Ticks and the diseases they carry are on the move, rapidly expanding into new territories once considered inhospitable.
While many factors are to blame, the U.S. government affirmed with "high confidence" in a report that one reason is warmer weather connected to climate change.
In the last decade, the number of cases of Lyme disease in the U.S. have tripled, according to the Centers for Disease Control and Prevention. The risk of this tick-borne disease was historically concentrated in the Northeast and upper Midwest, but a recent study by lab giant Quest Diagnostics found cases of Lyme have been detected in all 50 states and the District of Columbia.
"Lyme disease is a bigger risk to more people in the United States than ever before," said Harvey W. Kaufman, M.D., senior medical director for Quest Diagnostics. "Our data show that positive results for Lyme are both increasing in number and occurring in geographic areas not historically associated with the disease. We hypothesize that these significant rates of increase may reinforce other research suggesting changing climate conditions that allow ticks to live longer and in more regions may factor into disease risk."
Named after the coastal Connecticut town where it was first identified in the mid ‘70s, Lyme disease emerged from obscurity to become the leading vector-borne disease in the U.S.
Lyme disease is caused by the bacterium Borrelia burgdorferi, which typically lives in white-footed mice, chipmunks and birds — all animals that ticks feast on. The disease is transmitted to deer and humans through the bite of an infected tick.
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Lyme can cause fever, skin rashes, fatigue, arthritis-like joint pain and in some cases nervous system complications and brain fog.
Lyme isn’t the only disease that’s spreading. The CDC said state and local health departments reported in 2017 a record number of cases of other tick-borne diseases, including anaplasmosis, spotted fever group rickettsia (Rocky Mountain spotted fever), Babesiosis and Tularemia (rabbit fever).
The agency has also reported an explosion in the population and geographic range of ticks, particularly the blacklegged tick, the primary transmitter of Lyme disease in the U.S. Also known as the deer tick, these blood-sucking arachnids have extended their reach north, south and west — and with it, their illnesses.
Tick Migration and Survival
The deer tick has a two-year life span that is divided into three main developmental stages: larva, nymph and adult. These tiny arachnids require a bloodmeal in every one of these stages for their development, and each of these bloodmeals provides an opportunity for the tick to contract or spread Lyme disease.
A tick’s survival is also dependent on climate.
"If they don’t have a long enough season to find a host, they’ll use up their reserves and drop dead,” said Rick Ostfeld, an ecologist at the the Cary Institute of Ecosystem Studies in Millbrook, New York.
Deer ticks can't reproduce or seek out a host to feast on if the temperature drops below 45 degrees Fahrenheit. Warmer seasons, earlier springs and longer summers in broader parts of the country mean more ticks stay alive through the winter, remain active for longer periods of time and travel further and further north to look for their food.
Ticks' survival are so dependent on environmental factors that the Environmental Protection Agency (EPA) uses the number of cases of Lyme disease as an indicator of climate change.
“Studies provide evidence that climate change has contributed to the expanded range of ticks, increasing the potential risk of Lyme disease, such as in areas of Canada where the ticks were previously unable to survive,” the agency reported. “The life cycle and prevalence of deer ticks are strongly influenced by temperature… Thus, warming temperatures associated with climate change are projected to increase the range of suitable tick habitat and are therefore one of multiple factors driving the observed spread of Lyme disease.”
Across the Northeast, where, over the last three decades, average winter temperatures have risen by almost 4 degrees Fahrenheit, according to NOAA, the cases of Lyme disease have skyrocketed. States from Pennsylvania and northward to Maine are becoming warmer and more humid, creating a favorable environment for ticks to thrive. In 2017, Connecticut, Maine, Massachusetts and New Hampshire had their warmest autumn since record keeping began, the agency reported.
That same year, health officials in Connecticut also discovered that the Lone Star tick, the most common human-biting tick in the southeastern U.S. and Texas, which causes a food allergy to red meat, had reached their shores. In a 2017 press release announcing the findings, Connecticut’s Agricultural Experiment Station (CAER) said their northern range “may be increasing due, in part, to the milder winters the northeast has been experiencing over the past few years.”
