Lyme disease, foxes, and coyotes

A gray fox swaggered across University Ave this morning, its bushy tail bouncing as it trotted. It was headed to the patch of woodland behind Otey Parish Hall and the Duck River Electric building. I’ve seen fox scat on the road there, so I think this must be a resident, perhaps the same animal that I saw last summer with a rabbit in its mouth.

This has been a busy few months for fox and coyote sightings in Sewanee. Now, a paper in the Proceedings of the National Academy of Sciences has added some insight into the tangle of interactions among these wild dogs and their prey. Apparently, tick abundance and Lyme disease risk is affected by the numbers of foxes and coyotes. The paper examines data from the northeast, but its results may also be relevant here.

Lyme disease (caused by the bacteria Borrelia burgdorferi) is transmitted to humans by tick bites (especially bites from nymphal stage of Ixodes scapularis, the black-legged tick) But humans are not the main host of these ticks, so the abundance of ticks is determined by the abundance of their other mammalian hosts, especially mice. Foxes and coyotes both prey on mice, so you’d think that more foxes and more coyotes would mean fewer mice, and therefore fewer ticks, and therefore fewer Lyme disease cases. But things are not quite so simple.

The abundance of foxes is indeed correlated with a decreased risk of Lyme disease. Foxes love to eat mice and fox populations can get quite dense, so mice fare poorly in areas with healthy fox populations. Coyotes also eat mice, but coyotes live at lower population densities than foxes. Coyotes also drive out foxes. So the overall effect of coyotes on Lyme disease is a positive one: more coyotes = fewer foxes = more mice (despite the few that get eaten by coyotes) = more ticks = more Lyme disease. And deer? There was no correlation with Lyme disease; mouse abundance drives the dynamics of the disease and deer abundance seems to have little effect (except in areas that have no deer — an unusual situation these days — that do have lower incidences of Lyme).

Excerpt from one of the paper’s figures, showing correlations (or lack thereof) between Lyme disease and either coyotes per fox (positive correlation), foxes (negative correlation), or deer (no correlation).

An important caveat: this paper examined correlations among estimates of the abundance of different animals. But correlations are slippery things. They seem to imply that we’ve discovered a cause-and-effect relationship, but this is often misleading. So I’m sure this story will evolve as scientists tease out the subtleties (does the effect of foxes depend on the availability of other prey?) and alternative explanations (might some unknown causative third variable be correlated with coyotes and Lyme?). I wonder how domestic cats play into all this. They are major predators on mice and often live at densities well above what could be supported without the subsidy of store-bought cat food. Do they also suppress Lyme?

Some notes for blog readers in the south:

1. We’re well outside the “hotspot” of Lyme disease, as shown on this CDC map. The disease does not seem to be increasing in many parts of the south. But, in Virginia and other areas that are close to the center of Lyme’s activity, the disease is increasing quite rapidly.

2. The south is blessed with other tick-borne diseases. Southern Tick-Associated Rash Illness is one; Rocky Mountain Spotted Fever is another.

3. Although we do have black-legged ticks here, lone star ticks and dog ticks are more common. These are not prime carriers of Lyme, but they can transmit the other tick-borne diseases.

For more info about ticks, the CDC site has some good links and great pictures (which will make your skin crawl). Obviously, if you have medical concerns about a bite, check in with an MD, not a Rambler.

12 thoughts on “Lyme disease, foxes, and coyotes

  1. Mark

    I love your blog! Emery and I just saw a video on this at the Museum of Natural History in NY! It is complicated. I may not remember correctly, but it seems rabbits can also be vectors, but possums and raccoons not!
    BTW, the red fox has gotten two of our hens in the last week. I, ironically, had them out eating ticks. Now, they are safely in their yard, not eating as many ticks.

  2. batesvillian

    Very interesting to me on multiple levels. Thanks for posting. Looking at the CDC map, I’m struck by how prevalent the disease is in the densely settled Mid-Atlantic-Northeast urban corridor, and other urban hotspots like the Pittsburgh area. So more mice in the cities and suburbs, perhaps because of fewer predators combined with favorable habitat, equal more incidents of Lyme disease? If you live in a rural area, you’re less likely to contract the disease it would seem.

