Climate Disruption, Forest Fragmentation: How Reduced Biodiversity leads to Increased Disease Load from Zoonosis.


Recently, an exchange on social media concerning Lyme disease and ticks prompted a good deal of disinformation, which was followed by several text messages to me asking such questions as, “Can ticks really carry rabies?” or, “Is there a vaccine for Lyme disease?”

At a time of climactic change, we are seeing a rapid rise in infectious diseases, in particular, zoonosis, that are appearing in novel locations. This is caused by changes in the environment that are at once simple to predict, yet difficult to fully comprehend. As the weather patterns become unstable, and as the Northeast starts to warm up with higher average temperatures, species that are long-time inhabitants of the Mid-Atlantic States have begun to survive in more northern areas. Deforestation and fragmented woodlands magnify the effect, particularly among the vectors of disease.

In parts of sub-Sahara Africa, logging and mining have made inroads into areas not regularly inhabited by humans. This has brought humanity into direct contact with species that were previously rare. As a result, some novel zoonotic diseases have resulted including Ebola. In parts of upstate New York, the housing market is infringing on native woodlands and forests as people seek to get away from it all. As many New Englanders will tell you, you need not travel to Africa to be infected with a zoonosis. In North America, as the climate continues to warm, and deforestation continues, expect once novel infections like Lyme Disease, Powassan virus, Tularemia, Anaplasmosis, Tick-borne relapsing fever, and 364D Rickettsiosis, to become far more common. Perhaps the most significant threat from these tick-borne diseases comes in the form of forest fragmentation.

While it is well known that Lyme disease is spread by ticks, there is a significant amount of disinformation and outright fabrications on social media concerning this and other conditions that stem from contact with arthropods. While the popular press is concerned with Deer as the harbingers of Blacklegged ticks, responsible for the spread of Lyme, the reality is that Deer (like humans) are a dead-end species, and benefit the Tick little. For Burgdorferi bacteria to profligate, there must be a reservoir animal that is either unable, or unlikely, to shed itself of the disease vector, and where the bacteria can live and spread to a new host. The reservoir species must be an animal that can harbor the infection and pass it on to other Blacklegged ticks that come to feed on the blood of the host. In much of North America, that reservoir is the cute little Whitefooted Mouse.

Any animal researcher will tell you that Whitefooted mice are not particularly effective at self-grooming, nor are they particularly likely to socially groom. Chipmunk, Opossum, and Red Squirrel are all subjected to Tick infestation. However, each of these animals are excellent self-groomers and can dislodge and shed the ticks. Another apparently novel reservoir is the shrew, a small insectivores animal that is also (apparently) not a particularly effective self-groomer. Given that Shrews are also quite aggressive and territorial, opportunities for social grooming are probably rare if not nonexistent. We have effectively identified the principal reservoir for Lyme disease, and we know well the vector (Blacklegged Ticks), all that is necessary for a Lyme disease explosion is the opportunity for both the mice and the ticks to reproduce.

In normal eastern Woodlands, a high degree of biodiversity exists, complete with competition species for the Whitefooted Mouse, including chipmunks, squirrels, and other small rodents, and a host of predators. However, many of these species require a diverse habitat with several varieties of plants to consume, and these are kept in balance by predators including raptors, owls, and small to medium-sized carnivorous mammals (think Marten, Fisher, Coyote, and Fox). In fragmented forest and deforested areas, the old growth forest necessary for biodiversity of predators is largely absent. As a result, the loss of both competing species (that are effective at self-grooming and disposing of ticks), and predatory species (in particular Owls, Hawks, and carnivores), the mice are free to breed like, well, like mice. And they do.

The more mice there are on an acre of land, the higher the chance that ticks living off the blood of those mice will bite a human. In areas of biodiversity, like big forested areas, the wide range of animals that prey upon mice, or compete with them, keep the mouse population in check. When we fragment the forest, we drive off the predators, and many of the competition species, and mice have proved to be extremely resilient living close to human habitations. As roads dissect forest, and housing pushes into the woodlands, biodiversity rapidly drops off leaving only opportunistic species, which often thrive.

Although Lyme carrying ticks can and do bite several mammal species, many, like deer and humans, are a dead-end species. Other species can be bitten by Lyme carrying ticks but not become infected, which leads some researchers to believe that mice are a relatively new reservoir for tick-borne disease in North America.

How to avoid both Ticks and Tick-Borne Diseases.

Be conscious of ticks, especially between May and November. Use an insect repellent; it is best to choose an EPA product that is listed as both safe and effective against tick bite. I use products that contain permethrin which can be applied to clothing such as pant legs, socks, boots, shirt sleeves, and gloves. Do not apply permethrin directly onto your skin. If you feel you must apply an insect repellent directly to your skin, products that contain DEET should be used, however, be certain that the product you are using contains no more than 30%. According to the American Academy of Pediatrics, this concentration is safe for children as well, however, I would never use DEET in any strength for infants or children under six months of age. Above all, follow the manufacturers instructions for all repellent application.

Check for ticks anytime you are outdoors. Remember, ticks must remain attached for 24 to 48 hours before they can transmit the spirochete that causes Lyme disease. You should remove the tick as soon as you discover them, however, as Anaplasmosis, or Powassan may require significantly less time. Search your entire body closely, especially any areas that are hard to see. A tick may appear like a speck of dirt, so check carefully, especially children. If you discover a tick, don’t panic. Remove it as soon as possible to lower your risk of infection. Use a pair of tweezers to grasp the tick by the head as close to the skin as possible. Pull the tick outward slowly and gently with a steady motion. Clean the area of the bite with soap and water thoroughly. Please remember that folk remedies like applying Vaseline to the tick, applying nail polish remover, or burning the tick with a match are not safe or effective methods of tick removal.

If you are uncertain how to remove the tick, please view the following videos, and always check with the CDC websites to learn more about infectious agents.

Here are two excellent videos showing effective tick removal.

From the University of Manitoba:

From the Tick Encounter Resource Center:

Finally, if you are concerned about the risk of infection, contact your healthcare provider.

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