Global Warming and Tick-Borne Diseases in MAINE

In response to the last post we got a super-insightful comment from “Jeff”:

“The level of Lyme disease diagnosis is more than anything, proportional to the publicity and near hysteria generated by shows like Dr Oz and Oprah.”

I know Oprah is powerful, but does she really have mind control over Maine’s physicians and the CDC? How did she muster the “publicity and near hysteria” for other tick-borne diseases like anaplasmosis and babesiosis?

Not to be undone, Jeff tried again:

You certainly cherry picked Maine.
Some neighbour states to Maine (about the same latitude) have Lyme disease notifications falling over the decade.
New York 2006 = 4460 , 2016 = 2623 (confirmed) + 1259 (probable) = 3882
New Hampshire 2006 = 617, 2015 = 436
And southern Texas increases 29 to 71 over the decade

Don’t they get the “Oprah” show in New York and New Hampshire? The horror …

Lest others miss the point, let me make it explicit. Global warming is changing where diseases (like Lyme disease) are common, by changing where disease vectors (like ticks) are common. Lyme disease has been with us quite a while, but only recently has it been common in Maine. The reason? Global warming.

But wait — didn’t I suggest it was temperature increase (specifically, overnight low temperatures in winter) that brought this about? Aren’t New York and New Hampshire at about the same latitude? Haven’t they also seen increases in overnight low temperatures in winter? Yes, yes, and yes. Then why, oh why, haven’t they also shown dramatic increases in Lyme disease?

Maybe … just maybe … it’s because they were already warm enough (even overnight during winter) to sustain a tick population (specifically, the deer tick which transmits Lyme disease) and the hosts on which they prey. Perhaps … perchance … Maine wasn’t … but now is.

Shall we investigate? Here’s the average overnight low temperature during winter in New York, New Hampshire, and Maine:

It looks like Maine is colder than New York and New Hampshire, at least on average during winter nights. This, despite its being at about the same latitude. Funny that.

Temperature in Maine has crossed a threshold, the one which has allowed the tick population to explode. The frequency of Lyme disease (and other tick-borne diseases) has exploded right along with it.

As for Texas, I think that’s Jeff’s attempt to suggest that you can get a dramatic rise in Lyme disease without global warming. Maybe you can — but not if you go by Texas. Lest anyone think the “increase” in Texas is a worrisome trend like it is in Maine, here’s the data (which Jeff used) for Texas, showing those fluctuations from year to year that we all expect.

But it doesn’t show any trend unless the blue dashed line is your idea of a trend, Jeff. My idea of a good way to estimate a trend is the red dashed line, from least-squares regression. But don’t rejoice too soon, Texans, it’s not statistically significant. For all we know, Texas’ Lyme disease cases are fluctuating but don’t show any trend at all.

Just to refresh everyone’s memory, this is what a trend looks like:

Do I really have to do least squares (or other type of) regression and quote a test statistic?

Maine gives us an excellent example of the spread of disease brought about by climate change. That was the point, lest anyone miss it. Lyme is just one disease, from one disease vector, in one state. Don’t doubt that there are others.

Which diseases are coming for your state?

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35 responses to “Global Warming and Tick-Borne Diseases in MAINE

  1. That is awesome. Deniers don’t need to do the leg work, they just have to sow doubt and most people would probably say Jeff has a point, because they don’t want to do the leg work either and it’s comforting to know that it may not be global warming that is the problem.

    Thanks so much for doing the leg work, Tamino, and showing those deniers up for what they are … deniers.

  2. I would suggest caution in the interpretation of this data. I do not know what is the actual background situation on the ground in these american states however I do know about Lyme’s epidemiology.
    You have to account for land use practices, livestock and wildlife dynamics. It is not as simple as vector dynamics only. Although the tick vector is undoubtedly important.
    To draw more firm conclusions I would suggest looking at what has happened to the deer population in these states (is hunting in decline for example?), what has happened to woodland vs pasture areas, has there been a decline in the sheep grazing population for example? What about leisure activities? Are there more people in outdoor pursuits that before? What about the rat population? Lyme within wildlife circulates between deer and rodents. Has there been any alteration there?
    Looking at the emergence of disease in areas previously disease free is perhaps, in the absence of the information mention above, a more robust proxy for tick population influence in the overall picture.
    This argument is applicable as well to babesiosis and anaplasmosis with slight corrections to account slight differences in their epidemiology.
    Whilst I certainly agree that temperature is an important risk factor for Lyme’s epidemiology, I would respectfully suggest caution interpreting this data and expanding the scope of variables examined before drawing specific conclusions.

