It's Lyme Disease Season in Ontario. You can Blame the Housing Crisis.

Aerial photo of Ottawa suburbs. From flickr.

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Between 2012 and 2017, the incidence of Lyme disease in Ontario grew by a factor of ten. This rapid increase has only continued since then and has prompted public health authorities in the province to develop new strategies for monitoring and preventing the tick-borne disease. Lyme disease as a phenomenon in Ontario is very recent, and very spatially heterogeneous.

Lyme can be very serious if left untreated but first manifests as a fever, fatigue, and a characteristic bull’s-eye rash called an Erythema Migrans. The rash appears wherever the patient was bitten by the pathogen’s preferred vector, the black-legged tick, a flesh-eating critter that gets its dose of the disease while hitching a ride on the white-footed mouse. When talking about diseases that spread from mice to ticks to humans, it’s hard to identify any systemic causes beyond Mother Nature’s whims. But Lyme disease’s rise in Ontario is more than an unfortunate coincidence, and alongside climate change, one unlikely culprit stands out: the housing crisis.

Everything we humans do to nature has consequences for our own health -- particularly in the case of zoonotic (animal-borne) diseases, where disruption of an ecosystem can lead pathogens to “make the leap” and spill over into humans. Although not everyone agrees, Covid-19 is thought to be one such disease, and has been linked to bats forced out of their natural habitat and into close proximity with human settlements.

With Lyme disease, the story of human meddling is similar: our expanding settlements have given Lyme the perfect conditions to develop into a fact of life in Southern Ontario. Research published in Conservation Biology has shown how Lyme disease risk is affected by forest fragmentation: If a forest is dissected into so many small parts, smaller animals that require less forested land area are comparatively less disturbed than their larger predators. Run a highway through the middle of a forest, and the foxes might have trouble crossing, while rodents are content to stay on either side. Fewer predators means more white-footed mice, more black-legged ticks, and more Lyme disease. Research on the I-95 corridor in the Eastern United States has shown that urban and suburban expansion is spatially correlated with Lyme disease risk.

Ontario is currently in a period of suburban and exurban expansion – the kind that puts humans in proximity with elevated tick populations. Research from the Toronto-based not-for-profit Evergreen has shown that growth in Ontario’s mid-sized cities has mostly been in suburbs and exurbs, while denser downtown cores are declining. This is in part because of local zoning regulations that prevent the densification of existing urban areas. While demand for housing in Ontario has skyrocketed, new supply is all but impossible to build in urban areas.

Some commentators have latched on to this fact and used it to promote sprawl as a solution to the housing crisis, arguing that conservation land like the Greater Toronto Area’s Greenbelt should be repurposed. Others have argued that instead of de-regulating conservation land, we should untangle the regulatory spaghetti of “Not-in-my-Backyard” laws that serve only to protect property values. When debating which approach to take, it’s worth remembering that Lyme disease is only one of the environmental consequences of sprawl.

As mentioned, Lyme disease prevalence in Ontario is spatially heterogeneous. Based on this map from Public Health Ontario, we can see that Lyme disease risk areas tend to be around the suburbs of cities like Toronto, Ottawa, Hamilton, Kingston, or Niagara. The nation’s capital, for example, has an enormous footprint, and not surprisingly it is there that Lyme disease prevalence has increased the most.

Lyme disease’s increase can be attributed to factors other than sprawl, namely climate change. Black-legged ticks’ range has expanded northwards as Southern Ontario warms, which is doubtlessly part of the reason Lyme disease incidence has shot up so quickly. But climate change too is linked to sprawl. Not only are single-detached homes less energy efficient, but sprawl creates more commuters and more transportation emissions.

All of these issues are tightly intertwined, necessitating a holistic approach that sees the trees for the forest (pardon the pun). How we treat the environment has ramifications for human health, most notably in the form of zoonotic diseases. And as Ontario’s housing crisis demonstrates, how we treat each other has ramifications for the environment. We cannot have a sustainable society that is not also an equitable one.