Winter Tick in Moose and Other Ungulates


Last Update: August 2000

Author: F. A. Leighton

Reviewer: W.M. Samuel (August 2000)

Cause of Winter Tick Disease

This disease is caused by a species of tick known variously as the Winter Tick, Moose Tick, or Elk Tick. Its scientific name is Dermacentor albipictus

Affected Species

Animal Species Parasitized by the Winter Tick
The Winter Tick has been found on many different species of mammals. However, it is principally a parasite of ungulates. In Canada, it regularly is found to parasitize:
Moose**White-Tailed Deer *
Caribou**Mule/Black-Tailed Deer *
Elk**Bison *
CattleHorses
** Causes mild to severe disease in these species
* Disease caused by the Winter Tick has not been recognized in these species, although each is regularly parasitized by the tick.

Occurrence in Canada

Severe disease in moose has been reported from time to time throughout most of the range of the moose in Canada south of about 60 North Latitude. Winter tick is present but is not common on moose inthe southern Yukon; it is absent from the island of Newfoundland. Disease from Winter Tick also has been reported in woodland caribou northern Alberta. Mild to moderate hair loss in Elk associated with Winter Tick infestation has been observed in Alberta.

Importance to Human Health

None: The Winter Tick rarely if ever parasitizes humans, nor is it known to carry and transmit agents that may cause disease in people.

Ecology of Winter Tick in Canada

Life Cycle

The life cycle of the Winter Tick has been studied most completely on moose, and the life cycle on moose in the southern boreal forest of Canada is described here. It is essentially the same on the other host species in Canada.

Stages of Winter Tick
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Life Stages of the Winter Tick (W.M. Samuel)*
  • 1. Engorged Larva
  • 2. Nymph
  • 3 - 5. Engorged Nymphs
  • 6. Adult Male
  • 7. Adult Female
  • 8 & 9. Engorged Adult Female
* (From: Samuel, W.M. 1988. Use of age classes of winter ticks on moose to determine time of death. Canadian Society of Forensic Science Journal 21: 54-59. Used with permission.)

The Winter Tick parasitizes only one individual host animal during its one-year life cycle. Eggs are laid on the ground in May. Larval ticks hatch from the eggs in late summer. Larvae are dormant for a few weeks, then climb up vegetation (tall grass, small trees, shrubs) in September and October, and attach to host animals that brush against them. They live the rest of their lives on the host animal's skin.

Larvae of Winter Tick

Larvae of Winter Tick on the Tips of Spruce Needles (D. Fraser)

The larvae take one blood meal from their ungulate host and molt to become nymphs. During November and December, the nymphs are dormant. Then, in January and February, the nymphs take a blood meal and molt into adults. In Canada, adult Winter Ticks will not be found on moose until January or February.

Winter Tick Attached to Moose

Adult Winter Ticks Attached to Moose Skin (February-March)

Adult ticks take a blood meal and mate while attached to the moose during March and April. The blood-engorged ticks then detach from the moose's skin and fall to the ground. All Winter Ticks detach during late March to early May, and none remain on moose after this time until new larvae are picked up in the fall (There may be other species of ticks on moose during the summer, but the Winter Tick does not parasitize its ungulate hosts in summer).

Female Winter Ticks on the Ground

Engorged Female Winter Ticks on the Ground in Spring (E. Addison)

Adult female ticks remain dormant on the ground for several weeks after they detach, and then lay eggs in early summer and die. A single female tick can lay as many as 3000 eggs. Larvae hatch from the eggs in late summer and are then dormant until September, when they climb up vegetation and attach to suitable ungulate hosts that brush against them.

Hatching eggs of Winter Tick

Larval Winter Ticks Hatching from Eggs (J. Robinson)

Occurrence of Epidemic Disease in Moose

The number of Winter Ticks on moose varies greatly from year to year. Small numbers of ticks on a moose do not cause recognizable disease. In some years, however, epidemics of disease in moose caused by the Winter Tick have occured over very large geographic areas in Canada, for example, from British Columbia to New Brunswick in 1999. Diseased moose generally have 30,000 ticks or more on their skin, often twice this many (The record for numbers of ticks appears to be held by a captive reindeer in Alberta which had approximately 400,000 ticks on its skin). Calf moose carry higher densities of ticks [number of ticks per square centimeter (cm2)] on their skin than do adults. Mooring and Samuel (1998a) reported the average density of ticks on the skin of diseased moose in late winter to be 2.33/cm2 for calves, 1.44/cm2 for bulls and 0.97/cm2 for cows.

