Chagas, dengue, and chikungunya are likely to become more familiar words in the United States in coming decades. Once limited to climates more tropical than our own, these infectious diseases are now increasingly affecting Americans.
There have always been a handful of Americans who return from tropical travel infected with these diseases. But experts think it’s likely that they will take root here.
As the climate continues to warm, the insects that spread these diseases from one person’s bloodstream to another’s will inhabit larger swaths of the map, according to the United Nations International Panel on Climate Change (IPCC). The habitats of the mosquitoes that carry dengue and chikungunya are already expanding in the United States.
“Climate change is definitely having an effect on parasitic disease worldwide,” said Patricia Dorn, Ph.D., a Chagas expert at Loyola University New Orleans. But, she said, we’re also getting better at testing for these diseases, so part of what looks like an uptick in cases is really an uptick in diagnoses.
For patients, the difference is academic: They’re being diagnosed with diseases they may have never heard of — and their doctors may not know to look for them.
Chagas Disease: It’s Not Just in Rural Areas
Chagas is the latest addition to the list of tropical diseases that Americans have to fear. The disease initially has few symptoms — possibly a fever, rarely some swelling at the site of the bug bite — but if left untreated the parasites accumulate in the cardiovascular system and cause heart damage in 1 in 3 patients.
Triatomine, or “kissing,” bugs spread Chagas disease — though their nickname puts an overly positive spin on what they do: suck the blood of a mammal host and then defecate. The feces spread the parasite that causes the disease.
Chagas is widespread in Latin America and has rarely been considered endemic to the United States, but experts say that’s because we weren’t looking for it.
In 2007, Chagas was added to the list of tests performed on donated blood. Those who tested positive were contacted and interviewed. It suddenly it became clear that at least a few of the 300,000 Americans who tested positive for Chagas disease had not traveled to Latin America.
Bugs in the triatomine family capable of spreading the disease inhabit the bottom two-thirds of the United States. The now-outdated conventional wisdom said that the bugs, which bite at night, only fed only on humans living in thatched huts in rural areas.
“The dogma for years was, ‘We live in improved housing; we have air conditioning — yeah, the bugs are here but they live in wooded areas and we’re just not in contact with them.’ Our most recent study shows that that dogma is wrong,” Dorn said.
A forthcoming study co-authored by Dorn documents that triatomine bugs in Louisiana often feed on human blood. Among the bugs that had fed on humans, 4 in 10 tested positive for the Chagas parasite.
A Texas study published in October used blood donation data to identify five people who had been infected with the virus in Texas.
As the climate continues to warm, triatomine bugs will likely push further northward. And as forested lands are cleared, the bugs are likely to feed more often on humans.
Just this week, a study suggested that bedbugs could spread the parasite, too. But Dorn and Melissa Nolan Garcia, M.P.H., who co-authored the Texas study, said those findings don’t mean much on the practical level.
Although bedbugs are seemingly capable of transmitting the parasite, they probably don’t, based on empirical data. Among different triatomine species, some are far more efficient at transmitting the Chagas parasite, and even the most efficient do so only once in a thousand bites.
“We think that our bugs have better manners,” said Dorn, referring to U.S. triatomine bugs. “In Latin America the better vector takes a blood meal and defecates at the same time. The studies that we did showed that the bug took the blood meal — it was from a mouse — and left the host and defecated later.”
What does all this mean for patients? It means that those who hunt and camp in the South and those who have traveled to Latin America should get screened for Chagas.
“It’s a simple blood test,” Nolan Garcia said.
However, U.S. doctors don’t necessarily know what to do about Chagas. Dorn told the story of a California woman who got a “really scary letter” telling her that she’d been permanently banned from blood donation after testing positive for Chagas. But when she went to her doctor, he didn’t know how to treat her.
The only two drugs to treat Chagas disease are available through the Centers for Disease Control, which has dubbed Chagas a “neglected parasitic infection” and targeted it for public health action.
