Vector-borne Diseases Growing as Threats to U.S. Public Health: Climate Change, Travel Linked to Illness

Kim Krisberg

Disclosures

Nations Health. 2010;40(7) 

Abstract and Introduction

Introduction

Last fall, an old resident returned to the beaches of Florida, though it certainly was not welcome and officials are determined to see its visit cut short.

The unwanted visitor is mosquito-borne dengue, which made headlines this summer after public health officials found that 5 percent of Key West residents showed recent exposure to the virus. The problem began late last year with 27 reported cases, prompting immediate responses from local health and mosquito control officials. Winter came and went with no more reported cases, and health officials hoped that it meant the end of dengue's stay. But April saw another case with more after that, finally leading to the headline-grabbing Key West study, which began after the initial cases in 2009.

"These people had not traveled outside of Florida, so we need to determine if these cases are an isolated occurrence or if dengue has once again become endemic in the continental United States," said Harold Margolis, chief of the dengue branch at the Centers for Disease Control and Prevention.

With the last reported Florida dengue outbreak in 1934, its recent re-emergence has health officials worried that with the right conditions, the virus could gain a new foothold in its old stomping grounds. Health officials also worry that outbreaks such as the dengue one are part of a growing trend that could be poised to get worse — that changing climates, more travel and urbanization mean vector-borne, once-tropical diseases such as dengue and West Nile virus are not only here to stay, but are on the move.

"This is not going away," said Peter Hotez, president-elect of the American Society of Tropical Medicine and Hygiene. "I actually think that this is a bigger threat than many of the biodefense pathogens that we're spending huge amounts of money on. Dengue and other vector-borne diseases are a true homeland security threat."

Experts have long warned that warmer temperatures due to climate change will make it easier for vector-borne diseases to spread. For example, a study in the April issue of Environmental Health Perspectives on the northward expansion of a species of tick and the emergence of Lyme disease in Canada said the tick's movement is "possibly facilitated by a warming climate."

Vector-borne diseases are also taking advantage of easy travel, both domestically and globally. A 2007 study in the New England Journal of Medicine on mosquito-borne chikungunya fever, which affects a person's joints, said the disease was diagnosed in an "unprecedentedly" large number of people returning to Europe and the United States from islands in the Indian Ocean where a 2006 outbreak occurred. The authors noted that the "chikungunya outbreak is an example of the abrupt expression of vector-borne diseases in the global village."

And in some cases, researchers cannot yet find the reason for a vector-borne outbreak. For example, a 2005 study in the New England Journal of Medicine on the tick-borne Rocky Mountain spotted fever said the disease was in the midst of its third emergence since 1920, with more than 1,500 cases reported in 2004. While the authors said Rocky Mountain spotted fever can no longer be considered only a rural or southern disease, theories attributing its emergence to "suburban development, changes in recreational activities or long-term changes in climate have not withstood careful investigation."

Back in Florida, Carina Blackmore, DVM, PhD, state public health veterinarian with the Florida Department of Health, said the source of the recent dengue outbreak is "certainly" travel-related. She said the strain found in Key West ultimately originated in Mexico, with the likely scenario being a person transporting the virus to the Keys and infecting the local mosquito population. Blackmore noted that dengue was once endemic in Florida, but with housing and mosquito control advancements in the 1950s, the disease lost its hold.

"We have the environmental conditions for these diseases," Blackmore told The Nation's Health. "But it's human behavior that's different now than 50 years ago, so it's difficult for these diseases to get re-established."

Blackmore said she is more concerned with vector-borne diseases that have animal hosts, such as Rift Valley fever, which infects livestock and can be transmitted to humans via contact with an infected animal's blood or organs. The mostly Africa-based virus, which has human symptoms similar to West Nile virus, has not been detected in Florida, but she said officials are watching and preparing for it.

Other U.S. communities have been confronting dengue for a while now. In Brownsville, Texas, health officials are constantly on watch for the disease, said Art Rodriguez, director of the health department in Brownsville, which sits on the U.S.-Mexico border. A study in a 2007 issue of CDC's Emerging Infectious Diseases journal found that past dengue infection had been detected in 40 percent of Brownsville residents. The last reported cases in which health officials believe residents acquired dengue locally was in 2005, Rodriguez said, noting that the health department has significantly restructured its vector-borne disease surveillance in the last three years. Central to the department's strategy is trapping mosquitoes after rainfalls and sending specimens to the state health department for testing.

Rodriguez said the trapping not only provides data on the species and density of mosquitoes, but helps pinpoint hotspots and gauge if, when and where to use chemical pesticides. Regarding susceptibility to dengue, Rodriguez said there tends to be a disparity between those with central air conditioning and heating and those without. But because Brownsville is home to many low-income residents, prevention education is targeted citywide, from TV spots to leafleting neighborhoods.

"It's under control until the next rainfall, and then you can scratch all that hard work we did and we're back at ground zero," Rodriguez told The Nation's Health. "It's a constant state of alert."

Low-income communities are much more susceptible to vector-borne diseases, often due to poor housing conditions and weak community infrastructures. In a 2008 article in the Public Library of Science Neglected Tropical Diseases journal, Hotez, of the American Society of Tropical Medicine and Hygiene, called the spread of certain tropical and vector-borne diseases inside the United States "neglected infections of poverty."

Regarding the dengue outbreak in Florida, Hotez said "it's not too much of a stretch to believe that it could emerge along the Gulf Coast," however, he said the biggest determinant of its spread will be poverty.

"My concern is the poorest people on the Gulf Coast," Hotez told The Nation's Health. "First we had (Hurricane) Katrina, then the oil disaster, and dengue, in my opinion, could be the third threat."

Despite data that show vector-borne diseases pose a threat, President Barack Obama in his 2011 fiscal year federal budget proposed eliminating funding for CDC's vector-borne disease program. In a letter to key members of the House and Senate, APHA and fellow public health advocates said the proposal has the "potential to leave America vulnerable to some of the world's deadliest diseases."

For more information on vector-borne diseases, visit www.cdc.gov/ncidod/dvbid/index.html. For more news from The Nation's Health, visit http://www.thenationshealth.org

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