The factors that result in epidemic disease from Winter Ticks in some years and only relatively mild parasitism without disease in other years are not fully understood. DelGiudice, Peterson and Samuel (1997) argue convincingly that precipitation and temperature in April have a major affect on the survival of female Winter Ticks after they leave their ungulate host and before they lay eggs. Thus, April weather may be a major factor determining the proportion of female ticks that survive to lay eggs and, hence, the number of larvae that hatch and seek ungulate hosts in the fall and the occurrence of disease in moose the following February and March, almost a year later. If this is true, then the density of moose in an area would have very little influence over the occurrence of epidemics, as has seemed to be the case. Dependence of Winter Tick survival on April weather also would explain why epidemics occur in some years simultaneously over large parts of North America; large areas often share basic climatic variations from year to year. Warm temperatures, low precipitation and absence of snow cover in April favour survival of adult ticks. Thus, these weather conditions may predict epidemic disease in the following winter and spring.

Winter Tick on Different Host Species

Disease versus Benign Parasitism

Large numbers of ticks are able to parasitize individual moose, and, when this happens, serious disease can result. Much less is known about the relationship between the Winter Tick and other host species. Large numbers of Winter Ticks, and consequent disease, have been observed in wild woodland caribou, but there is very little information about the occurrence of Winter Tick disease in this species of host. Mooring and Samuel (1998b) found an average of 1,791 Winter Ticks on individual elk in Alberta, and also observed some associated mild disease in this species. Winter Ticks parasitize white-tailed deer and mule/black-tailed deer, and also American Bison, but do not appear to cause significant disease in these species.

Grooming Behaviour and Winter Ticks

Mooring and Samuel (1998a, b,c) studied the response of moose, elk and bison to Winter Tick and concluded that the high susceptibility of moose to high levels of parasitism and severe disease compared to the much lower susceptibility of the other ungulate species studied could be explained, in large measure, by the grooming behaviour of moose compared to that of other species. Winter Ticks infest their ungulate hosts in September and October, but moose do not respond to the presence of the larvae or nymphs with intensive grooming. It is only when adult ticks begin to feed, in February and March, that moose groom intensively and in proportion to the number of ticks on their skin. Thus, moose go into winter carrying most of the Winter Ticks that infested them in the fall, and may have vast numbers of adult ticks on their skin, beginning to feed, before they start to respond to them in any protective way, by grooming. In contrast, white-tailed and mule/black-tailed deer, bison and elk groom more intensively in fall and early winter. This early grooming effectively removes a substantial proportion of the Winter Tick larvae and nymphs before they can become adults. Thus, these species seldom have large numbers of adult Winter Ticks on their skin and, therefore, seldom suffer significant disease. (More Information on grooming behaviour and Winter Tick)

Signs of Disease in Affected Animals

Moose

Moose show signs of disease only in late winter and spring, and only when they are infested with large numbers of ticks. The principal signs of disease are loss of hair and emaciation (very thin body condition). Affected moose are sometimes called "ghost" moose because the appear very light coloured; the areas of hair damage and loss are pale grey instead of the dark brown of the normal, intact hair coat.

Aerial View of 'Ghost Moose'

'Ghost Moose' with Extensive Hair Loss (W.M. Samuel)

Hair loss in moose is most evident on the neck and shoulders, but can occur nearly anywhere on the body that moose can groom or rub against objects.

Moose with Hair Loos Typical of Winter Tick

Hair Loss on Moose Over Neck and Shoulders Associated with Winter Tick Infestation (M.D. Wood)

Affected moose groom excessively, and grooming behaviour may be seen frequently if moose are observed for a period of time.

Affected moose often are thin or emaciated. This is particularly true of affected calves. This may or may not be evident in the living animal, but is quickly evident in moose that die from Winter Tick and are examined.

Elk

Affected elk may have small areas of hair loss along the sides, produced by oral grooming, and at the base of the neck at the shoulder, which results from grooming or scratching this area with the hind hoofs. Emaciation is not reported as a consequence of infestation in elk. Generally, disease in infested elk is mild or not apparent.

Hair Loss in Elk from Winter Tick Hair loss in Elk from Winter Tick

Three views of Elk with Hair Loss on the Neck Associated with Winter Tick Infestation. (W.M. Samuel)

The upper two photos show extensive hair loss on the lower neck, dorsally. The lower photo shows small patches of hair loss along the sides and a large area at the base of the neck.

Caribou

Little is known about disease in caribou caused by Winter Tick. A small number of emaciated Woodland Caribou with extensive hair loss and heavy infestations have been observed in northern Alberta.

How Does Winter Tick Cause Disease?

Animals that die because of heavy infestations with Winter Ticks appear to die because they do not eat enough food to supply the energy they require. Thus, they appear to have starved to death. Heavy infestations with the Winter Tick both increase the amount of energy the host animal requires and cause the animal to spend inadequate amounts of time feeding. The ticks have these effects on the host animal in two different ways: 1) by feeding on the animal's blood and 2) by causing severe irritation of the skin.

