R.A. Bram and J.H. Gray
Introduction. Ticks, generally regarded as the ectoparasites that cause the greatest economic losses to livestock production in the world today, adversely affect livestock hosts in several ways (Snelson, 1975): they contribute to unthriftiness and anaemia by exsanguination; they damage hides and subject livestock to secondary infection; they cause toxicoses and paralysis by the injection of their salivary secretions; and, most importantly, they transmit pathogenic agents that cause diseases, many of which result in debility and death. Of the many tick-borne livestock diseases, four are of particular concern: bovine anaplasmosis, bovine babesioses, theilerioses, and heartwater. The economic impact of ticks and the diseases they transmit is enormous. Not only is the annual global cost of ticks and tick-borne diseases estimated to run to thousands of millions of dollars, but also mankind is deprived of a significant amount of animal protein that cannot be replaced from other sources (Bram, 1975; Callow, 1975; Drummond et al. 1978; Snelson, 1975).
To counteract the adverse effects of ticks and tick-borne diseases, a variety of tick-control programmes have been integrated into modern livestock management practices. Foremost among these is chemical tick-control, which not only decreases the detrimental effects of the ticks themselves, but also interrupts the transmission cycle of pathogenic agents and, thus, reduces the incidence of tick-borne diseases. Depending on the tick species involved, the livestock-management practices followed, the tick-borne diseases present, and the environmental conditions of an area, cattle are treated at periodical intervals with any one of over 30 available acaricides. The frequency of acaricide treatment may vary from every 3 or 4 days in East Africa for the protection of cattle against East Coast fever transmission by Rhipicephalus appendiculatus to intervals approaching 6 months for the reduction of Boophilus microplus populations. In addition to the chemical control of ticks, increased efforts are being made to introduce tick-resistant breeds of livestock that, though they continue to support tick populations, are not conducive to massive tick infestations. The control of tick-borne diseases is also accomplished by the use of live, attenuated vaccines. Noteworthy are the vaccination programmes for tick fever (a disease complex caused by Babesia bovis, B. bigemina, or Anaplasma marginale) in Australia (Callow, 1978) and the development of an East Coast fever vaccine (consisting of strains of Theileria parva, and T. lawrencei) by an FAO regional project located at Muguga, Kenya. In addition, live attenuated vaccines have been used in programmes to control heartwater (caused by Cowdria ruminantium) and tropical theileriosis (caused by Theileria annulata) (Uilenberg, 1975; Wilde, 1978).
Quarantine is an essential tool in tick eradication.
Certainly, any investments in organized tick and tick-borne disease control are economically sound. A recent cost-benefit analysis of cattle-fever tick (Boophilus spp.) control in the United States resulted in a 1:98 ratio. That is, for every $1 spent on tick control, there is a $98 return. Howver, tick control itself implies a perpetual annual investment that imposes a continual, and sometimes unnecessary, drain on livestock production. A control philosophy, therefore, is a philosophy of living with ticks and tick-borne diseases.
Dr Bram is Principal Staff Officer for Vector Biology, Surveillance and Control, Veterinary Services, Animal and Plant Health Inspection Service, US Department of Agriculture, Hyattsville, Maryland 20782, USA; he was formerly Tick Control Officer, Animal Production and Health Division, FAO, Rome. Dr Gray is Regional Tick Epidemiologist, Veterinary Services Animal and Plant Health Inspection Service, US Department of Agriculture, PO Box 2091, Austin, Texas 78768, USA.
NOTE: Mention of a proprietary product or a pesticide in this article does not constitute an endorsement or recommendation by the US Department of Agriculture or by FAO.
