Ticks: What you don't know may hurt your practice (Proceedings)

Controlling tick infestations is important not only because ticks are nuisance parasites of dogs and cats, but also because they are vectors of a variety of bacterial and protozoal diseases. Tick control is therefore, a major component of disease prevention and should begin with an understanding of the ecology of the tick(s) encountered in your practice area.1 Veterinarians need to be educated on the various aspects of tick biology and control so that they can then educate their staff and the pet owners. Likely the most important aspects of client education are the explanations on changing tick distributions, our inability to control wildlife tick hosts and expectation differences between flea and tick control. These three factors likely cause most real and perceived product failures as will be discussed in this paper.

Tick Ecology

It must be remembered that most ticks infesting dogs in North America are considered 3-host ticks, with each successive stage feeding on separate hosts after molting. The adult female feeds until engorged and then drops off and deposits her eggs in the environment and then dies after depositing an egg mass typically numbering in the thousands. The tick species most commonly encountered infesting dogs in the U.S. are Amblyomma americanum, Amblyomma maculatum, Dermacentor variabilis, Dermacentor andersoni, Ixodes scapularis, and Rhipicephalus sanguineus.1 Other tick species may occasionally be recovered from dogs including, the one host tick Otobius megnini. While not as commonly infested as dogs, cats can be parasitized by Amblyomma americanum, Dermacentor sp. and Ixodes scapularis.1

Historically Dermacentor sp. were some of the most common ticks infesting dogs in North America. In the typical life history of Dermacentor variabilis (American Dog Tick) larva usually feed on small mammals such as various rats and mice.1-3 Nymphs can be found feeding on dogs, rabbits, raccoons, opossums, and other medium to small sized mammals. The adult tick will feed dogs, horses, cattle, and other large mammals, including man. Dermacentor variabilis occurs in the Central and Eastern US from Florida to southern New England and from the Atlantic coast to the eastern sections of the plains states. Populations also occur along the pacific west coast. A closely related species Dermacentor andersoni (Rocky Mountain Wood Tick) is found in the Rocky Mountain States and in southwestern Canada. Dermacentor sp. ticks are of importance because they can vector Rocky Mountain Spotted Fever (Rickettsia rickettsii) Tularemia (Francella tularensis) and Cytauxzoonosis (Cytauxzoon felis).1 These species can also cause tick paralysis.

One tick species that is increasing in importance across much of the southern, midwestern and eastern states is Amblyomma americanum (Lone Star Tick). Once considered primarily a southern tick, its range appears to have expanded.4 Several environmental and host factors may have contributed to its increased geographic range and frequency of occurrence.

Amblyomma americanum prefers woodland habitats with dense underbrush and reforestation in urban and rural habitats has therefore, increased areas of suitable habitat for this tick. In addition while A. americanum has a wide host range parasitizing a number of mammalian and avian hosts, increased populations and range expansion of two wildlife hosts may have contributed greatly to A. americanum range expansion.4 Range expansion and increasing populations of white-tailed deer has had a major impact upon A. americanum populations.4 The white-tailed deer is considered a preeminent host for A. americanum, because all three-life stages of the A. americanum will feed successfully upon the white-tailed deer and ticks generally fall off into wooded habitats after engorgement.4 Another host that utilizes similar habitats and is an excellent host for larval and nymphal A. americanum is the wild turkey. As wild turkey populations have increased across much of North America, they have reintroduced and increased populations of A. americanum.4 Numerous animals can serve as host for this tick including, rodents, rabbits, dogs, cats, fox, coyotes, humans, deer and a variety of birds serving as hosts for larvae and nymphs. Adult ticks feed on numerous hosts including deer, cattle, horses, sheep, dogs, cats, and humans.1

While A. americanum is considered a major nuisance parasite, it is also a vector of Ehrlichia chaffensis (Human Monocytic Ehrlichiosis), E. ewingii & Borrelia lonestari Southern Tick-Associated Rash Illness.1 It has also been implicated in the transmission of Tularemia. While not as common, the Gulf Coast Tick, Amblyomma maculatum will infest dogs and carries Hepatozoon americanum, the etiologic agent of American Canine Hepatozoonosis. The transmission of this disease is unique in that dogs need to ingest the ticks to become infected.

A tick species that is of considerable importance because it can inhabit kennels and human dwellings is Rhipicephalus sanguineous (Brown Dog Tick).1-3 It also is a 3-host tick where every stage (larva, nymph and adult) prefers to feed on dogs. Infestations of homes or kennels are distressing to pet owners and are extremely difficult to eradicate. It is intolerant of cold and persists in temperate regions within kennels and homes. This tick species often crawl up walls in homes and kennels and can be found in false ceilings. Rhipicephalus sanguineus can vector Ehrlichia canis (Canine Monocytic Ehrlichiosis) and Babesia canis (Canine Babesiosis).1

Another tick that has seen a well documented range expansion is Ixodes scapularis (Blacklegged Tick also referred to as Deer Tick or Lyme Disease Tick).5 It is found throughout the Midwest and Eastern US. The distribution of I. scapularis is associated with the distribution and abundance of white-tailed deer as is the case with A. americanum.1,6 While adult females feed primarily on white tail deer, they will also occasionally parasitize dogs, raccoons, and other wildlife. Larvae feed on a variety of occasionally small mammals, including mice, squirrels, voles, shrews, and raccoons.1-3 Nymphs feed on mice, squirrels, chipmunks, raccoons, opossums, shrews, cats, and man. Ixodes scapularis is the primary vector of Borrelia burgdorferi (Lyme disease) in the central and eastern United States, and is the vector of Anaplasma phagocytophilum (formerly Ehrlichia phagocytophila; HGE). In the western US., I. pacificus is the vector of B. burgdorferi and in parts of the Rocky Mountains I. neotomae can vector B. burgdorferi.7

Tick Control

Numerous studies have been conducted to evaluate the efficacy of various acaracides against ticks infesting dogs with current data suggesting that of the compounds registered in the U.S. amitraz (impregnated collar), fipronil (spray and spot-on formulations) and permethrin (spray and spot-on formulations) have the greatest activity against ticks on dogs.8-14 For cats fipronil spot-on is a highly effective and safe acaracide.

