How climate change is affecting Lyme disease incidence
“Lyme disease” is a phrase you’ve likely heard more often lately. It is, as some researchers have called it, a “ticking bomb”—a pun with a sense of urgency folded in. The last twenty years have charted a striking increase in the incidence and geographic range of Lyme disease across Canada and the United States. This trend, strongly associated with climate change, is only expected to accelerate in the years to come.
So, let’s start with an overview. Lyme disease is the most common tick-borne disease in North America. It is caused by the bacteria Borrelia burgdorferi. It’s a zoonotic disease, meaning that it’s transmitted to humans from an animal reservoir. Ticks feed on animal hosts that carry B. burgdorferi, and then on humans, passing along the bacteria from animals. Different tick species are responsible for transmitting Lyme disease in different parts of the world, but in Ontario and most of eastern North America, the primary vector is the black-legged or deer tick (Ixodes scapularis). Immature ticks, called nymphs, are the main culprit in Lyme disease transmission, because they are smaller and harder to detect.
If left untreated, Lyme disease progresses through three stages, with symptoms varying widely from person to person. Stage one occurs a week or two after the bite. It usually manifests as the infamous “bullseye” rash: a red ring surrounding a smaller red circle centered on the bite site. The next stage occurs three to five weeks after the bite. At this point, multiple rashes are seen across the body and muscle and joint pain, fatigue, and various cardiac and neurological symptoms will be experienced; for example, headache, facial paralysis, chest pains, and heart palpitations. The final stage sets in after three months and can last for years. It often involves recurring arthritis, and chronic neurological symptoms which range from mild difficulties concentrating and remembering things to inflammation of the brain and spinal cord. Cardiac manifestations often resolve themselves but can also be fatal.
But there’s no need to panic. Treatment for Lyme disease is readily available and highly effective, provided it is caught early. Prognosis hinges on quick recognition, since it takes at least 24 hours for a disease-carrying tick to transmit Lyme disease. If a tick is removed within this window, medical intervention is rarely necessary— although it’s always a good idea to check with a doctor. If you find a tick, the best removal strategy is to place tweezers right where the tick has burrowed into the skin and pull it straight out, ensuring removal of the head along with the body. If possible, it’s useful to keep the tick to bring to the doctor, as it’s much easier to test the tick for B. burgdorferi than it is to test you. When identified early, a single course of antibiotics is usually enough to prevent disease progression. Once Lyme disease has reached later stages, multiple courses of different antibiotics are administered, depending on the symptoms displayed. Some symptoms will persist even after treatment.
Despite the ready availability of treatment, there is still growing cause for concern. As our climate changes, Lyme disease is becoming both more frequent and more widespread. This association is strong enough that the US Environmental Protection Agency is now using Lyme disease incidence itself as an indicator of climate change. Even within the most optimistic warming estimates, scientific models unanimously predict large increases in Lyme disease cases and distribution within the next 50 years.
A tick’s life cycle, activity, and survival are all climate dependent. Below minimum temperatures, if the weather is cool enough, ticks remain dormant underground. Temperature is the main limiting factor in the northward spread of their habitat—without enough warm periods, ticks cannot establish a population. In warmer climates, the tick life cycle accelerates, increasing the likelihood that a tick survives to maturity, and ultimately resulting in higher reproductive rates. Increasing seasonal temperature averages are also pushing the beginning of Lyme disease season earlier in the spring, increasing its duration. Ticks are also highly sensitive to humidity. In more humid environments, ticks can spend a longer time seeking hosts, and forage higher above ground-level, increasing their chances of encountering hosts (like humans).
Lyme disease incidence is also dependent on the density of host animals for ticks to feed on, and the percentage of these hosts competent in passing on B. burgdorferi infections. In Canada, the white-footed mouse, a highly significant source of B. burgdorferi infection in ticks, has already been observed migrating northward alongside the ticks. In a few years, these new populations will begin to spread Lyme disease farther and farther north.
Fortunately, there is another pivotal factor affecting Lyme disease incidence: human behaviour. We can reduce our exposure by avoiding tick habitats or taking protective measures like wearing long clothing and applying DEET-formulated bug repellent. Check for ticks after being outside in long grass or woodlands, especially in local high-risk areas like the Toronto Islands and Rouge Valley. Moving from the individual to a larger scale, scientists are working to develop vaccines, faster and more accurate diagnostic tools, and better treatments for late-stage Lyme disease. Education, too, plays a vital role: when it comes to Lyme disease, awareness is the key to safety.
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