The main reasons for tick control are to protect hosts from irritation and production losses, formation of lesions that can become secondarily infested, damage to hides and udders, toxicosis, paralysis, and of greatest importance, infection with a wide variety of disease agents. Control also prevents the spread of tick species and the diseases they transmit to unaffected areas, regions, or continents.
These measures can be directed against both the free-living and parasitic stages of ticks. The free-living stages of most tick species, both ixodid and argasid, have specific requirements in terms of microclimate and are restricted to particular microhabitats within the ecosystems inhabited by their hosts. Destruction of these microhabitats reduces the abundance of ticks. Alteration of the environment by removal of certain types of vegetation has been used in the control of Amblyomma americanum in recreational areas in southeastern USA and in the control of Ixodes rubicundus in South Africa. Control of argasid ticks such as Argas persicus and A walkerae in poultry can be achieved by eliminating cracks in walls and perches, which provide shelter to the free-living stages.
The abundance of tick species can also be reduced by removal of alternative hosts or hosts of a particular stage of the life cycle. This approach has occasionally been advocated for control of three-host ixodid ticks such as Rhipicephalus appendiculatus, Amblyomma hebraeum, and Ixodes rubicundus in Africa, and Hyalomma spp in southeastern Europe and Asia.
Rotation of pastures or pasture spelling has been used in control of the one-host ixodid tick Rhipicephalus (Boophilus) microplus in Australia. The method could also be applied to other one-host ticks, in which the duration of the spelling period is determined by the relatively short life span of the free-living larvae. However, it has minimal application to multihost ixodid ticks or argasid ticks because of the long survival periods of the unfed nymphs and adults.
Predators, including birds, rodents, shrews, ants, and spiders, play a role in some areas in reducing the numbers of free-living ticks. In the New World, fire ants (Pheidole megacephala) are noteworthy tick predators. Engorged ticks may also become parasitized by the larvae of some wasps (Hymenoptera), but these have not significantly reduced tick populations.
Zebu (Bos indicus) and Sanga (a B taurus, B indicus crossbreed) cattle, the indigenous breeds of Asia and Africa, usually become very resistant to ixodid ticks after initial exposure. In contrast, European (B taurus) breeds usually remain fairly susceptible. The tick resistance of Zebu breeds and their crosses is being increasingly exploited as a means of control of the parasitic stages. The introduction of Zebu cattle to Australia has revolutionized the control of R microplus on that continent. Use of resistant cattle as a means of tick control is also becoming important in Africa and the Americas. In Africa, infestations of ixodid ticks on livestock and wild ungulates may also be reduced by oxpeckers (Buphagus spp), which are birds that feed on attached ticks.
Also see Ectoparasiticides.
Control of ticks with acaricides may be directed against the free-living stages in the environment or against the parasitic stages on hosts. Control of ixodid ticks by acaricide treatment of vegetation has been done in specific sites (eg, along trails) in recreational areas in the USA and elsewhere, to reduce the risk of tick attachment to people. This method has not been recommended for wider use because of environmental pollution and the cost of treatment of large areas. Dog kennels, barns, and human dwellings may also require periodic treatment with acaricides to control the free-living stages of ixodid ticks such as the kennel tick, Rhipicephalus sanguineus.
The free-living stages of argasid ticks, which infest specific foci such as fowl runs, pigeon lofts, pig sties, and human dwellings, are more frequently and more effectively treated with acaricides.
Treatment of hosts with acaricides to kill attached larvae, nymphs, and adults of ixodid ticks and larvae of argasid ticks has been the most widely used control method. In the first half of the century, the main acaricide was arsenic trioxide. Subsequently, organochlorines, organophosphates, carbamates, amidines, pyrethroids, and avermectins have been used in different parts of the world. The introduction of new compounds, such as the phenylpyrazoles, has been necessary because of the development of resistance in tick populations.
Acaricides are most commonly applied to livestock by use of dips or sprays, with dips being considered the more effective. In recent years, several other means of acaricide application have been developed, including slow release of systemics from implants and boluses, slow release of conventional acaricides from impregnated ear tags, pour-ons (which are applied on the back and spread rapidly over the entire body surface), and spot-ons (which are similar but have less ability to spread). Acaricides are usually applied as dusts or washes on cats, and as dusts on dogs and fowl.
For many years, pyrethroids and organophosphates formulated as dusts, dips, or collars were used on dogs and cats to control ticks. With the advent of the phenylpyrazoles, long-lasting spray and convenient spot-on formulations were introduced. Pyrethroids are available as highly concentrated spot-on products that are labeled only for dogs because of their toxicity in cats. Use of these concentrated pyrethroids is not advised on dogs if a cat is even in the same household.
An advance of potentially great importance has been the production, using biotechnology, of a promising vaccine against R microplus. The immunizing agent is a concealed tick antigen, not normally encountered by the host. The immune mechanism it stimulates is different from that stimulated by exposure to ticks (ie, tick feeding). The antigen was derived from a crude extract of partially engorged adult female ticks. It stimulates the production of an antibody that damages tick gut cells and kills the ticks or drastically reduces their reproductive potential.
Prospects of developing similar vaccines against other ixodid tick vectors of cattle diseases of major veterinary importance are not clear. Rhipicephalus ticks are good candidates for such vaccines in that they are one-host ticks and show a marked preference for bovine hosts, which act as the principal reservoir of perhaps the most important group of disease agents (Babesia spp) these ticks transmit. By contrast, most other tick vector species of agents that cause important cattle diseases (eg, anaplasmosis, heartwater, theileriosis) are three-host ticks, which infest not only cattle but also wild ungulate species, for which vaccination is not feasible. Moreover, many wild ungulate hosts of the vector ticks serve as reservoirs of these disease agents. For these reasons, vaccines against nonboophilid vector ticks may be unable either to eradicate the ticks or to eliminate important sources of the disease agents they transmit.
Initially, the main uses of acaricides were for tick eradication, prevention of spread of ticks and tickborne diseases (quarantine), and eradication and control of tickborne diseases. The eradication programs were successful in some ecologically marginal subtropical areas, such as southern USA and central Argentina where Rhipicephalus spp and babesiosis were eradicated, and southern Africa where East Coast fever (caused by Theileria parva parva) was eradicated. The programs were less successful in the ecologically more favorable tropical areas of northeastern Australia, Central America, the Caribbean Islands, and East Africa.
In the areas where eradication was not achieved, costs of maintaining intensive tick control programs often have become prohibitive. For this reason, integrated biologic and chemical control strategies are being adopted. The effectiveness of these cost-containment strategies requires better knowledge of the dynamic associations among the disease agents, their vertebrate hosts, the tick vectors, and the environment. Strict quarantine measures to prevent reintroductions are enforced in countries from which ticks and tickborne diseases have been eradicated. Climate-matching models, geographic information systems, and expert systems (models based on expert knowledge and artificial intelligence) are being used to identify unaffected areas in which tick pests could become established if introduced.
Control of these diseases will require use of the principles of endemic stability and development of improved recombinant vaccines. A current, promising strategy is the identification of receptor sites on the midgut of vector ticks and the development of antibodies that bind with these sites, thereby blocking tick-ingested tickborne pathogens from infecting the tick. Cattle injected with receptor-site antigens may produce antibodies that feeding ticks ingest.