Many tropical adventures lead divers through foreign lands, where they can be exposed to vectors of diseases not commonly encountered in North America. The most worrisome of these afflictions is malaria.
Malaria is caused by infection with one of four microscopic parasites: Plasmodium falciparum, P. vivax, P. malariae, or P. ovale. These are transmitted in the wild by the bite of an infected Anopheles mosquito. While malaria is present in more than 90 countries, most cases of malaria acquired by U.S. citizens are contracted in sub-Saharan Africa, with most of the remainder linked to travel to Southeast Asia, Central and South America, the Indian subcontinent, the Middle East, and the Central and South Pacific. The disease is felt to be “on the rise” in tropical countries, many of which are bordered or surrounded by oceans that attract divers.
TRANSMISSION
On a “live-aboard” dive boat moored out at sea, mosquitoes are generally not present. However, when the boat is harbored close to land, during transit to and from shore, and during inland explorations, mosquitoes may be found in great numbers. In such circumstances, it is important to take precautions, including the use of mosquito netting during times of sleep.
When an infected mosquito bites a human, it releases malaria sporozoites (an immature form of the parasite) into the bloodstream. These travel to the liver, where they invade the liver cells and mature into merozoites. During this process, a single sporozoite can produce tens of thousands of parasites. From this location, the merozoites invade red blood cells. Traveling in the bloodstream, the organisms continue to multiply and can penetrate the vital organs, such as the brain, lungs, liver, and kidneys. The incubation period between acquisition of the parasites and the onset of symptoms is eight to 40 days, depending on the species. Typical symptoms include a flulike illness, with headache, chills, sweats, fatigue, loss of appetite, muscle aches, nausea, and vomiting. These are soon followed by episodes of headache, intense chills, high fever, and sweating. Jaundice and anemia may occur. The episodes last one to eight hours and are separated by two to three days.
Persons infected with falciparum malaria may be significantly more ill, with episodes of fever and chills at closer intervals lasting for more than 30 hours. Severe malaria can lead to anemia, heart and kidney failure and/or coma, or even be fatal; untreated infections can cause recurrent illness for years. Falciparum malaria causes more than 2 million deaths worldwide each year. Deaths from all forms of malaria may exceed 2.7 million per year, out of a total of 500 million episodes of illness.
Identification of the specific plasmodium is accomplished by observing the parasites under the microscope in blood smears. Persons infected with P. falciparum are treated with quinine sulfate in combination with pyrimethamine-sulfadoxine (Fansidar) or tetracycline. In recent studies, artemether (an artemisinin [quinghaosu] derivative) has been shown to be as effective as quinine in the treatment of severe P. falciparum malaria. Persons infected with P. vivax, P. malariae, or P. ovale are treated with chloroquine and primaquine phosphate.
PREVENTION
Avoidance of mosquito bites is key to prevention. Because the Anopheles mosquito tends to feed during the evening and nighttime, it is particularly important to sleep under nets or screens, spray living quarters (e.g., with a pyrethrin-containing product) and clothing (e.g., with permethrin 0.5 percent, Duranon or Permanone; concentrated Perma-Kill 4 Week Tick Killer, diluted and applied to clothing), and wear adequate clothing and insect repellent (N,N-diethyl-3-methylbenz-amide, called DEET) at these times.
If travel is undertaken to a region where P. falciparum is resistant to chloroquine and pyrimethamine-sulfadoxine, then prophylaxis (a drug taken to prevent disease) can be accomplished with mefloquine. The adult dose is 250 mg (salt) weekly. The pediatric dose varies according to the weight of the child: weight 15 to 19 kg, 63 mg; weight 20 to 30 kg, 125 mg; weight 31 to 45 kg, 188 mg; and over 45 kg, 250 mg. (For estimating purposes, 1 kg = 2.2 lb.) Mefloquine should be started one to two weeks before travel, then administered once a week during travel in malarious areas and for four weeks after a person leaves such areas. Mefloquine should not be taken during pregnancy.
An alternative drug for travelers who cannot take mefloquine is doxycycline (adult dose, 100 mg a day beginning one to two days prior to travel and for five to six weeks after; pediatric dose [age greater than eight years], 2 mg/kg a day, up to adult dose). Doxycycline is not advised for pregnant women or children under the age of eight years, and may cause increased skin sensitivity to sunlight.
