How forest fragmentation and biodiversity loss increase Lyme disease risk
Of course I worry about Lyme disease. Lately I find myself hiking in areas with Lyme disease warnings – I know that if I’m bitten by an infected tick, with prompt antibiotic treatment I’ll recover, but I worry that I won’t see the characteristic bulls-eye rash and will mistake the flu-like symptoms for just that: the flu. If Lyme disease is left untreated there is a risk of complications such as joint pain, memory loss and other long-term symptoms. Many of us who love the outdoors know someone who’s had it, and may still struggle with it.
The explosive spread of Lyme disease is at least partly driven by climate change, and it is likely to continue, but there’s more to it than that. The story of Borrelia burgdorferi’s rise from an unnamed bacterium in the 1970’s, to now causing the most important vector-borne disease in the United States, is a Rocky-like comeback story, but with an anti-hero bacterium in the place of the lovable washed-up Philadelphia boxer. There are now an estimated 300,000 cases of Lyme disease in the United States each year – in Canada cases number in the hundreds, but doubled from 2014 to 2015, and there’s little reason to be optimistic.
Lyme disease is of course carried by ticks, which move the bacterium between animal hosts, including humans (more on tick safety below). Ticks that carry Lyme disease are now found in half of US counties, and parts of Canada – the primary culprit is the black-legged tick or deer tick (Ixodes scapularis), but the western black-legged tick also transmits the disease. If Lyme disease is new in your region, you can expect it to get worse* with ongoing climate change. A 2014 study predicts that climate conditions will be favourable for black-legged ticks as far north as Temagami or Sault Ste-Marie by 2040, and to date Lyme disease has followed the ticks wherever they go. But the risk of Lyme disease is certainly not equal in all places. In fact, the environment ticks live in can have a huge effect on the likelihood that they will carry the disease.
A 2000 study in southeastern NY state found strong significant relationships between forest fragmentation and prevalence of Lyme-disease carrying ticks. You would be seven times more likely to encounter a Lyme-disease infected tick in the smallest forest fragments (<1 ha) compared to large forest fragments in the study. This relationship isn’t linear; very large continuously forested areas have about half as many infected ticks per area as small fragments, still a very significant difference. So what’s going on here?
Ticks are not born carrying Lyme disease. When the larvae of ticks hatch out in the spring, they are tiny – about the size of a period on a printed page. The larva needs a meal of blood in order to grow and molt into a nymph, and which animal it happens to climb aboard for this first blood meal is critically important for the transmission of Lyme disease. And, though Ixodes scapularis is sometimes called the deer tick, the first meal of this tiny larval stage is likely to be something much smaller than a deer.
Of mice and men
White footed mice are the most common small rodent in the mixed forests of eastern North America. If a black-legged tick larva encounters a white-footed mouse, this is good news for Borrelia burgdorferi, the Lyme disease bacterium. For one thing, mice are bad groomers. Almost half the ticks that grab onto a white-footed mouse take their full blood meal and drop off. Compare an opossum, where only 3.5 % of ticks manage to feed and drop off – the rest are groomed off, and eaten by the opossum. Many other animals, such as chipmunks, are intermediate between the two.
More importantly for us, white-footed mice are also great incubators of the Lyme disease bacterium. Up to 90% of larval ticks that successfully feed on a white footed mouse will end up being infected with Lyme disease, and ready to inject it into the next host they bite. Eastern chipmunks and shrews infect a moderate number of ticks, whereas most other mammals, birds and reptiles have low infection rates (e.g. 2.5% for opossums, 1% for deer). The net effect is that white footed mice will groom off and kill about 1,000 ticks per hectare, but another 900 ticks will successfully feed and become infected with Lyme disease. Opossums will groom off and kill over 5,000 ticks per hectare, while only five ticks will successfully feed and be infected with Lyme disease. Opossums and a number of other animals act as “ecological traps” for Lyme disease, and their very presence in a forest greatly reduces the load of infected ticks questing for a meal. As forests become more fragmented and disturbed, these other species disappear from the increasingly simplified systems, and almost all that’s left are mice, and the associated Lyme-disease carrying ticks waiting for a meal. White footed mice actually thrive in fragmented disturbed forests, where there is little competition or predation; a study near Ottawa Ontario, for example, found that mouse abundance increased with greater road density.