Mary Beth Pfeiffer, an investigative journalist and author of the book "Lyme: The First Epidemic of Climate Change," said climate change is abetting the spread of ticks to new frontiers, including Canada.
"They are climbing mountains. They are also climbing latitudes. They are going up the Scandinavian peninsula. They have even been found in parts of Siberia,” Pfeiffer told NBC in a phone interview. "And we’ve had lots of big changes in the ecosystem that's causing ticks to spread."
Lyme Disease Cases Rising
One of those changes is the increase in the developmental rate of ticks due to rising temperatures — and in turn, an uptick in the number of tick-borne illnesses.
A 2015 study found that, as the climate warms, it is pushing the feeding timing of nymphs to earlier in the spring, potentially influencing transmission dynamics.
"When nymphs emerge months before larvae, they inoculate the host community with pathogens that the later-emerging larvae can then contract," said Taal Levi, a biologist at Oregon State University and lead author of the study. "The Lyme disease pathogen is long-lived — it will remain in the host. So an increasing gap between the nymphs feeding in the spring and the next cohort of larvae feeding in late summer will give the nymphs more time to infect the hosts with bacterium that can then be passed to the next generation of tick larvae."
Researchers led by Levi and Ostfeld, the New York ecologist, analyzed nearly two decades worth of data to tie changes in tick emergence directly to climatic changes.
"The climate has clearly warmed," said Ostfield. "That’s not even slightly controversial." And in this one location, at least, ticks have shifted their lifecycles accordingly.
Since 1995, the number of cases of Lyme disease through tick bites reported to the CDC has tripled from less than 10,000 a year to over 30,000 annually. The CDC estimates that the number of infections is actually closer to 300,000 due to underreporting from state health agencies and doctors.
And while the majority of cases of Lyme have historically been concentrated in a cluster of states in the Northeast, the analysis by Quest Diagnostics found that in 2016 and 2017, California and Florida saw the “largest absolute increases in positive test results.” The New Jersey-based lab testing company found infection in California increased 194% over 2015 levels. In Florida, it rose 77% over the same period.
In Connecticut, where a team of scientists around the state are tracking and monitoring the growing tick population, about 50% of the arachnids tested for organisms that cause human diseases have Borrelia burgdorfei, according to Theodore Andreadis, the New Haven-based director of CAER.
Andreadis said this year's tick season is expected to be bad because the winter wasn't severe enough to knock down the population.
"It's always bad here," Andreadis told NBC. "We have so much habitat in the northeast — 60% of the state is forested — so it's a prime environment for ticks to thrive."
More Research Is Needed
The CDC notes that while the exact reason for the geographic spread of ticks and the diseases they carry is unclear, a number of other factors also contribute.
One key driver in the Northeast is the reforestation of land that was once used for farming. Another is the proliferation of the deer population in the Northeast, thanks to stricter hunting laws, fewer predators and the deer-friendly landscape of New England.
Human encroachment into wildlife zones is also factor. With suburbanization, more people are living near the animals that carry Lyme.
“The irony is that we have set this epidemic in motion,” Pfeiffer said. “A warmer world is hospitable to more ticks in more places. Broken bits of forest sustain mice and deer, on which ticks feed and breed. And human development abuts landscapes devoid of predators to curb infection. We’ve created the perfect storm of conditions for ticks to move around the planet.”
The CDC urges people to protect themselves from getting a tick bite by avoiding areas with high grass and leaf litter, walking in the center of trails when hiking, using EPA-registered insect repellents and by wearing long clothes. Shower as soon as possible after coming indoors to wash off and more easily find crawling ticks before they bite you. The agency said people should do full body checks using a hand-held or full-length mirror to view all parts of the body upon returning from the outdoors.
“The likelihood of picking up a tick is quite high so people should be checking for ticks when they come outside from any outdoor activity, including your backyard,” Andreadis said.