    [Response: There are other factors which likely contribute in addition to climate change. But to deny the contribution from global warming is tantamount to saying that temperature doesn’t affect the range of disease-bearing ticks.

    Recent research from the Maine Medical Center Research Institute in collaboration with the Univ. of Maine’s Climate Change Institute has solidified the link between climate change and the increase of Maine’s tick population. Continuing research is investigating how global warming’s influence has its effect, considering not just fewer very cold nights, but warmer and shorter winters and hotter summers, since tick larvae can’t survive without enough summer heat.]

  3. Reblogged this on Don't look now and commented:
    Brilliant analysis.

  4. We don’t have to infer an increased tick population from just the increase in these diseases. There have been news stories that talk about a massive increase in tick infestation of the moose population in Maine, to the point that many moose are seen extremely debilitated because of the loss of blood from so many ticks on them. E.g.:

    “Ticks devastate Maine, N.H. moose populations”

  5. Dear Tamino
    I am not denying the impact of global warming in the epidemiology of vector-borne diseases, neither in general nor in the specific examples you point to in these last 2 posts. You may very well be right in the assertions you make in your posts.
    What I am saying however is that there are other factors in the particular equation of these vector-borne diseases which may be have locally more impact in disease dynamics than temperature and therefore in the current observations.
    Without these factors considered I just feel that assertions about the link between global warming and the increase in cases you report require more caution.
    On top of of what I have mentioned above already, note that a simple explosion of tick numbers with no other changes, in a certain area would increase the parasitic load on the reservoir wild hosts, making them weaker and increasing general disease burden on those populations of hosts. This is not a credible scenario in my opinion.
    More likely is a situation similar to what I happening in Scotland where Lyme disease is also on the increase where rural policies have changed the landscape of the rural countryside have promoted an increase of habitats for wild populations, an increasingly tertiary economy is leading to a decline in agriculture with a reduction in livestock populations, reduction in managed land and increased turism. So you have an increase in the reservoir population, a decrease in human management of tick populations, an increase in potentially infectious contacts AND

  6. AND increased temperatures which improve survivability of the vector.

    [Response: I didn’t mean to imply that you deny the influence of climate change on tick population in Maine. Nor should I; your comments are measured and appropriate. My “admonition” was a warning to others (like Jeff) of the folly of attempts to dismiss global warming’s impact.

    To those studying the issue in detail, the handwriting is on the wall.]

  7. Thank you for thorough work on the numbers. You do a great service with your time on these questions. Thank you, thank you, thank you!

  8. Andy Lee Robinson

    Ticks are scary. They are no joke.
    I got bitten by one a couple of years ago just a few minutes after sitting down on a garden bench. I didn’t feel it, I just happened to notice it on my bare leg.
    It was already quite dug in but luckily I could get my nails around its mouthparts and it came off without needing too much force. I put some betadine on the hole and kept an eye on it for any rash over the next few days.
    I think I got away with it this time, but it certainly made me more vigilant.

  9. Climate change is definitely a factor, but something else is going on as well. Deer and moose populations are stable to falling in Maine. Mice and small rodents were always found throughout Maine. Have their populations exploded and if so why? What host mammals have moved into Maine that could support the tick explosion? The University of Maine lists insects, fish and crabs as invasive species, but no mammals. Ticks can survive cold weather in leaf litter beneath snow, where it remains relatively warm. Most tick migration over the last 25 years has been from the coastal areas of Maryland through Maine, to inland areas. Not a south to north migration.
    Its puzzling.

