Dermacentor andersoni: the Rocky Mountain wood tick

Dermacentor andersoni, the Rocky Mountain wood tick,  is a large reddish-brown, ornate hard tick.  In Canada, it is found primarily in the southern half of Alberta and British Columbia and is increasingly restricted to focal remnants in the southwest of Saskatchewan.  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 wildernis or urban green space.  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 produce a salivary neurotoxin that causes tick paralysis in pets, livestock, and people, primarily in British Columbia. Depending on the owner’s risk tolerance, there are 3 tiers to tick control on pets; the first is simply to avoid tick habitat, especially in the spring and early summer. The second is to do thorough tick checks and remove ticks from pets within 6-24 hours of being active in tick habitat.  The third tier involves administration of topical or oral tick preventatives, which serve as repellents or systemic products that rapidly kill ticks within hours of infestation.  Systemic isoxazolines are rapidly becoming the treatment of choice for tick prevention in dogs and cats, while older repellent type products  (i.e. iImidacloprid with permethrin, and some other pyrethrin/pyrethroid-based products) may still be used in dogs, but are unsafe for cats. Dermacentor andersoni  will readily feed on people.  This tick is known to transmit the causative agents of Colorado Tick Fever, Rocky Mountain Spotted Fever (rickettsia) and tularaemia to people.  although very rarely in western Canada.

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 (white colouring on the scutum), has festoons, distinctive eyes on the scutum, and the basis capituli (at the base of the hypostome) is rectangular. Species of Dermacentor can be differentiated by the shape of the spiracle located ventro-laterally behind the last pair of legs, and the size and number of goblet cells in the spiracle. 

In Canada, Dermacentor andersoni is found in southwestern Saskatchewan, where it has largely been displaced by D. variabilis, and the southern half of Alberta and British Columbia. Since the 1970s, the distribution of Dermacentor andersoni has shifted west and north, likely due to competition with D. variabilis and climate change, respectively. 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 but peak activity seems to be in May and June.

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 find 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.  With continued climate change, earlier, warmer springs may lead to adult ticks questing earlier in the year, but this may be countered by hotter summers and regional variation in precipitation.

Dermacentor andersoni is a three-host-tick and both small (hosts for larvae and nymphs) and larger (hosts for adults) mammals are required for the tick to complete its life-cycle. Domestic animals acquire this infestation when habitat is shared with free-ranging hosts.

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.

Dermacentor spp. ticks are most active on sunny, windless days in warm spring or moderate summer temperatures.  They tend to be found on tall grass or low brush, along the edges of trails, pastures, and wooded areas with deeper litter for environmental stages.  While ticks tend to be associated with rural and wilderness regions, they are increasingly found in urban green spaces (river valleys, parks, off leash areas, etc).

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 produce a salivary neurotoxin that causes tick paralysis in pets, livestock, and people, primarily in British Columbia.  Even one tick can cause a serious and progressive paralysis that can be fatal if the tick is not found and removed.

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) in ungulates.  These pathogens are rare in dogs in Canada, RMSF is an issue in dogs in the United States.

Owners may observe engorged ticks on their livestock or pets.  Dermacentor andersoni spp. ticks are quite large and easily recognized by white markings on the scutum, and by their posterior festoons (scalloped edges).  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. Most veterinary personnel should be able to identify ticks to genus level, but owners in western Canada can also seek identification by submitting digital photos of ticks through the eTick App or website (https://www.etick.ca/) or to their provincial surveillance program.  Some owners may want the tick or their pet tested for exposure to pathogens, and PCR screening panels are increasingly available for these purposes.  Results should be interpreted carefully, since ticks may test positive for pathogens present in blood recently taken from a host, but this does not mean that they are competent to transmit the pathogen.

Depending on the owner’s risk tolerance, there are 3 tiers to tick control on pets; the first is simply avoiding tick habitat, especially in spring and early summer. The second is to do thorough tick checks and remove ticks from pets within 6-24 hours of being active in tick habitat.  This timing is based on the minimum amount of time for ticks to attach, feed, and transmit pathogens.  Ticks can be carefully removed with tweezers (or fingers)  by grasping the mouth parts as close to the skin as possible (ticks secrete a glue patch and embed their mouthparts deep into the skin, so use slow and steady pressure).  Bites should be washed and monitored for signs of local bacterial infection 

 

The third tier involves administration of topical or oral tick preventatives, which serve as repellents or systemic products that rapidly kill ticks within hours of infestation.  Systemic isoxazolines are rapidly becoming the treatment of choice for tick prevention in dogs and cats, while older repellent type products  (i.e. imidacloprid with permethrin, and some pyrethrin/pyrethroid-based products) may still be used in dogs, but are unsafe for cats

 

Dermacentor andersoni  will readily feed on people, although they generally seem to prefer animal hosts.  This tick is known to transmit the causative agents of Colorado Tick Fever, Rocky Mountain Spotted Fever and tularaemia to people. although very rarely in Canada.  Some female D. andersoni produce a salivary neurotoxin that causes tick paralysis in pets, livestock, and people, primarily observed in central British Columbia.  Even one tick can cause a serious and progressive paralysis that can be fatal if the tick is not found and removed.  People can minimize tick exposure by avoiding tick habitat at peak times of year, finding and removing ticks from themselves and their children within 6-24 hrs after being active in tick habitat, and wearing long, light coloured pants and tops.  People can also use repellents on their skin or wear repellent-impregnated clothing.