On a sunny afternoon in May a few years ago my brother-in-law Rob and I paddled our canoe across a northern Saskatchewan lake to a wilderness area. We set up a temporary campsite, cooked our lunch, photographed wildlife, and paddled back across the lake. That evening as we were sitting down to dinner my sister-in-law said “what’s that little black dot on your neck?” I tried to remove it, but it was stuck to my skin. On closer inspection I realized it was a tiny wood tick. I got a pair of tweezers and pulled it off. Relief!
But then the penny dropped. Maybe I had more ticks. I pulled off my shirt and sure enough I found a tick in my underarm, then one on my side, and before long I had found twenty-six ticks on my body! But Rob beat that record with a total of thirty-two ticks on him.
We had all these ticks on us, yet we hadn’t felt a thing. Fortunately for us, wood ticks (Dermacentor variabilis) don’t transmit Lyme disease. Yet thousands of people in the US and Canada every year aren’t so fortunate. The ticks that they encounter are the blacklegged tick (Ixodes scapularis) that can carry the Lyme disease bacterium, Borrelia burgdorferi. According to the Center for Disease Control (CDC) almost 30,000 people are confirmed to have Lyme disease each year, but they estimate that the number of infections is likely 10 times higher than reported - nearly 300,000 new cases per year! A lot of people apparently don’t get too sick from Lyme disease, but some people get very sick and every year some people die.
If you don’t have Lyme disease in your area, it might not be long before you do. The CDC has reported that Lyme disease has spread tremendously over the past fifteen years and they expect that trend to continue. It’s not clear why, but global climate change might be partly to blame. Whatever is causing its range to expand… it might soon be appearing in a landscape near you.
The CDC provides good advice for people on how to avoid coming in contact with the blacklegged tick, but of course not everyone is likely to read and follow the CDCs advice. That’s where landscape architects can help. There are things that can be done to minimize the potential for people to come into contact with the ticks that cause Lyme disease.
A few years ago one of my Master of Landscape Architecture advisees, Sarah Ward, studied Lyme disease in the landscape. She investigated the ecology of Lyme disease so that she could identify critical interactions between people and the blacklegged tick. She was able to identify times of year, locations, and landscape conditions where people are most likely to come in contact with ticks.
It turns out that humans are ‘accidental’ hosts of the blacklegged tick. Two of its preferred hosts are white-tailed deer (Odocoileus virginianus) and white-footed mice (Peromyscus leucopus). The ticks can’t move more than about 3 metres on their own, and rely on hosts to carry and disperse them. This is a critical point. Ticks will tend to occur in habitats that favour their hosts – deciduous woodlands. But don’t think that removing deer and mice from the landscape will solve anything. The blacklegged tick is equally willing to feed on at least 27 other mammals and 36 species of birds. A better approach is to identify where and when the ticks will be looking for a host, and design the environment so that the host is not likely to be a human.
Sarah and I published a paper called “A framework for incorporating the prevention of Lyme disease transmission into the landscape planning and design process” in Landscape and Urban Planning. You can access it through the journal website but I’ve also put an open-access copy of the paper on the University of Guelph Atrium. You can download a PDF at:
http://hdl.handle.net/10214/4615
I’ll summarize four of the key points here.
1. Blacklegged ticks are extremely sensitive to desiccation so they are found almost exclusively in moist habitats with a minimum of direct sunlight. Path surfaces can be made inhospitable to ticks by constructing them of gravel or other xeric material.
2. Ticks can’t jump and don’t drop out of trees. They crawl along stems of plants and wait at the tip for a host to come along and then they grab on. That means that paths should be wide enough to allow people to walk along the path without brushing against adjacent vegetation.
3. Seating should be located at least 3 metres away from dense brush, wooded areas, and heavy groundcovers and should be installed on a bed of gravel or other xeric material.
4. A metre-wide strip of xeric material should be installed between lawns and wooded areas and around play areas and patios. Ticks will tend to avoid these dry barriers.
These are pretty simple things to incorporate into your site-scale designs, and while they won’t eliminate the threat of Lyme disease, they will reduce the potential for people to come into contact with the tick that carries the bacterium.
Our journal article has been cited by many other studies. Much of the current research is focused on the larger landscape and the results will be of considerable interest for work at the landscape planning scale. Sarah became more interested in medicine than landscape architecture and studied to become a physician. She went on to become an orthopaedic surgeon. Rob and I now tuck our pants into our socks when we walk in natural areas, and check ourselves for ticks when we get home.
Reference:
Ward, S. E. and R. D. Brown. 2004. A framework for incorporating the prevention of Lyme disease transmission into the landscape planning and design process. Landscape and Urban Planning 66: 91–106