Blood Loss:

Winter Tick nymphs on moose in Canada take a blood meal in January-February, and adult ticks take a blood meal in February and March. Nymphs and adult male ticks each take a fairly small amount of blood, but the total volume of blood consumed must be substantial. Females take much more blood and become engorged and distended with a thick, paste-like concentrate of blood that represents several times its volume in actual whole blood. The host animal's blood is not removed all at once; blood feeding by the population of adult ticks on a moose is spread out over a two-month period. Nonetheless, the host animal must produce new blood to replace the blood that is consumed by the ticks. The total amount of blood lost is considerable. If each female tick takes a total of 2 millilitres of blood, and half of the 40,000 ticks on a diseased moose are females, then the females alone will consume 40,000 millilitres, or 40 litres, of blood. A 400 kilogram moose has a total blood volume of about 32 litres (8% of body weight). Thus, 40,000 ticks will consume, and the moose will have to replace, all of its blood during the 2-month feeding period. Blood is an energy-rich substance, and there is a high cost to the moose in energy and materials to replace the lost blood.

Irritation:

Moose are severely irritated by feeding adult ticks. They respond to this irritation by intense grooming behaviour and by rubbing against firm objects such as trees. This behavioural response to the irritation of ticks has two results that contribute to disease associated with Winter Tick: 1) hair loss and 2) distraction from normal feeding behaviour.

Hair Loss:

Moose with heavy tick burdens groom and rub the hair off of large proportions of their bodies. 40% hair loss is common and 80% occurs frequently. This loss of hair must increase loss of body heat, which, in turn, will require the moose to expend more energy to maintain body temperature. It is likely that calf moose, which have more body surface area per kilogram of body weight than do adults, use more energy to compensate for hair loss than do adults with the same degree of hair loss.

Distraction from Feeding

Heavily infested moose spend large amounts of time grooming and less time feeding. This distraction appears to result in lower intake of food by moose with heavy tick burdens.

Total Effect:

The combination of blood loss, hair loss and distraction from eating place severely affected moose in a negative energy balance or worsens the negative energy balance the animals commonly experience anyway at the end of winter. In much of Canada, weather in February and March is very cold. High-energy foods are not usually available, and wild ungulates have just experienced three months of cold weather, often with restricted food supplies. Thus, fat stores usually are low. Heavy tick burdens require expenditure of energy to replace blood and maintain body temperature after loss of insulating hair. Normal food intake is reduced because of distraction. The overall result is starvation and, probably, increased susceptibility to predation.

It is important to note that heavily infested moose with extensive hair loss may nonetheless survive the effects of Winter Tick. The severity of disease produced by Winter Tick ranges from fatal to unimportant. The nutritional condition of an animal at the beginning of winter and its food supply during November-January may greatly influence the severity of the impact that a heavy burden of Winter Ticks will have on an individual host animal.

Pathology: Changes in Organs and Tissues Caused by Winter Tick

Moose that die because of heavy burdens of Winter Tick typically have extensive hair loss, reduced muscle size, prominent ribs and other bones, and no body fat. They also have large numbers of adult and nymph Winter Ticks on their skin.

Emaciate Moose - Winter Tick

Thin, Emaciated Moose Dead of the Effects of Winter Tick (F. Matejka)

Healthy moose in late winter should have some visible fat around the kidneys and at the wide end of the heart. The bone marrow should be white or pink, opaque and greasy. The bone marrow of ungulates that have starved to death has a very low fat content, has a jelly-like consistency, and is translucent.

Starvation - no fat in bone marrow

Jelly-Like Bone Marrow from a Thin, Dead, Emaciated Moose(F. Matejka, E. Addison)

Identification and Diagnosis of Winter Tick Disease

It is not possible to demonstrate, with absolute certainty, that an animal has died because of the effects of Winter Ticks. However, emaciated animals with heavy tick burdens, hair loss and no fat stores are likely to have died from the total of effects of Winter Ticks. Careful examination of the carcass by someone with the appropriate expertise should be undertaken to rule out other possible causes of death.

To measure the fat content of bone marrow

To count the number of ticks on a dead animal

Further Reading

DelGiudice, G.D., R.O Peterson, W.M. Samuel. 1997. Trends of winter nutritional restriction, ticks, and numbers of moose on Isle Royale. Journal of Wildlife Management 61(3): 895-903.

Mooring, M.S. and W.M. Samuel. 1998. The biological basis of grooming in moose: programmed versus stimulus-driven grooming. Animal Behaviour 56: 1561-1570.

Mooring, M.S. and W.M. Samuel. 1998. Tick-removal grooming by elk (Cervus elaphus): testing the principles of the programmed-grooming hypothesis. Canadian Journal of Zoology 76: 740-750.

Mooring, M.S. and W.M. Samuel. 1998. Tick defense strategies in bison: the role of grooming and hair coat. Behaviour 135:693-718.

Samuel, W.M., M.S. Mooring and I.O. Aalangdong. 2000. Adaptations of winter ticks (Dermacentor albipictus) to invade moose and moose to evade ticks. Alces 36:183-195.

Additional Literature

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