Diseases once thought to be rare or exotic in the United States are gaining a presence and getting new attention from medical researchers who are probing how immigration, limited access to care and the impacts of climate change are influencing their spread.
Illnesses like schistosomiasis, Chagas disease and dengue are endemic in warmer, wetter and poorer areas of the world, often closer to the equator. According to the World Health Organization, almost 1 billion people are afflicted with more than one tropical disease.
Caused by bacteria, parasites and viruses, these diseases are spread through bites, excrement and dirty water stemming from substandard housing and sanitation. Consequently, the United States has been largely isolated from them.
But Americans are traveling more, and as tropical vacationers return home, they may unwittingly bring back dangerous souvenirs. Immigrants from endemic regions are also bringing in these diseases, some of which can lie dormant for years. All the while, the flies, ticks and mosquitoes that spread these illnesses are moving north as rising temperatures make new areas more welcoming.
In 2009, dengue emerged in south Florida and infected more than 60 people, the first outbreak since 1934, according to the Centers for Disease Control and Prevention (CDC). Dengue is caused by four closely related viruses spread by mosquitoes. It results in joint and muscle pain, severe headaches and bleeding.
The outbreak was first detected in a Rochester, N.Y., woman who traveled to Key West, Fla., for one week, with several Key West residents subsequently reporting infections. The infection rate rose to 5 percent, which CDC said indicated "a serious risk of transmission."
According to the Monroe County Health Department, there hasn't been a confirmed dengue case in the Florida Keys since November 2010. "We keep the public aware that they need to be dumping standing water and wearing mosquito repellent," explained Chris Tittle, public information officer at the health department. The outbreak may have been linked to travel from Latin America and the Caribbean, where the disease's incidence has risen fourfold over the past 30 years. In 2010, Puerto Rico faced the largest dengue epidemic in its history.
However, not every outbreak is imported, and future epidemics may come from within. "There's a substantial but hidden burden of tropical disease in the United States, particularly among people in poverty," said Peter Hotez, founding dean of the National School of Tropical Medicine, the first such school in the United States, at Baylor College of Medicine in Texas. Diseases like leishmaniasis often are not tracked rigorously in this country and are classified as neglected, unlike vector-borne illnesses like Lyme disease that are monitored.
Little data or public awareness
Since there is a dearth of data, it is hard to distinguish to what extent neglected tropical diseases are actually endemic in the United States or are brought by travelers and immigrants. It is also hard to tell if the number of infections is rising or if people are just noticing them more.
"In most cases, we don't know. We're just really getting our arms around how pervasive the disease is," said Hotez, who is studying these diseases in communities along the Gulf Coast. "People jump to the conclusion that it must be immigration coming up from Mexico or Central America, but we don't think that's the case."
Hotez believes some of these diseases may be spreading indigenously, though other infections do have stronger links to immigration. For example, Chagas disease is a parasitic infection caused by Trypanosoma cruzi, a single-celled parasite. It causes swelling at the infection site and, if left untreated, develops into a chronic illness that can be asymptomatic or unfelt in most people and can cause digestive, heart and nervous system failures in others.