma species transmitting cowdriosis and Boophilus microplus and B. decoloratus transmitting babesiosis and anaplasmosis. However, the use of acaricides is constrained by their high cost, resistance to them, concerns about residues in food and in the environment and high mortality due to diseases that follow failure to sustain intensive acaricide application.
Possibilities now exist for control of ticks and TBD by less intensive acaricide application. Reduction in intensive acaricide treatments are feasible because immunisation techniques against the diseases above are available, and there is evidence that indigenous and, to a lesser extent, crossbred cattle can develop immunity to Boophilus and other tick species. Consequently, the intensity of application of acaricides will depend on whether the aim is to prevent disease transmission or to reduce tick damage and/or worry. Thus a choice can be made as to whether to adopt intensive or strategic/threshold acaricide application or other systems.
Proposals for strategic/threshold acaricide treatments evoke fears of the possibility of speeding up the emergence of tick populations resistant to specific acaricides. Resistance of ticks to acaricides has been reviewed by Whitehead (1965), Drummond (1977), Baker (1978), Solomon (1983), FAO (1984), Nolan and Schnitzerling (1986), Nolan (1990) and Kunz and Kemp (1994). In summary, Boophilus ticks (B. microplus and B. decoloratus) have developed resistance to all of the currently used organophosphorous (OP), synthetic pyrethroid (SP) and amidine acaricides, although resistance to amidines (e.g. amitraz) is not widespread. In contrast, three host Rhipicephalus and Amblyomma species of importance in Africa have so far developed resistance to OP but not to SP or amidine acaricides. There is speculation that the less intensive treatments used for the control of Boophilus ticks in Australia might be responsible for resistance being a greater problem there than in Africa. However, the lack of facilities and of detailed studies to detect and quantify resistance in Africa means that we do not know the extent or magnitude of the problem here. The emergence of acaricide resistance is complex; it involves some or all of the following: frequency of resistance alleles, dominance, acaricide application methods (i.e. treatment intervals, acaricide concentration, mixtures or rotation of acaricides) and other issues.
Country reports on tick
control policy, acaricide
use and resistance problems
Data on tick control policy and acaricide use and resistance problems were obtained from Ethiopia, Kenya, Malawi, South Africa and Tanzania, which were selected as representative countries for the range of policies and practices in the region.
In Ethiopia there is no tick control policy but one has been drafted by the government and is being debated. There are no functional dips and control is by spray application or pour-ons. The acaricides used are OPs, carbamates and SPs such as spot-on (deltamethrin) and pour-on (flumethrin) formulations. Resistance to OPs is suspected. The aim of the policy is to regulate importation of acaricides, regulate movement of cattle, apply strategic tick control such as treatment at the time of maximum threat of ticks and TBD and introduce vaccines for TBD.
Since 1968 no acaricide can be applied legally to cattle within Kenya unless it has been approved for the purpose by the Department of Veterinary Services. Dipping or spraying with any approved acaricide must be carried out at intervals of not more than seven days in order to control R. appendiculatus under the Cattle Cleansing Act. Most areas with high producing dairy or beef cattle have been gazetted as Cattle Cleansing Areas. In gazetted areas it is compulsory to dip or spray cattle at weekly intervals although this is not fully implemented. Commercial farms have been able to comply with compulsory dipping regulations, with farms paying the full cost of tick control. For small-scale farmers, tick control is subsidised by the government but the government is in the process of withdrawing subsidies. In non-gazetted areas tick control is voluntary and there is no subsidy. Where ECF is not a problem no tick control is enforced.
Most dips (about 6000) use OP Group II acaricides such as chlorfenvinphos but a few privately owned dips on commercial or private farms use amidines. Chemicals such as synthetic pyrethroids as emulsifiable concentrates and pour-on have been evaluated for tick control purposes but are reserved for future use. Pour-on or spot-on formulations of synthetic pyrethroids have been used for control of tsetse in the non-compulsory areas where human trypanosomiasis is a risk or in areas where tick-borne diseases are not the main problem but they are not gazetted for tick control.
