Moving farther north each year, ticks are spreading in North America as the climate changes, bringing dangerous diseases with them. Health officials are drawing on the past to forecast where the illnesses might go, but new trends keep emerging, forcing researchers to adapt and revise their models.
One of the most notorious tick-borne infections is Lyme disease, spread by the vector Ixodes scapularis. Initially uncovered in 1975 in Lyme, Conn., the disease spreads from mice and deer to humans via tick bites, leading to fever, joint pains and rashes.
Caused by bacteria from the Borrelia genus, the pathogen leads to about 30,000 reported cases in the United States, making it the most common vector-borne disease in the country, according to the Centers for Disease Control and Prevention. However, the CDC acknowledged that the true number could be as high as 300,000 cases, since the disease is difficult to recognize and is underreported.
Now Canadians are worried, too. The Public Health Agency of Canada added Lyme disease to its list of reported diseases in 2009.
"Risk is emerging in parts of Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia and has been present in southern British Columbia for several years, due to spread of populations of the ticks that carry the bacterium that causes Lyme disease," the agency said on its website.
Six Pacific Coast Ticks (Dermacentor occidentalis) clustered on a blade of grass, ready to attach themselves to hikers at Harmony Headlands State Park, Harmony, CA, 12 June 2011. (Credit: Flickr/Mike Baird)
Canada's tick population to explode
In a study published in advance last month in the journal Environmental Health Perspectives, scientists tracked how hospitable regions for ticks shifted between 1971 and 2010, with an eye toward changes in temperature and rainfall.
"The novelty of this study is that we were able to quantify the potential effects that climate change will have on the capacity of the blacklegged tick (the tick that transmits Lyme disease) to reproduce," said Nicholas Ogden, a co-author and a senior research scientist at the Centre for Food-borne, Environmental and Zoonotic Infectious Diseases in Canada, in an email. "Further, the study explored how future climate change may [affect] the risk of acquiring Lyme disease as well as other vector-borne diseases in Canada."
The researchers found that climate change will push tick-borne diseases into new regions and will increase infection rates in areas where the disease has already settled in. Vector reproduction will increase as much as fivefold in Canada and twofold in the United States.
Though other environmental factors like urbanization and deforestation also influence these problems, warming temperatures between spring and autumn push more ticks through their life cycles faster, increasing bite risks for humans.
"Climate may affect ticks in a number of ways as their survival depends on the environment being humid and warm enough," Ogden said. "We believe this is a key factor in determining where tick populations can become established in Canada."
In another study, published last month in the journal Emerging Infectious Diseases, scientists uncovered Lyme disease as well as a new pathogen spread by ticks in the San Francisco Bay Area, indicating that the illness spreads in areas beyond New England and the Midwest in the United States.
"Several people in the neighborhood tested positive for Lyme disease, and people were nervous they were in an area of increased risk," said Daniel Salkeld, a disease ecologist at the Stanford Woods Institute for the Environment and a co-author of the report.
Hunting for the unknowns
Dragging white flannel blankets through some of the region's parks and picnic spots, the team collected ticks and studied what bacteria they carried. Analyzing their DNA, the researchers confirmed the presence of the Lyme disease bacterium, Borrelia burgdorferi, and a new pathogen, Borrelia miyamotoi.
"I think it's been in the background for a long time," Salkeld said. "We went looking for it, and we found it."
The findings highlight how much researchers still have yet to understand about tick-borne disease dynamics while raising concerns for doctors who might not have these infections on their radar.
"What you get is the appearance of the infected ticks in [parts of the country] where there's been little, if any, history of Lyme disease and no one knows what to look for," said Richard Ostfeld, a senior scientist at the Cary Institute of Ecosystem Studies.
Part of the reason is that the disease itself is new. "We only have data going back a few decades," Ostfeld explained.
With the benefit of hindsight, researchers are evaluating their past predictions for Lyme disease. In 2012, Ostfeld projected a high outbreak risk, based on converging variables like a warm preceding winter and a crash in the mouse population that commonly hosts ticks (ClimateWire, March 23 2012).
"Our prediction was partially but not fully borne out," Ostfeld said. "We had a record hot, dry spring in 2012. Very warm and very dry conditions in spring are known to have a negative impact on the tick population. This might have nipped a potential outbreak in the bud."
Getting better forecasts is critical because there are still many people who aren't familiar with tick-borne diseases, so health officials need to target their information campaigns to get people to be careful around wooded areas and check their clothing and pets for ticks. Climate-based projections are also important for interventions that are more aggressive. "We haven't figured out where to apply [pesticides] and over what spatial area to apply them to really get a protective impact on public health," Ostfeld said.
The goal now is to develop models that track ticks based on their feeding, reproduction and movement mechanisms instead of just using correlational data from the past. Researchers are also looking into the reservoirs for Lyme disease, namely mice and deer, to see how their behavior under climate change influences outbreaks.
Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500.
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