Dermacentor variabilis — horses

Myrna,  won't transfer

Phylum: Arthropoda

Subphylum: Chelicerata
Class: Arachnida
Subclass: Acaria
Superorder: Parasitiformes (Anactinotrichidea)
Order: Ixodida (= Metastigmata)
Family: Ixodidae

Among the arthropods, ticks are most closely related to mites and spiders. Other hard ticks of veterinary importance in Canada include Rhipicephalus sanguineus, Dermacentor andersoni, D. albipictus, and various Ixodes spp. including I. scapularis and I. pacificus, the vectors of Borrelia burgdorferi - the cause of Lyme disease.

Note: Our understanding of the taxonomy of helminth, arthropod, and particularly protozoan parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on that in the seventh edition of Foundations of Parasitology by Larry S Roberts and John Janovy Jr., McGraw Hill Higher Education, Boston, 2005.

Dermacentor variabilis is a large (unfed adult females are 5-7 mm long, fed females can be double in size) reddish-brown to gray-brown tick.  All life stages are dorsoventrally flattened. The larvae have six legs while the nymphs and adults have eight legs.  Dermacentor variabilis  is an ornate tick (colouring on the scutum), has festoons, distinctive eyes on the scutum, and the basis capituli (at the base of the hypostome) is rectangular.

In Canada, Dermacentor variabilis is found from eastern Saskatchewan, where it overlaps with the eastern limit of the range of D. andersoni, through to Nova Scotia, primarily in the southern portions of each province.  Larvae and nymphs prefer small mammal hosts primarily mice, but occasionally rabbits,gophers as well as others) while adults prefer larger mammals including wild ungulates, domestic livestock, dogs and people.

Dermacentor variabilis is a three-host-tick with each stage of the life cycle (larvae, nymphs and adults) feeding on a separate host. The life cycle of D. variabilis follows a basic pattern but there can be great variation in the timing of the cycle across its broad geographic range.

Adult D. variabilis overwinter off the hosts and begin to emerge from hibernation (exit diapause) in early spring (April through May depending on the location within the tick’s geographic range) and actively search (quest) for hosts by climbing to the tips of grass blades and other low vegetation waiting for appropriate hosts to brush past.  Numerous mammal species can serve as hosts for adult D. variabilis.  They are generally larger animals, primarily dogs but also deer, elk, coyotes, horses, cattle, sheep and people.  As the weather warms the ticks become active in greater numbers, reaching a peak of activity in the late spring (mid May to late June). As days get hotter and drier the number of active ticks declines rapidly although some ticks will remain active all summer.  Most adult ticks that have not found hosts by this time seek protection under ground debris and generally will not become active again the following spring. These unfed ticks can survive for at least two years and possibly longer, and become active each spring.  Adult ticks that have been successful in finding a host will feed for about a week, mate during that time, and drop off the host into the environment.  Females lay eggs about seven days later and then die.

Eggs hatch approximately a month after being produced.  Although a small percentage of D. variables larvae will begin to actively quest shortly after hatching, the majority of these newly hatched larvae will settle into the ground debris until the following spring (May and early June) at which time they will actively seek hosts, generally small mammals, primarily mice and voles.  If successful in finding a host they will feed for 2-14 days and fall off into the environment where they moult and become nymphs.

Most nymphs of D. variabilis begin questing shortly after moulting.  They will search for small mammal hosts, almost exclusively mice and voles and, if successful, feed for between 3 and 10 days.  They fall off the host and moult to the adult stage.  D. variabilis nymphs can be found from March to October across their North American geographic range, with peak numbers seen in June.  In general, newly emerged adults generally do not feed until the following spring.

The entire life cycle can occur in one season (54 days under ideal laboratory conditions) but it is generally completed in two years.  Factors that influence life cycle completion include temperature, humidity and host availability.  Conditions supportive of tick development can advance the timing of the cycle and increase tick abundance.  Unfavourable conditions have the opposite effects.   

Dermacentor variabilis is a three-host-tick and both small (hosts for larvae and nymphs) and larger (hosts for adults) usually free-ranging mammals are required for the tick to complete its life-cycle. Domestic animals acquire this infestation when habitat is shared with free-ranging hosts, for example grazing by livestock or hiking with dogs in wild or semi-wild areas.

Dermacentor variabilis is seasonal in its activity with adults (the life stage primarily seen on domestic animals) actively questing for hosts in spring and early summer. Climatic factors such as temperature, wind and moisture levels will affect tick activity.

Dogs with light to moderate infestations of ticks generally do not display any clinical signs.  Animals may be pruritic around the bite sites.  Hair loss may be seen in heavy infestations.  Some female D. variabilis are associated with tick paralysis.

Dermacentor variabilis transmits the causal agents of Rocky Mountain Spotted Fever (RMSF) (Rickettsia rickettsii) and tularaemia (Francisella tularensis).  These pathogens are believed to not occur or to be extremely rare in dogs in Canada, although RMSF is of concern in dogs in the United States.  Dermacentor variabilis is also a candidate vector for cytoauxzoonosis (Cytauxzoon felis), which does not occur in Canada. In cattle, D. variabilis is a vector for Anaplasma marginale.

If large numbers of ticks are present then owners may observe engorged ticks on their livestock or pets.  Dermacentor variabilis engorged females are quite large and easy to recognize.  Dermacentor variabilis is, however, similar in appearance to the other Dermacentor species found in Canada and is differentiated by its seasonal occurrence, geographic location and by the size and shape of the goblet cells within the spiracle plates.  These are located ventro-laterally behind the last pair of legs. 

While the differentiation of D. albipictus from the other two species on the basis of morphology is relatively straightforward, variations of the spiracle plates means that in areas of Saskatchewan where the two species are sympatric distinguishing D. variabilis from D. andersoni can be difficult.  In these areas the situation is further complicated by the possibility of inter-breeding between the two species. 

There are few products that are approved for use in dogs in Canada for the treatment and control of Dermacentor variabilis. It is important to remember to control both the infestation on the animal as well as avoiding tick contaminated areas. This is especially important when walking dogs in the spring and early summer. Imidacloprid with permethrin, and some other pyrethrin/pyrethroid based products have a claim of efficacy against ticks in general.

There are a number of products approved for use on cattle for the treatment and control of this tick. These include permethrin based products, some malathion products and one product containing lambda-cyhalothrin (Saber^®).

There are some permethrin/pyrethroid based products available for use on horses.

Additional information on the products mentioned is available from the Compendium of Veterinary Products  (Twelfth Edition, 2011), or from the manufacturers.

Adult Dermacentor variabilis  will readily feed on people.  Individuals become infested when walking in wild or semi-wild areas frequented by wildlife.  This tick is known to transmit the causative agents Rocky Mountain Spotted Fever and tularaemia to people