Insect-Borne Diseases Have Tripled. Here's Why.
The year 2004 was a simpler time to be an infectious disease doctor in the US. Zika and chikungunya hadn’t yet emerged. Mystery RNA viruses weren’t spreading by tick bite around America’s heartland, killing farmers and ranchers. Certainly no one was on the lookout for a meat allergy caused by a tick with a white splotch on its back the shape of Texas. But that was then.
Since 2004, the number of people who get diseases transmitted by mosquito, tick, and flea bites has more than tripled, according to a new report released by the Centers for Disease Control and Prevention on Tuesday. Between 2004 and 2016, about 643,000 cases of 16 insect-borne illnesses were reported to the CDC—27,000 a year in 2004 (the year in which the agency began requiring more detailed reporting), rising to 96,000 by 2016. At least nine such diseases have also been discovered or introduced into the US in that same timeframe. Most of them are found in ticks. Many of them are potentially life-threatening.
What’s to blame for the surge in reported cases? Warmer weather for one thing, said the agency’s director of vector-borne diseases, Lyle Petersen, during a media briefing. Warmer temperatures allow tick populations to expand into new ranges and set up disease reservoirs where none existed before. Earlier springs and later falls also extend the length of tick season, exposing more people to risks longer. And the warmer it gets, the faster mosquitoes can breed and the higher the viral loads they carry around; outbreaks tend to occur when temperatures are higher than normal.
But the CDC report made no mention of climate change, and Petersen, its lead author stopped short of connecting warmer temperatures to the larger global phenomenon. “I can’t comment on why there’s increasing temperatures, that’s the job of meteorologists,” Petersen told reporters on the call. “What I can tell you is increasing temperatures have a number of effects on all these vector-borne diseases.”
Well, what I (with the help of the meteorologists and climate scientists at NOAA) can tell you is that since 1901 the average surface temperature across the contiguous 48 states has risen at an average rate of 0.14 degrees per decade. Eight of the top 10 warmest years on record have occurred since 1998; 2012, 2015, and 2016 were the three warmest. And 97 percent of actively publishing scientists attribute this trend to human activities. So, I think it’s safe to say we do know why there are increasing temperatures.
When asked why the report did not include potential causative factors for the increase in disease transmission, including the impacts of climate change, a CDC spokeswoman said the purpose of the report was only to examine trends in occurrence. "In the US it remains unclear exactly how much climate and weather change may affect the distribution and timing of infectious diseases, as well as the introduction of new diseases," she said. "Research and surveillance is underway that will help address this important question."
It's true that there isn't a scientific consensus linking known climate change trends to observed increases in vector-borne disease transmission. There are a lot of complicating factors at play. Suburban sprawl, for instance: As people have developed wildlands they’ve fragmented habitats for predators like bears, cougars, foxes, and coyotes. With their natural enemies driven out, blood hosts like deer and rodents are thriving, creating large disease reservoirs exactly where people are mostly to be exposed—at the wildland-urban interface.
There’s also an increased volume of international travel and trade coming overseas to the US. That’s how Zika arrived first in Puerto Rico, and then Florida and Texas in 2016.
Then there’s a bunch of biology that is still far from being understood. “Most of these diseases are super gnarly, complicated systems,” says A. Marm Kilpatrick, a disease ecologist at the University of California, Santa Cruz. “And climate change doesn’t affect them as simply as people make it out.”
It’s not a straight line between temperature and disease temperature, he explains. Warming may increase biting rates and how fast diseases can move through a bug into their salivary glands. But it also shortens their lifespan. At some point, as you go from warm to hot, that effect swamps out the others. So you wind up with a “hump-shaped relationship” instead.
And temperature isn’t the only way weather changes the equation. Last year Kilpatrick’s lab conducted a nationwide study using CDC records and local mosquito data to examine how weather influenced outbreaks of West Nile virus. They found the biggest climate change driver of disease epidemic was actually drought. Even though less water meant fewer mosquitoes, the drought also stressed out birds—the primary virus host. So the net result, however counterintuitive it may be, was an increase in infection prevalence. In fact, Kilpatrick's models suggest increased drought severity could double the number of annual West Nile cases in the near future. “That suggests a simple warming equals more disease story was not borne out,” he says. He thinks it's more likely that local weather and land use changes are combining with other factors to alter the ecology of the underlying transmission cycle, in both vectors and their hosts.
Another confounding factor could be reporting bias. The CDC study suggested that for diseases like Lyme, only one out of 10 cases get reported. When Kilpatrick surveyed a dozen Lyme disease researchers last year, many of them worried that recent surges didn’t reflect rising incidence so much as increased physician awareness. To support this they point out that while ticks are expanding their ranges up into Canada and across the Midwest, those areas aren’t the only places seeing increased transmission.
But others say we’re making a mistake by not responding to the impacts of climate change right in front of us. Goudarz Molaei runs the state of Connecticut’s unique and long-running tick-testing program. Residents who get bitten send in their ticks and Molaei screens them for diseases like Lyme. The research station where he works began this collection service back in 1990. But the biggest changes he’s seen have come in the last few years.
“We’ve had a string of exceptionally warm winters that have resulted in a surge in tick abundance as well as prevalence of Lyme and other tick-associated diseases,” says Molaei. “These things cannot be explained without getting into the issue of climate change.”
That's the bad news, he says. The good news is that this year is actually shaping up to be unterrible, tickwise. Those April and May snowstorms that cold-shocked the Midwest and Northeast means the disease-transmitting season is off to a slow start. People in Connecticut are just beginning to find their first ticks. This time last year Molaei’s office was already inundated; people sent in close to 1,000 specimens in the first four months of 2017.
But that doesn’t mean you shouldn’t take precautions this spring and summer. If you live in the Southeast or in California, places disease-carrying mosquitoes have infiltrated, take steps to keep them out of your home. Replace or repair damaged window screens. Overturn, cover, or throw away items that can hold water where mosquitoes lay their eggs—things like planters, kiddie pools, bird baths, or trash containers. If you’re going into areas where deer live or graze wear long-sleeved shirts and long-pants.
And don’t shy away from insect repellant. It doesn’t have to be DEET, eucalyptus oil will do the trick too. Just do something. Even if scientists don’t agree on what’s causing the uptick of vector-borne diseases, you can still fight back.
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