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Tick Discovery Highlights How Few Answers We Have About These Pests In The U.S.

by Melinda Wenner Moyer: As Lyme and other tick-borne diseases worsen in the U.S., new efforts seek to fill in the gaps…

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Last August Tadhgh Rainey, division manager at the Hunterdon County Division of Public Health Services in Flemington, N.J., received an odd phone call. An assistant in a nearby building excitedly explained to him that a woman had walked into their office covered in tiny ticks—far more than they had ever seen on one person. “She’s really scared—and now we’re kind of scared, and her pants are in our freezer,” the assistant said.

The woman, a local farmer, had been shearing her sheep when she realized she was covered in black, crawling spots and booked it to the health department. She had a change of clothes in her car and gave the health office staff her bespeckled pants; they threw them in the lab freezer to kill the ticks. Later that day Rainey headed over to his colleague’s building, where the staff took the pants back out of the freezer and shook the clinging ticks out into a pan. They saved about a thousand—although the ticks were dead, Rainey wanted to keep them for further testing. “I took a look at them, just thinking, ‘Well, that’s an awful lot of ticks,’” Rainey recalls. “It was almost exhilarating—to the point where you wanna blow up the building, though.”

The collected ticks were larvae, the youngest stage in their two-year life cycle, which are too tiny for humans to see in any detail. Yet to Rainey they did not seem like the black-legged ticks common in New Jersey, because those do not travel in packs of thousands and do not usually feed on sheep. Rainey reached out to colleagues but they also had no idea what type of ticks they were. A month and a half later, Rainey ran into his acquaintance Andrea Egizi, a biologist at Rutgers University’s Center for Vector Biology, at a conference, who offered to sequence the ticks’ DNA. It came back as a perfect match for Haemaphysalis longicornis, a species of tick Egizi had never heard of. “The first thing I did was go to Google,” she says. “The more I read about it, the more I realized: This could be big.” She had confirmed an invasive, exotic tick species had found its way to the U.S., seemingly for the very first time.

Approximately 90 tick species call the U.S. home but H. longicornis, called the long-horned tick—the Latin longicornus translates to “long-horned”—has never been one of them. It’s usually found in east Asia, Australia and New Zealand where it primarily hurts cattle—causing considerable blood loss and limiting their milk production. The species is not known to carry Lyme disease. Nobody seems to know how the long-horned tick wound up in New Jersey: Neither the farmer nor any of her sheep have ever traveled to those locales. Other than Hunterdon County, H. longicornis has also been spotted in three other New Jersey counties as well as in West Virginia, Virginia and Arkansas.

It’s unclear how concerned we should be that these migrant ticks are now calling the U.S. home. As of yet there is no evidence they have bitten any humans or harbor diseases, but “pathogens can adapt to new vectors,” Egizi says, so it is possible the ticks could start carrying and transmitting endemic tick-borne diseases such as Lyme. In Asia this tick has also been known to transmit potentially deadly viruses including ones that cause Japanese spotted fever and severe fever with thrombocytopenia syndrome—conditions that can cause fever, vomiting and multiple organ failure.

Rainey’s long-horned tick story is a “serendipitous discovery of an exotic, dangerous tick that apparently landed here and spread years ago but went undiscovered until some poor sheep got plastered by it,” says Richard Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies, who was not involved with the long-horned tick discovery case. Among other things, the tale emphasizes the country’s dire need for more and better tick surveillance. Scientists also know very little about which interventions actually reduce tick-borne disease risk and many have not been given the funding needed to address these key data gaps. But the long-horned tick discovery and other mounting tick concerns are inspiring a much-needed surge in tick research interest and funding.

Many U.S. states have active surveillance programs for mosquitoes—and have had them for a long time. In fact, the U.S. Centers for Disease Control and Prevention was established in 1942 specifically to prevent the spread of malaria. And in 2016, in response to the Zika threat, the CDC launched MosquitoNET, a program in which state or local government employees use traps to capture mosquitoes and then report the location and species of those they identify. The CDC provides these entities with recommendations on where, when and how to use traps, and how frequently to check them; then it uses their data to regularly update mosquito distribution maps and guide additional surveillance and control efforts. But “right now there’s not a single organized coordinated effort at tick surveillance,” says Ben Beard, chief of the CDC’s Bacterial Diseases Branch in the Division of Vector-Borne Diseases.

That’s not to say that there’s no data at all—it’s just that the information is incredibly limited. For instance, the CDC’s online tick distribution mapfor black-legged ticks, the only vector for Lyme disease, is based mainly on data from a 2016 study. In it CDC researchers combed through published studies and surveyed U.S. counties to determine if health department employees or other local tick investigators had identified at least six black-legged ticks or seen at least two of the three black-legged tick stages in the previous year. If they did, the authors categorized the ticks as being “established” there; if at least one tick but fewer than six had been seen that year, they were categorized as “reported.” The map they created based on these data suggests black-legged ticks are now present in nearly half of all U.S. counties. They also found these ticks have spread and expanded since the last such surveillance study was conducted in 1998. Yet still the study provides only a snapshot of tick presence and does not provide any information on how many there are, how the populations are changing or what life stages are most common (which is important, as it is the tick nymphs, not the larvae or adults, that are primarily responsible for transmitting Lyme to people). Other ticks, such as lone star ticks, carry diseases, too, including Rocky Mountain spotted fever, ehrlichiosis and tularemia; yet we know even less about where these ticks are or how their populations are changing.

