Winter Ticks

Winter ticks (Dermacentor albipictus) are a common parasite of moose across most of North America except far northern Canada and Alaska.

Winter ticks are absent from the island of Newfoundland in Canada because moose were stocked on the island at a time of year when they are free of the tick parasite. Winter ticks are common in Minnesota and on Isle Royale and in most years probably every moose carries at least a few winter ticks. Winter Tick

Winter ticks seem to prefer hoofed animals, especially members of the deer family, and have been found on elk, deer, woodland caribou, bison, horses and cattle. However; they seem to most effectively parasitize moose.  For whatever reason, winter ticks seem to be uninterested in using humans as a host.

Winter tick larvae climb vegetation in the fall and wait for a passing host.  Studies have shown they most often congregate at a height equivalent to chest height on a moose, and the larvae are most active during the period of the moose rut (mid –September to early October) when the moose are most active and ranging widely looking for mates. The tick larvae congregate in clusters of individuals numbering into the 100s, and the larvae in a cluster interlock their legs.  Because of this behavior, when a suitable host passes, it usually picks up 100s of larval winter ticks at a time.  A study from western Canada estimated an average of 33,000 ticks per moose with some individuals having over 100,000.  Larval winter ticks that don’t find a host, don’t survive the winter.

The larval winter ticks began taking a blood meal from their host in November and molt to the nymph stage. The nymphs are dormant for a couple of months, but begin taking another blood meal in late January and molt to the adult stage in February and March. Adults take their final blood meal in March and April, and then the female drops off to lay her eggs on the soil surface. Winter ticks are absent from moose after late May. Engorged ticks

The height at which larval tick clusters aggregate on vegetation and the time of their greatest activity, make moose the most susceptible host for acquiring large numbers of winter ticks.

 In addition, moose appear to do a poorer job than other species of removing ticks through grooming. Therefore moose suffer the greatest impacts from winter ticks. By early spring moose may be showing large patches of broken or missing hair where they have tried to rub away ticks. Moose with large patches of broken hair are sometimes referred to as “ghost” moose because the white base of the hair shaft is all that remains. Ghost calf

Winter ticks take their final blood meal at a time of year when moose are nutritionally and energetically stressed anyway at the tail end of winter prior to the spring green up. Ticks are thought to negatively affect moose through increased exposure to the elements through loss of the hair coat, increased energy expenditures because of extra time spent grooming away ticks instead of foraging and increased stress due to substantial blood loss from thousands of feeding ticks.  Skin with ticks

In years with high ticks numbers, many moose, especially calves and older adults, may die as a result of heavy tick loads. Necropsy results of moose carcasses heavily infested with ticks often show the moose was anemic and in poor physical condition.

Tick larvae are most active during periods of warm dry weather in the fall and adult survival is greatest if the engorged female tick drops off the moose onto bare ground in the spring.  Wet, cold weather in the fall reduces larvae activity and fewer larvae find a host.  Adult ticks that drop off moose in the spring and land on snow cover have a poorer survival rate.  Climate change can be predicted to improve conditions for winter ticks through longer, warmer falls and earlier snowmelt in the spring.

Further Reading

White as a Ghost: Winter Ticks & Moose, by Dr. Bill Samuel tells the story of winter ticks and moose. It was published in 2004 by the Federation of Alberta Naturalists.