Nolan Garcia compared treatment with these drugs to “chemotherapy.”
The parasites settle in the heart tissue, and to eliminate them, the drugs have to kill off some healthy tissue too. The treatment is recommended for patients younger than 50 and on a case-by-case basis for those older than 50.
Dengue Fever: It’s Creeping up from Latin America
Dengue is a mosquito-borne viral infection so painful that it’s also called “breakbone fever.” Found throughout the world in tropical and subtropical regions, it’s rarely fatal. Those infected a second time with a different strain of the virus risk developing a more severe form of the disease called dengue hemorrhagic fever; without good medical care, 1 in 5 patients die from this form of the disease.
Dengue has become more common since the 1950s. In the Americas, it’s become at least five times more prevalent since the 1980s. But only in this century has it crept into the United States. Hawaii saw a cluster of infection in the early 2000s, but in recent years Texas and Florida have been U.S. hotspots.
Last year, there were 49 confirmed cases of locally transmitted dengue in the United States. So far this year, all of the hundreds of reported dengue cases along the U.S.-Mexico border have come in from Latin America, Texas public officials told Healthline. However, Florida has confirmed six local cases.
Some counties in Mississippi may also be at “extreme risk” for domestic transmission, experts say.
The mosquitos that carry the dengue virus, Aedes aegypti and Ae. albopictus, are expanding their range, in part due to climate change. As they do, more Americans will be at risk.
“I do think it’s a real possibility that we could have sustained transmission in parts of the U.S., in the southern U.S. in particular, and in parts with lower socio-economic status,” said Crystal Boddie, M.P.H., at the Center for Health Security at the University of Pittsburgh Medical Center in Baltimore.
Window screens, air conditioning, and an indoor job all diminish one’s risk of infection.
The United States won’t be hit as hard as developing countries, where screens are rare and air-conditioning is a luxury reserved for the very rich. But as dengue becomes more common, U.S. doctors will have to learn to diagnose it and provide the right kind of palliative care.
“The initial symptoms are pretty similar to the flu. It’s hard to distinguish if it’s dengue or flu,” Boddie said. “It’s difficult to diagnose if you’re not looking for it specifically.”
The good news is that, because dengue is so widespread in other parts of the world, researchers have been trying to develop a vaccine. Five candidates are currently being tested in advanced clinical trials.
Chikungunya has symptoms similar to dengue and is spread by the same two types of mosquito. The two diseases often arise in the same places, but more people get sick from chikungunya because infected mosquitos are about three times as likely to transmit it.
The disease is less likely to be fatal than dengue, but it’s painful enough to have earned its own colorful name. Chikungunya means, roughly, “writhing disease” in a local language of East Africa, where it first emerged.
“It’s not a trivial disease,” said Roger Nasci, Ph.D., the chief of the Arboviral Diseases Branch at the CDC. He described “prolonged, debilitating joint pain where people just can’t get out of bed.”
About 1 in 3 chinkungunya patients will experience joint pain for months or even years after they recover from the initial infection. There are currently no medications to treat chikungunya and there is no vaccine to prevent it.
Until chikungunya entered India in 2005 and infected 2 million people, it wasn’t on the global health community’s radar. Last year, it earned still more attention when it jumped the Atlantic, sickening people on several Caribbean islands. This year, there have been 11 cases of locally transmitted chikungunya in Florida.
Public health officials say more domestic infections are “inevitable.”
The CDC is trying to educate doctors to make them aware of chikungunya and dengue. Beginning in 2010, doctors were required to report cases of dengue. On Jan. 1, 2015, they’ll be obligated to report chikungunya cases as well.
Mosquitoes breed in pools of standing water, so removing these breeding sites is a top priority. Nasci said the CDC is working with local mosquito control programs to try to limit the number of tires, buckets, and other outdoor sites that collect rainwater. In that one respect, climate change may actually help curb mosquito populations, as California and other Western states experience record droughts.
Healthline News | Cameron Scott | November 20, 2014