Dermacentor variabilis

This section will provide information about Dermacentor variabilis, which is often called the wood tick in Wisconsin but is called the American dog tick in other parts of the country.  This tick has not been associated with disease transmission here, although it can transmit the bacteria causing Rocky Mountain spotted fever and tularemia–also known as rabbit fever– in other areas of the United States. 

DER VAR MALE FEMALE & NYMPHFigure 1.  Wood tick or American dog tick.  Top=nymph.  Bottom left=adult female.  Bottom right = adult male.

Demacentor_Passive surveillance

Figure 2.  Surveys for Dermacentor variabilis (wood tick or American dog tick).  Veterinary clinics, human societies and animal rehabilitators collected ticks and sent them to UW Madison for identification.  Counties in white were those with no participating organization.

Summary of surveys:  Wood ticks were common on dogs and cats.  Raccoons, coyote, opossums, porcupines, woodchucks, red fox, and one rabbit also hosted the adult wood ticks. Small mammal trapping resulted in detection of larvae and/or nymphs on white footed and deer mice.

Description and Identification: American dog or wood ticks (Dermacentor variabilis) have three life stages after hatching from eggs. Wood ticks develop from the egg stage to larvae, nymphs and finally the adult. Their size and coloration vary depending on the life stage. Wood tick adult females generally are about 1/4 inch (5-6 mm) in length when unfed and are reddish brown in color. Adult females have a  “dorsal shield”  behind their head that covers only the upper half of their body and has creamy-white to silvery-gray markings or striations. Females will vary in size depending on whether they have taken a blood meal. Blood fed or engorged females can enlarge to up to 1/2 inch in length or 15 mm long and 10 mm wide (Chan & Kaufman 2008).

Adult male ticks are slightly smaller in size (3.6 mm in length) than females, and are also reddish brown with cream to silvery-gray colored vertical markings or lines over the entire back.  The head and mouth parts of wood ticks are more rectangular in shape compared to the deer tick (Keirans and Litwak 1989). 

Nymphal wood ticks are very tiny and are approximately 0.035 inches (0.9 mm) long and pale, yellowish brown before taking a blood meal. Nymphs become light gray in color when engorged. Larvae are smaller than nymphs or about 0.024 inches (~0.6 mm) in length, also yellowish brown in color before a blood  meal and then turn grey in color when engorged (Smith and Whitman 1992). Unlike the deer tick, nymphs (and larvae) of the wood tick are very uncommon on humans.

Distribution in United States and Wisconsin: This tick is widely distributed east of the Rocky Mountains and is also found in Canada east of Saskatchewan and in limited areas along the Pacific coast (west of the Cascade and Sierra Nevada Mountain ranges) (Chan & Kaufman 2008). In Wisconsin the tick can be found distributed throughout the entire state particularly associated with woodland, shrubby or tall grass habitats.  

Seasonality: In Wisconsin, all life stages of Dermacentor variabilis are most active throughout the warmer months (May-August). Peak abundance and activity of D. variabilis varies within North America with adults being most abundant in June and July in temperate zones and July and August in sub-tropical zones (Cilek & Olson 2000).

Hosts: Dermacentor variabilis larvae typically feed on small mammals, and nymphs feed on small-to medium-sized mammals. Adults will occasionally feed on humans but are more commonly found on domestic dogs and other medium-sized mammals (Merten and Durden 2000; Sonenshine 1993; Campbell & MacKay 1979). 

Life Cycle: 

The wood tick is a 3-host tick which means that it requires three different hosts (such as an animal or human) and at least 54 days to complete the life cycle from egg to adult. The wood tick develops from the egg stage, to the six-legged larva, to the eight-legged nymph and finally to the adult. For the cycle to continue, the tick requires a blood meal before developing from larva to nymph, from nymph to adult and by the adult female for egg production. The completion of the life cycle can vary and may take up to two years to complete depending on availability of appropriate hosts to obtain a blood meal, host location and environmental factors such as temperature and humidity (Chan and Kaufman 2008; Sonenshine 1993). 

Life cycle of a 3-host hard tick, based on Dermacentor variabilis (American dog tick).
Illustration by: Scott Charlesworth, Purdue University, Medical Entomology Extension

Medical and Veterinary Significance: 

The wood tick is capable of transmitting the bacterial organism that causes Rocky Mountain spotted fever, Rickettsia rickettsii, (Azad & Beard, 1998) but this disease is rare in Wisconsin (WI-DHS, 2012). Cases that are reported in Wisconsin residents are usually associated with travel to other states where Rocky Mountain spotted fever more commonly occurs.

Dermacentor variabilis is also known to transmit the bacteria Francisella tularensis which causes the disease tularemia or rabbit fever. Other known tick vectors include the Rocky Mountain wood tick (Dermacentor andersoni) and the lone star tick (Amblyomma americanum) as well as deer flies, (Chrysops spp.) (Goethert & Telford 2009). This disease is also rarely reported in Wisconsin. There were a total of three human cases of tularemia in Wisconsin from 2001-2010 reported to the Centers for Disease Control and Prevention. Most cases occur in the South Atlantic and West Central states (CDC, 2011).

