Dermacentor andersoni -- horses

Dermacentor andersoni is a large reddish-brown to gray-brown tick.  In Canada, it is found from central Saskatchewan and west through Alberta and into British Columbia.  Dermacentor andersoni is a three-host-tick with each stage of the life cycle (larvae, nymphs and adults) feeding on a separate host. 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 andersoni is seasonal in its activity with adults (the life stage primarily seen on domestic animals) actively questing for hosts in spring and early summer.  Adult ticks prefer larger mammals including wild ungulates, domestic livestock, dogs and people.  Dogs, horses and cattle with light to moderate infestations generally do not display any clinical signs. Hair loss may be seen in heavy infestations.  Some female D. andersoni are associated with tick paralysis.  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. Dermacentor andersoni  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 of Colorado Tick Fever, Rocky Mountain Spotted Fever and tularaemia to people.  It can also cause tick paralysis.

Phylum: Arthropoda
Subphylum: Chelicerata
Class: Arachnida
Subclass: Acaria
Order: Ixodida (= Metastigmata)
Family: Ixodidae

As arachnids, ticks are closely related to mites and spiders.  Hard ticks of veterinary importance in Canada include Rhipicephalus sanguineus, Dermacentor variabilis, D. albipictus, and various Ixodes spp. including I. scapularis and I. pacificus, the vectors of Borrelia burgdorferi - the cause of Lyme disease.

Dermacentor andersoni 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 andersoni  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 andersoni is found from central Saskatchewan, where its range overlaps with the western limit of that of D. variabilis, through Alberta and into British Columbia.  Larvae and nymphs prefer small mammal hosts (mice, gophers etc) while adults prefer larger mammals including wild ungulates, domestic livestock, dogs and people.

Dermacentor andersoni is a three-host-tick with each stage of the life cycle (larvae, nymphs and adults) feeding on a separate host.

Adult ticks overwinter in the environment and begin to emerge from hibernation (exit diapause) in early spring shortly after snowmelt (a period of days above 5°C). They will 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 mammals have been reported as hosts for adult D. andersoni.  They are generally larger species including deer, elk, coyotes, dogs, horses, cattle and people.  As the weather warms the ticks become active in greater numbers, reaching a peak in the late spring (temperatures between 16°C and 19°C and above 20% humidity) or possibly early summer (in Saskatchewan). As days get hotter and drier (a period of days above 20°C and below 20% humidity) the number of active ticks declines rapidly.  Most adult ticks that have not found hosts by this time seek protection under ground debris and generally will not become active again until the following spring. These unfed ticks can survive for at least two years and possibly longer and will become active each spring.  Questing adult D. andersoni have been reported from February to November in regions of their North American geographic range.

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 with the emerged larvae dispersing from the egg mass and actively seeking hosts, generally small mammals  including mice, squirrels, gophers, chipmunks and small carnivores.  If successful in finding a host they will feed for 2-6 days and fall off into the environment where they moult and become nymphs.  It is possible to finds questing larval D. andersoni in regions of their North American geographic range from March to October, with peak numbers found in June and July. Larval ticks can survive, unfed, for at least a year and will overwinter.

Most nymphs do not feed during the summer in which they have developed and will be inactive, under ground cover, until the following spring or early summer.  When they do begin questing they will search for small mammal hosts and, if successful, feed for about a week.  They then fall off the host and moult to the adult stage.  Active D. andersoni nymphs can be found in regions of their North American geographic range from March to October with peak numbers seen in May through June.  They may be able to survive a second winter beneath ground debris.  Newly emerged adults generally do not feed until a number of weeks after moulting and therefore generally do not feed until the following spring.

The entire life cycle of D. andersoni can occur in one season (68 days under ideal laboratory conditions) but it is generally completed in two and occasionally three 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 andersoni 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 andersoni 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. andersoni are associated with tick paralysis. 

Dermacentor andersoni transmits the causal agents of Rocky Mountain Spotted Fever (RMSF) (Rickettsia rickettsii) and tularaemia (Francisella tularensis), as well as the Colorado Tick Fever (CTF) virus (which seems not to occur naturally in animals).  Dermacentor andersoni might also be a significant vector for Anaplasma (e.g. A. marginale) species of ungulates.  These pathogens are believed to not occur or to be extremely rare in dogs in Canada, although other species of Anaplasma (e.g., A. phagocytophilum) are a concern in dogs in this country and in the United States, as is RMSF in dogs in the United States.

If large numbers of ticks are present then owners may observe engorged ticks on their livestock or pets.  Dermacentor andersoni engorged females are quite large and easy to recognize.  Dermacentor andersoni 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 on 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 andersoni.  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 (K9 advantix® - NOT FOR CATS), 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.

 

Dermacentor andersoni  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 of Colorado Tick Fever, Rocky Mountain Spotted Fever and tularaemia to people.  It can also cause tick paralysis.