Can America Cope with a Resurgence of Tropical Disease?

Carrie Arnold, Medical Xpress, August 11, 2015

One rainy Friday morning in March 2015, Dr Laila Woc-Colburn saw two patients with neurocysticercosis (a parasitic infection of the brain) and one with Chagas disease, which is transmitted by insects nicknamed ‘kissing bugs’. Having attended medical school in her native Guatemala, she was used to treating these kinds of diseases. But she was not in Guatemala any more–this was Houston, Texas.

For half a day each week, one wing of the Smith Clinic’s third floor in Houston is transformed into a tropical medicine clinic, treating all manner of infectious diseases for anyone who walks through the door. Since it opened in 2011, Woc-Colburn and her colleagues have treated everything from dengue and chikungunya to river blindness and cutaneous leishmaniasis. Their patients are not globetrotting travellers, bringing exotic diseases back home. The Smith Clinic is a safety net provider, the last resort for healthcare for people on low incomes and without insurance. Many of their patients haven’t left the Houston area for years.

This suggests that what Woc-Colburn sees in the clinic may be just the leading edge of a gathering crisis. Diseases once associated with ‘elsewhere’ are increasingly being found in the southern states of the USA. Infectious disease physician Peter Hotez was so concerned that he founded a school of tropical medicine at the Baylor College of Medicine in Houston, well within the territory that Hotez argues is one of the world’s ten hotspots for so-called neglected tropical diseases.

“While we were calling them neglected tropical diseases, the ‘tropical’ part is probably a misnomer,” says Hotez. “Most of the world’s neglected tropical diseases are in wealthy countries. It’s the poor living among the wealthy.”

Poverty is a critical factor. But the American South’s hot and humid climate, the influx of insects that carry diseases, and the ever-increasing movement of humans have combined with the region’s high poverty levels to create a perfect storm of disease. While we might already have seen some of the storm’s effects, the worst has yet to arrive.

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As tropical diseases faded from medical consciousness in the rich countries of the world, they also vanished from public awareness. By the dawn of the millennium, most tropical diseases had become profoundly neglected, although many of them were still rife in low-income countries. Perhaps this entrenched the idea that they were distant problems: if Americans thought about them at all, it was as diseases of other countries far away, not something to be found in their own backyards. Although travellers and immigrants occasionally needed treating, tropical disease was essentially a thing of the past for the USA.

For those in the know, there were warning signs. Not enough to set alarm bells ringing, but indications nevertheless that something serious was in the offing.

Paediatrician Morven Edwards, a soft-spoken petite woman in her 60s, is an infectious disease consultant at Texas Children’s Hospital in Houston. She’s the doctor other doctors call when they can’t figure out what’s going on with their patients. In 2010, she took a call from a local paediatrician whose 17-year-old patient had tested positive for Chagas disease after donating blood for her high school blood drive.

Edwards knows a few things about Chagas: it is caused by the parasite Trypanosoma cruzi and spread by a group of insects called Triatoma, or ‘kissing bugs’ (because they like to bite near the mouth). Most people with Chagas don’t know they’ve been infected but that doesn’t mean damage isn’t occurring. Over time, the T. cruzi parasite can chew through cardiac muscle and cause heart failure. Chagas is a major problem in Latin America, where an estimated 8 million people are infected. It’s also one of the leading causes of heart failure in the region and causes an estimated 11,000 deaths each year.

Edwards’ new patient, however, had never been to Latin America. She had never even left the USA. She was, Edwards explains, “your All-American girl”, hardly a candidate for a disease that mainly infected poor, rural populations in Central and South America. Edwards repeated the blood test, just to be sure. Again it turned up positive for Chagas. Given the girl’s young age and the high risk for heart failure if the disease is untreated, Edwards decided to treat her with anti-parasitic drugs.

The case has stayed with Edwards for several reasons, the main one being the mystery of how the girl became infected if she’d never left the country. But south Texas is home to the same kissing bugs that transmit Chagas. The answer, then, had to be this: she had been infected with Chagas in Texas. And she was unlikely to be the only one.

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Results from studies that tested donated blood for Chagas support the idea that it is a major problem in Texas. A 2014 study showed that one out of every 6,500 people who donated blood screened positive for Chagas–almost 50 times more than the CDC’s estimate that one in 300,000 Americans was infected. Although the results can’t prove where someone was infected, interviews by Baylor College of Medicine epidemiologist Melissa Nolan Garcia revealed that a substantial portion had no known risk factors for Chagas disease and had never spent an extended time in another area where Chagas was known to be present.

In 2010, disease ecologist Sahotra Sarkar at the University of Texas published some of the first work showing that much of southern Texas was indeed at “very, very high risk” for Chagas disease. A physicist and philosopher by training, Sarkar transferred his skills to modelling disease risk across geographic areas. He teamed up with Edward Wozniak at the Texas Department of State Health Services to determine the distribution and density of triatomine bugs across the state and the proportion of them that were infected by T. cruzi, the Chagas parasite.

A trio of studies published beginning in the early 2000s had suggested that more than half of the triatomine bugs in Texas were infected with T. cruzi. Sarkar’s work used this infection rate, along with the population density of both triatomine bugs and humans, to create a Chagas disease risk map for Texas. The southeastern tip of the state, curved in a ‘C’ shape and bordered by the Rio Grande in the south and the Gulf of Mexico to the east, was at the highest risk for Chagas. Other areas, such as the bayous of Louisiana and San Antonio, Texas, also saw unusually high risk.

To Sarkar, the message was clear: the mixture of parasite-infected bugs and lots of people meant that Chagas disease was endemic in Texas. But, he says, “the response from physicians was almost complete silence because they didn’t even know this disease could be a serious issue in Texas.”

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It is the nature of these diseases–neglected diseases, diseases of poverty, call them what you like–that they can go unnoticed for years, chewing away at the health of individuals and communities. As poverty, geography, climate and social factors combine to bring tropical diseases out of hiding once again in the US South, physicians, politicians and the general public have to take the warning signs seriously and recognise that the tools available for tackling tropical diseases are sorely lacking. With diseases like Chagas now known to be prevalent and transmissible within the USA, better awareness, better tests and better treatments are all urgently required. Otherwise, as Hotez says, the number of people affected and infected will only continue to rise as this perfect storm grows ever stronger.

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