Winter Ticks

NWDC is in the process of updating this fact sheet. 

Cause

Winter ticks (Dermacentor albipictus) are one-host ticks that primarily affect moose and elk populations in North America. 

Significance

In all areas in which the two occur together, winter ticks are having a significant impact on North American moose populations. In New Hampshire, moose populations have dropped more than 40% over the last decade. Shorter, warmer winters may have lead to huge increases in tick populations, causing moose to succumb to severe tick infestations. Winter ticks do not commonly take humans as hosts and are not known to transmit disease to humans.

Species Affected

Winter ticks have been found on several species of North American mammals. The main hosts generally belong to the deer family, which includes moose, white-tailed deer, mule deer, elk, and caribou. Other wildlife hosts include mountain sheep and goats, beavers, bison, pronghorns, coyotes, black bears, and wolves. Domestic hosts of winter ticks are primarily horses and cattle. 

Distribution

Currently, winter ticks are found in all moose populations of North America that reside south of 60° north latitude. They have been found on other ungulates in the United States as far south as Mexico. 

Transmission

The female tick lays thousands of eggs on the ground in May-June, which then hatch sometime in Augus- September. In late August- October, these larvae climb nearby vegetation to wait for a passing host. The larvae congregate in interlocking clusters to ensure that the hosts pick up large numbers of ticks at a time. If they do not find a host, they will die during the winter. Larvae that have attached to a host take their first blood meal in October- November and molt to the nymph stage. They remain dormant (not feeding) but still attached to the host until late January. Their second blood meal is taken between January and March. The nymphs then molt a final time to become adults and take their third and final blood meal from the host in March or April. After their final blood meal, the males search for females, reproduce, and then die, while the females immediately drop to the ground to lay their eggs, continuing the cycle. If females fall onto snow rather than soil, they will die before they are able to lay eggs. By May, few surviving moose have ticks remaining on their bodies. Studies have shown that winter tick larvae congregate in vegetation in the greatest quantities at heights that are approximately at moose chest level, making it easy for the larvae to attach to passing moose. The larvae are also most active during the moose rut, the time period when moose are actively looking for mates and are thus most mobile. Moose also tend to have a harder time removing ticks via grooming than other species, especially when ticks are in the larval stage. In addition, calves do not actively groom throughout the tick cycle and thus have little ability to dislodge these ticks. Because of this combination of factors, moose suffer the greatest impacts from winter tick infestations.

Clinical Signs

Engorged female winter ticks are about 6.5mm in size are easily visible to the eye. An infested moose can have an average of 33,000 winter ticks on its body, though some moose have been found to host upwards of 150,000 ticks at a time. Heavily infested moose suffer significant blood loss. Moose attempt to remove the ticks by grooming their body with their hooves and teeth and rubbing against trees and shrubs, causing significant hair loss and thus damaging their outer thermal protective coats (beginning in March). As a result, the moose lose their dark brown terminal shaft color and appear light gray or white, giving them a ghostly appearance. This constant grooming also prevents the moose from normal activities such as resting or eating, leading many moose to become lethargic, malnourished, and to rapidly lose their fat stores. These moose, especially calves, are more susceptible to cold and are less likely to survive the winter.

Diagnosis

Winter ticks are easily diagnosed through visual examination of the parasite.  

Treatment

There are no practical treatment options for wild animals. Domestic animals may be treated with anti-tick medication to prevent tick infestations. Any ticks found on domestic animals should be removed and destroyed. 

Management

It is impossible to eliminate winter ticks from the wild and unlike most parasites, they appear to be designed to kill their host. Climate change and warmer winters may be causing both a northern shift in the winter tick range and larger, more viable tick populations. In order to control tick populations in moose habitat, policies should be actively pursued to address climate change. Until that can happen or have an effect, there are management options that may help control tick populations. These include:

1. Limit moose populations by hunting. This will serve to limit tick numbers.

2. Utilize controlled burning in late winter and early spring to kill adult females, eggs, and in the fall to kill larvae.

3. Manage moose habitat wherever possible to insure a high nutritional plane and fat reserves in the fall.   

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