Biodiversity, ecosystem services, and infectious disease transmission
This limited perspective that considers only four species—spirochetes, mice, ticks, and deer—is entrenched by a reductionist paradigm, one that belies the nature of a complex ecological system involving numerous other plant and animal species and influenced by patterns of human behavior, including land use and development. This complex of forces ultimately lead to loss of biodiversity—a term that refers to the variety of life, including the diversity of genes, species, and ecosystems on earth—and also to increased disease in humans. Yet few physicians are aware of the importance of biodiversity as a powerful force influencing some infectious diseases, including Lyme disease.
The Lyme disease system is an example of a healthy, diverse forest providing an important ecosystem service for humans. The simplest definition* of ecosystem services is “the benefits people obtain from ecosystems.” There have been efforts to assign economic values to these services, equivalent to the economic cost that would be incurred if the service was lost, or a replacement cost for the service; however understanding and valuing ecosystem services, which many have argued have an infinite value, has proven challenging.
As our understanding of Lyme disease increases, it reminds us how much we still have to learn about ecosystems, and how much we still depend on them despite all our technological abilities. As an isolated case perhaps this disease system wouldn’t tell us much about ecosystem services. However a 2010 review in Nature examined the larger context and concluded that “in recent years, a consistent picture has emerged—biodiversity loss tends to increase pathogen transmission and disease incidence. This pattern occurs across ecological systems that vary in type of pathogen, host, ecosystem and transmission mode.”
Another familiar example to North Americans is west Nile virus, which is transmitted by mosquitoes with birds as primary hosts of the virus. Several studies have shown that low bird diversity is correlated with higher risk of transmission to humans. I think more research is needed in the case of west Nile virus – but it should nevertheless give us pause. We should be considering the growing evidence of the many values of biodiversity and healthy ecosystems when making decisions about land use or habitat restoration, whether it be in urban, suburban, greenbelt or rural areas. As I argued in my last post, we now have the knowledge to maintain or restore biodiversity in our populated areas, and we have increasingly compelling reasons to act on that knowledge
Lyme disease prevention
Lyme disease is going to continue to spread, and climate change may increase infection rates in many areas where it’s already established*. Learn good tick prevention and management practices: the CDC is a good resource. Remember that it takes at least 24 hours for Lyme disease to begin transmission after a tick climbs onto you – so checking for ticks after walking in infested areas is an important step in disease prevention.
Lyme disease is advancing quickly, be aware of Lyme disease risk areas. Here is a map for Ontario.
Some promising control strategies are being tested by the Cary Institute: learn more.
Afterword: the origins of Lyme disease
Lyme disease was first described in Old Lyme, Connecticut in 1975, and the spirochete that causes it was isolated in 1981. As it turns out, its sudden appearance and relatively rapid spread makes it a good candidate for a conspiracy theory: Lyme disease was reputedly created by the US government in a secret biological weapons lab on Plum Island, off the tip of Long Island NY. It seems to me that locating your biological weapons facility immediately adjacent to your nations largest city is a dubious proposition – but aside from that, DNA from the Lyme disease bacterium was found in the “iceman,” a 5,300-year-old mummy uncovered in the Italian alps (which we can safely say predates the US biological weapons program), and in ticks preserved in museums in North America and Europe. Genetic analysis of Borrelia burgdorferi tells us that it probably originated in Europe, but has been present in North America for a “long time,” likely millions of years. Accounts from early North American settlers suggest they may have encountered Lyme disease, but as forests were widely cleared and deer were nearly eliminated from the landscape, the black-legged tick, which transmits the disease, also became rare, and the disease faded into obscurity.
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