Tick biology and behavior (Proceedings)

Basic tick biology

The common ticks on dogs and cats in North America are all three host ticks. Accordingly, the larvae, nymphs, and adults of each species must each quest, attach to a host, and feed before leaving the host to molt or deposit eggs in the environment. Most of the common ticks of dogs take as long as 2 to 3 years to complete this developmental cycle, becoming dormant and sheltering during adverse environmental conditions and emerging again to actively seek a host when conditions become favorable. One notable exception is Rhipicephalus sanguineus, the brown dog tick or kennel tick, which can survive inside homes and may complete development from egg to egg laying adult in as little as two months. Effective control of ticks and prevention of tick-borne diseases requires (1) recognition of the tick species active in a given area at a particular time of year, (2) understanding of the habitat and reservoir hosts required to support that species, (3) awareness of the reproductive capacity and strategies of ticks, and (4) consistent use of effective acaricide control products on the dog and, when necessary, in the environment.

Identification of common tick species

Although not often pursued, adult ticks can be easily identified by veterinarians and veterinary technicians with minimal effort. Several excellent pictoral keys are available to aid in identification. Knowing the tick species infesting a given dog or active in an area at a particular time of the year enhances our ability to recommend effective control measures, allowing adjustments to be made when necessary. For example, persistent R. sanguineus infestations on a dog suggests a home or kennel infestation may present, whereas A. americanum ticks are more likely acquired from wooded areas around the home where the dog spends its time. Identification of ticks on dogs can also provide valuable information about tick-borne diseases of concern in the area because different ticks transmit different pathogens to dogs.

Rhipicephalus sanguineus

The brown dog tick or kennel tick, Rhipicephalus sanguineus, is the most widely distributed tick on dogs worldwide (Dantas-Torres, 2008), and dogs infested with this tick can be found in every state in the USA although infestations are more commonly reported in the South. Brown dog ticks are unique among the ticks of dogs in that they use dogs as a strongly preferred, almost exclusive host in all life stages. Although individual ticks occasionally feed on another species, such as people or cats, populations of R. sanguineus are supported almost entirely on dogs. In addition, they can tolerate the low humidity inside homes and kennels and can complete their life cycle in as little as 2 months, thus this tick can establish in large numbers and thrive indoors so long as adequate dogs are available for feeding (Dantas-Torres et al., 2006). Home infestations with R. sanguineus, which may be found in climate controlled environments at any time of the year, are particularly troubling because they increase the risk of transmission of zoonotic disease agents, such as R. rickettsii and E. canis (Goddard et al., 1989; Demma et al., 2005). This tick can also thrive outdoors in warmer areas, usually in an area immediately surrounding a home, with peaks of activity occurring late spring to early fall (Goddard 1989; Louly et al., 2007).

Dermacentor spp.

Adults of several Dermacentor spp. ticks can be found parasitizing dogs and cats in North America, including D. variabilis, D. andersoni, and D. occidentalis. Dermacentor variabilis, the American dog tick, is the most commonly found on dogs and is present throughout much of the eastern US as far west as the Plains States, with populations also established on the West Coast (Easton et al., 1977). Dermacentor spp. prefer to quest in grassy meadows and along trails and roadways (Burg, 2001) and are most often found on dogs in spring and summer. Immature Dermacentor spp. usually feed on rodents and other small wild mammals, and dogs become infested when they encounter questing adults in areas where rodents have deposited fed immature stages (James et al., 2006).

Amblyomma spp.

Amblyomma spp. ticks in North America, including the lone star tick, Amblyomma americanum, and the Gulf Coast tick, A. maculatum, were traditionally considered primarily ticks of southern states. However, in recent years populations of these aggressive ticks have spread dramatically. For example, lone star ticks now inhabit a range from southwest Texas up to Iowa and across the northeast as far as Maine (Paddock and Childs, 2003). Seasonal activity varies, with adults becoming active as early as February in the deep south or as late as May further north (Davidson et al., 1994). Nymphs and larvae follow becoming active in spring to early summer or late summer to fall, respectively. Gulf Coast ticks were historically found only along the coast of the Gulf of Mexico, but in recent decades this tick has become established throughout the southeastern quadrant of the US, as far north as Kansas and as far east as the coastal mid-Atlantic states (Paddock et al., 2008). Although most common in forested habitats, Amblyomma spp. ticks also thrive in fragmented wooded areas around suburban developments (Hair and Howell, 1970).

Ixodes spp.

Two species of Ixodes are commonly found parasitizing dogs in North America: Ixodes scapularis, the eastern black legged tick, which is distributed widely throughout the eastern US, and Ixodes pacificus, the western black-legged tick, which is primarily found on the West Coast and in focal populations in some southwestern states (Keirans et al., 1996; Dennis et al., 1998). Immature I. scapularis ticks feed on rodents and other small mammals, whereas immature I. pacificus may feed on lizards or occasionally small rodents. Adults of both species feed on deer and are also thought to be the stage that most commonly infests dogs; cats are commonly infested with nymphs and adults. The resurgence of the white-tailed deer population throughout the eastern US is thought to be largely responsible for the explosive increase in populations of this tick in recent decades. In the northeastern US, nymphs are found primarily from May through July. However, in the South, nymphs may be active from January through September depending on location and local climatic factors (Dennis et al., 1998; Wilson and Spielman 1985). Adults occur most commonly from October through December with those not finding a host questing again early the next spring (Lane et al., 1991).

Tick reproduction

Ticks are quite fecund, with a single fertilized female able to produce a clutch of thousands of eggs. Clutch size, and thus larval hatch, ranges from 3,000-4,000 eggs produced by a Dermacentor female to as many as 8,000 eggs from an Amblyomma female. Metastriate ticks (Amblyomma spp., Dermacentor spp., Rhipicephalus sanguineus) mate on the dog, with males requiring a short blood meal prior to mating. The males may move between females, mating multiple times, and can even change hosts in order to fertilize additional female ticks (Little et al., 2007). In contrast, male prostriate ticks (Ixodes spp.) are able to mate prior to feeding and often remain in copulation with the female tick throughout her feeding and even after she detaches from the host in a strategy referred to as "mate guarding." With the exception of Rhipicephalus sanguineus, the ticks that parasitize dogs feed and reproduce largely on wildlife reservoirs, and thus we are not able to control tick populations in the way that we can control flea populations. Wildlife hosts in areas frequented by dogs provide a constant source of new ticks to allow re-infestations. For this reason, effective tick control requires a combination of restricting the time a dog spends in a tick infested environment together with monthly application of acaricides year-round to all dogs (CAPC, 2010).

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