Recent findings published in the Journal of Medical Entomology carry critically important implications for Lyme Disease detection in Montana and other states currently not considered Lyme endemic.
Although North Dakota was considered to be outside the range of deer ticks (Ixodes scapularis), researchers sampled nine locations in North Dakota and found deer ticks present at six of them, including ticks infected with Lyme spirochetes (Borrelia burdorferi), Anaplasmosis, and Babesia.
This is a good reminder for states that ‘don’t have’ deer ticks or Lyme Disease that you are less likely to find what you are not looking for. States that haven’t looked recently, or haven’t looked hard enough, are treading a dangerously slippery slope in stating that a tick species is ‘absent’ within their borders.
This is important for all non-endemic states, and residents of those states to remember. Assumptions regarding the distributions of tick species drive assumptions about the presence of Lyme Disease, because protocols driving testing, diagnosis, and care are built around the notion that only black legged ticks transmit Lyme Disease to humans. This notion has been disproved by numerous peer reviewed studies and documented accounts to the contrary, but for the time being the notion continues to preclude proper consideration of Lyme Disease in many states.
Numerous other wrong assumptions are inhibiting proper diagnosis and care of this disease, but artificial geographic boundaries are an almost insurmountable obstacle for patients who contract Lyme Disease in states not currently considered Lyme endemic.
Until recently, the tick distribution maps posted on the CDC’s websites (which still appear in some locations and on some state agency websites) displayed Lyme Disease and black legged tick distributions as hand-in-hand, and also displayed black legged tick distributions that did not account for survey effort. Shaded areas or areas containing many dots representing the presence of ticks confirmed the presence of black legged ticks in those areas, but really said nothing at all about the unshaded areas. Unshaded areas, therefore, are just as likely to represent areas without sufficient survey effort as they are to represent the absence of a given tick species.
Proof of Absence vs. Absence of Proof
Since the CDC tick distribution maps still do not display survey effort (the map below is one of multiple maps now), it should also be assumed that other areas lacking black legged tick occurrence on the maps (as was the case in North Dakota) may or may not have been surveyed extensively in recent years or decades.
We cannot reasonably state that black legged ticks do not occur within states are geographic areas where adequate recent survey efforts have not occurred. Recent improvements to CDC tick distribution maps include the separate mapping of species, no longer displaying maps in a way the directly links Lyme Disease occurrence to black legged tick distribution, and more approximate labeling of the maps overall. They still lack a description of survey effort or uncertainty that would be necessary for assessing the relevance/probability of their predictions, and the risk and geographic extent of Lyme Disease and some tick species are still under-represented.
A local example of the inherent difficulty of predicting presence, absence, or distribution of a species is that of grizzly bears, which are relatively easy to see when present. It took multiple agencies millions of dollars and multiple decades to reliably estimate the geographic distribution of grizzly bears in Montana and surrounding states, let alone their population levels or presence vs. absence in specific locations.
This is notable because survey efforts for grizzly bears are considerably better funded and grizzly bears are considerably easier to locate than ticks, and considerably more likely to be seen, noticed, and correctly identified by the general public. So you will more likely get public reports of grizzly bears (but the general public would not know of a need to ‘report’ a tick sighting, is unlikely to know tick taxonomy, and would not necessarily know who to report it to).
Yet in many cases we are making far more precise predictions for tick species, including unsubstantiated presence/absence statements, despite our lack of comparable effort expended toward their detection, and without addressing the difficulties that would preclude our ability to do so.
Another example in my area is the Couer de’ Alene Salemander. We did not ‘have them’ in this part of the state of Montana until someone looked for them. The crew sent to survey for them within predicted habitat found them immediately. Not only did they find them their first day surveying, but they found them in the stream beside where they parked before they had even geared up or begun to work.
It is not appropriate to state that a species is not present in an area where it has not been adequately looked for, let alone where it has not been looked for at all.
Tick Occurrence and Basic Sampling Statistics
The idea that absence of proof is not proof of absence is a very basic (let me say that again–VERY basic) tenet of biology and of sampling design. We must demand more of our public health agencies if they are making assumptions counter to very basic biology.
I would need to un-learn a great deal of basic statistics and basic biology in order to accept that the gaps of data on the CDC’s distribution map are adequate evidence of deer tick distributions. Though again, this is moot since species other than deer ticks or even black legged ticks transmit Lyme Disease. We are forced to discuss, belabor, and dwell on black legged tick distribution anyway, unfortunately, because it currently remains central to the argument against Lyme Disease in Montana and many other states.
