Lyme disease may surge this year in the northeastern United States and is already spreading into Canada from a confluence of factors including acorns, mice and the climate.The illness is transmitted from mice and deer to humans via bites from the black-legged tick, Ixodes scapularis, usually in forested areas. According to the Centers for Disease Control and Prevention (CDC), Lyme disease is the most commonly reported vector-borne illness in the United States.
A deer tick perched on a blade of grass, waiting for a warm-blooded animal to come by.
Ninety-four percent of cases have been concentrated along the Eastern Seaboard and in Wisconsin and Minnesota. There were more than 20,000 confirmed cases in the United States in 2010, according to the most recent data available.
But now the disease is spreading in unprecedented ways, and public health officials from the United States and Canada are investigating methods to anticipate where it will spring up next.
The bacterium Borrelia burgdorferi causes Lyme disease, and infected individuals experience symptoms like chills, fatigue, muscle aches and often have a bull's-eye-shaped rash. Patients usually recover following antibiotic treatments.
"Many studies have shown that the ticks which transmit Lyme disease bacteria are affected by temperature and moisture," said Rebecca Eisen, a vector-borne disease researcher at the CDC, in an email. "As a result, changes in the geographic distribution and onset of human Lyme disease cases could occur."
The climate also has effects on other organisms involved in Lyme disease transmission. Richard Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies, studied the interplay among oak trees, white-footed mice, ticks and Lyme disease. He forecasts higher infection risks this year based on several factors.
Start with 2010's acorns and 2011's mice
The process generally starts with acorns -- the seeds for oak trees. Producing acorns is very energy-expensive, explained Ostfeld, so an oak tree will only make them every couple of years or so. A bumper acorn crop leads to more mice several months later. Acorn production at the research site reached a record high in 2010 and was followed by a mouse population boom in the summer of 2011.
The mice carry the Lyme disease pathogen that then infects ticks at various stages in their life cycle. Ticks drink blood as larvae, as nymphs and then as adults, so if a baby tick gets infected, it can spread the disease as it matures. "It's a cycle where the pathogen gets transmitted back and forth between wildlife hosts and ticks," said Ostfeld.
Mice and deer are the main hosts for ticks. For Lyme disease, mice are particularly effective carriers because the disease doesn't harm them and their immune systems don't bother fighting off the bacteria. Mice also don't groom or aggressively fight off ticks, making them prone to bites. According to Ostfeld, this means ticks are at least twice as likely to pick up Lyme disease from mice as they are from other animals. However, ticks will gladly sample humans should the need or opportunity present itself. And this year, they might.
Though mice were abundant in 2011, acorn production hit a record low, which will lead to a mouse population crash this year. In addition, this past winter was warmer than usual, so normally dormant ticks are active. Now infected maturing ticks are crawling through forests in the Northeast in search of their next meal. "Humans unwittingly are going to be hosts of a whole heck of a lot of infected nymphs," said Ostfeld.
Add 2012's record warm spring
In Canada, researchers and health officials are more concerned about how these unwelcome migrants from the south are moving further north and into broader areas. "[Lyme disease has] been around for a fair while, but in isolated pockets," said Patrick Leighton, a scientist at the Faculty of Veterinary Medicine at the University of Montreal. "It's really emerging now as more of an actual problem."
Leighton and his team studied Lyme disease patterns in Canada to find out what variables they could use to anticipate outbreaks. He published his findings earlier this month in the Journal of Applied Ecology.
"Currently, the areas where we are seeing a larger tick population is in the eastern part of the country along the U.S. border," said Leighton, who observed that ranges for ticks are expanding by roughly 45 kilometers per year. He said the spread was linked to established Lyme disease hosts like white-tailed deer, suitable forest habitat and warmer temperatures.
"If you look historically, increases in temperature have been important [for Lyme disease]," he said. "The main thing that our study showed was that under warmer climate conditions, ticks move faster."
However, the disease itself doesn't move as quickly as its hosts. "Lyme disease risk lags behind the establishment of ticks by a number of years," Leighton said, noting that newly observed populations could serve as an early warning system for infections.
And you get proliferating infections
With this information, public health officials hope to prevent illnesses. Though Lyme disease has not been very prevalent in Canada in the past, with about 150 cases annually over the past several years, infections are on the rise. "That increase is consistent with what we've seen in the ecology of Lyme disease," said Nicholas Ogden, a senior research scientist at the Centre for Food-borne, Environmental & Zoonotic Infectious Diseases in Canada.
Ogden said eradicating ticks is expensive and difficult, so officials are concentrating on educating people about the disease, how to recognize it and how to prevent infections. "The ticks are living in woodland environments, so by and large, it's people involved in outdoor pursuits and outdoor activities [who are at risk]," he said. These individuals should wear long clothing, tuck pants into socks, wear pest repellents like DEET and check for ticks on their bodies and on their pets when operating in the wilderness.
Over time, disease-carrying ticks are likely to continue spreading, according to Andrea Smith, a biologist at York University in Toronto. "With warming temperatures and increased precipitation in the Arctic, disease vectors will have increased survivability and shorter generation times," said Smith. "On many different levels, climate change is making many different areas inviting to invasive species."
Human movement and development is also likely to accelerate spreading vectors by altering ecologies, according to Smith. Expanding cities and suburbs are pushing against forests, creating more opportunities for infections. By 2080, ticks may move northward by 1,000 kilometers, according to Smith.
However, she acknowledged that it's difficult to predict exactly how these changes will affect humans. Using bioclimatic modeling, Smith said it is possible to prioritize infection risks as Lyme disease slogs north. Still, she said the only real way to stop the illness is to stop animal movement. "We have to be ready to prevent the species from arriving, because once they arrive, it's too late to do anything," she said.
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