Guest essay by Eric Worrall
Deer tick invasion
Encounters with ticks didn’t always cast a dark shadow over North American summers. Cases of Lyme disease first appeared in 1976 in the woodsy suburb of Lyme, Connecticut. At that time, deer ticks were found only in a hotbed encircling Long Island Sound, along with a small area in Wisconsin.
Since the 1970s, deer ticks have rapidly extended their reach north, west, and south. The most recent map shows that deer ticks now roam throughout the eastern coastal states, from Maine to Florida, and across the Midwest. They are now established in 45 percent of US counties. That means the deer tick has more than doubled its reach in the 20 years since the previous map was published.
The spread of Lyme disease has closely followed the spread of the forest nymphs. Lyme disease is now the most common disease transmitted by a vector — a mosquito, tick, or other bug — in United States. More than 30,000 cases are reported each year, and the Centers for Disease Control and Prevention estimates that 10 times as many Americans develop the disease.
Ticks spend the majority of their lives on the forest floor. They are vulnerable to changing local climates and death by freezing, drowning, or desiccation. Warmer winters and longer summers let more ticks survive and thrive further north each year. Warmer temperatures quicken the tick life cycle, too. Tick eggs hatch sooner and ticks spend more time questing for blood, and so are increasingly likely to feast on a human and pass on a disease-causing pathogen. Because more ticks survive and mature more quickly, diseases can be transmitted faster.
What happened in the early 1970s, which might have caused a sudden rise in the rate of dangerous insect borne disease infections?
In 1962, Rachel Carson published the book Silent Spring. It cataloged the environmental impacts of widespread DDT spraying in the United States and questioned the logic of releasing large amounts of potentially dangerous chemicals into the environment without understanding their effects on the environment or human health. The book claimed that DDT and other pesticides had been shown to cause cancer and that their agricultural use was a threat to wildlife, particularly birds. Its publication was a seminal event for the environmental movement and resulted in a large public outcry that eventually led, in 1972, to a ban on DDT’s agricultural use in the United States. A worldwide ban on agricultural use was formalized under the Stockholm Convention on Persistent Organic Pollutants, but its limited and still-controversial use in disease vector control continues, because of its effectiveness in reducing malarial infections, balanced by environmental and other health concerns.
The timing fits. DDT was banned because of fears about its long term persistence in the environment – its ability to spread and continue killing insects, well beyond the location where it was originally sprayed.
But there are other issues which might have contributed to the “rise” in Lyme disease rates.
Nobody actually knows how many people are infected every year with Lyme disease. Estimates are statistical models, based on the number of people who are diagnosed, which the CDC admits are only a tiny fraction of the number of people they believe are actually infected. The bacterium which causes Lyme wasn’t identified until 1982, when it was described by Willy Burgdorfer. Given the variability of Lyme disease symptoms, any diagnosis prior to 1982 must be considered circumstantial at best. Changes to the quality of diagnosis, of which there have been a number in recent years, could potentially have a significant impact on the number of reported cases.
Each year, approximately 30,000 cases of Lyme disease are reported to CDC by state health departments and the District of Columbia. However, this number does not reflect every case of Lyme disease that is diagnosed in the United States every year.
Surveillance systems provide vital information but they do not capture every illness. Because only a fraction of illnesses are reported, researchers need to estimate the total burden of illness to set public health goals, allocate resources, and measure the economic impact of disease. CDC uses the best data available and makes reasonable adjustments—based on related data, previous study results, and common assumptions—to account for missing pieces of information.
… the number of people diagnosed with Lyme disease based on medical claims information from a large insurance database. In this study, researchers estimated that 329,000 (range 296,000–376,000) cases of Lyme disease occur annually in the United States.
Changes to land management may have also contributed. The ticks which transmit Lyme disease to humans hide in leaf litter. During much of the 20th century, controlled burning was unfashionable – fire departments sought to suppress fires, rather than burn off excess fuel. Policies have varied in different places at different times, so it is difficult to match management policies to tick prevalence, but controlled burning when it occurred likely had a massive impact on the tick lifecycle, by burning the leaf litter in which they hide.
It is not impossible that climate has influenced the distribution of Lyme disease. However, concluding that climate has caused a rise in Lyme disease infections in US and Canadian forests seems dubious, without considering the likely significant impact of other contributing factors, or the very real possibility that much of the apparent rise is a statistical artefact, caused by poor historical diagnosis and reporting.
Update (EW) – Katharine also mentions the rapid rise in deer population as a contributing factor
In part, ticks are following the spread of one of their favorite sources of blood: deer. As deer populations exploded over the last sixty years, thanks to strict hunting laws and the largely predator-free and deer-friendly landscapes in New England and the Midwest, deer ticks followed. However, the steady crawl of ticks north into Canada can’t be explained by deer alone.