If you want to state that a species is ‘not present’ in a given area, you are out of luck, statistically speaking. You can state that a species is unlikely to occur, but for this to be credible, there are multiple numbers you should know, or at least be able to estimate or address, and these are numbers that are hard to come by. This is concisely explained in a related article abstract as follows:
In ecological studies, it is useful to estimate the probability that a species occurs at given locations. The probability of presence can be modeled by traditional statistical methods, if both presence and absence data are available. However, the challenge is that most species records contain only presence data, without reliable absence data. Previous presence-only methods can estimate a relative index of habitat suitability, but cannot estimate the actual probability of presence.
At the very least, you should be able to state the probability of it’s occurrence, which is a function of probability of detection, which circles back to needing both presence and absence data. Proving absence is not possible, but even demonstrating a low probability of occurrence, with reasonable statitistical power, requires a very large number of sampling sites—especially for very small organisms that may be difficult to detect and may have diverse or complex micro-habitat needs, making their occurrence irregular on multiple scales.
Significance/Context for Montana
The confirmation of I. scapularis (especially those carrying B. burgdorferi) findings are very meaningful for Montana, not only because of our proximity to these surveys, but also because Montana’s basis for stating that Lyme disease can’t be contracted here is based primarily on a stated lack of deer ticks (without referencing any recent survey efforts).
As for other states, the assumed lack of deer ticks is relied upon by Montana doctors (as stated to me personally by numerous doctors, and as stated on numerous agency websites and in media articles) as the basis of excluding Lyme disease diagnosis, even though the presence, absence, and distribution of deer ticks in Montana is not known.
If recent survey efforts have occurred they have not been published, distributed, or posted in any location I have found. Informal discussions with professionals in the fields of Entomology, Forest Ecology, and Wildlife Biology anecdotally support that if surveys have occurred, they have not been published or widely circulated among agencies, and their likely location is not intuitive.
It is therefore still assumed by this author that recent surveys have not occurred. Based on factors discussed earlier, it is certainly safe to assume that surveys extensive enough to discount the presence of deer ticks in the entire state of Montana have not occurred.
Having contracted Lyme disease in northwest Montana, the assumptions about our tick distributions delayed my diagnosis and treatment by more than three years. These are unfounded assumption being made my medical doctors that have life altering ramifications for patients. I am astounded that no data would be required to preclude an organism and a disease that can end and destroy lives.
I contracted Lyme, Babesia, and Bartonella while working as a professional Wildlife Biologist for one of the agencies that contributed to the costs of the years and years of data needed to speak to distributions of grizzly bears. I have a very hard time believing that any state agency (or federal agency, for that matter) is funded or staffed well enough to have recently surveyed enough locations throughout Montana to support non-occurrence of any species of tick.
A critical reminder is again necessary here, which is that deer ticks are not the only species of ticks that transmit Lyme to humans. Multiple tick species (whose presence are not disputed in Montana) also transmit Lyme to humans. Evidence is increasing that other insects, including mosquitos, mites, and fleas, also transmit Lyme to humans.
Why This Matters
An entity, agency, or medical doctor willing to emphatically state that there are no deer ticks in Montana or any state (especially where no surveys are occurring) exposes their lack of basic biology credentials and identifies themselves as unqualified to make life or death health decisions hinging on related principles.
The fact that deer ticks were found at a high proportion of sampling sites, even (or especially) with such a small number of sites sampled strongly suggests that deer ticks are present in other areas in the state as well. The researchers were able to expand predicted distribution range in the sate of North Dakota because they found the deer ticks in new areas. With only nine sites sampled, it would not be in any way accurate or adequate to state that the effort demonstrated or suggested a lack of deer ticks if deer ticks hadn’t been found based on so small a sample size.
Keep in mind that this is an oversimplification of what would be required for the state of Montana, or any state, to be able to claim to know that deer ticks (or any organism) is ‘absent’. The probability of absence is also decreased by multiple additional factors such as migratory wildlife, climate change, and human travel. The mapped presence (or actual presence) of deer ticks in the area where one lives (even if they do not travel) is not a valid prerequisite for contracting Lyme disease.
You can find further statistical examples and explanations (that are not oversimplified) at the following locations, if you are into that sort of thing:
- Likelihood analysis of species occurrence probability from presence-only data for modelling species distributions
- Absent or undetected? Effects of non-detection of species occurrence on wildlife–habitat models
- Can we model the probability of presence of species without absence data?
You can read more about the survey and results here:
- Ticks that Vector Lyme Disease Move West into North Dakota
- Survey of Ticks (Acari: Ixodidae) and Tick-Borne Pathogens in North Dakota