Eradication - an alternative. An alternative philosophy is one of living without tick-borne diseases and their vectors through a major eradication effort. Whereas the cost-benefit ratio for cattle-fever tick control in the United States was determined to be 1:98, the cost-benefit ratio for eradication was 1:140. Thus, resources devoted to successful eradication in this case would result in approximately a 40 percent greater return on investment than would tick control.1
The progress of Boophilus tick eradication in the United States, 1907 to 1943
To be successful, certain preconditions must be met before eradication can even be considered. Foremost is the need for a broad base of scientific knowledge about the biology and host relationship of the tick species in question, the epidemiology of the disease complex to be eradicated, and the techniques for tick eradication. In addition, an eradication philosophy requires strong support from the livestock industry (usually stimulated by economic considerations), a favourable legal and political climate, and a longterm commitment of manpower and financial resources. Finally, whereas tick control can be accomplished with varying degrees of effectiveness by the efforts of individual livestock producers working independently, eradication depends upon the existence or creation of a comprehensive animal health infrastructure with programme policies that are uniformly applied to all segments of the livestock industry. If none of these conditions are fulfilled, an eradication effort could fail.
A number of tick-eradication efforts have been attempted in several parts of the world (Graham and Hourrigan, 1977). One successful example that can be cited is the eradication of cattle-fever ticks, Boophilus spp., and bovine babesiosis from the continental United States. In 1907, an area of approximately 1 813 000 km2 was infested with Boophilus spp. At that time, a Federal/State cooperative programme to eradicate cattle-fever ticks was initiated with a general strategy of beginning on the northern edges of the infested area and moving southward. Through a centrally planned and coordinated programme of quarantine, compulsory dipping of cattle, and/ or pasture vacation, the campaign gradually progressed to a successful conclusion by 1943. The success of this effort has been attributed to several factors: the fact that B. microplus and B. Annulatus are one-host ticks with limited host range; a strong animal health infrastructure at the Federal, State, and country levels; a sound manpower, financial, political, and legal base of support; and innovative research before and during the eradication campaign. Although all these factors contributed to eradication, in the final analysis credit for the success must be given to the cattle owners themselves, who enthusiastically demanded, supported, and participated in tick eradication. The benefit to the livestock industry of the United States has been a savings estimated to exceed $1 thousand million per year.
The most thorough, effective method of acaricide treatment is the total immersion of livestock in a dip-vat.
Programme planning and application. In planning and executing a national tick-eradication campaign, five major programme components contribute to a coordinated effort: surveillance, extension and training; quarantine; treatment; and research and development.
A tick-eradication campaign begins and ends with surveillance. Surveillance defines the initial area of tick infestation and determines the prevalence and distribution of the diseases ticks transmit. Surveillance monitors campaign progress and provides the day-to-day information upon which eradication decisions are based. And surveillance guards against the reestablishment of tick infestations in eradicated areas. The techniques of tick surveillance naturally vary with the tick species in question. With onehost ticks, such as Boophilus spp., the detection of all parasitic stages is accomplished by manual inspection of the host. This technique, known as “scratching”, is applied to all animals in an eradication effort, not just to a statistical sample. Other surveillance techniques that have been employed, particularly for two- and three-host tick species, include dragging, livetrapping, manual examination of mammalian or avian alternate hosts, and trapping with carbon dioxide bait (Gladney, 1978). After collection, tick specimens must be officially identified by competent authorities at a central laboratory where centralized records are maintained for analysis and programme evaluation.
Extension and training are the foundations of eradication. “Extension” includes all educational efforts directed toward the livestock producer and the industry in general; “traning” covers the instruction of eradication campaign personnel. Thus, to achieve success through the cooperation, support, and participation of the stockman (even when temporary inconveniences or financial hardships are encountered) an extension effort usually begins prior to actual programme implementation and continues throughout the eradication campaign. The livestock producer will cooperate best when he understands why eradication is necessary, how eradication will be accomplished, and what eventual eradication will mean to the industry in general and to himself in particular. Similarly, personnel at all levels of the eradication organization can function effectively only with a clear understanding of the importance of their own responsibilities and how they relate to both the eradication campaign and the livestock industry. Training should strive to achieve efficient execution of duties and the uniform application of programme policies. It should also emphasize to programme managers the necessity of utilizing all available operational resources in the most effective manner through conscientious supervision.