Although the bacteria which causes Lyme disease, Borrelia burgdorferi, has been found in wood ticks (Dermacentor variabilis), this tick has not been proven to be a competent vector (it does not efficiently pass the bacteria from inside the tick to humans or other hosts) and the wood tick (Dermacentor variabilis).  At this point, there is no scientific evidence that wood ticks are able to transmit the Lyme disease bacterium (Piesman and Happ 1997; Mukolwe et al. 1992; Mather & Mather 1990; Soares et al. 2006). 

The wood tick is also known to cause tick paralysis. There are several other species of ticks in North America which are capable of causing tick paralysis but the two species that cause the most human cases in the United States and Canada are the wood tick (Dermacentor variabilis) and the Rocky Mountain wood tick (Dermacentor andersoni). Tick paralysis is not caused by a disease pathogen such as a bacteria or virus but is thought to be caused by a toxin produced in the tick’s saliva. Tick paralysis can often be confused with other acute or rapid onset neurological diseases or disorders. The onset of symptoms of tick paralysis occurs only after a female tick has attached and begun feeding and clinical signs do not typically develop until the tick has fed for at least four to seven days. Symptoms include acute, progressive flaccid paralysis which usually begins in the extremities such as the arms or legs. If the tick is not removed, death may occur from respiratory failure. Paralysis typically subsides within 24 hours of removing the tick. Tick paralysis may occur in a variety of domestic and wild mammal species and birds. Although rare, tick paralysis can occur in humans and most commonly occurs in children (Sexton 2012; Merck Veterinary Manual 2011). 

References:

Azad AF and Beard CB. (1998). Rickettsial pathogens and their arthropod vectors. Emerg. Inf. Dis; 4:179–186.

Burg JG. (2001). Seasonal Activity and spatial distribution of host-seeking adults of the tick Dermacentor variabilis. Medical and Veterinary Entomology 15: 413-421.

Campbell A and MacKay PR. (1979). Distribution of the American dog tick. Dermacentor variabilis (Say), and its small-mammal hosts in relation to vegetation types in a study area in Nova Scotia. Can. J. Zool. 57: 1950- 1959.

Centers for Disease Control and Prevention. (2011). Reported tularemia cases by state, United States, 2001 – 2010. Accessed June 27, 2012. http://www.cdc.gov/tularemia/statistics/state.html

Chan WH and Kaufman PE. (2008). American dog tick: Dermacentor variabilis (Say) (Arachnida: Ixodida: Ixodidae). Featured Creatures: University of Florida Institute of Food and Agricultural Sciences; Dept. of Entomology and Nematology. http://entnemdept.ufl.edu/creatures/urban/medical/american_dog_tick.htm                     Accessed June 27, 2012. 

Cilek, J.E. and M.A. Olson. (2000). Seasonal distribution and abundance of ticks (Acari: Ixodidae) in northwestern Florida. Journal of Medical Entomology 37:439-444.

Goethert HK and Telford SR III. (2009). Nonrandom Distribution of Vector Ticks (Dermacentor variabilis) Infected by Francisella tularensis. PLoS Pathog 5(2): e1000319. doi:10.1371/journal.ppat.1000319

Keirans JE and Litwak TR. (1989). Pictorial Key to the Adults of Hard Ticks, Family Ixodidae (Ixodida: Ixodoidea), East of the Mississippi River. J. Med. Entomol. 26(5): 435-448. http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA233445

Mather TN and MAther ME. (1990). Intrinsic competence of three ixodid ticks (Acari) as vectors of the Lyme disease spirochete. J Med Entomol; 27(4): 646-50. 
Merck Veterinary Manual. (2011). Tick Paralysis. Accessed July 17, 2012.  Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc.Whitehouse Station, NJ USA. http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/102600.htm
Merten, H. A. and L. A Durden. (2000). A state-by-state survey of ticks recorded from humans in the United States. Journal of Vector Ecology. 25(1): 102-113.
Mukolwe SW, Kocan AA, Barker RW, Kocan KM, Murphy GL. (1992). Attempted transmission of Borrelia burgdorferi (Spirochaetales: Spirochaetaceae) (JDI strain) by Ixodes scapularis (Acari: Ixodidae), Dermacentor variabilis, and Amblyomma americanum. J Med Entomol; 29(4): 673-7.
Piesman J and Happ CM. (1997). Ability of the Lyme disease spirochete Borrelia burgdoferi to infect rodents and three species of human-biting ticks (blacklegged tick, American dog tick, lone star tick) (Acari:Ixodidae). J Med Entomol; 34(4): 451-6.
Sexton DJ. (2012). Tick Paralysis. Up to Date. Accessed July 17, 2012. http://46.4.230.144/web/UpToDate.v19.2/contents/f36/23/37610.htm
Smith EH and Whitman RC. (1992). Field Guide to Structural Pests. National Pest Management Association, Dunn Loring, VA.
Soares CA, Zeidner NS, Beard CB, Dolan MC, Dietrich G, Piesman J. (2006). Kinetics of Borrelia burgdorferi infection in larvae of refractory and competent tick vectors. J Med Entomol; 43(1): 61-7.

Sonenshine DE: Ecology of non-nidocolous ticks, in Biology of Ticks, vol 2. New York, Oxford University Press, 1993, pp 3–64.

Wisconsin Division of Public Health -Bureau of Communicable Diseases and Emergency Response. June 2012. Rocky Mountain Spotted Fever and typhus fever group. (Tickborne rickettsial diseases). http://www.dhs.wisconsin.gov/communicable/Tickborne/RockyMountainSpottedFever.htm