Scientists tackle the ‘difficult problem’ of how warming spreads Lyme disease

By Brittany Patterson | 05/04/2015 08:12 AM EDT

Climate change is increasing not only the range in which Lyme disease-carrying ticks can survive — ticks are moving into warming Canada and other northern locations — but the amount of time in which ticks can feed, according to a recent study published in the journal Philosophical Transactions of the Royal Society B.

In 2008, Rachael French, a then-healthy 22-year-old, vacationed with friends in a cabin in the woods near Gaylord, Mich.

On the last day of the trip, while at a local water park, French said she felt the sting of chlorine on an open wound, looked down and noticed a small scab on her thigh. She figured it was a spider bite.

Within hours, she remembers feeling nauseated, sore and exhausted, but chalked it up to having a busy vacation. Things became a bit foggy from there, she said.


When she was admitted to urgent care, French had a 103-degree temperature, was lethargic and in pain all over her body, and had trouble forming sentences.

"The doctor asked if he could look at the bite, and when I showed him I noticed there was a ‘bulls-eye’ ring around the bite, which I hadn’t noticed before," French said in an email. "It was so large it covered about half of my thigh."

The doctor removed the scab and showed French where a tick had buried itself into her skin.

"Everything happened so fast," she said. "One minute I’m in a cabin with a few friends having a great time, the next I’m being diagnosed with Lyme disease — an incurable disease."

Blacklegged ticks may be coming to the woods near you.
| Photo courtesy of Wikipedia.

Most people will make a full recovery after a course of antibiotics, but between 10 to 20 percent of patients — including French — report lingering symptoms in a condition often called "chronic Lyme disease."

The number of confirmed cases of Lyme disease in the United States has been on an upward trend, according to data from the Centers for Disease Control and Prevention. In 1995, there were 11,700 confirmed cases. In 2013, there were 27,203 confirmed cases, as well as another 9,104 probable cases. Ninety-five percent of reported cases in 2013 came from 14 states — located primarily in the Mid-Atlantic, Northeast and Upper Midwest. Lyme disease is the most commonly reported vector-borne disease reported in the United States.

Climate change is increasing not only the range in which Lyme disease-carrying ticks can survive — ticks are moving into warming Canada and other northern locations — but the amount of time in which ticks can feed, according to a recent study published in the journal Philosophical Transactions of the Royal Society B.

This finding — a result of an extensive 19-year field study of blacklegged ticks — has led researcher Richard Ostfeld to conclude that Lyme Disease Awareness Month should be moved up from May to April.

"In order to get the greatest efficacy from Lyme disease public awareness campaigns, we want to do them at the right time," said Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies. "We’re arguing that ticks are on the move and becoming active earlier and earlier as the climate warms."

A disease for which timing is everything

Ostfeld and a team of researchers conducted an emergency analysis using observations of tick life-cycle behaviors from more than 50,000 mice, 12,000 chipmunks, 403,000 larval ticks and 44,000 nymphal ticks collected in the 2,000 acres of forest that is part of the Cary Institute’s campus in Millbrook, N.Y., which happens to be a tick hot spot.

The researchers concluded that in warmer years, ticks emerged as much as three weeks early, which poses a risk to public health.

To understand how climate change is influencing ticks, first researchers needed to document the species’ life cycle.

"We think about these ticks because we find them crawling on our bodies," Ostfeld said. "We see them in little snapshots, but they have long and complicated lives."

The parasite’s life is consumed by feasting on blood just three times in its 2-year life span.

The cycle begins when tick larvae hatch out of eggs, typically around August. Ticks are born free of the bacterium Borrelia burgdorferi, which causes Lyme disease.

At the larval stage, they eat one blood meal from a host — they especially like white-footed mice — for about three to four days. It’s the host that larval ticks feed from that is the carrier of Lyme disease, and infects ticks.

Larval ticks become nymphs and eat another single blood meal, this time for between four and five days. This is the stage at which almost all human cases of Lyme disease are transmitted because the blood meal in this case is often from a human. The nymphal stage typically occurs between May and June, and if the tick’s meal isn’t a human host, chances are it is some other creature, which would then be infected with Lyme disease.