    • ecosystem is in a free fall. opportunistic parasitic and predatory beings may find niches and situations that work well for them. Complex systems, but not hard to spot that our species has triggered the sixth great extinction event and many, many things will be changing over the next few thousand years. I am only going to be around for the very start of that process, but I am thankful to be here. Spark of life is great thing. Ticks and lyme disease? not so much.

    • “Most tick migration over the last 25 years has been from the coastal areas of Maryland through Maine, to inland areas. Not a south to north migration.”

      Perhaps the crucial direction in that instance is “up,” as in thermoclines moving uphill under warming. That’s a dynamic that’s been observed often enough, for instance:

  10. @ATTP, @Lowlander,

    I lean towards the complexity @Lowlander suggests, although the regional temperature dependence is evidence which is difficult to explain away. To do so requires positing a bunch of straw men, like more human-wilds contact because of warmer weather. Even so, that, indirectly, could be a climate contribution.

    The places I’d go slow on is tick vector-host dynamics as influenced by infrastructure development and the like. There is a threefold connection, for example, between prevalence of ticks, hosts (which are principally small rodents, not deer), and good oak acorn years, with year-long lags in between. If these are influenced in some way by more building of homes and the like, then you could get a backdoor coupling between a monotonically increasing amount of development and monotonically increasing temperatures. Of course, the devilish part is establishing that development-tick link, even if there are plenty of examples with other species and what people consider pests.

    Causation is hard. Y’need something like convergent cross-mapping to establish it for these kinds of systems. It’s been applied to climate questions before. Related is the prediction of coastal algal blooms.

  11. Hank Roberts

    One suggestion that has worked for us at a place we regularly camp:
    Do this in the fall before the snow starts:
    Collect toilet paper cardboard rolls. Get a permethrin spray (the kind meant to be sprayed onto clothes, not onto skin); spray paper towels then once they dry tear up (gloves!) and stuff into the cardboard tubes and scatter those around the area where you want protection, going out ten or twenty yards or more.

    The cardboard tubes keep rain or snow from washing the permethrin away.

    Field mice will ‘steal’ the treated paper to line their nests for the winter, and the permethrin will kill off the ticks that usually overwinter with them.

    We did this for several years running at a place we often camped; we’d had a lot of tick problems and after doing this permethrin trick no more ticks.

    A farmer friend tells me that livestock and deer populations can be protected by putting “horse collar dispensers” with permethrin over food bins, so the animals will pick up a permethrin treatment by sticking heads through the collars to get at the food.

    That would probably work for moose as well ….

    • If you have a clothes dryer, you can do this using the lint collected in the dryer filter. Spray it with permethrin, let it dry, stuff it in the tubes, and put them out.

  12. While blame for the Southern New England Lyme plague may be laid in modet proportion on gunshy green suburbanites letting the exploding deer population roam free, and failing to poison voles in their holes, the practical folk down East and up North have no such excuse

  13. @Hank Roberts,

    I’m sure importing (raising and then releasing) families of possums would work well, too. That kind of measure does not appear to occur to people.

  14. Jeffrey Davis

    I live in Kentucky and contracted Lyme Disease after hiking in Acadia National Park. A local dermatologist announced that the “scaly” spot on my back that I couldn’t see was a tick. Nothing vaguely hysterical about the diagnosis. Ironically, before we went on the trip my wife had dismissed the likelihood of Lyme Disease. Fortunately, if the disease is diagnosed early, it usually responds to a single round of antibiotics.

  15. One hopes some pharma compant will summon the courage to see if anti-tick dog & kitty collars can be produced in child-safe anklet editions

    • Also ideal for those children separated from their parents at the border.

    • Ha!

      We won’t even endanger our cats and late dog with those.

      Just my opinion, but perhaps it’s partly we don’t allow them to roam free (they are a danger to other fauna as well as tasty to local coyotes), and it doesn’t take that long to give them a good checkover and grooming once they’ve been out — on a leash.

  16. Good luck grooming, Hyper- our free range up-island deer ticks average about 300 microns across, are well, kind of dog-colored.

    As they hang out on leaves overhanging foot and bike paths, victims human and canine are often bit long before they get home.

