Public health vs. the mosquito: Pushing back against dengue

by Kim Krisberg

A couple weeks ago on the southern-most tip of the continental United States in Key West, nearly 70 residents gathered at a town hall meeting to talk about mosquitoes. And not just any mosquito. A special, genetically modified mosquito designed to protect people's health.

While the modified mosquito has yet to make the two-mile wide island its home, local mosquito control officials are busy making the case that its intentional release will help safely contain the risk of mosquito-borne dengue fever, which made a startling reappearance in Key West in 2009. The male mosquito is genetically altered so that when it mates with wild Aedes aegypti females, the main carrier of the dengue virus, the offspring die as larvae and never become full-grown mosquitoes.

The mosquito control strategy is so new that Key West officials are struggling to find a state or federal agency with the appropriate authority to sign off and oversee the plan as well as a risk assessment. So for now the prevention strategy is in limbo.

"We were looking for a new tool because although we use some of the most cutting-edge and environmentally friendly techniques to control this mosquito, it's not enough," said Coleen Fitzsimmons, a biologist with the Florida Keys Mosquito Control District. "We're here to protect public health, so we felt like we needed something new."

Whether or not genetically modified mosquitoes get added to Key West's mosquito control toolkit, the recent re-emergence of dengue have officials looking for new ways to control the resilient bug. After decades without a single documented human case of locally acquired dengue, 27 such cases were found in Key West in 2009. In 2010, the locally acquired caseload jumped to 67, Fitzsimmons told me. A locally acquired case means the person was infected by a local mosquito and not due to travel outside the country.

And while no cases have been identified since, Fitzsimmons said it's no time scale back control efforts. Because hotter temperatures speed up the development of adult mosquitoes, cause females to bite more often and shorten the amount of time the dengue virus needs to develop inside the mosquito, "whether or not dengue comes back can be the difference between a two- or three-degree change in the temperature," she said. And the female mosquito that spreads dengue is particularly difficult and costly to control. The Aedes aegypti is well suited to human environments, laying it eggs in flower pots, buckets or birdbaths -- "they don't swarm, they just linger close to home," Fitzsimmons said.

She noted that although the species of mosquito that transmits dengue accounts for less than one percent of the Florida Keys' entire mosquito population, it takes up about 10 percent of the agency's budget.

"We can't just say our efforts are working," Fitzsimmons said. "We have to be on all the time."

According to the World Health Organization, incidence of dengue has increased 30-fold in the last five decades, and between 50 million and 100 million infections are estimated to happen every year in more than 100 countries. That means that half the world's population is at risk for dengue. Researchers also predict that warming temperatures related to climate change could help the disease spread more efficiently in the future.

Even in countries like the United States, where vigilant mosquito control and public health efforts have reduced the risk so much that dengue barely registers on the public radar, the tropical disease is poised to make a comeback. In fact, experts say that as dengue spreads in South and Central America, it's only a matter of time before it becomes a bigger problem in the United States.

Tiny bug, big problems

Dengue is a viral infection primarily transmitted via the bite of an infected female mosquito known as the Aedes aegypti, which contracts the virus after biting an infected human. The Aedes aegypti bites during the day, likes to feed indoors and its eggs are particularly hardy. Dengue is caused by one of four related viruses and oftentimes presents itself via flu-like symptoms. Some people with the virus don't even realize they have it. However, a more serious form of the virus, known as dengue hemorrhagic fever, can be life-threatening if not treated.

Dengue is a quintessential public health disease -- its control and prevention is directly tied to environmental conditions, sanitation and human behavior. When the outbreak happened in Key West, health officials blanketed the island in handouts and public health service announcements, encouraging residents to protect themselves from mosquito bites and get rid of standing water where mosquitoes breed, said Vincent Conte, senior physician and deputy director of Epidemiology, Disease Control and Immunization Services at Miami-Dade County Health Department.

