June 01, 2017If you've ever combed a tick from your hunting dog's coat, you know about the involuntary shiver that creeps up your back. There are worse feelings, like flicking away a tick crawling over your own skin or trying to extract a blood-engorged hitchhiker without breaking off the parasite and making the problem worse.
Sorry if you've just eaten lunch. I could change the subject, but you should know that blacklegged ticks (the ones that spread Lyme disease and are commonly called deer ticks) are increasing in the Northeast and Upper Midwest. Researchers are also watching a Southern population of the same species, which—if they close ranks and join up with their Northern kin—could create the perfect storm for a pandemic.
This scenario would not bode well for those of us who love the outdoors.
Lyme disease is now the most common vector-borne illness spread by parasites in the United States. The Center for Disease Control (CDC) estimates nearly 300,000 people are diagnosed with Lyme disease each year. In 2015, of the roughly 10 percent of these cases reported to the CDC, 95 percent came from 14 states in the Northeast and Upper Midwest, including Wisconsin and Minnesota. Although Michigan was exempt from this list, ticks are increasing here, too.
Three components must be present for anyone to contract Lyme disease: (1) the vector or parasite (the blacklegged tick), (2) the bacteria (Borrelia burgdorferi), and (3) a competent host able to harbor the bacteria. In northern states the primary host is usually small mammals such as white-footed mice. In the South the primary host is usually reptile.
Deer are not competent hosts because they can't carry the bacteria that cause the disease. Deer, however, are important to the spread of Lyme disease because they feed the adult stage of the tick.
Life cycle of the blacklegged tick include larvae, nymph and adult stages. Lyme disease is not passed from mother tick to her eggs; rather, it is maintained throughout the life stages once a juvenile tick takes a blood meal from an infected host. Each stage must involve a blood meal.
How Lyme Disease Spreads
Starting with adults, here's how the disease propagates itself over a two-year period: After gorging on its host (deer, dog or person), the female drops off and may wait all winter until spring to lay 1,000 to 2,000 eggs in leaf litter. In the laboratory, eggs take about 40 days to hatch. In the wild, however, larvae may not appear until summer and perhaps as late as September. The tiny larvae, which are not infected, (again Lyme disease spreads through digestive and not reproductive organs), climb vegetation to a favorable height where they might encounter a host. Here they "quest" with legs extended upward in hopes of hitching a ride and getting their first blood meal.
After feeding, the engorged larva drops off the host and prepares to molt for 40 days or longer when it emerges as a nymph (engorged larvae can also overwinter to become nymphs the following spring). As nymphs, the parasite repeats the scenario—attaches itself to a host for a second blood meal before dropping off and returning to litter duff.
When the adult finally emerges, as early as February, it waits to cling to whatever is available. Peak adult questing occurs in spring and again in fall. During the nymphal or adult stages, if the tick has acquired the bacteria from a previous blood meal, it can infect its human host.
The Michigan Connection
How often does infection occur? That was one of the questions I had for Isis Kuczaj, a former PhD candidate in the Dept. of Fisheries & Wildlife at Michigan State University. She studied Lyme disease ecology in an effort to understand why ticks behave differently depending upon where they live. According to 10 years of research at VanBuren State Park in southwest Lower Michigan, 17 to 20 percent of nymph populations could be infected.
The infection prevalence for adults jumps to as high as 40 percent. According to the CDC, most cases involving humans occur in June, July and August when people tend to be more active outdoors. However, spring turkey hunters and mushroom pickers are not exempt, nor are hunters in the fall. Unusual patterns of warm weather, such as we sometimes experience in spring and fall, extend the periods when ticks are active.
No doubt Lyme disease has been around for hundreds, perhaps thousands of years. Otzi, the famed Austrian hunter whose 5,000-year-old frozen body was discovered in 1991, carried evidence of the disease.
In Michigan, Lyme disease was discovered in Menominee County in 1990 and is still concentrated there. There is a population of blacklegged ticks on North Manitou Island in Lake Michigan, but no one can say why with certainty. In recent years ticks have spread up the eastern Lake Michigan coast from Berrien Springs all the way to Sleeping Bear Dunes National Lakeshore.
Hotspots exist in Wisconsin and Minnesota and in Maine and other Northeast states. There are no similar Lyme disease hotspots in the South where blacklegged ticks, although plentiful, are not evenly distributed. Kuczaj is involved in a National Science Foundation research project from Massachusetts to Florida that is looking at reasons why. She also wonders what will happen if the growing northern and southern populations of blacklegged ticks merge one day.
Why Ticks are Increasing
Northern ticks are more visible, Kuczaj believes, because they quest above the leaf litter. Southern ticks don't often appear above the leaf litter, possibly because of the difference in available hosts. Using a one-meter square, white piece of cloth, Kuczaj and other researchers drag sample vegetation looking for ticks. Nymphs are the focus of her research because they are the most important to understand. Their small size makes them harder for people to detect, and their spring-summer seasonality coincides with human outdoor activity.
Why is the incidence of Lyme disease increasing? The obvious answer is that tick populations and ranges are increasing. Modern game management that includes reforestation is another reason, as is the increase in deer densities that may be creating the perfect storm.
In addition, Kuczaj thinks climate change may help to create more favorable conditions for tick growth along with an informed public reporting more cases of the disease.
Don't be a Victim
The CDC website http://www.cdc.gov/lyme/ has excellent information about how to prevent getting Lyme disease and what to do if you contract it. Myths are dispelled and half-truths explained.
"Awareness is the best tool for prevention," Kuczaj believes. "Ticks know how to hide, so check your body (and that of your dog) after being in the woods. It takes 24 hours for infection to occur; if you find a tick, pull it straight out with tweezers. Save the tick in rubbing alcohol. If you feel symptoms (a bullseye rash around the bite, headache, nausea, joint pain), take the tick to your doctor for lab analysis."
You can vaccinate your dog but it will need an annual booster shot. Consider a flea and tick prevention collar like Frontline, too. I carry a topical repellent containing Permanone, which I spray on pant cuffs and my dog's legs and belly before going afield.
The lesson is clear: Be aware and be vigilant.