A final alternative drug prophylaxis against malaria is accomplished with chloroquine phosphate (adult dose 300 mg of the base once a week; pediatric dose, five mg/kg of the base, up to adult dose, once a week), which should be taken one to two weeks before a person enters a malarious region and continued until one month after the journey. Chloroquine is recommended for travelers who cannot take mefloquine or doxycycline, particularly pregnant women and children who weigh less than 15 kg. If a person uses chloroquine for prophylaxis, he should carry three tablets of Fansidar to be taken in the event of a flulike illness or other unexplained fever, assuming the absence of an allergy to sulfonamide antibiotics.
Proguanil (Paludrine) is a drug that may be used for antimalarial prophylaxis in areas where P. falciparum is resistant to chloroquine. The drug is available without prescription in parts of Europe, Scandinavia, and Africa, but is as yet unavailable in the United States. It is administered in an adult dose of 200 mg daily (pediatric dose: under two years, 50 mg; age two to six years, 100 mg; age seven to 10 years, 150 mg; over 10 years, 200 mg), along with weekly chloroquine (the latter to protect against other forms of malaria). It may be used by persons who will spend more than three weeks in rural areas of East Africa (particularly Kenya and Tanzania), but does not appear to be useful in Papua New Guinea, West Africa, or Thailand.
TREATMENT
Pyrimethamine plus dapsone (drug combination: Maloprim) is prescribed in many malaria-endemic regions outside the United States. This drug cannot be used by pregnant women and can also cause bone marrow suppression.
Thus, if a person is stricken in an area where the malaria organism(s) is felt to be sensitive to chloroquine, but he has not been taking prophylaxis, begin treatment with chloroquine (adult dose, 600 mg of the base immediately, followed with 300 mg at six hours and once a day on days two and three; pediatric dose, 10 mg/kg of the base up to 600 mg immediately, followed by five mg/kg at six hours and once a day on days two and three). In a region where P. falciparum is resistant to chloroquine, administer quinine sulfate (adult dose, 650 mg every eight hours for three days; pediatric dose, eight mg/kg up to 650 mg every eight hours for three days) plus tetracycline (adult dose 250 mg four times a day for seven days; pediatric dose, five mg/kg up to 250 mg four times a day for seven days) or Fansidar (adult dose three tablets; pediatric dose: weight 5 to 10 kg, one half tablet; weight 11 to 20 kg, one tablet; weight 21 to 30 kg, one and one-half tablets; weight 31 to 45 kg, two tablets; weight over 45 kg, three tablets). In all cases of suspected malaria, seek the advice of a physician as soon as possible.
Halofantrine (Halfan) is used to treat chloroquine-resistant P. falciparum infections in a dose of 500 mg every six hours for three doses, with repeat therapy in seven days.
To determine the malaria risk within a specific country and to learn of the most recent recommendations for prophylaxis and drug therapy, you can call the Malaria Hot line sponsored by the Centers for Disease Control (CDC) in Atlanta, Georgia at (404) 332-4555.
Unfortunately, there is not yet a useful vaccine against malaria. This is because experimental vaccines thus far have not stimulated a sufficient immune response in recipients to prevent infection. However, a recent report in the New England Journal of Medicine is encouraging. In an article entitled “A preliminary evaluation of a recombinant circumsporozoite protein vaccine against Plasmodium falciparum malaria,” the authors reported that they had developed a vaccine in which only one of seven immunized persons exposed to malaria became infected. The authors recommended that field trials be conducted. The effectiveness of this and other vaccines in such a setting will determine whether or not we can expect to see a decline in the incidence of this devastating disease.
MALARIA Areas of Risk:
• Sub-Saharan Africa
• Southeast Asia
• Central America
• South America
• Indian subcontinent
• Middle East
• Central and South Pacific
Cause:
• Parasites transmitted by the bite of an infected mosquito
Prevention:
• Use nets or screens at night.
• Spray living quarters and clothing with an insecticide.
• Wear insect repellent and adequate clothing.
• Use preventative drugs as appropriate.