  17. Since I just pulled a deer tick off me the other day, I thought I’d add a few comments to this thread. I’m a retired meteorologist and have lived in the central Maine town of Garland for the last 37 years. I grew up in the New Bedford area of southeastern Massachusetts back in the 1950s and 60s and just occasionally dealt with the occasional dog tick which were much easier to deal with that the tiny deer ticks that are causing so many problems now. When my wife and I moved to Garland in 1982, all ticks were unheard of in this area and we never had to worry about such things while working in the woods and fields that surrounded our homestead. I’ve been keeping detailed records of weather conditions at our house since 1985 when the average winter high temperature (Dec, Jan, Feb) was about 27F. Since then the average has risen to about 30F and this trend is certainly reflected in Tamino’s graphs for average Maine temperature over the last few decades. I should also point out that both dog ticks and deer ticks has gradually made an appearance in central Maine over the last 10 years as the average winter temperatures have also gradually risen, especially the extreme mins which used to frequently drop into the -30s F back in the 1980s, but which now never do and some winters don’t even make the -20s F.

    I would also like to comment on the idea that similar latitudes imply similar weather conditions. In all my years as a meteorologist, I have found this misconception quite common among the general public who seem to think that going north means cold and going south means warm in all situations. I’m sure that Icelanders would beg to disagree when they occasionally walk around green grass in the middle of winter as I did when I lived there back in 1972! In reality, latitude just has a relatively minor roll to play in the temperature regime of a particular area, especially in the mid-latitudes with its very transient and volatile weather patterns. Much more important is the prevailing wind direction which tends to transport air from other places that have certain levels of warmth and humidity depending on their origin. More specifically, Maine tends to be the coldest of all the northeastern states because we tend to have more air from both eastern Canada and from the north Atlantic that the other New England states plus New York, New Jersey and Pennsylvania. Any surges of warm air that can affect most of the rest of the Northeast tend to get cut off before they can spread into most of Maine at all times of the year. Our weather conditions tend to be more like those of southeast Quebec and the Canadian Maritimes than the rest of the Northeast and so our average temperatures at all times of the year tend to be noticeably colder than even New Hampshire and northern New York, even though our latitudes are similar (though the northern half of Maine sticks up into Quebec and New Brunswick and is by far the furthest northern part of the Northeast).

    So, to summarize, Tamino certainly knows what he’s talking about concerning the increase in ticks in Maine brought about in large part by the global warming that is occurring and even accelerating with each passing year, and his graphs and explanations shout this out loud and clear!

    • “I would also like to comment on the idea that similar latitudes imply similar weather conditions.”

      Yes, compare for instance Clyde River, Nunavut, Canada, with Illulisat, Greenland, which are pretty close in latitude, fairly directly across Baffin Bay from one another. Says here that for the former “Over the course of the year, the temperature typically varies from -23°F to 47°F and is rarely below -36°F or above 57°F,” whereas on the Greenlandic side “the temperature typically varies from -3°F to 52°F and is rarely below -24°F or above 58°F.”

      If you don’t like the cold, you’d clearly prefer Illulisat to Clyde River, similar latitudes notwithstanding. (And you’d clearly also prefer a better set of choices.)

    • @Ed Hummel,

      Ticks are not going to be the only health threat release by the migration of the tropics north. Moreover, Lyme disease isn’t the only pathogen ticks carry. The Ecology literature has been on this consistently since the Kevin Lafferty-led special issue in Ecology in April 2009. See Wilson’s summary of the issue where he writes:

      The rationale for this prediction is simple: the geographical ranges of most species, including many vectors of infectious diseases, are constrained by their climate tolerances (especially temperature); as climate change is likely to expand the range of areas potentially suitable for habitation, so we should expect to see changes in the geographical distribution of infectious diseases and their vectors. The idea that infectious diseases will expand their ranges under climate change is intuitively appealing and has spread. As a consequence, the scientific literature in this field has grown rapidly, with more than 4000 papers being published in 2008 alone. Indeed, a number of high-profile papers published over the past 10–15 years have provided compelling evidence that global climate change has resulted in an increase in a number of infectious diseases, ranging from malaria in humans to fungal infections of amphibians. In his Concepts and Synthesis paper, however, Lafferty argues that the situation is more complicated than many authors have appreciated and that very different predictions will emerge with a deeper understanding of the complexities constraining species distributions and host–parasite interactions. Lafferty questions whether we really should expect to see a net increase in infectious disease prevalence, rather than simply a shift in their distributions, especially for vector-transmitted and water-borne human infections. Indeed, he argues that because past or ongoing disease control efforts have eliminated or reduced many widespread infectious diseases from developed countries, any climate-induced shifts of infectious diseases into these areas are likely to be thwarted by renewed or existing control activities. As a consequence, Lafferty suggests that we might even see a net reduction in the ranges of some infectious diseases.

