MINNEAPOLIS — Deer ticks are expanding their range in the upper midwestern part of the United States and southern Canada, new ticks are moving into the area and existing ticks are picking up new diseases, increasing the threat of illness to hikers tramping through the region’s woods.
Minnesota health officials last week reported the state’s first death from Rocky Mountain spotted fever, as well as the state’s second-ever case of brain inflammation from the Powassan virus — similar to West Nile, but spread by ticks instead of mosquitoes.
Officials are also watching to see if the lone star tick, which can spread a Lyme-like disease, establishes itself permanently. The tick until recently wasn’t often found north of southern Iowa, but about a dozen have been identified in Minnesota this year. So far, no infections have been reported.
Scientists aren’t sure why tick populations are expanding, but many suspect one factor could be that subtle changes in the climate are tipping the ticks’ complicated ecosystems toward expansion. The Minnesota Health Department has applied for a grant to study how climate change is affecting the state’s tick population, and Wisconsin is seeking funding for its own study.
“We think about climate change all the time,” said David Neitzel, a Minnesota department expert in insect-borne diseases. With climate change, he said, “There is going to be a change in all the diseases we work on.”
Deer ticks are a well-known threat to infect humans with their bite. Lyme disease, the best known infection spread by ticks, can result in fever, headache, fatigue and rash, and if left untreated can linger and spread damage to joints, the heart and the nervous system.
Rocky Mountain spotted fever is spread by the American dog tick, also called the wood tick. While the tick is common in Minnesota, the bacteria is considered very rare here. Nationally, most cases are in the southeastern U.S. The symptoms include fever, vomiting, severe headache and a distinctive spotted rash.
While none of the Upper Midwest states do a comprehensive tick census, they track the spread of ticks by following up on confirmed disease cases, doing some of their own sampling and identifying ticks sent in by the public.
In Minnesota, deer ticks have expanded to the northwest from the traditional high-risk areas of eastern and central Minnesota. Neitzel said he’s seen it personally on his property in Becker County.
“I’ve been stomping around up there since the early 1970s, but in just the last few years we’re seeing black-legged ticks up there,” he said. Deer ticks are also known as black-legged ticks.
Canadian health officials report a growing deer tick population just north of Minnesota’s northern border, where there were none in the early 1990s. In Wisconsin, the ticks are marching from the northwest to the southeast. And in Iowa, the deer ticks are moving from the northeast toward the southeast and the centre of the state.
“We’re watching it, and certainly we’re concerned,” said Dr. Patty Quinlisk, Iowa state epidemiologist.
The number of Lyme disease cases has been climbing in each state since 1993, according to figures from the Centers for Disease Control and Prevention. The number of cases more than doubled from 2000 to 2007 in all three states, reaching 1,814 cases in Wisconsin, 1,238 cases in Minnesota and 123 cases in Iowa.
In Canada, government health researcher Nicholas Ogden found that in the early 1990s there was only one area infested by the ticks. Now, there are tick populations all along the northern border of Minnesota and the northeastern United States.
“Recent studies have suggested that the risk of exposure to Lyme disease is emerging in Canada because the range (of the black-legged tick) is expanding, a process that is predicted to accelerate with climate change,” he wrote in a June 2009 article of the Canadian Medical Association Journal.
Melissa Kemperman, an expert in tick-borne diseases with the Minnesota Department of Health, said a changing climate can affect many variables for the ticks. “It’s not just the heat and humidity,” she said.
For example, if birds and mammals move into new territory their ticks could hitch a ride. If that new habitat has plenty of food and shelter — and doesn’t get too cold in the winter — the ticks could get established year-round, she said.
Also, humans can change tick habitat. To cite just one example, Neitzel said, when timber companies cut down an older section of a forest the new growth is better for ticks. There are more ground-level shrubs to live in and more mice and deer on which to feed.
Public health officials say the changes mean anyone who goes into the woods needs to be extra vigilant. It also means that more doctors need to be on the lookout for tick-borne diseases in their patients.