"We're lucky enough in the U.S. that we have the money and public funds to have adequate and efficient mosquito control measures," he said. "But the frequency of occurrence is on the rise, and unless it's dealt with in appropriate ways, we could be looking at a gradual increase in cases."

Miami-Dade County has had three locally acquired cases of dengue since 2010 -- before that the last locally acquired case in the south Florida area was in the 1940s, Conte said. The health department also documents about 12 to 15 cases of imported dengue a month. Fortunately, Conte said the Miami area is home to many physicians familiar with tropical diseases and astute at recognizing dengue symptoms.

Unfortunately, however, dengue and vector control come with same funding problems as other public health efforts. People don't consider dengue to be a major threat at the moment and so Miami-Dade County has seen cuts to mosquito control and staffing services.

"With dengue, we do have the opportunity to prevent its rise," Conte told me. "It's just a matter of legislators making the commitment so we can be more proactive rather than reactive."

Jim Kazura, president of the American Society of Tropical Medicine and Hygiene, said "it's hard to get money for mosquito control when there's no problem going on." A professor and director of the Center for Global Health and Diseases at Case Western Reserve University, Kazura said cases of dengue are probably under-reported in the United States -- "clinically, the disease is not always easy to distinguish from myriad of other illnesses," he said. He noted that a dengue vaccine combined with effective mosquito control has the potential to eliminate the risk of dengue around the world. Clinical trials on five dengue vaccine candidates are now underway.

"Mosquito control is a perfect example of public health always working in the background," Kazura said. "And unfortunately, public health doesn't become noticeable until we have a problem."

No borders for dengue

In Brownsville, Texas, Art Rodriguez can practically see Mexico from his office window. The public health director for the Brownsville Public Health Department, Rodriguez told me that while Brownsville hasn't had a locally acquired case of dengue since 2005, it's a year-round challenge to prevent the disease and control the mosquitoes that spread it.

"We always know that the danger is lurking," Rodriguez said. "Just footsteps from us people are getting sick and dying from dengue hemorrhagic fever."

Rodriguez works with his counterparts across the border to maintain mosquito control and reports that surveillance efforts haven't uncovered a mosquito carrying the virus in three years. The health department holds annual trainings with local physicians about vector-borne diseases, and Rodriguez said that most physicians who come to Brownsville are surprised the community is at risk for tropical diseases. Because of the barriers to diagnosing dengue in Brownsville residents, he said it's hard to say if or how many residents are carrying the virus.

Rodriguez said dengue is only an emerging threat in Brownsville because of the constant work of public health and mosquito control officials.

"Public health is a never-ending battle to maintain a balance and we always have to be proactive," he told me. "Outbreaks occur too quickly for us not to be prepared for them."

To the northwest in Laredo, Texas, which also sits on the border with Mexico, there hasn't been a locally acquired case of dengue in five years, and across the border in New Laredo, there hasn't been a case in three years, said Hector Gonzalez, director of the City of Laredo Health Department.

Like Rodriguez, Gonzalez works with his counterparts in Mexico to contain mosquito populations. The city employs an integrated pest management approach that includes insecticide spraying, sanitation improvements and community education, Gonzalez said. Minnows have also been employed against dengue: For the past eight years, thousands of minnows, which have a short life span, have been distributed into retention ponds, creeks and lakes to eat mosquito larvae.

"Everything combined is working in an integrative manner to prevent dengue," Gonzalez told me.

Despite the success, Laredo public health workers are bracing for funding cuts that could impact dengue prevention. For years, the federally funded Early Warning Infectious Disease Surveillance Program has been operating at the border, facilitating the exchange of data and information that can serve as an early warning for disease. When a case of dengue popped up across the border in New Laredo three years ago, Gonzalez said he learned about it through the surveillance program. Next year, funding for the program has been zeroed out, he said.

"We have to sustain basic public health surveillance," said Gonzalez, who noted that the Laredo department now monitors for more than 200 diseases. "It's critical for a disease like dengue."

To learn more about dengue, visit www.cdc.gov/dengue or www.who.int/topics/dengue/en.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for almost a decade.