Quarantine, never a popular component of any animal health programme, is an essential tool in tick eradication. Since ticks spread to new areas primarily by the movements of their mammalian hosts, the control of livestock movements through effective quarantine can render a dynamic distributional pattern static while tick eradication proceeds. Both area and premises quarantines have their place in an active campaign. As the name implies, area quarantines prohibit livestock movements out of large, infested areas without prior inspection and precautionary treatments in order to prevent the spread of ticks. Premises quarantines apply to individual ranches or pastures that are defined by recognizable physical barriers that prevent livestock and ticks from crossing under ordinary circumstances. Premises quarantines must be strictly enforced until ticks within the quarantine boundaries have been unquestionably eradicated. The utilization of quarantine techniques implies the existence of an adequate legal foundation and sufficient manpower to monitor or enforce, when necessary, the controlled movement of livestock.
The routine determination of dip-vat concentration and contamination is vitally important to proper dip-vat management. Here a tick inspector is carrying out these tests in his office.
The actual killing of ticks is usually accomplished by applying acaricide directly to the domestic host of the ticks. The most thorough, effective method of acaricide treatment is the total immersion of livestock in a dipvat (Drummond, 1975). Although other methods of acaricide application, such as spray races, spray-dip machines, hand spraying, and hand dressing, have been employed under special circumstances, the dip-vat continues to be the principal eradication tool in most situations. In fact, utilization of dip-vats is so important that the success or failure of an eradication effort may depend on the quality of dip-vat management practices. Foremost among these management practices is the systematic treatment of all livestock in a quarantine area on a regular schedule determined by the biology of the tick species and the local environmental conditions.
Whereas effective tick control can be accomplished by a well-planned dipping schedule (basically, dipping livestock at intervals in order to kill the most ticks with the least effort), eradication is based on a rigid, systematic dipping schedule that can compensate for the biological variation within the tick population as well as for occasional instances of human error. For example, the Boophilus eradication programme in the United States prescribes dipping all livestock on infested premises at 14-day intervals for 9 months. In addition to a systematic dipping schedule, the acaricide within the dip-vat is continually maintained at the maximum safe concentration to kill ticks. Again, using the United States Boophilus eradication programme as an example, an official dipping with one of the permitted acaricides, coumaphos, requires a dipvat concentration of from 0.125 to 0.250 percent active ingredient (in most parts of the world, coumaphos is used for tick control at a concentration of approximately 0.03 percent). Furthermore, the acaricide concentration within the vat is sampled before, during, and after each dipping operation. There is also the requirement that the dip-vat be emptied, cleaned and recharged when there is over 10 percent suspended matter in the acaricide, when the number of animals treated exceeds two per 4 litres (approximately) of acaricide in the dip-vat, or when the acaricide has been in the dip-vat for longer than 240 days.
For best results, dip-vats must be emptied, cleaned and recharged at prescribed intervals.
Technical support of dip-vat management is provided by a chemical laboratory that not only conducts routine confirmatory analyses of acaricide concentrations, but also develops the essential data that are used by programme managers in selecting approved acaricides and formulations. In small-scale efforts to eradicate two- or three-host ticks, techniques of ground application of acaricides have been employed to kill non-parasitic stages. However, such procedures are usually only considered with newly introduced, exotic species that have not spread appreciably beyond the point of original introduction.
Livestock management practices have also been incorporated into campaigns to eradicate ticks. In the case of a one-host tick such as Boophilus spp. pasture vacation (or pasture spelling) has been applied to good advantage to kill larval ticks by starvation due to the absence of their host for approximately a 9-month period. This technique of tick eradication assumes the existence of well-fenced pastures and the absence of suitable alternate hosts. As with dip-vat treatments, pasture-vacation schedules must be rigidly enforced.