Rapid kill or prevention of attachment and feeding is important in the prevention of tick transmitted diseases.15 While it is generally accepted that killing ticks within 24 to 48 will prevent disease transmission, this may not always be correct. More research is needed to determine the exact transmission times of the various pathogens to dogs and cats.

When dogs were infested with adult I. scapularis containing B. burgdorferi, seven days after placement of amitraz-impregnated collars none of the dogs seroconverted to B. burgdorferi.16 Fipronil spray prevented transmission of B. burgdorferi after dogs were exposed to adult I. scapularis 7 and 28 days after treatment.17 A permethrin-imidacloprid combination spot-on prevented seroconversion to B. burgdorferi or A. phagocytophilum when dogs were exposed to infected adult I. scapularis ticks 7 days after treatment.18,19 In addition a field study demonstrated that fipronil spot-on could be used to markedly reduce seroconvergence to E. canis.20

While such studies demonstrate the efficacy of various products against ticks and prevention of disease transmission, pet owners should be advised that a particular product might not completely prevent disease transmission. Too many variables exist once a particular product has been dispensed such as frequency of bathing and water exposure, reinfestation pressure, potential acaracide susceptibility differences between different tick species and frequency and timing of treatments by pet owners.

Occasionally label recommended application of a product may not appear to control the problem. This may be real or perceived, based upon pet owner expectations of product performance and reinfestation rates. Occasionally additional control measures are needed. If possible start by destroying tick habitat. Refuge areas for ticks and wild mammals that can also serve as hosts for ticks, such as grass, weeds, and brush piles, between runs and along buildings, should be removed. Controlled burning of forest canopy or grass lands has produced at least short term reduction in populations of A. americanum and Ixodes scapularis.

If brown dog ticks are the problem acaracides should be sprayed into cracks and crevices, behind and under cages and along the boards in the ceiling. Effective compounds for environmental tick include cyfluthrin, permethrin and s-fenvalerate. Following application, make sure the acaracide is dry before you allow animals or humans back into the premises to minimize toxicity problems.

These same compounds can also be used outdoors. Rarely is broadcast application of acaracides necessary in the yard. Spot treatment along fences, kennels, or shaded areas is preferred. Finally, restricting pet access from tick-infested environments may be necessary.

It is apparent that the range and local density of certain tick species has increased over the past 20 to 30 years.3,5 The cause for these changes are likely multifactorial and may include changes in climate, vegetation, agricultural practices and wildlife host abundance. Whatever the factors it must be recognized that tick infestation pressure may be much higher and associated tick transmitted diseases may be more prevalent in some locations today than in the past. The increase in tick populations means that pets are encountering ticks more frequently, are exposed to more ticks per encounter and clients may be seeing more ticks on their pets than in the past. Since tick products do not kill or repel all ticks instantly, clients may get the false impression that the products are not performing as well as in the past. These situations necessitate that clients be educated that more stringent adherence to control and prevention programs is warranted. Additional control measures may be needed including premise treatments, tick and tick host habitat destruction or exclusion of pets from tick habitat during the peak tick season. It also must be mentioned that due to these climatic, host and habitat changes year round tick prevention programs may be necessary in many locations. Such programs can be tailored along with heartworm, flea and other broadspectrum parasite prevention programs.

The other common problem that this author encounters is that pet owner perceptions of the success of tick control are often more adamant than that for flea control. First it must be remembered that while we often can control the in home premise or yard flea infestation we may have minimal long term impact on environmental tick populations. The other problem is that there is a major perception difference between a couple fleas on a dog and a couple ticks on a dog. I will explain using the following hypothetical example. Your client applied a broadspectrum flea-tick product to your dog three weeks ago. Due to the natural half-life of the compound the product is now at 95% efficacy. The dog gets out of the back yard and runs through the neighborhood, crawls under a flea infested porch or two and encounters 50 fleas. The product kills 95% of those fleas within the next 24 hours leaving 2 - 3 fleas on the dog. Most clients likely will not even notice those fleas. However, the same dog takes off across a field and runs through a local woodlot and encounters 50 ticks. What are the chances that same client notices 2 – 3 engorged ticks a few days later? It is this author's opinion that the same level of efficacy can give completely different perception differences between flea and tick control. What can be done about this problem? Currently the answer lies with education, education of yourself, your staff and your clients.

Finally it must be remembered that treatments aimed at eliminating existing tick infestations are frequently too late to prevent disease and distress in pets. Many dogs and cats would benefit greatly by being placed on preventative flea and/or tick control products throughout their life.

Since substantial geographic differences occur in flea and tick prevalence and seasonality, prevention programs should be tailored to individual situations.

References

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