The Mosquito Coasts

Malaria and other diseases lurk in many vacation spots. We look into where a daytrip to the jungle is risky, and what to do about it

A vacation to a warm climate might be the best cure for wintertime blues. But increasingly popular, far-flung destinations, from Tanzania's beach resorts to Cambodia's Angkor Wat temples, pose some health risks to travelers from mosquito-borne illnesses such as malaria and dengue fever.

Concern has grown in recent months that an avian-flu pandemic could emerge in Asia and spread world-wide. But other, more common diseases in tropical and subtropical zones may be a more immediate threat to tourists.

Malaria kills at least one million people a year world-wide, most of them children in sub-Saharan Africa, and it sickens hundreds of millions more, according to the World Health Organization's Web site. More than 10,000 tourists fall ill from malaria every year. Dengue fever, a potentially fatal virus carried by mosquitoes, infects tens of millions of people annually, according to the WHO. Incidence of dengue fever, found primarily in and around urban areas, has grown rapidly in recent years and poses a growing menace to tourists. There are no vaccines for either malaria or dengue fever, though a number of drugs exist that combat malaria infection when taken preventively.

For tourists traveling to unfamiliar areas, deciding where to go and whether or not to take prophylatic medication is complicated by a dearth of information about local risks. The U.S. Centers for Disease Control publishes a Web site that many travelers consult to assess risks of mosquito-borne illnesses, including malaria, dengue fever, Japanese encephalitis, filiarisis and yellow fever. But the site often lacks detailed advice for individual cities and regions.

So how far off the beaten track can a traveler go without worrying?

In the Dominican Republic, for example, the countryside that surrounds the popular resorts of Punta Cana carries a malaria risk. In Mexico's Yucatan, tourist hot spot Cancun is considered safe, but malaria is present in nearby rural and jungle areas where many visitors venture on excursions.

"Sometimes it does get confusing," says Phyllis Kozarsky, an Emory University professor of medicine and consultant to the CDC. "That message about the Dominican Republic is going to be changed, because we realized it posed a question for the public." According to a CDC official, the revised information on the Dominican Republic will recommend taking antimalarial drugs even for travelers only going to coastal resorts, including Punta Cana.

Tourists ride an elephant at Angkor Wat in Cambodia. Malaria is present in the region.

Several experts said tourists who return from resort vacations with malaria often have taken daytrips to nearby rural areas. "People go to the Cancun area, and there isn't any transmission there, but they take side trips into the Yucatan Peninsula and do take a small risk," says G. Richard Olds, chairman of the department of medicine at the Medical College of Wisconsin. "Just inland from Cancun and Cozumel is jungle. That's when they enter a malaria-endemic area, and don't realize it."

Decisions about whether to take prophylaxis or other preventative measures might be clear-cut for a business traveler who won't leave major cities, or a volunteer worker who will spend an extensive amount of time in the countryside. But vacations often involve a mix of urban and rural destinations.

"I often skip taking antimalarials on short business trips in cities," said Neal Donahue, a 36-year-old economic consultant in Watertown, Mass. "If I were going camping, I'd take it. But to move between offices and a Hilton, I don't bother."

Just how risky a trip is will vary even within areas where diseases are present. Someone who suffers from chronic illness such as diabetes might be more vulnerable to complications. And antimalarial drugs don't offer 100% protection against the disease.

There are four malaria parasites; the most dangerous is plasmodium falciparum, prevalent in sub-Saharan Africa and other areas very close to the equator. Other malaria parasites typically cause less-severe illness and are rarely fatal.

P. vivax, p. malariae and p. ovale are usually not life-threatening. Physicians and companies that specialize in travel medicine sometimes can provide more specific advice. The International Association for Medical Assistance to Travelers, a nonprofit group in Ontario, publishes a malaria risk chart with information on viral strains in different regions of the world. The chart offers detailed recommendations on whether to take antimalarial drugs for brief visits to popular tourist sites. For example, travelers in Mexico making only a daytime excursion from Cancun to the Mayan ruins of Chichen Itza 100 miles inland don't need antimalarials. But for overnight stays, the association recommends a full course of medication. (The chart is available at: http://www.iamat.org/pdf/WorldMalariaRisk.pdf) The World Health Organization's International Travel and Health Publication also offers country-by-country guidance on malaria and other risks to travelers.

