Mystery Surrounds Isolated Dengue Fever Outbreaks In United States
November 15, 2013

Mystery Surrounds Isolated Dengue Fever Outbreaks In United States

Lawrence LeBlond for - Your Universe Online

Dengue fever, an infectious tropical disease carried by several species of mosquito in the genus Aedes, has been showing up in Key West and the Florida mainland, potentially exposing millions of people to a disease that can lead to life-threatening illness.

While the dengue virus is mysteriously working its way up into Florida, researchers are equally as puzzled as to why the dangerous illness has not yet become common in and around Tucson, Arizona, which is close to Mexico, where outbreaks routinely occur. Mosquitoes in the state have been discovered to be commonly carrying the pathogen, as well.

"Key West and Tucson share a lot of risk factors. Even in arid Tucson we have a large population of mosquitoes that can carry dengue, and people here spend a lot of time outdoors, but we have yet to see evidence of locally-acquired infections," sttated Kacey Ernst, PhD, an infectious disease epidemiologist at the University of Arizona in Tucson.


Presenting new research at the annual meeting of the American Society of Tropical Medicine and Hygiene (ASTMH), Ernst and study coauthor Mary Hayden, PhD, a scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, conducted a survey of residents in two cities – Key West and Tucson – separated by two thousand miles and each having unique climates. Their research has turned up some astonishing similarities.

In the surveys, the two researchers found that residents in both cities report spending at least an hour outside each day. And survey participants from both cities report nearly an identical amount of time using central air conditioning – 55 percent in Key West vs. 56 percent in Tucson – which is typically believed to lower the risk of dengue. However, the authors report that AC systems in Tucson are less dengue-friendly.

The duo noted that many people in Tucson rely on evaporative cooling systems – collectively known as “swamp” coolers – which increase humidity in the home and may in fact increase survivability rates of mosquitoes, particularly due to the otherwise arid desert environment.

"It is still a mystery as to why dengue infection has not shown up here," Ernst said. "When researchers looked at why dengue is not more common along the Texas side of the Mexico border, they cited factors limiting contact with mosquitoes, like people spending a lot of time in sealed, air-conditioned buildings. Those issues are extremely important considerations, but we don't think they fully explain why Key West has dengue and Tucson doesn't."

While there exists the likelihood that dengue is sporadically infecting people in Tucson, it is possible that those infections go undetected, or perhaps there is some yet unknown factor in the mosquitoes’ life cycle that inhibits them from transmitting the disease. This conundrum is multiplied by the fact that Tucson should be viewed as much more vulnerable to dengue because the mosquito-eradication campaigns in this region are far less intensive than they are in and around Key West.

The researchers have also moved their studies to Nogales, a city in the Mexican northern state of Sonora, where Aedes aegypti, the primary vector of the dengue virus, is most common. The research team has mysteriously found no dengue infections in this area as well, despite a similar social and economic structure to that of other Mexican cities where dengue is endemic.

This research has led the team in the search for a better understanding about the human and biological factors that allow dengue to affect some regions but not others. The ultimate goal of the research is to prevent dengue from moving farther into the US than it already has, preventing a potentially dangerous epidemic. Dengue, which at one time was viewed as obscure in nature, now threatens more than 40 percent of the world and has become endemic to Mexico.

Symptoms of dengue fever, which is also known as “break-bone fever” due to its ability to cause debilitating joint pain, include fever, headache, muscle aches and skin rash similar to measles. Dengue infections can often progress to a potentially fatal form of hemorrhagic fever. While there are currently no drugs available to treat or cure infections, the team discussed new efforts to develop a vaccine at the ASTMH meeting.

"I think you see researchers in the United States and around the world mounting a full-court press to try to keep dengue in check," said ASTMH President David H. Walker, MD. "One of the reasons we hold an Annual Meeting is to allow scientists who are focusing on different aspects of the disease—the biology of the virus itself, the spread of the vector, and the role of human behavior—to share their latest findings and ultimately curtail the suffering from this disease."

Dengue, which is believed to have been non-existent in the continental US for longer than the past half century, has slowly been working its way into the country’s southern states. In the past nine years there have been reports of small, isolated dengue outbreaks in southern Texas (2004 and 2005) and in Key West (2009 to 2011). The most recent outbreak occurred in August of this year, where 20 “locally-acquired” cases of dengue were confirmed in the Jensen Beach are of South Florida’s Treasure Coast, which provides some evidence that dengue may in fact be moving north.

But as of yet, apart from the Florida and Texas outbreaks, there have been no reports of dengue infection elsewhere in the US.


Aedes aegypti, a mosquito species that is commonly found throughout the southern United States from Florida to Arizona, has recently made its way to California. Now one expert on vector-borne diseases is discussing the future threat posed by this menacing insect and the pathogens it carries.

Lars Eisen, Ph.D, from Colorado State University, told the ASTMH meeting this week that the threat from Aedes aegypti on human health is high. This mosquito has made a steady comeback across the Americas after the discontinuation of an intense control campaign that was initiated in the mid-20th century by the Pan-American Health Organization (PAHO), which had eliminated the species from many countries.

Eisen noted that Aedes aegypti has likely existed for hundreds of years in the far southeastern US and was able to thrive, as the PAHO campaign had not included the US as part of its mosquito extermination efforts.

But despite evidence of Aedes aegypti now showing up in California, Eisen said it will take a lot more than just the insect’s presence there to cause a dengue outbreak.

Air conditioning and window screens do a good job at keeping mosquitoes at bay, reducing the threat of indoor biting. Also, Eisen noted that regular trash removal and access to piped water reduces breeding opportunities for these pests. Things like these significantly limit the opportunities for a virus to transmit through communities.

And even when dengue does occur in the US, these factors seem to keep virus transmission levels at a minimum, he added. However, rising temperatures due to climate change could bring the mosquito even farther north into the US, adding a potential threat of millions of infections. Still, there are many elements that influence the mosquito and the transmission of dengue, and climate is only one factor in assessing the threat of this disease.


Eisen said at the meeting that new efforts are underway to help detect areas that may be at risk of dengue outbreaks.

Using satellites from a collaborative effort between Science and Technology in Atmospheric Research (STAR) and NCAR – both located in Boulder, Colorado, researchers are trying to develop better imagery to detect “container” habitats preferred by Aedes aegypti, according to Paul Bieringer, Ph.D, a scientist with NCAR's National Security Applications Program.

Unlike mosquitoes that carry and spread the West Nile Virus, which prefer ponds, sewers and other sources of still, often stagnant water, Aedes aegypti typically seek out water-filled containers, such as cans, buckets and old tires that collect rain water. This preference is why this particular mosquito is referred to as a “container breeder.”

The latest technology now allows satellites to provide clear, close-up images of relatively small objects where water can build up, allowing researchers to employ sophisticated computer programs to scan these images and hopefully identify hotspots capable of harboring large populations of mosquitoes, ones that may be carrying the dangerous dengue virus, explained Beiringer.

"Our goal is to develop maps that can simulate the abundance of this dengue virus carrier in a particular area, and then use these insights along with demographic and economic data, to predict where disease outbreaks are most likely to occur," he said.