Tick infestations of dogs and cats are important not only because ticks are nuisance blood-sucking parasites but also because they can transmit a variety of diseases. Pet owners often get frustrated when they use a tick product yet still find ticks on their pets.
A number of factors contribute to problems with ticks, such as an increase in tick populations, changing tick distributions and our inability to control native wildlife tick hosts.
The tick species that most commonly infest dogs in North America are:
While we often use the same products to combat ticks as we use to combat fleas, there are substantial differences between flea and tick control. One of the major differences is in the number of tick species that confront dogs and, less frequently, cats.
While there is one predominant flea species that infests dogs and cats in North America—the cat flea (Ctenocephalides felis)—there are at least nine tick species that may be encountered.
Because these ticks often feed on different hosts and can survive in different habitats, there can be remarkable regional variability in the number and diversity of tick species that infest dogs. While dogs in Hawaii may only deal with one tick species, dogs in New Mexico may encounter three different species, in California six different species, New England four different species and in Kansas and Oklahoma up to seven species of ticks.
This wide diversity in tick species means that ticks occur at different times of the year and carry and transmit different diseases. The list of tick-transmitted diseases continues to grow with Anaplasmosis, Babesiosis, Cytauxzoonosis, Ehrlichiosis, Lyme disease and Rocky Mountain spotted fever being the most important.
Over the past few decades, there has been a change in the distribution and abundance of several tick species in North America.
A number of factors have contributed to an increase in tick populations, including changes in agricultural practices, reforestation, wildlife conservation, relocation and restocking, climate fluctuations and decreased environmental pesticide application.
The expansion of two tick species, Amblyomma americanum (Lone Star tick) and Ixodes scapularis (Black-Legged tick; often referred to as the “deer tick” or Lyme tick) in the eastern half of North America are the best documented.
Since both these ticks are important vectors of disease, changes in distribution and abundance have had a marked effect upon both human and animal health. Specific factors that have contributed to the increased range of these two ticks include increased habitat via reforestation and their wide host range, which includes deer, small mammals, birds, dogs, cats and humans.
The white-tailed deer is considered the preferred host for the Lone Star tick and all life stages will feed successfully upon them. Another wildlife host that utilizes similar habitats and is an excellent host for this tick is the wild turkey.
Areas with high white-tailed deer and wild turkey populations and a deciduous forest canopy can have remarkably large populations of Lone Star ticks. Similarly, the distribution of the well-known Lyme tick in eastern North America is also linked to the distribution and abundance of the white-tailed deer. Yes, both of these tick species are “deer ticks.”
In the early to mid-1800s, white-tailed deer were numerous and widespread throughout North America. But throughout the 19th century, unregulated hunting and extensive loss of habitat decimated the deer populations.
By the beginning of the 20th century, an estimated 300,000 to 500,000 deer remained in North America. Then during the early and mid part of the 20th century, restrictions were placed on deer hunting, there was a loss of natural predators, a number of states began restocking efforts and there was an increase in natural habitat.
These combined effects contributed to a marked resurgence in deer populations to an estimated 27.7 million by 2005. As deer populations increased and their range expanded, there was a corresponding increase in the tick species closely associated with white-tailed deer.
While pharmaceutical advances have been made in control of fleas, such advances in tick control are lacking. Many modern flea products not only kill fleas but also control flea reproduction by either killing fleas before they can reproduce or killing flea eggs and larvae. By controlling flea reproduction, we can effectively eliminate or prevent flea infestations.
However, it is not just because we have effective flea products that we are successful; it is also due in large part to the fact we can often target the primary reproductive host, the flea-infested dog or cat.
When dealing with most ticks the problem is that the majority of the reproducing ticks are on their natural wildlife hosts such as coyotes, deer, rabbits, opossums, raccoons, woodchucks and squirrels or some other wild mammal or bird. We are limited in our ability to manage ticks on wildlife, so reinfestations are a common occurrence and the seasonal or year-round use of tick products as preventives has become routine in many areas.
Because tick control can be extremely difficult and because ticks are vectors of a variety of bacterial and protozoal diseases, veterinarians should have an understanding of the ticks encountered in their areas. Veterinarians need to know the various aspects of tick ecology, disease transmission and control methodologies so that they can then educate their staff and pet owners.
A number of studies demonstrate the high level of efficacy of the various acaracides but the residual activity is rarely 100 percent and the efficacy of products varies between and within species, even in the same laboratory. Evaluations of acaracides under natural or field conditions further illustrates that while efficacy is good, it is not 100 percent.
If a dog is treated with one of these highly efficacious products and encounters just a few ticks, it is likely all those ticks will be killed. However, if tick exposure is considerably larger, we can expect a few ticks to be observed on these dogs and owners may perceive a lack of efficacy.
Therefore, in areas where tick populations are increasing, the perception may be that the products are not as effective as they once were.
We might also expect that efficacy in real-world situations might be lower due to such factors as bathing and swimming, differences between dog breeds and haircoat types and frequency and correctness of product application. While it is true that these products can dramatically reduce the chances a dog or cat will acquire a tick-transmitted disease, because these products are not always 100 percent, we cannot expect these products to completely prevent disease transmission.
With many three-host ticks, destruction of tick habitat can reduce exposure pressure. Areas that serve as refuge for ticks and wild mammals, such as grass, weeds, brush and wood piles, can be eliminated or treated with an approved acaracide. Removal of leaf litter from under bushes allows the ground to dry, which can kill many ticks as well. In some very heavily tick-infested areas, restricting a dog's access from tick-infested environments may be necessary.
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.
Dr. Dryden, MS, Ph.D., is a professor of veterinary parasitology at the College of Veterinary Medicine at Kansas State University.