    That said, Lyme disease is a reality here in rural central Virginia. I came down with it a couple of years ago, right about this time of year. Found the bull’s-eye rash after a couple of bouts of mysterious flu-like symptoms. Fortunately, my wife, who’s a physician, took one look at the rash and placed me on an antibioic regimen. I knocked it out pretty easily.

    David, when you said that coyotes “drive out” foxes do you mean that coyotes, being bigger and stronger, kill and eat foxes as they do domestic animals, or are the foxes displaced from their former territory? What’s the correlation between fox and coyote populations?

    Also, that gray fox you’ve seen near Otey and the Duck River office may be the one (or one of its offspring) that my parents saw denning in the culvert along the service entrance road next to SPS a couple of years ago. You might check it out sometime to see if anything’s still there.

    Finally, a couple of immunologists here at U.Va. have discovered a link between Lone Star tick saliva and the sudden onset of red meat allergies:

    Scary stuff. Lately, however, we’ve had other unpleasantries to keep us up at night:

    The, um, scat continues to hit the fan at Thomas Jefferson’s Academical Village.
    — William

    1. David George Haskell Post author


      I’m glad to hear that the antibiotics knocked out the Lyme. Scary.

      My understanding of the coyote-fox connection is that coyotes directly harass foxes (eating them if they can catch them) through aggressive interactions. As coyotes move in, fox populations decline (as do feral cats).

      One thing to bear in mind about the map is that human population density varies, so rural areas may have just as many ticks and Lyme, but fewer human cases because there are just fewer people there. When I was giving a talk at U of Richmond I learned that black-legged ticks are spreading rapidly in VA and the causes of this spread are not understood. They are showing up places (the mountains) where the models say they should not. So much for theoretical models… (although these models do have the advantage of telling us where we need more work).

      The Univ VA story has been all over Facebook and the news. Sounds like a real mess — scat hitting the fan indeed. Does not sound fun.

      Best wishes, David

      1. Milton

        This is a complicated issue. This review is omitting a huge factor: outdoor cats and dogs. It is understandable that Lyme does not occur very often in rural areas because the feral/outdoor cats are pretty much removed. Suburbs and urban areas have high outdoor cat populations, and cats prey on the mice, the mouses’ ticks jump on the cats, and the cats bring the ticks inside the home = humans contract Lyme. The same is true for outdoor unleashed dogs in any setting. Coyote and fox flourish by consuming these Lyme-carrying mice. However, development of rural and suburban areas have fractured landscapes so that the small mice-eating predators like the coyote and fox have been pushed out, allowing the mice populations to explode, despite the presence of feral/outdoor cats and dogs. (Chickens, pheasant, wild turkey grouse, even quail will snack on ticks-that’s a good thing! These birds will also consume mice if given the opportunity-I have witnessed it!) People need to keep their cats and dogs inside the house to help prevent Lyme in humans. We also need to help keep the coyote and fox populations healthy (but NEVER provide/give food). Also stop the incessant development and fragmentation of open space that drives these small predators out.

  3. dale swant

    I am surprised that deer do not positively show up in this study. Look at a deer with binoculars or very freshly dead this time of year and you will find about a hundred ticks behind each ear. I know of no other mammal that supports any where near as many ticks as an individual. The extensive daily movement of deer would seem to add positively also. I guess there are more and more traveling rodents than I would guessed.

    1. David George Haskell Post author

      I agree — they are infested with ticks. But in the NE it is mostly tick nymphs that drive the Lyme dynamics and these nymphs generally feed on rodents. In addition, although deer carry a lot of ticks, there are relatively few deer compared to the vast numbers of mice, chipmunk, and shrews in many forests. I learned this the hard way when I lived in a cruddy old cabin in the woods in NY — tons of mice showed up inside every winter, eager to share the warmth of the hearth. (Luckily no deer ever invited themselves inside.)


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