Tick-borne Lyme disease exploding into Michigan; human cases up 5-fold
Excerpted from the Detroit free press: (02/23/2017)
They’re already back.
All it took was an unusual February warm spell this past week for the tiny insects causing an increasingly big problem in Michigan to become active once again, beginning their hunt for blood.
“A student in the medical entomology lab just brought in six adult blacklegged ticks — three male and three female — that were collected by his dog (Tuesday) in Lansing,” said Jean Tsao, an associate professor in Michigan State University’s departments of fisheries and wildlife and large animal clinical sciences.
The ticks are of interest because of what they often carry with them: the bacterium Borrelia burgdorferi. When the ticks bite an animal, seeking a blood meal, that bacteria can transfer. And that bacteria, in dogs, horses and humans, can cause Lyme disease, a serious affliction that can be permanently debilitating for people when it’s not treated early and well.
Tsao is a co-author of a recently published research paper documenting Lyme disease’s rapid spread into Michigan in recent years. There were fewer than 30 human cases of Lyme disease reported in Michigan in any year between 2000 and 2004. By 2009, the number had jumped to 90 reported cases. By 2013, it was 166 cases.
That number could be very low. The federal Centers for Disease Control and Prevention estimates the number of Lyme disease cases nationwide could be 10 times higher than what’s actually reported. Because of Lyme disease’s relative newness in Michigan, and because its early symptoms often mimic what feels like the flu,infected people — and even their doctors — often don’t test for it. Doctors also don’t always report finding Lyme disease to their local public health department, said Erik Foster, an entomologist with the Michigan Department of Health and Human Services and a co-author of the recent study.
The Lyme disease spike in Michigan correlates with the spread of blacklegged ticks here. In 1998, the ticks were established in only five counties — Berrien County in the southwestern-most Lower Peninsula, and four counties in the Upper Peninsula — and reported in 22 other counties. By 2016, however, the ticks were established in almost five times as many counties — established in 24 Michigan counties and reported in 18 others. The ticks have overtaken the entirety of the Lake Michigan shoreline in the Lower Peninsula, from Charlevoix to St. Joseph. But tick populations are not staying confined to coastal counties, becoming established increasingly to the east in the southern part of the state.
Tsao is always on the hunt for blacklegged ticks. In spring and summer, she can often be found dragging white corduroy cloth through Michigan forests, checking for the presence of the tiny bugs — usually only 3 to 5 millimeters in size, but that can grow more than twice that when engorged on a host’s blood. In the fall, Tsao and her students often check for the ticks on harvested deer at check stations during hunting season.
These days, the more Tsao looks, the more she finds.
“We know this is a real invasion,” Tsao said.
Why the increase?
Why the increase is happening now, scientists can’t say for sure. Lyme disease and the ticks that carry it have long been established on the East Coast. In the upper Midwest, states such as Wisconsin and Minnesota have more than 10 times as many human cases of Lyme disease as Michigan.
Tsao’s theory is that Lake Michigan served as a natural barrier holding the blacklegged ticks back. But the ticks are capable of being carried across the lake on birds, she said.
“Over time, as the Wisconsin, Minnesota and Upper Peninsula populations grew, that pressure of ticks coming down to places where birds like to stop over would have increased,” she said. “In the past, they could have come and died. But eventually, enough of them have arrived, survived the various life stages and began mating.
“Mom, if she gets a nice blood meal, she can lay 2,000 eggs. Nineteen-hundred and 99 can die; so long as one survives to meet another tick from another female, you have the next generation going.”
The ticks thrive in sandy areas with varied plant life, which is why they raced up Michigan’s Lake Michigan shoreline once established here, Tsao said. The ticks go through three life phases — larva, nymph and adult, and each stage needs a blood meal, she said. They perch on plants and branches, waiting for a host animal to brush by.
“It finds a host, takes its blood meal, falls off into the leaf litter, evolves and finds its next host,” she said.
The process can take two to three years for ticks in Michigan, the larvae feeding in year one; the nymph and adult in the second year, Tsao said. Each winter, the leaf cover and snow serve as an insulating blanket until spring.
While croplands farther east in Michigan’s Lower Peninsula aren’t as optimal for blacklegged ticks, all it takes are patches of woods and herds of deer to continue the in-state spread, Tsao said.
“If you’ve got nice woods around, and have nice habitat for deer, mice, birds and other species, you could have a little island for ticks,” she said.
Ticks have been found in East Tawas on the Lake Huron side of Michigan, as well as in the Thumb, Tsao said.
“I’m not trying to be fear-mongering,” she said. “But if you look at Michigan and look at other states where Lyme disease-causing ticks are found, what kind of vegetation is there? What kind of animals are there?
“There’s no reason we would not have ticks everywhere.”