"Those nymphs go around biting the next generation of mice and small animals and inoculating them," Ostfeld said.

Nymphal ticks then molt into adults and feed for between a week and 10 days. Then, they drop off their host and reproduce. The females lay eggs, and then the adults die.

The timing of the tick life cycle is why Lyme disease exists, Ostfeld said, and if warming temperatures change the timing, climate change could have a multiplying effect on the spread of the disease.

"If larvae and nymphs fed at the same time — which is mostly the case in parts of Europe — then a lot of the new baby larvae ticks would be feeding on mice that hadn’t been exposed," he said.

But with a warming climate, Ostfeld’s data showed, what is happening currently is opposite of what’s going on in Europe. Larval ticks are coming out earlier and infecting more hosts, and are thus increasing the overall number of Lyme disease-carrying creatures.

As climate warms, ticks move north

To understand the link between climate warming and Lyme disease, it’s also important to understand the disease’s history. In the fall of 1975, two mothers from Old Lyme, Conn., reported what appeared to be cases of rheumatoid arthritis in children.

Something wasn’t right with a rheumatoid arthritis diagnosis, and the mothers reached out for help. Researchers from the Yale School of Medicine began cataloging the children’s symptoms and realized that they first appeared in summer, the height of tick season.

Several children reported having had a skin rash just before developing the arthritis, and many of them recalled having been bitten by a tick where the rash appeared, according to an article in the Yale Journal of Biology and Medicine about the history of Lyme disease.

The term Lyme disease was coined to help doctors diagnose patients, and then in 1981, researchers with the National Institute of Allergy and Infectious Diseases at Rocky Mountain Laboratories in Hamilton, Mont., identified the bacterium that causes the disease and discovered the connection between the deer tick and the disease.

The ticks originally came from Long Island, N.Y., although they had probably lived on the mainland United States before colonists cleared the forests (and with them the deer) in the Northeast, said Durland Fish, a professor of epidemiology at the Yale School of Public Health who studies the disease.

When deer were reintroduced along the northeastern coast, the ticks came with them.

"The reason we have an increasing incidence of Lyme disease every year is because tick populations are expanding and exposing more people to tick bites," Fish said.

Fish was a co-author of one of the first studies to examine the effects of climate change and Lyme disease risk, published in 2005. Using a climate-based logistic model, the research team determined that by the 2080s, blacklegged ticks would see a 213 percent increase in suitable habitat in Canada. Ticks would also shrink in range, away from the southern and central United States.

The challenge to mapping the exact range and timing of the coming tick invasion is that most models doing this work are just based on temperature, Fish said, adding that there hasn’t been enough funding from the federal government specifically aimed at connecting the effects of climate change with infectious diseases like Lyme disease.

Ostfeld said one challenge he sees to charting how exactly climate change is going to affect Lyme disease is that it is nearly impossible to separate what are and are not climate effects within these ecosystems, which are very complex.

What makes ticks tick? A host of researchers want to know

"These ticks are known to be expanding their ranges in a bunch of different directions, and so some of their spread doesn’t seem to be correlated with climatic conditions," Ostfeld said.

And as far as ways in which the climate is affecting ticks, Ostfeld said, researchers are still working to determine what combination of climatic variables is driving the changes.

"Is climate change important because extreme conditions kill them outright or something more general like it affects the length of the growing season?" he asked. "It’s a difficult problem." But it’s one, he added, that a host of smart people are working on.

For French, the disease remains a daily struggle. For example, no matter how many hours of sleep she gets, she says, she is always exhausted, and her short-term memory is terrible. Sometimes the pain is so bad that "if I were to brush up against someone, I would shudder in pain," she said.

"With this disease, I’ve become a great actor," French said. "I’m very good at smiling and pretending I’m fine, while inside my body and spirit are at war with one another."

Yet raising awareness of this disease has been a priority. Often, she wears a bright green awareness ribbon pinned to her clothing, a symbol of her fight with the disease. One day, she hopes to attend the annual Mayday rally in Washington, D.C., to stand with other "Lymies," as she calls them, to rally for more awareness of the disease.

"It’s alarming how many people have never heard of this debilitating and growing epidemic," she said. "Six years ago, I was one of those people."