Killing deer not the answer to reducing Lyme Disease, says HSPH scientist

November 23, 2010 — Andover and Dover are among towns in Massachusetts and throughout New England that introduced or expanded limited deer hunting this fall, in large part to help curb the spread of tick-borne Lyme disease.

Prevalence of the disease, the most common tick-borne illness in North America, has increased dramatically in the region over the past few years. Massachusetts now has the country’s fourth highest infection rate, according to the most recent Centers for Disease Control and Prevention data. Lyme disease produces flu-like symptoms in those bitten by infected ticks and, if not treated promptly, can cause more serious complications including arthritis, neurological symptoms, and even cognitive defects and heart rhythm irregularity.

Tamara Awerbuch, instructor in the Department of Global Health and Population at HSPH and a specialist in emerging epidemics, has done research on the life cycle of the deer tick. Based on her field studies in the 1990s, she argues that hunting deer won’t effectively combat Lyme disease because ticks also depend on another key host animal: white-footed mice. Ticks do not actually get Lyme disease from deer, as is commonly believed—rather, ticks contract it as larvae when they feed on infected mice. Adult female ticks need the deer to lay their eggs and for food, but the deer do not become infected.

Awerbuch spoke about her research and the decisions by some communities to allow deer hunting.

Q. Can you explain, based on your studies, why you think the deer hunts aren’t the right strategy?

A. The tick that transmits Lyme disease is called the deer tick. The adult tick takes a blood meal from deer, lays eggs and then dies. In Crane Beach [in Ipswich, MA], where I conducted my study, people thought that if they killed deer they would reduce the number of ticks. Deer were reduced [from around 400 in 1983 to just over 100 in 1991], but Lyme disease kept growing. The question was why? We killed deer but people still got Lyme disease.

So I did a study using a mathematical model to capture the life cycle of the tick. Because the ecology of Lyme disease is so complex, it is very hard to look at deer and tick, mouse and tick, one by one. You have to link all the factors together in a way that lends itself to mathematical analysis.

Q. And what did you find?

The deer do not carry the bacteria. They are needed to continue the life cycle of the tick, but they are not infected. So as you killed deer, you would simply have more ticks per deer because the surface area of each is enough to support many ticks. Just killing deer won’t do the job.

Q. According to a recent Boston Globe article, state wildlife officials consider 8 to 10 per square mile the target density for a healthy deer population. Is there a threshold at which you start to see a decline in the tick population?

A. Our research showed that if you leave fewer than eight deer for the whole Crane Beach area, the tick population will start to decrease, but it will take many, many years. Another important thing I found is that the tick population oscillates, which is part of the insect’s natural life cycle. Sometimes people go to the same site in different years and see a reduction in ticks and think, oh, we don’t have to do anything. But it’s the natural dynamic and their numbers increase again within a few years.

–Rachel Johnson

photo: iStockphoto/brm1949