Resistance to OP Group II acaricides has been suspected but only where dipping is compulsory. This localised resistance is cleared using more effective chemicals such as amidines. Resistance is a problem with OP Group I (dioxathion, quintifos) and carbamate acaricides.
Although weekly dipping remained compulsory until recently, the law was not strictly enforced and only 2040% of cattle were being dipped regularly. The low dipping percentage, dip management problems (a shortage of acaricide or water or tank breakdown) and presumed resistance of some ticks to arsenic trioxide meant that a large proportion of Malawis zebu cattle population was not intensively dipped even though the law required it. All communal dips have used arsenic trioxide with the exception of 20 dip tanks and two spray races where chlorfenvinphos has been used for approximately 10 years on crossbred dairy animals of smallholder farmers. The dipping service is essentially free except for an annual fee (veterinary service fee) levied per head of cattle owned. Government farms currently use chlorfenvinphos or amitraz. Commercial farmers mostly use chlorfenvinphos, flumethrin and, to a lesser extent, deltamethrin and amitraz. Tick resistance on commercial farms is not perceived to be a problem although there have been a few studies. In a survey conducted on commercial farms no evidence of resistance in Boophilus microplus to organo- phosphorous was found (Kamvazina 1994).
It is unlikely that Malawi has a widespread problem of resistance to the newer generations of acaricides because of their very limited use. Consequently, the need for acaricide rotation or mixtures is not seen.
Recently, the Chief Veterinary Officer issued the following tick and tick-borne disease control policy for communally grazed Malawi zebu cattle:
From mid-November 1993, all government dip tanks, except those in Ngabu Agriculture Development Division (ADD), will only operate once every two weeks through the rainy season, i.e. mid-November until the end of April. There will be no dipping in the dry season, i.e. from May to mid-November. Veterinary assistants will provide alternative tick control measures on a cost-recovery basis during the dry season (e.g. tick grease, hand spray, pour-on). The fortnightly dipping will be offered on a non-compulsory basis in all ADDs, except Ngabu, where there will be no dipping. Chlorfenvinphos, which is a highly effective organo- phosphorous acaricide and less damaging to the environment than arsenic, will be used. This policy is based on consideration of recent research work, existing epidemiological information and expert advice. Initially, the policy will be for a trial period of one year while TBD incidence and tick burdens are being monitored and the effectiveness of this approach is assessed.
The department recommends that crossbred and exotic dairy cattle should be zero grazed to minimise the risk of contracting TBD and that these cattle should be protected by vaccination against TBD and other methods of tick control instead of communal dipping. The department will provide advice to owners of these cattle on tick and tick-borne disease control methods.
South Africa can be divided into two main groups as far as dipping policy is concerned: the commercial farming areas and the communal farming areas.
No dipping policy is laid down in the commercial farming areas and the farmers are responsible for dipping their cattle. Most commercial farmers are using amidine, amitraz or one of the pyrethroid dipping materials.
The communal farming areas are divided up into the independent states and the self-governing states. Each of these states is responsible for its own dipping policy and the choice of dipping material used. For example, in Kwazulu the policy is weekly dipping in the summer months and fortnightly dipping in the winter months. Animals are dipped free of charge by the government and the Department of Agriculture builds and maintains all dipping facilities. Tick resistance to both amitraz and pyrethroids has been reported from Kwazulu. At present, the policy is to change a tank to pyrethroid as soon as both laboratory and clinical resistance is seen. The plan is to introduce cost-recovery as soon as possible, with the dip tanks becoming community dip tanks and the community eventually taking over all financial responsibility for the dipping programme. The aim is eventually to reduce dipping frequency to extend the life of acaricides and to increase the level of endemic stability in the national herd.
Approval, registration and monitoring of the quality of acaricides or pesticides is a legal mandate of the Tropical Pesticide Research Institute (TPRI), established by act of Parliament No. 18 of 1979. Alongside this, the Animal Diseases Ordinance (and Dipping Regulation) 1940 empowers the Director of Veterinary Services (DVS) to approve, authorise, regulate and monitor the quality and quantity of acaricides to be used as deemed appropriate for protection of livestock health within the Tanzanian mainland.