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A software program called Travax, developed by travel-health company Shoreland Inc., amasses guidelines from a range of medical and governmental sources. Shoreland also operates a free Web site, www.tripprep.com, that has pared-down information.

Experienced tour operators also can be resources for local information about disease risk. Butterfield & Robinson, a Toronto company that runs high-end biking and walking trips in more than two dozen countries, says some of its clients inquire in advance about mosquito-borne illness. "We have people that work for us in the places we offer trips," says Butterfield spokeswoman Cari Gray. "So at any given time, they can say that there hasn't been an incident of malaria where they are in a year, or that they anticipate heavy rain in the next month and there may be more mosquitoes."

Chloroquine was once the prophylaxis of choice, but resistance to the drug has increased since the 1960s. Chloroquine-based medication is now only effective in Central America and some island communities. As chloroquine resistance has grown, travelers began using drugs such as mefloquine, sold under the brand name Lariam. Mefloquine can cause side effects ranging from nausea and vomiting to anxiety and hallucinations.

The latest iteration among malaria drugs is Malarone, which is recommended by the CDC but typically costs substantially more than other treatments. The antibiotic doxycycline is another alternative.

"For a short visit, I'd most likely use Malarone. If you're in a big city in meetings and an air-conditioned hotel, chances [of getting malaria] are small," says Dr. Rajiv Narula, a travel-medicine specialist in New York. "But you can pick up malaria in the waiting lounge of an airport. It's better to be overcautious."

Tourists who contract malaria can become severely ill because they lack any immunity to it. In contrast, local residents often are infected with malaria a few times during their lives, and develop a partial immunity to the disease. About 1,500 cases of malaria are reported annually in the U.S., according to the CDC, nearly all believed to have been contracted overseas.

Dengue fever, a viral infection transmitted by the aedes aegypti mosquito, can cause symptoms ranging from a flu-like virus with fever and chills, to severe and fatal hemorrhagic disease in its most aggressive form. There is no vaccine for dengue fever, and no drugs that specifically target the virus. Treatment is limited to addressing symptoms, which can last for months.

Dengue fever, sometimes also called breakbone fever, occurs in most tropical countries in the South Pacific, the Caribbean, the Americas and Africa. During the past five or six years, dengue fever has been on the rise in the Caribbean, says Dr. Olds. Severe or fatal dengue tends to afflict people from endemic areas who have previously been infected with a different strain of the virus, he says -- not tourists.

"It's not uncommon to get a case of dengue in a person on a Caribbean vacation," Dr. Olds says. "They tend to be relatively mild, but even so, it's a fairly debilitating disease."

Many Caribbean resorts aim to remove breeding sites for mosquitoes on their properties. Beyond that, travelers can protect themselves by using insect repellent and other protective measures.

Yellow fever is endemic in tropical South America and parts of sub-Saharan Africa. Some countries in yellow-fever zones require proof of vaccination to enter, even for travelers who have flight layovers and don't leave the plane. Because an effective vaccine has been available for several decades, cases of yellow fever among tourists are rare.

Japanese encephalitis occurs in rural areas of Southeast Asia at certain times of year. While some infections are mild and asymptomatic, severe cases can cause neurological damage. The vaccine for Japanese encephalitis is only recommended for those spending a month or longer in a high-risk area. "I would recommend [the] vaccine if you're spending a long time in a rural area, or if you're going back and forth to the country," says Dr. Narula.

For all mosquito-borne illnesses, the best and sometimes only prevention is protection from mosquito bites. The CDC recommends using insect repellent that contains the chemical DEET. Picaridin, a repellent that has been used in Europe for years, recently became available in the U.S., but in much lower concentrations, so more frequent applications are necessary.

Wearing roomy, long-sleeve shirts and loose pants, and using bednetting or clothing with permethrin, an insecticide, also helps, says Dr. Kozarsky. In her view, natural products such as eucalyptus oil aren't as reliable.

Write to Annelena Lobb at annelena.lobb@wsj.com