As originally published in the April 12 issue of The Mountain-Ear newspaper.

Irene Shonle, CSU Extension Gilpin County.

There was a moose with silver shoulders in my yard the other day. At first, I thought this was just variation in fur color or spring shedding, but closer inspection with binoculars showed that the moose was suffering from the dreaded winter tick (Dermacentor albipictus). The silver color on infested moose comes from the white color at the base of the hair shaft – the dark outer part of the shaft gets rubbed off as the moose tries to get rid of the ticks. The silver coloration is normally most obvious on the neck and shoulder area. Moose with extensive silver patches are called ‘ghost moose.’ Silver ‘socks’ on a moose’s legs, however, are a normal color variation.

Winter ticks have, along with other factors such as climate change, contributed to a sharp decline in the moose population in Canada and the Northeast. It’s hard to imagine a little tick being able to bring down a giant moose, but some moose have been found with up to 100,000 ticks on them. Calves are especially vulnerable, as they have less body mass to support all the parasites. The cause of mortality can be loss of blood, hypothermia due to loss of hair, or starvation when the severe itching causes them to forage less than is necessary to feed themselves. The winter tick issue seems to be more of a problem when moose densities are fairly high. With Colorado moose populations increasingly rapidly, it remains to be seen as to what will happen with our moose and the ticks.

The winter tick is a one-host tick, with all three active life stages (larvae, nymphs, and adults) feeding on the same individual. Their preferred hosts are moose and other ungulates, including deer and elk. Deer are very good about grooming off the ticks, and elk are less so. Moose have the most difficult time grooming off the ticks, and so are the most susceptible. Occasionally, in years and locations where ticks are particularly numerous, they may try to live on cattle and horses. But these species provide marginal habitat for winter tick, so the number of ticks per individual remains low and they rarely mature to the adult stage. Winter ticks are rarely found on dogs and cats, and they cannot live for long on these hosts. They rarely bite and feed on humans. Unlike the Rocky Mountain Wood tick or the American Dog tick (both of which will readily attach to dogs and humans), winter ticks do not spread Lyme disease or other tick-borne diseases.

Winter ticks produce one generation per year. The ticks will feed and remain on their host during their subsequent molts until spring when adult females detach and drop to the ground. She will lay her eggs and then die. In late summer and early fall, the larval ticks climb vegetation and wait for the moose to walk by so they can jump on and burrow in.

Adult survival is greatest if the female tick drops off the moose onto bare ground in the spring. Adult ticks that drop off moose and land on snow cover have a poorer survival rate. Wet, cold weather in the fall reduces larvae activity and fewer larvae find a host. Climate change is predicted to improve conditions for winter ticks through longer, warmer falls and earlier snowmelt in the spring. If we don’t get some snow this spring, it will probably be a good year for winter ticks – and perhaps a hard year for some moose.