      Lafferty had two articles in that issue, but this one

      K. D. Lafferty, “The ecology of climate change and infectious diseases”, Ecology, 2009, 90(4), 888-900.

      is the main, and is accompanied by others, including one by Randolph which I liked:

      S. E. Randolph, “Perspectives on climate change impacts on infectious diseases”, Ecology, 2009, 90(4), 927-931.

      It was presented in a Forum setting. Randolph cited “alarmism” not in the sense of climate alarmism, but in the without-a-doubt lack of scientific criticism that accompanied public and even scientific response to projected climate-related risks of malaria as depicted by Martens, et al in the mid-1990s. In particular, while Randolph thoroughly acknowledges links in range and migration to climate change, the need to account for other factors is underscored. In particular, Randolph notes:

      The studies reviewed by Lafferty (2009) provide clear evidence that intuitive assertions based on the undeniable sensitivity of VBDs to climate, and therefore climate change, are not a reliable basis for either explaining the past or predicting the future. The mercurial epidemiology of each VBD is the system-specific product of complex, commonly nonlinear, interactions between many disparate environmental factors. These include not only climate but also other abiotic conditions (e.g., land cover) and the physical structure of the environment (e.g., water sources), and furthermore biotic factors such as host abundance and diversity.


      The upsurge of tick-borne diseases within preexisting endemic regions in central and eastern Europe thus appears to be an unforeseen consequence of the fall of the iron curtain and the end of the cold war, a sort of political global warming. The emergence of VBDs in lands far removed from endemic regions is more commonly recognized as being contingent on economic, commercial, or social events. The most general of these concerns global movements of people and trade goods, happily no longer one and the same.

      More currently, and to the point, there is

      R. S. Ostfelda, T. Levi, F. Keesing, K.y Oggenfuss, C. D. Canham, “Tick-borne disease risk in a forest food web”, Ecology, 8 May 2018.

      The Abstract reads:

      Changes to the community ecology of hosts for zoonotic pathogens, particularly rodents, are likely to influence the emergence and prevalence of zoonotic diseases worldwide. However, the complex interactions between abiotic factors, pathogens, vectors, hosts, and both food resources and predators of hosts are difficult to disentangle. Here we (1) use 19 years of data from six large field plots in southeastern New York to compare the effects of hypothesized drivers of interannual variation in Lyme disease risk, including the abundance of acorns, rodents, and deer, as well as a series of climate variables; and (2) employ landscape epidemiology to explore how variation in predator community structure and forest cover influences spatial variation in the infection prevalence of ticks for the Lyme disease bacterium, Borrelia burgdorferi, and two other important tick-borne pathogens, Anaplasma phagocytophilum and Babesia microti. Acorn-driven increases in the abundance of mice were correlated with a lagged increase in the abundance of questing nymph-stage Ixodes scapularis ticks infected with Lyme disease bacteria. Abundance of white-tailed deer two years prior also correlated with increased density of infected nymphal ticks, although the effect was weak. Density of rodents in the current year was a strong negative predictor of nymph density, apparently because high current abundance of these hosts can remove nymphs from the host-seeking population. Warm, dry spring or winter weather was associated with reduced density of infected nymphs. At the landscape scale, the presence of functionally diverse predator communities or of bobcats, the only obligate carnivore, was associated with reduced infection prevalence of I. scapularis nymphs with all three zoonotic pathogens. In the case of Lyme disease, infection prevalence increased where coyotes were present but smaller predators were displaced or otherwise absent. For all pathogens, infection prevalence was lowest when forest cover within a 1 km radius was high. Taken together, our results suggest that a food web perspective including bottom-up and top-down forcing is needed to understand drivers of tick-borne disease risk, a result that may also apply to other rodent-borne zoonoses. Prevention of exposure based on ecological indicators of heightened risk should help protect public health.