Research and development precede the initiation of eradication and continues until its successful conclusion. It has been estimated that the level of research and development support should approach 10 percent of the operational programme budget. Probably the major research commitment would be concentrated prior to and during the meticulous planning necessary for eradication, whereas developmental studies and field applications would increase during the course of the campaign. Major areas of research and development have included: all aspects of tick biology, particularly under the variety of environmental conditions found in an eradication zone; ticks and tick-borne disease epidemiology which, in effect, is a combined developmental and operational function; eradication technology, particularly as it relates to new acaricide products, formulations and methods of application; and the continual monitoring for acaricide resistance in the tick populations. In order to apply results of research and development to best advantage, the campaign should be sufficiently flexible to incorporate new information into operational procedures and policies.
Eradication and control. Although eradication is one alternative to the control of ticks and tick-borne disease, it is not necessarily the best option in all circumstances. In fact, in many situations, a well-planned and supported control effort precedes eventual eradication. In other instances, decentralized, individual control activities are most appropriate.
Thorough agitation of a dip-vat before treating cattle is an important factor in proper dip-vat management.
Certainly, an eradication campaign for ticks and tick-borne disease requires an effort that far exceeds the resources needed for control. It is based on a substantial, long-term financial and manpower commitment. It is also based on a strong animal health infrastructure supported by the regulatory authority to enforce quarantine measures and to require the systematic treatment of all livestock. In addition, after success has been achieved in a geographic area, sufficient authority and resources are necessary to prevent the re-introduction of the eradicated tick species and disease(s). As eradication approaches realization, large livestock populations become highly susceptible to tick-borne diseases. Should the campaign falter or be abandoned at this point, an epidemic of major proportion would ensue due to enzootic instability. Eradication does not always require a national effort. On the contrary, areas of eradication could be limited to regions, states, or even individual holdings provided that the capability exists to prevent the reintroduction of ticks after eradication has been achieved.
All livestock leaving an infested premises must be treated with acaricide to prevent the spread of ticks.
Varying degrees of control can be accomplished either on an individual-producer basis or by a centrally planned and coordinated programme. The utilization of live vaccines (when available) can also be successfully incorporated into a control scheme. Regrettably, by their very nature, tick-control programmes seem inevitably to meet problems associated with tick resistance to acaricides. As a result, farsighted control-planning would, hopefully, include provisions for reducing dependence on acaricide treatments by consideration of alternative tick-control methods (Wharton, 1976).
The decision, therefore, to initiate the eradication of ticks and tick-borne disease is influenced by a complex array of interacting parameters: the biology of the tick species and the epidemiology of the diseases involved; the economic impact of the ticks and tick-borne disease(s); and the availability of government financial and other resources over a number of years; the existence or creation of a functional animal health infrastructure; the legal authority to enforce strict quarantines and systematic treatmentschedules; and the enthusiastic commitment of the livestock industry itself. Perhaps a phased approach to national control and eradication of tick-borne disease is most logical. For example, the Mexican tick campaign (Fideicomiso Campaña Nacional Contra la Garrapata) is designed in three phases - the promotion phase, the control phase and the eradication phase. Thus, different areas of the country are designated as promotion zones, control zones, eradication zones, or free zones, and the programme policies and activities in the different zones vary depending on the objectives. As objectives are successfully attained, the zone status is progressively changed to a new phase with the application of appropriate policies, activities and resources.
A spray-dip machine in operation. This method of treating livestock conserves water and the equipment is portable.
Conclusions. The eradication of tick-borne livestock diseases and their vectors is a viable alternative to the control of ticks and tick-borne disease. However, the assessment of this alternative deserves the consideration of a variety of interrelated parameters that will eventually influence the success or failure of a campaign. Although in many situations eradication may not be an attainable national goal, successful examples of the eradication of ticks and tick-borne diseases illustrate that under the proper conditions, with the necessary resources and commitment, it is possible and economically sound. Perhaps the most logical approach to national control and eradication of tick-borne diseases is a progressively phased programme that can accommodate zones of no control, promotion, control, eradication and, finally, free zones.
References
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