Deer Control & Lyme Disease

Lyme disease is a significant and growing problem in the Dunes Region. The chart above shows that diagnosed cases are on the rise in Porter County, home to Beverly Shores (Data from the U.S. Centers for Disease Control and Prevention, Lyme Disease Data and Statistics). Porter County accounts for around one fifth of all Indiana Lyme disease cases in most years.

Lyme disease is caused by a bacterium transmitted to humans when bitten by a blacklegged tick, commonly known as a deer tick. The deer tick does not acquire the Lyme disease bacterium from deer, but instead from biting “reservoir hosts, ” small mammals such as mice or chipmunks. But deer are important “reproductive hosts” for deer ticks. Immature stage ticks feed on small rodents and acquire the bacterium, but adult ticks only feed on large mammals like deer (and humans). As the University of Rhode Island's TickEncounter Resource Center notes:

[A]dult stage blacklegged ticks commonly blood feed on deer, becoming engorged and obtaining all of the protein necessary for laying their clutch of 1,500-2,000 eggs. For various reasons (host density and distribution, grooming and development of tick resistance, tick control, etc), few other animals are as efficient in providing the population of adult blacklegged ticks with this all important blood meal.

Indiana has nowhere near the number of Lyme disease cases found in the Northeast, and far fewer than observed in Minnesota and Wisconsin, but the numbers are increasing. Given the importance of deer as reproductive hosts for the ticks that carry the disease, one major reason to control local deer populations is to inhibit the spread of Lyme disease in our area.

It is worth noting that a new and nastier species of bacteria has emerged in the midwest, carried by the same deer ticks that spread the more common form of Lyme disease observed to our east. See this CBS News story for details. It would seem to be just a matter of time before these new bacteria are observed in our area.

Note also that ERG's efforts to control deer and invasives have related impacts. We've noted elsewhere the close relationship between deer and garlic mustard, but there is another relationship of note, one that involves Lyme disease. See Barberry, Bambi and bugs: The link between Japanese barberry and Lyme disease, a guest blog for Scientific American.

And here’s the kicker for those of you who’d still consider planting [barberry] in your backyard: The prevalence of ticks infected with the Lyme disease–causing spirochete (Borrelia burgdorferi) is greater in areas with Japanese barberry than areas without.

“Deer eat everything but barberry, and because they don’t eat barberry, they’re weeding out forests. They’re helping promote the invasive species,” explains Jeff Ward, chief scientist for the Department of Forestry and Horticulture at the Connecticut Agricultural Experiment Station (CAES).

Thanks to Bambi’s distaste for its harsh chemicals and spiny branches, Japanese barberry is left alone to thrive while other plants such as red trillium get shaded out or nibbled to the ground.

Japanese barberry has denser foliage than most native species. As a result, the plants retain higher humidity levels. Ticks need humidity and become desiccated when levels drop below 80 percent. Relative humidity under a barberry is about 100 percent at night.

“The plant exists in an umbrella-like form, so the daytime humidity drop is much more subtle under the canopy of barberry than under other plants,” says Scott Williams, a research scientist at CAES. The shrubs also provide nesting areas for white-footed mice and other rodents, which are primary sources for larval ticks’ first blood meal, and reservoirs for Borrelia burgdorferi.

In the open, ticks can only be active for 15-16 hours per day, but when they’re protected by Japanese barberry, that number increases to 23 or 24. “There’s only an hour or two when they would have to retreat into the soil,” Williams explains. “Instead, they sit and hang out and wait for a host like you or me or a raccoon.” Since the majority of Lyme disease cases occur from nymphal tick bites, and nymphs are most active in the summer, risk is highest during the warm months when we’re all happily tromping around wearing shorts and sandals. In forests. In backyards. In parking lots. Day and night.

ERG will continue to be active in our efforts to control both deer population and invasives in our region. The benefits from doing so go beyond the significant value of protecting our diverse natural environment. There are potential and increasingly important health benefits as well.