      People can speculate all they want, but there’s nothing quite like experiment. The story is still being written, but, once again it looks like pushing predators out, in this case smaller ones, removes the ecological services of controlling Lyme-bearing ticks.

      Building McMansions and expanding suburbs doesn’t only have climate consequences.

  18. Hyper notes that:

    ” spraying wide areas to suppress Equine Encephalitis in mosquitos is silly when people get into these 1-2 tonne things and drive at 30 meters/second on congested asphalt.”

    Quite right– enjoyable as coachingis, galloping clydesdales should be kept off the public highways .

  19. @RussellSeitz, @Doc Snow,

    It’s good to know that a report of hard Science reduces critics to being comics with questionable talent.

    • Oh, my questionable talents as comic don’t actually reduce my talents as ‘critic’–those were already quite questionable to start with.

      (Though I confess I really don’t think of myself as ‘critic’ in relation to “Open Mind”; I come here to learn. It’s true that one of my learning modalities is to ask questions, however feckless they may be, on occasion.)

      On the other hand, I may also throw some interesting sidelights from time to time. I hope so, anyway.

    • I should add, though, that my levity does not mean that I failed either to read, or to appreciate, your nuanced comments on the science. Thanks to taking the time and the trouble.

  20. One of the references I cited in my comment about Rocky Mountain Spotted Fever on the previous page noted that one factor contributing to the increased risk is that ticks seek hosts and attach more aggressively as temperatures rise. Even if the population of infected ticks were to remain the same, the increased T level and variability will increase the chance of infection. Tamino can probably tell us off the top of his head what the ratio of what were 3.5 sigma days in the fifties is to temperatures now. You might live in a location where the weather hasn’t made tick born diseases riskier, but you might live in an area where the opposite is true, You’ve got to ask yourself one question: “Do I feel lucky?” Well, do ya, punk? And if you are smart, you’ll ask experts questions about how to reduce the risks. Many won’t – what a friend of mine calls “evolution in action”
    BTW I got a tick bite, a rash, a (mild)fever, and a course of antibiotics a few years ago. My physician said that blood tests in NC show 9 out of 10 people with my symptoms are positive for RMSF, and virtually all the others are Lyme disease, with a few negatives. The treatment was the same for RMSF and Lyme, and everybody would start treatment before lab results were back , so I didn’t bother with a test. It was a non event for me – some people, fortunately few, aren’t so lucky –
    I wonder if homeless/uninsured people die uncounted?

  21. I live in the Canadian prairie province of Saskatchewan, and the population of dog ticks hereabouts has been increasing steadily in recent decades. Black-legged (deer) ticks are very rare, but the provincial surveillance program has turned up a few over the last few years. It’s thought that there isn’t an endemic population of black-legged ticks but they hitch a ride from the US on migratory birds (we are on a convergence of two major waterfowl flyways.
    Details in this report:

    • @Phil L,

      I gotta say that if, per page 9, and assuming the sampling is representative, if the rates are 4 positive ticks in 5112 ticks collected (for 2017), those aren’t bad odds … A bite doesn’t necessarily transmit but even if it does, I think my chances of being hit by a stupid driver while I’m out on a run are considerable higher.

      These odds don’t necessarily transfer to Massachusetts, of course.

      To transmit, a tick needs to be attached a good long time, and, even then, it’s estimated probability of transmission is 5%.

      Moreover, even among ticks tested (in Massachusetts) only 30% or so carry the principal pathogens and I have some doubts of the representativeness of this testing, since many of these ticks, from what I understand, are submitted to the Zoological Lab from physicians who have removed them from patients. These are taken over 2 years.

      For recent one year intervals, in Westwood, where I live, there is essentially no prevalence data about pathogen-carrying ticks. In Acadia National Park, there is more, but it is small.

  22. It’s hard to imagine a killbot hellscape without deer ticks.