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  • Alex Cloherty

Why do some diseases have home turf?

The topic of today's post was suggested by my cousin. He wrote, "How about regionalization of certain viruses/diseases?", and commented that Lyme disease, Zika virus, and other infectious diseases are moving further north. At the root of the movement of Lyme disease and Zika is a concept that we haven't really touched on before: vectors.

Taking a human-centered approach, the World Health Organization defines vectors as "living organisms that can transmit infectious diseases between humans or from animals to humans". However, taking a broader view, we can also consider organisms that transmit infectious diseases between non-human animals to be vectors. To explore this, let's start with the plague as an example. You probably remember hearing about the plague, also known as the "black death" or, scientifically, as the disease caused by Yersinia pestis bacteria, in your high school history class. Maybe you also remember that the plague was carried into new regions by rats. In fact, the plague was actually transmitted by a species of flea that targets rats, as was discovered by Dr. Paul-Louis Simond in the late 1800s. From both a human point of view and a rat point of view (rats can also die of the plague), the fleas were the true problem, as vectors of the black death.

This story of Yersinia pestis, rats, fleas, and humans, shows how complicated the spread of disease can be. In the case of the plague, the home turf of the causative bacteria depended on where rats carrying plague-infected rat fleas were able to travel. The travel plans of the rats in turn depended on where human ships were traveling, and if the rats could jump on board and hitch a ride. The spread of the plague was therefore tied up with not only human sanitation and living conditions, but also human trading routes and economies.

Today, the plague is not nearly as worrisome to humans as it used to be, but other vector-transmitted diseases have replaced its role in human society. To go back to Lyme disease, Borrelia burgdorferi, the responsible bacteria, are carried by ticks. In a similar manner to how plague-infected fleas were carried by rats in the middle ages, the ticks that transmit Lyme disease can be introduced to new regions by migrating birds. Some scientists have posited that recent increases in Lyme disease diagnoses in Northern communities, for example in Canada and in the Netherlands, could be due to the warmer winters. These scientists argue that birds have always been dropping ticks as they migrate, but now the ticks, and as a result the Borrelia, survive the winters and infect more people the following spring. However, other scientists argue that there is no hard evidence for increased tick survival, and the reason for increased Lyme disease diagnoses could just be increased awareness of the disease. As far as I could find, the jury is still out on this.

The take-home message for Lyme disease here is that where the ticks go, Borrelia burgdorferi and Lyme disease can follow. This holds true for all vector-transmitted diseases. Where the Aedes aegypti mosquito can live, Dengue and Zika viruses can follow. Where the infamous Tsetse fly can travel, so can sleeping sickness. Where the flea-ridden rats walked, the plague crept behind.

For scientists, it can be rather difficult to sort through all of the different ways in which humans are altering their environments – from air travel to climate change to destruction of ecosystems – and find a primary reason for the movement of disease vectors. To make it even more difficult, all of these factors are intertwined. But it does hold to reason that if humans change the environment sufficiently, we could open up new turf for these disease vectors. This is a topic that I'll return to in the future, as soon as I find some new research on the changing habitats of ticks and mosquitoes... But in the meantime, it can't hurt to do your part for the environment!

Until next week,

- Alex

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