Case study: Lyme disease
A little-known but common problem
Lyme disease is the most common arthropod-borne disease in Europe and North America, yet one we still know little about. It is carried by ticks, tiny arachnids (like spiders), and caused by bacteria of the genus Borrelia – the disease is also known as borreliosis. The problem is that it’s hard to diagnose, with several stages to the infection and symptoms that mimic other diseases.
It was ﬁrst identiﬁed in the 1970s around the town of Lyme in Connecticut, though it is probably much older. If caught early on, while the infection is localised to the site of the tick bite, antibiotics do a good job of clearing it up. But if the bacteria spread to other sites in the body, an infection may become harder to detect and treat.
All this uncertainty means that nobody knows just how prevalent Lyme disease really is and how best to treat it. But all indications are that it’s on the increase in both Europe and North America.
Like all diseases that are zoonotic with a wild animal reservoir, the transmission cycle of Lyme disease is complex. In Europe the tick that transmits the disease will feed on a range of wild animals, including rodents, large mammals and birds.
The different species of Borrelia bacteria circulate between the ticks and the various host animals. One type occurs mostly in woodland birds, and another in small mammals. They even compete with each other within the tick. The resulting infection in humans depends upon the type of Borrelia transmitted, and therefore the types of wild animals present in people’s neighbourhoods are important. There are other types of Borrelia that don’t cause any disease in humans.
There are about 165,000 diagnosed cases of Lyme disease in Europe each year and that number has been increasing, as it has in the USA too. There are those that suspect climate change could have a hand in this increase, with milder winters increasing the number of ticks. There is some evidence from the Alps and Scandinavia that ticks are now found at higher altitudes and latitudes.
In the USA Lyme disease is already an indicator of climate change, meaning its prevalence is used as a measure of the effects of increasing temperatures. However, the number of deer and other host animals in a region may be equally as important. One 2014 study found that cases of Lyme disease in a Connecticut community decreased substantially in the period following a deer hunt.
The best advice remains to avoid getting bitten in the ﬁrst place, for example by not exposing bare ﬂesh when walking through woodland areas or bracken.Lead image:
David Gregory and Debbie Marshall/Wellcome Images CC BY NC ND
- US Centers for Disease Control and Prevention: Lyme disease data
- Lyme borreliosis in Europe (2011)
- NHS: Lyme disease
- US National Institute of Allergy and Infectious Diseases: Finding the cause of Lyme disease
- Scientific American: Has climate change made Lyme disease worse?
- The relationship between deer density, tick abundance, and human cases of Lyme disease in a residential community (2014)