Mosquito Thrives; So Does Dengue Fever

By JANE E. BRODY
14 may 2008--On a recent visit to Cambodia, outside a children’s hospital a block from my hotel, I saw a large red-and-white sign that warned of a severe epidemic of dengue hemorrhagic fever. Years ago, the disease killed our tour guide’s 5-year-old brother.
My tripmates and I managed to escape even the milder form of this mosquito-borne viral infection — we all slept in an air-conditioned hotel and each day applied insect repellent with 30 percent DEET on our exposed skin. But I have since learned that I could have been infected on several previous trips abroad and even in parts of the United States.
Dengue (pronounced DEN-gee) fever has increased rapidly in tropical and subtropical areas worldwide in recent years, thanks to factors both natural and manmade.
Among the countries that have experienced recent epidemics are Cambodia, Costa Rica, India, Indonesia, Malaysia, the Philippines, Singapore, Thailand and Vietnam. In the Western Hemisphere, outbreaks have also occurred in some Caribbean islands, Cuba, northern Mexico, Nicaragua, Panama, Puerto Rico and Venezuela.
This year, dengue fever has ravaged Rio de Janeiro, infecting more than 75,000 people in Brazil’s Rio state, including Diego Hypólito, a world champion gymnast and gold-medal favorite in the Beijing Olympics this summer. More than 80 people in Rio have died from dengue.
Though most North Americans who receive a diagnosis of dengue fever were infected while traveling to countries where the disease is endemic, including Mexico, it has also struck residents of Hawaii and Texas who had not left American shores. And last summer a related mosquito-borne disease, chikungunya, afflicted more than 100 residents of a village in Italy, Castiglione di Cervia.
The disease is not contagious; rather, it is passed from person to person through the bite of a virus-carrying mosquito.
Epidemiologists say that global warming is allowing the tiger mosquito, Aedes albopictus, a vector of both chikungunya and dengue fever, to survive in areas that were once too cold for it. This mosquito now thrives across southern Europe and even in France and Switzerland. All it takes is one infected traveler to bring the dengue virus home, where the bite of a resident tiger mosquito could transmit it to others.
The primary vector for dengue fever is Aedes aegypti, a daytime biter that is especially active during the early morning and late afternoon. (Unlike the Anopheles mosquito, which transmits malaria, it is not active at night.)
While dengue fever is not as serious a threat as malaria, which afflicts up to 500 million people and kills one million each year, both diseases have flourished since DDT, the pesticide that controlled mosquitoes more effectively and inexpensively than any other, fell out of favor in the 1960s. Uncontrolled urbanization and its accompanying population growth, along with inadequate water management systems, have also played a role in the spread of dengue fever.
The Virus and Its Effects
Dengue fever is caused by any of four variants of a flavivirus, DEN-1, DEN-2, DEN-3 and DEN-4. Other flaviviruses cause West Nile, yellow fever and Japanese encephalitis. While infection by one of the dengue variants confers lifetime immunity to it, a person can still be infected by any of the other three.
The evidence strongly indicates that it is the second infection (though not the third and fourth) that can lead to a far more serious form, dengue hemorrhagic fever, in which the capillaries leak fluid. If not treated soon enough, the hemorrhagic form can result in a life-threatening loss of blood volume and death from dengue shock syndrome.
There is no vaccine for dengue fever and unlikely to be one anytime soon. In 2003, the Bill and Melinda Gates Foundation pledged $55 million over six years to foster the development of a vaccine for dengue fever and stop its global spread. Vaccine trials are under way in several tropical areas, but approval of an effective vaccine is not expected for perhaps a decade.
The bite of an infected mosquito is followed by an incubation period of 3 to 14 days, most commonly 4 to 7 days, before symptoms might appear.
Many people experience only mild flulike symptoms, or none at all. In others, the characteristic symptoms of dengue fever are the sudden onset of high fever, a severe headache in the front of the head and excruciating pains in the joints and muscles — leading to its colloquial name of break-bone fever.
The fever typically resolves in three to five days, but in 1 percent of patients the disease progresses to the hemorrhagic form. Even if a first infection causes no or few symptoms, a second infection can increase the risk of the hemorrhagic form.
Since none of the known antiviral drugs are effective, treatment is symptomatic. Acetaminophen is given to reduce fever and pain. But all aspirin-like drugs, including ibuprofen, must be avoided because they can cause bleeding and make matters worse. As with other viral diseases, children with dengue fever who are given aspirin can develop Reye syndrome.
Patients should rest and drink lots of fluids. In most cases, symptoms resolve in a week or two. The disease is likely to have progressed to its more dangerous form if fever is followed by a low body temperature, severe abdominal pain, prolonged vomiting and mental changes like confusion, irritability and lethargy. Immediate hospitalization and intravenous fluids are then essential. Full recovery to a normal energy level can take months.
Prevention
Based on studies of military and relief workers, the Centers for Disease Control and Prevention estimates the risk to those who visit a dengue-endemic area as one illness per 1,000 travelers. This is likely to be an overestimate for ordinary travelers, most of whom stay only a few days in air-conditioned hotels with well-kept grounds.
Though many trips to Brazil this year were canceled during the epidemic in Rio, travelers do not need to avoid dengue-endemic areas if they are willing to take precautions to avoid mosquito bites.
The disease control agency recommends staying in well-screened or air-conditioned areas whenever possible (not a realistic option for sightseers and adventure travelers like me); wearing clothing that covers the entire body, including long sleeves and pants with tight cuffs; and covering exposed skin with insect repellent containing DEET at a concentration of 20 percent or 30 percent, applied three times a day. Though neither I nor my tripmates could tolerate full-body attire during Cambodia’s humid, 90-plus-degree days, we all doused ourselves daily with repellent.
In very sunny places, sunscreen should be applied first, followed by the repellent. It also helps to spray clothing with a repellent.
Since the mosquito breeds in small amounts of fresh water, eliminating standing water in places like flower pots and old tires can reduce exposure to this dengue carrier.