Vector-Borne Illnesses Like Lyme Disease On the Rise as Forests Heat Up

A common deer tick, also known as Ixodes Scapularis. Photo: Image by Erik Karits from Pixabay

Lyme disease, an infectious disease caused by the Borrelia bacterium, is a dangerous illness generally spread by the bite of deer ticks (Ixodes scapularis).

The disease makes its presence known at first with an expanding red rash at the site of the tick bite. The rash, while annoying, usually comes without the side effects other rashes carry, such as pain or itchiness. What does come with that rash early on are relatively minor symptoms of fever, headache, and tiredness. Because none of these suggest a serious problem, a high percentage of those who have been infected by Lyme disease are unaware they have the illness, at least at the beginning.

The problem with this illness, which early on is treatable but later is more difficult to stop, is that it can grow far more serious over time. Long-term symptoms of Lyme disease include loss of the ability to move one or both sides of the face, joint pain, neck stiffness, headaches, and heart palpitations. While those are bad enough, roughly one in five people suffering from the illness will develop more serious joint pains, a period of tiredness which could last six months or longer, and brain dysfunctions such as memory problems and an inability to concentrate fully. In some, the illness will eventually manifest with shooting pains in their arms and legs.

At the root of why the risk of contracting Lyme disease is increasing is the long-term global heating of the planet due to fossil fuel emissions and trapped solar energy close to the Earth’s surface.

Deer ticks thrive mostly in the warm forests of the Northern Hemisphere. They also die off in freezing temperatures, a phenomenon which explains why areas such as Canada had remained relatively tick-free for most of modern human history. Those days are long gone.

A joint research project led by Serge Olivier Kotchi of Canada’s Public Health Agency, and backed up with specialized research data provided by NASA, has uncovered the connection between rising temperatures and the likelihood that one could contract Lyme disease from a tick bite, particularly in Canada.

Past measurements of deer tick estimates used be carried out by a process known as “drag sampling.” In this method a cloth one square meter in size was dragged through the forest for a given region. Ticks that would fall onto the cloth during the process were trapped and counted, as kind of a census count of the ticks.

In the new approach, Kotchi’s team began by examining extensive land surface temperature and land cover information gathered from the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on NASA’s Terra and Aqua satellites.

Change in annual cumulative surface degree days >0° C (32° F) of forest habitats in Central and Eastern Canada from 2000 to 2015. Photo: Sergei Olivier Kotchi, et.al, Public Health Risk Sciences Division, National Microbiology, Public Health Agency of Canada, Saint-Hyacinthe, QC, CC

The researchers noted that large regions of central and eastern Canada now had 15 to 35 more days with above-freezing temperatures in 2015, compared to what was happening in 2000. They also found that some parts of southern Manitoba, Ontario, and Quebec experienced an increase of 35 more days above freezing compared to in the year 2000.

The researchers then used those increased warm day calculations in an algorithm based on other scientific data, to estimate the likely appearance of deer ticks for those regions.

Mapping of deer tick risk increases from 2000 to 2015. Photo: NASA Earth Observatory images by Lauren Dauphin, using data from Kotchi, Serge, et al. (2021).

What they came up with was that the number of adult ticks present per square kilometer almost doubled in Quebec during the 2000-2015 period. The exact numbers went up from 56 as the 21^st century began to 103 in 2015.

Reported Lyme disease cases in Quebec, increased even more dramatically during the period. Medical records data shows 144 people infected in Quebec in 2009, right in the middle of the research period. In 2016 the number was 992, a number almost 7X the number from only seven years earlier.

In Manitoba the increases in tick concentration were even more dramatic, from 36 per square kilometer in 2000 to 110 per square kilometer in 2015.

Both numbers are likely much higher now, given the accelerated rate of global heating.

If global heating contributed to minimum 2X increase in tick concentration during the period studied, and in turn resulted in a 7X increase in the number of Lyme disease infections in Quebec, the implication is that the 3X increase in tick concentrations in Manitoba could produce as much as a 10X or higher increase in Lyme infections for that province.

Look to other vector-borne diseases, carried by insects less common in the formerly much colder north, to increase their infection rates in the coming years as global heating continues.