Tanzania moved from use of arsenicals to organochlorines (chlorinated camphene) in the 1950s, then to organophosphates in the mid-1980s. The change in the use of these products was based on safety, efficacy and pest resistance. The use of chlorinated camphene (Toxaphane) was terminated as its active ingredients are no longer being manufactured. The use of OPs has been restricted to the Kagera Region since 1984 because of resistance to Toxaphene. The current policy is to replace Toxaphene and Group I OPs with chlorfenvinphos and other Group II OPs. In the future, Group III OPs, carbamates, amidines and synthetic pyrethroids should be used in succession.
Discussion and conclusions
Important points to emerge from the country reports were the wide diversity in tick control practices in Africa, the discrepancies between policies and actual practice in acaricide usage, and a trend towards reduced dipping in most countries, either deliberately or by default. Although resistance to specific acaricides in ticks was suspected, there was very limited supporting data presented. There was also a growing concern about contamination of meat for export with acaricide residues; the reality and magnitude of this problem needs to be investigated.
The meeting noted the lack of consistency in acaricide registration policy. It also noted a need for an acaricide registration system that would enable data to be shared internationally. The Tropical Pesticide Research Institute (TPRI) in Tanzania could be used as an international facility for the required testing. Participants supported a suggestion for an internationally harmonised system for registration, as this would help national authorities resist pressure from chemical manufacturers.
The meeting discussed the extent to which governments should be involved in the importation and use of acaricides under the new policies of liberalisation. Concern was expressed that all chemical groups used as acaricides are in use as pesticides in the crop sector. Pesticide use has already been liberalised in the crop protection sector. However, liberalisation of acaricide use should be recommended only after acceptance and establishment of international standards. It was agreed that there will be more resistance problems if liberalisation is not planned carefully.
Rotation or alternation of different groups of acaricides that have no cross resistance reduces the selection pressure for resistance to any one acaricide group. There is some successful experience with this strategy in South Africa and simulation modelling and experiments with rotation of insecticides in the crop sector have shown a delay in the emergence of resistance, but this has not always been consistent. Further work is required to determine the full benefit of rotation and alternation strategies. These strategies are also more applicable to spray races than to dip tanks because of the cost of re-charging the tanks.
The use of mixtures of acaricides is another strategy that has the potential to slow the emergence of resistance. It is based on the expectation that one individual is unlikely to have resistant alleles to two acaricides with different modes of action. This strategy also has been tried in South Africa and simulation modelling indicates its promise. However, there must be no pre-existing resistance to either acaricide in the mixture, the chemicals must be compatible and of equal persistence on the animal and they must be used at optimal (recommended) concentrations. The use of home-made acaricide mixtures by farmers (in South Africa) has led to problems, both with toxicity and with suspected acceleration of development of resistance. The technicalities required to produce the right chemical balance for an effective acaricide mixture are such that they require the competence of a pharmaceutical company and should not be attempted by farmers or unqualified people. The use of sub-optimal acaricide concentrations is thought to accelerate the emergence of resistance and the inefficient use either of mixtures, as suggested here, or other application methods such as impregnated ear tags or pour-on formulations must be avoided.
Strategic or threshold tick control in vaccinated cattle requires careful consideration but would reduce costs. The reduced frequency of treatments may also reduce selection pressure and delay the emergence of resistance. The relevance of the Australian and South American acaricide resistance experiences to Africa was questioned. It was agreed that the general principles and specifically the Boophilus spp situation are applicable to Africa. However, uncertainty was expressed regarding Rhipicephalus and Amblyomma.
The meeting concluded that the use of acaricides, acaricide resistance, testing for resistance, resistance management and preventing development of resistance are complex issues. It is difficult for governments to make decisions, considering the flow of sometimes contradictory information. Nevertheless, the meeting recommended that:
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