Winter Ticks

This fact sheet has been updated 10/29/20.

Cause

Winter ticks (Dermacentor albipictus) are one-host ticks that rely on moose, elk and caribou for sequential blood meals in order to complete their 8-9 month life cycle. All stages of the tick including the larvae, nymph and adult feed on a single host animal. D. albipictus cause a hairless syndrome where the severity of hair loss correlates with the number of ticks present. High numbers of winter ticks, especially on a young animal, can cause severe anemia and death. 

Significance

In all areas in which the two occur together, winter ticks are having a significant impact on North American moose populations. For example, in New Hampshire, moose populations have declined more than 40% over the last decade. In recent winters, more than half of the moose calves in the north eastern corner of Vermont have died primarily because of blood loss caused by winter tick infestation. Shorter, warmer winters may have led to increases in tick populations, causing moose to succumb to severe tick burdens.

While winter ticks can infect domestic cattle and occasionally horses, they do not commonly use 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 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

Winter ticks are widely distributed across North America. They have been found on ungulates as far south as Mexico but are strongly associated with the presence of moose.

Transmission

The female tick lays thousands of eggs on the ground in May-June, which then hatch sometime in August- 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 that then 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 that is cold enough 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. Due to differences in grooming behavior, moose 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

Winter ticks are approximately ¼ of an inch (6.5mm) in size and can be found all over the body but are commonly seen on hard to reach areas such as between the limbs and the body, on the shoulders and on the dewlap, as well as on the udder or scrotum. When fully engorged, the female ticks can be over ¾ of inch (1.27cm) long. Moose can become hosts to an average of 33,000 winter ticks, but some moose have been found to host upwards of 150,000 ticks at a time.

As moose attempt to remove ticks by grooming their body with their hooves and teeth and rubbing against trees and shrubs, they cause significant hair loss and damage to their outer thermal protective coats (beginning in March). As a result, moose lose the dark brown sections of the terminal hair shaft leaving the next light gray or white portion, giving them a ‘ghostly’ appearance. The entire hair shaft may be lost exposing the skin. Constant grooming also prevents the moose from normal activities such as resting or eating, leading to lethargy, malnourishment, and rapid loss of fat stores. These moose, especially calves, are more susceptible to cold and are less likely to survive the winter. Furthermore, heavily infested moose also suffer significant blood loss. The cumulative effect of heavy infestation can ultimately result in death of the individual animal.

Diagnosis

Winter ticks are diagnosed through visual identification 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 and have few natural predators. Climate change and warmer winters may be causing both a northern shift in the winter tick range and larger 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 by reducing the number of hosts.

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.