ByDr Lucy Gilbert, The James Hutton Institute. This article first appeared in the Winter 2013 edition of the RSGS’s magazine, The Geographer.
The tick, Ixodes ricinus, is a fascinating and unusual parasite because it is a true generalist, sucking the blood of almost every type of terrestrial vertebrate, including sheep, deer, mice, birds, dogs and humans. Like other generalists – gulls and crows come to mind – this tick species is highly successful and widely distributed around most of Europe and into Asia. One result of their catholic diet, unfortunately, is that they can be infected with a wide range of pathogens that can cause disease in humans, livestock, companion animals and wildlife. In fact they are the most important vector of zoonotic diseases (such as tick-borne encephalitis and Lyme disease) in Europe.
Lyme disease (or more correctly Lyme borreliosis) is the most prevalent of all vector-borne diseases in Europe. It is caused by spirochaete bacteria called Borrelia burgdorferi sensulato, which have similarities to the syphilis spirochaete. The disease is named after the towns of Lyme and Old Lyme in Connecticut, where there was a spate of illnesses in the 1970s, but it wasn’t until 1982 that the spirochaete was identified as the cause by Willy Burgdorfer and Alan Barbour. However, it is not a new disease: Borrelia originated in Europe before the Ice Age, and symptoms were officially described in 1883 in Germany. The disease often starts with a diagnostic bulls-eye rash, followed by flu-like symptoms. There is no vaccine, but it is usually treatable with antibiotics, although chronic and debilitating illness can occasionally develop if not treated soon enough. There are several different strains of Borrelia, each transmitted by a different host type (eg, one is transmitted only by birds, one only by rodents), each differing in ecology and distribution, and each causing different symptoms.
Ticks are increasing in abundance and distribution in much of Europe, as are cases of Lyme borreliosis. Data published on the Health Protection Scotland website suggest an enormous increase, from only a handful in Scotland in the late 1990s to 285 in 2008. Since then, reported cases have fluctuated around 230- 300. This may not seem a lot, but the majority go undiagnosed or unreported, so this is only the tip of the iceberg. The increase may be due in part to improved awareness, but environmental changes are certainly also impacting on ticks and Lyme borreliosis.
Ixodes ricinus ticks suck blood from their hosts for only a few days each year, so spend most of their lives in the ground vegetation exposed to ambient conditions. Like most invertebrates, ticks are increasingly more active as temperatures increase but are not active when it is cold: we rarely see them in a Scottish winter. However, climate change projections estimate that Scotland will warm by 3°C by 2080 – how will this affect ticks? You can mirror 3°C of climate warming by walking down a Munro into a Cairngorm glen at 450m in summer. Currently there are no ticks on the Munro tops; in fact almost no ticks until below 500- 550m, at which point they increase dramatically as you descend into the relative warmth of the glen. Therefore, in 2080, we may have to watch out for ticks while eating lunch on a Munro summit, and I shudder to think of the increase in tick numbers in the glens by then! Recent studies predict that ticks will be not only more common at higher altitudes but also active for one to two months longer in the year.
As a carbon sink to mitigate climate change and to enhance biodiversity, the Scottish Government aims to increase woodland cover to 25% over the next 15 or so years (current land cover is around 17% woodland). Woodlands provide a mild, humid micro-climate that improves tick survival and activity, and mixed/ deciduous woodlands in particular are great for deer, small mammals and birds. Deer are the most important tick host, while small mammals and birds are reservoirs for Borrelia spirochaetes. Recent Scottish research showed that unfenced semi-natural mixed/ deciduous woodlands generally had more ticks and Borrelia than other habitats.
In Scottish woods, 1-14% of ticks contain Borrelia, but in warmer continental Europe this percentage is much higher. As climate warming, woodland expansion and spreading roe deer increase ticks and Borrelia in Scotland, and as rising population growth and outdoor activities bring more people into contact with ticks, how can Lyme borreliosis risk be controlled? Research shows that controlling deer numbers or excluding deer from target areas (such as certain woodlands) using fencing can reduce ticks by up to 90%, which should greatly reduce disease risk. At a personal level, people can reduce disease risk by being tick-aware, checking themselves for ticks frequently and thoroughly, removing them as soon as possible with tweezers or a ‘tick tool’ and seeking treatment if symptoms develop.