The dearth of information on where ticks are and their activities is worrisome, considering that the burden of tick-borne disease in the U.S. is so high—much higher, in fact, than it is for diseases carried by mosquitoes. Only 2,149 Americans in 2016 were reported to have been infected with West Nile virus, which the CDC refers to as the country’s “major mosquito-borne disease.” (In epidemic years, however, case numbers have been as high as nearly 10,000.) Yet at least 300,000 Americans are estimated to contract Lyme disease each year, which is only contracted from tick bites.

Certainly, mosquito-borne diseases are a growing threat here partly due to climate change—but tick-borne diseases are increasing in the U.S., too, for various reasons including shifts in land use and climate change. A May 2018 CDC report found the incidence of tick-borne disease in the U.S. more than doubled between 2004 and 2016, and in some states the increase was more worrying. In Maine for instance, from 2006 to 2016, the incidence of Lyme disease quadrupled. And although most tick-borne diseases in Americans are thought to be acquired domestically, many of the mosquito-borne diseases that infect Americans, including more than 90 percent of Zika, dengue and chikungunya cases, are actually acquired outside the continental U.S.

Knowing where ticks are is just one piece of the tick-borne disease puzzle—yet researchers do not understand many other ecological aspects, either. In fact, tick researchers vehemently disagree on a number of crucial aspects of tick and tick-borne disease ecology, including which animal hosts—deer or small mammals including mice and chipmunks—are most important for the spread of Lyme disease.

There’s another big difference between how the U.S. approaches containment of ticks versus mosquitoes, too: State and county governments often intervene to control mosquito populations, yet they leave tick populations alone. “Local municipalities frequently have mosquito control districts funded by some sort of taxpayer base,” Beard says. “But the really interesting thing is if you have a problem with ticks in your backyard, typically you’re the one who deals with that. It’s not a public-owned problem; it’s a burden that’s borne on the shoulders of homeowners.”

And no clear solutions exist for such homeowners. The CDC recommends people who encounter ticks undertake a number of preventative measuressuch as using insect repellants and showering after being outdoors. Yet “we have very little data to support the fact that these interventions actually reduce human illness,” Beard says—a few studies have shown such interventions can reduce the number of ticks or tick bites, but it is unclear whether they actually keep people healthier. In a double-blind trial published in July 2016, for instance, researchers at the CDC and other institutions treated yards in tick-infested areas with insecticides while treating others with a placebo. Over the next two years, compared with the untreated yards, the treated ones had 63 percent fewer nymph-stage ticks—yet families living in the treated homes were no less likely to be diagnosed with Lyme nor did they get bitten by fewer ticks, perhaps because people tend to get bitten by ticks when they are off their property. “There are lots of things that kill ticks,” Ostfeld says, yet “none of them has been demonstrated to do it well enough and in the right places to reduce incidence of tick-borne disease.”

Finding such answers would require more funding support, experts say. A search of the National Institutes of Health’s Research Portfolio Online Reporting Tools shows there are 412 active NIH-funded research projects that can be found using the search word “mosquito,” but only 133 that can be found searching on “tick.” The NIH also awarded only $23 million in funding for research related to Lyme disease in 2018 yet gave nearly twice as much toward research on West Nile virus. Part of this problem might be historical: Mosquito-borne diseases have threatened the U.S. for centuries whereas tick-borne diseases have only been known as a menace in recent decades; the first case of Lyme disease was diagnosed in 1975. Lyme research tends to be skewed toward the clinical aspects, too—diagnosis, therapy and pathogenesis, notes Durland Fish, an entomologist and professor emeritus at the Yale School of Public Health. “Everything’s focused on humans, not on the environment where the diseases are contracted,” he says.

We have reasons to feel optimistic, though. Beard says the CDC is in the process of creating a nationwide tick surveillance program, which could be launched within the year; they are still working out the details of how, exactly, it will operate. The Department of Defense and some private donors have started funding domestic tick-borne disease research, too. With the aid of a $5-million grant bestowed by the Steven and Alexandra Cohen Foundation as well as additional funding from the CDC and other sources, Ostfeld and colleagues at Bard College are conducting a randomized controlled study in Dutchess County, N.Y.—an area which has one of the country’s highest rates of Lyme disease—to evaluate the efficacy of two tick control methods compared with placebos when they are applied over a four-year period. One intervention is tick boxes, which are placed outside and attract the small mammal hosts such as mice that feed young ticks. When the animals enter, they are doused with a small dose of a tick-killing chemical. The other intervention is a readily available natural fungus-based spray known as Met52, which also kills ticks. The study will evaluate how well these methods, used together or separately, reduce the risk of Lyme when deployed on a neighborhood-wide scale.

As for the long-horned ticks, they remain an ominous reminder of how much we have yet to learn about these blood-sucking parasites—despite efforts to eradicate them in New Jersey, they survived the winter just fine. “There’s a lot of really important and useful research that’s been done—it’s not as though there are a lot of slackers out there who haven’t been doing anything. It’s just that in my view it’s not enough,” Ostfeld says. Invasive, exotic ticks suddenly appear and scientists have no idea where they came from or what to do about them? “The depth of understanding,” he says, “leaves a bit to be desired.”

Source: Scientific American

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