Factors affecting epizootics of winter ticks and mortality of moose.

ABSTRACT: Die-offs of moose (Alces alces) associated with, or attributed to, winter ticks (Dermacentor albipictus) are widespread and have been reported since the early part of the last century. Extrinsic factors such as weather and vegetative structure, and host factors such as moose density and, indirectly, tick-induced damage to the hair coat, were examined in an attempt to predict related problems for moose. The proposal that warmer and shorter winters result in increased survival of adult female ticks dropping off moose in March and April, and increased tick populations on moose the following winter, was generally confirmed. Annual changes in hair damage and loss on moose, which are documented from the air, coincided with annual changes in numbers of ticks on moose, providing managers with a survey tool to monitor and estimate changing numbers of ticks. Tick numbers lagged 1 year behind moose numbers in Elk Island National Park over a 12-year period, and many moose died when numbers of both were high. Several widespread, concurrent die-offs suggest extrinsic influences play a role, possibly independent of moose density. The lack of objective and continuous data sets should guide future research efforts.

Key words: Alces, density, Dermacentor albipictus, epizootics, hair, moose, mortality, ticks, transmission, weather

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Die-offs of moose associated with, or attributed to, winter ticks are numerous and widespread across North America, having occurred since the early part of the last century (summarized by Samuel 2004). Almost all published and unpublished reports of such events indicate finding a certain number, sometimes few, often many, dead or dying moose covered with ticks. Tick-associated die-offs often are widespread and concurrent, involving many populations of moose in several-to-many jurisdictions. For example, the most recent widespread outbreaks of winter ticks, accompanied by losses of many moose, occurred in late winter and spring, 2002. There were reports from Isle Royale National Park, Minnesota, Maine, New Hampshire, and Vermont in the United States, and Alberta, Manitoba, Ontario, and Saskatchewan in Canada (Samuel et al. 2002, Peterson and Vucetich 2003, Samuel and Crichton 2003). Earlier concurrent widespread die-offs occurred in late winter-spring, 1999 and 1992, and one across Alberta in 1982 (Samuel 2004).

Die-offs are often attributed to winter ticks, perhaps because ticks are obvious and numerous on dead and dying moose. Unfortunately, direct evidence of the lethal effect of winter ticks on moose populations is lacking. There is, however, good information from experimental and field studies (reviewed by Samuel et al. 2000) that, at the least, suggests that winter ticks are a significant factor in rapid declines of moose numbers. Other factors, such as moose numbers or density, habitat, weather, and predation, likely also play a role.

The objective here is to review factors potentially contributing to tick-related die-offs of moose using published literature, reports, and observations from central Alberta, particularly Elk Island National Park and vicinity; then, to assess specific factors or influences that might be used to predict epizootics of ticks and die-offs of moose. In an attempt to determine if various weather parameters might be predictive of moose die-offs, I examined parameters of snow cover and temperature at the time of 5 die-offs in Alberta, 2 of which (late winter-springs of 1982 and 1999) killed many moose. I also examined the feasibility of using tick-caused damage to the winter hair coat of moose as an indication of tick numbers as done by Wilton and Garner (1993) and others.

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