Will McLellan takes us on a tour of his work on tick density in human-impacted environments.

What are you studying?

The word ‘tick’, to most people, would bring to mind a symbol on an exam answer sheet that says “well done!”, you got that question right “good job!”. For others, a tick might be a sign of nervous energy telling you that someone else sitting at your poker table is bluffing. For me, ticks are the reason I spent 3 months of my Summer in 2023 dragging a blanket through the forests of North-East Scotland. By this, of course, what I mean is that I spent my summer surveying for the tiny spider like creatures which feed on the blood of many vertebrate species, including humans.

Although they don’t know it, the dietary requirements of ticks cause a number of issues for public health professionals and livestock farmers throughout the northern hemisphere. It turns out that ticks are incredibly effective vectors for a variety of disease-causing pathogens, passing these tick-borne pathogens (TBPs) on to their unwitting hosts when feeding on them. As anyone who has experience of Lyme disease (LD) will tell you, some of these pathogens can be zoonotic, passing from animals to humans. The most abundant tick species in the UK, Ixodes ricinus, employ a tactic which mostly consists of sitting and waiting in order to find their next meal. By climbing to the top of ground vegetation though, they maximise their chances of being able to grab on to a deer or wood mouse when they wander or scurry past.

This behaviour is called questing and is the reason that dragging a blanket for 10 metres at a time (1 transect) through a Scottish woodland is an effective method for sampling tick populations and activity levels. Ticks don’t seem to distinguish between our nice woollen blankets and passing deer, and so, cling on for the ride to be counted and collected by a budding early career researcher like myself.

A good impression of what it’s like sampling ticks in the Scottish woodland.

What’s the big question and how are you answering it?

My fieldwork during Summer 2023 made use of this blanket drag method to answer the slightly strange sounding question: “Can humans scare away Lyme disease?”. I realise that this might need a bit of explanation…

The survival, fecundity, and activity of the ticks are dependent on the air temperature and humidity as well as host availability, whilst the pathogens depend on the animal species ticks feed on, whose population densities are dictated by habitat and interactions between species, including humans. It is because of the connection between TBPs and ecological interactions that we humans may play a major role in shaping our own disease risk without even knowing it. In my research hypothesis, I’m testing whether hubs of human activity, through a “landscape of fear”, may be causing a redistribution of wildlife species, particularly deer, the main I. ricinus tick host in Scotland. If deer spend less time close to places busy with humans (such as popular trails) we predict fewer ticks. However, if fewer deer means denser vegetation and more rodents, pathogen prevalence might be higher. I am interested in how these hypothesised opposing effects cascading through the ecosystem may reshape our own disease risk.

During my fieldwork I set out to sample tick populations, using the blanket drag method described earlier, at various distances from busy rural paths. Since tick densities and the proportion of ticks infected with a TBP (TBP prevalence) are both shaped by the host species and habitat, it was important to gather environmental information at the same time as sampling the tick populations. To estimate the height and density of ground vegetation we used sward sticks: bamboo poles with different coloured tape placed at 5cm intervals. We took note of signs of wildlife and tried to estimate deer space-use by counting the number of deer piles within our blanket drag transects (it doesn’t get much more glamourous than that!).

Counting and collecting ticks from a blanket (left). Deer dung and a sward stick (right).

Through statistical analysis, I hope to be able to contrast the environmental information and pathogen prevalence at different distances from the recreational paths and, therefore, build an understanding of how we might be shaping our own disease risk by affecting host animal behaviour.

What were the main challenges you faced during your fieldwork?

There were some major challenges during my fieldwork. Weatherwise, it was an unusual summer. When I initially tried to make a start on fieldwork, in mid-May, we were ready to go, but the ticks, unfortunately, were not. It was still just too cold for them to be consistently active in reliable numbers, so we were forced to delay until the following month. In June we experienced a major heatwave and drought, eventually forcing the ticks to hide to survive. Just to add to our weather-related issues, July that summer was one of the wettest on record – it just would not stop raining. Unsurprisingly, ticks don’t tend to be interested in grabbing on to a soaking wet blanket. This very selfish fair-weather attitude on the ticks’ part forced me into further delays. Still, I had 15 sites to visit twice each, and I wasn’t going to let the Scottish weather dampen my spirits and by mid-August my fieldwork was finally completed (although significantly later than I had intended). 

Along the way there were some other more minor challenges. It could at times be challenging to keep track of where all these ticks came from. Once collected, ticks were put in eppendorfs (small test tubes) and keeping track of which site, visit and distance each eppendorf came from involved a lot of labelling and admin, both in the field and at home in the evenings. For the three months of field work, ticks were everywhere: on me or my clothes, in the eppendorfs, and in my dreams (sometimes nightmares).

When it comes to tick related fieldwork, I’m very lucky in that I hardly ever seem to get bitten by ticks – I’ve never known why, maybe I just don’t seem very tasty to ticks. Unfortunately, the same cannot be said for a few of my volunteers last Summer. However, that kind of luck was very much offset when I got my first ever bout of hay fever midway through last Summer in the middle of forest. It took me quite a while to put two and two together and connect my constant sneezing and watering eyes with the forest full of tall grass and wildflowers around me. I don’t know if you’ve read or watched “Charlie and the Chocolate Factory”, where the character Violet Beauregarde samples an experimental chewing gum and, quite poetically, turns the colour violet, but that’s exactly what I looked like when I got home. Next Summer I’m bringing antihistamines with me.

Do you have any final pieces of wisdom from your field experience?

In the end, my advice for any other early career researchers who will soon be embarking on their first field seasons would be similar to the old scout’s moto: always be prepared. For me at least, summer fieldwork was a marathon and not a sprint. So, the endurance to keep going is incredibly important. Most importantly though, despite your best laid plans, things can change, and obstacles can come up very quickly, so be ready to adapt, and remember, there is a solution to most problems.

Acknowledgements

Banner and text photos by Fanny Olsthoorn and helpers.

Thanks to Will for his excellent contribution to the Fieldwork Diary series. Edits and formatting by Lucy Gilbert and Megan Griffiths.

If you are within the School of Biodiversity, One Health and Veterinary Medicine and would like to submit a fieldwork diary, please download the questions using the following link and follow the instructions in the document.

https://docs.google.com/document/d/1e5C8NUew5xFpEl_OPgH-yAjekmoc84OTLol0_eVoDL0/edit?usp=sharing