Tick ecology poses many challenges


To the Editor:

Duncan Caldwell raises some issues regarding the association of deer density and that of deer tick that require comment (Aug. 27, “The automatic association of Lyme disease with deer may be flawed”).

He is, of course, correct that I have always said “up to” or “more than 90 percent”…not “all” of the adult deer ticks feed on deer. After all, the host range of the tick is wide enough that they do feed on people. However, time and time again, experimental studies demonstrate that if deer are reduced, the density of deer ticks of all stages is reduced. In fact, his suggestion of using four-poster devices, which attract deer and paint them with insecticide, is one that I certainly do mention whenever I speak because the peer reviewed experiments demonstrating their efficacy in reducing deer tick density clearly prove that deer are critical hosts for the reproduction of these ticks. Paint the deer with insecticide and tick numbers fall. We cannot ask for a better demonstration of the association.

Yes, four-poster devices do attract other pests that might be treated — gulls, crows, raccoons, squirrels — but none of these, except raccoons, feed adult deer ticks. And adult deer ticks don’t particularly care for feeding on raccoons. Anyone who wants to question that statement should pick up a road-killed raccoon during mid October or November when the adult deer ticks come out. There just aren’t very many ticks, if any, on them. Road-killed raccoons in May or June, however, have hundreds of ticks on them, but they are dog (wood) ticks, not deer ticks. Why raccoons don’t have a lot of deer ticks when they are loaded with dog ticks remains an interesting question.

It is indeed a number game. The majority of adult deer ticks get their bloodmeal from deer. Yes, some bloodmeals come from other animals. If one were to magically eradicate all deer from Martha’s Vineyard, the deer tick would persist, but there would be orders of magnitude fewer, which would translate into proportionally fewer people encountering deer ticks. In public health, we seek to reduce risk; very few conditions can be eradicated (which means zero risk). Reducing the number of American smokers by 50 percent (1965-2006) was a significant public health achievement. Will we ever get to a goal of no smoking whatever? Doubtful, and perhaps not a goal that is worth spending limited public health funding. Similarly, reducing the incidence of Lyme disease by 50 percent, 1,000 cases each summer instead of 2,000 on Martha’s Vineyard, would be fantastic. One way to do this (Mr. Caldwell and I completely agree on a multidisciplinary approach, we call this integrated tick management) is to reduce the source of the ticks. The Martha’s Vineyard Tick Borne Disease Initiative clearly spells out the other means of reducing risk, mainly personal protection, but also habitat management, while we try to come up with ways to reduce the environmental contamination by deer ticks. Do check out our website.

Four-poster devices work when applied as specified by the manufacturer. Recent published work, some of which was done on Chappaquiddick, finds that if they are applied at a lesser deployment rate they are not effective. The four-poster has been proven to work, but it is expensive. I call the device a monkey on the back of the community. The Vineyard would have to spend more than half a million dollars a year, forevermore — once you stop using them the ticks come back within a year — to use four-posters effectively to reduce risk.

The perception out there is that the Vineyard is awash in money and surely this amount is a drop in the bucket, so why not use four-posters? As we should know, most of the year-round population barely scrapes by after tourist season ends, so this perception may be false. Hunting costs are not borne by the tax base and in fact can contribute to the local economy; a lead slug or arrow costs a few dollars. The sheer economics suggest that four-posters can only complement deer management. In fact, we might need fewer four-posters if we reduced deer first.

In addition, the potential unintended side effects of chronic four-poster use remain unexplored: increased numbers of fat raccoons, gulls, crows, squirrels, rats; and note that many of these animals serve as hosts for zoonotic infections that might increase in prevalence as a result. Continued exposure of ticks and mosquitoes (most mammal biting mosquitoes get their bloodmeals from deer) to insecticide might select for resistance. Permethrin is one of the chemicals used in four-posters. Do we really want to make it less likely to kill pest arthropods?

As for Mr. Caldwell’s observations on where one finds ticks There are many sites with lots of deer and few ticks; and some sites with little evidence of deer but lots of ticks. I have seen this too. In the latter instance, there is a paradoxical sampling issue. With few deer, the ticks accumulate and appear to be more dense. In the former, it is clear that there are habitat correlates. Deer ticks survive longer in moister habitats; abundant leaf litter and shade are critical. And yes, mice and other small animals (not just mice, but shrews, chipmunks, and even certain birds such as towhees) feed a large proportion of the immature ticks (larvae and nymphs). So even in a site with good habitat, perhaps there were few mice or other small animals the year before, and hence few host-seeking ticks the following year. This is an excellent example of how complicated studies of Lyme disease ecology are: all these observations may vary between months and years. One site may have tons of ticks this year, but go back next year and fewer may be found simply because vegetation and host associations are dynamic in time.

We can learn much from our backyard naturalists and I thank Mr. Caldwell for his observations and providing an opportunity to try to clarify what is known and unknown. A team of citizen scientists, comprised of residents such as he, can help us better understand our complex world, and even better, go out and educate the public with the evidence.

Sam Telford ScD

Professor of Infectious Disease and Global Health

Tufts University

Martha’s Vineyard Tick Borne Disease Initiative