INFECTIOUS DISEASES
Foreign travel is increasingly a component of the wilderness experience, and thus American travelers are exposed to numerous diseases that are not indigenous to the United States. In addition, domestic outdoor activities expose us to the vectors (carriers, such as mosquitoes or ticks) and microorganisms that generate diseases such as malaria, Rocky Mountain spotted fever, and Lyme disease. People who handle wild animals or ingest animal products are at increased risk. This section addresses some of the more common and worrisome infectious diseases associated with outdoor activities. Immunizations are discussed on page 449.
MALARIA
Malaria is caused by infection with one of four microscopic protozoan parasites: Plasmodium falciparum, P. vivax, P. malariae, or P. ovale. These are transmitted in the wild by the bite of an infected Anopheles mosquito. Of the nearly 430 species of Anopheles mosquitoes, only 30 to 40 transmit malaria. Most cases of malaria acquired by U.S. citizens are contracted in sub-Saharan Africa; most of the remainder are linked to travel in Southeast Asia, Central and South America, the Indian subcontinent, the Middle East, and Oceania (Papua New Guinea, Vanuatu, and the Solomon Islands).
Unfortunately, there is not yet a useful vaccine against malaria. 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 (with, for instance, a pyrethrin-containing product) and clothing (with, for example, permethrin 0.5%, Duranon, or Repel Permanone; or concentrated Perma-Kill 4 Week Tick Killer, diluted and applied to clothing); and wear adequate clothing and insect repellent (N,N-diethyl-3-methylbenzamide, called DEET) at these times (see page 390).
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 2 years, 50 mg; age 2 to 6 years, 100 mg; age 7 to 10 years, 150 mg; over 10 years, 200 mg), along with weekly chloroquine (the latter to protect against other forms of malaria). It can be used by those who will spend more than 3 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.
To determine the malaria risk within a specific country and to learn of the most recent recommendations for prophylaxis and drug therapy, you can seek information from one of many sources on the Internet, such as www.cdc.gov/malaria/.
YELLOW FEVER
Yellow fever is a viral disease transmitted in the jungle by mosquitoes of the genus Haemogogus and in urban areas by Aedes aegypti mosquitoes. “Jungle” yellow fever is seen in forest-savanna zones of tropical Africa, parts of Central America, forested areas of South America, and Trinidad. The “urban” variety is seen in South America and West Africa. The disease has not yet been noted in Asia.
Since yellow fever is so difficult to treat, it is essential to use yellow fever vaccine and mosquito control measures (see page 390). A live-virus vaccine is available. A single injection induces immunity after 10 days that is adequate for 10 years (see page 453).
DENGUE FEVER
The incubation period following a mosquito bite is 2 to 8 days. The disease is self-limited (5 to 7 days) and characterized in older children and adults by a sudden onset of severe headache, sore throat, fatigue, cough, high fever (greater than 39°C or 102.2°F), chills, muscle aches, sore throat, reddened eyes, enlarged lymph nodes, nausea, bone and joint pain (“breakbone fever”), and a fine, red, itchy skin rash that typically appears simultaneously with the fever on the proximal arms, legs, and trunk (it spares the face, palms, and soles). It may then spread to the face, and farther out on the arms and legs, becoming slightly darker and more solid. Although the fever usually remits spontaneously, an occasional victim will relapse. Some victims have a cycle of a few days of fever, then 1 to 3 days without fever, then fever again. It is not uncommon to suffer central nervous system manifestations, such as irritability, depression, seizures, or severe altered mental status. Children under 1 year of age appear to be particularly vulnerable to especially severe forms of dengue virus infection, associated with severe bleeding problems (dengue hemorrhagic fever: nosebleed, bleeding gums, severe abdominal pain, bloody vomit, darkened stool, restlessness, weakness, etc.) and circulatory problems that can lead to extremely low blood pressure (shock—see page 60). When this occurs, the victim may develop a diffuse, dark purple, blotchy rash caused by bleeding into the skin.
Treatment is supportive and based on symptoms. Fever should be treated with acetaminophen, and not with aspirin. There is no vaccine available against dengue fever. Insect repellents (particularly those containing DEET; see page 390) are critical for prevention.
WEST NILE VIRAL DISEASE
West Nile (named from the West Nile province of Uganda) viral disease (West Nile virus: WNV) is caused by a flavivirus (such as those that cause St. Louis encephalitis, Japanese encephalitis, and Murray Valley encephalitis) carried predominantly by mosquitoes (Culex pipiens in the eastern United States, C. pipiens quinquefasciatus in the southern United States, and C. tarsalis in the western United States, Aedes, Anopheles, and many other species) and at least 160 species of birds, although it has been found in small mammals and to an alarming degree in horses. The mosquitoes become infected by feeding on birds and many animals (e.g., bats, horses, chipmunks, dogs, rabbits, reindeer, squirrels, and even alligators). It appears to be transmitted to humans by mosquito bite and has been presumed to have arrived in the United States from the Middle East. In rapid fashion, it appears to have spread across the United States. The four top species of wild birds affected by WNV are American crows, Western scrub-jays, yellow-billed magpies, and Steller’s jays. Mosquitoes bite the birds and thus acquire the virus. West Nile viral disease is endemic in Africa, the Middle East, and West Asia. The virus has been spread to the recipient of an organ transplant from an infected donor, from a pregnant mother to a fetus, by blood transfusion, and possibly through breast milk. Otherwise, it does not appear to spread from human to human. While much of the clinical WNV activity is noted in summer and autumn, it is certainly possible to acquire the disease in winter from the bite of an infected mosquito.
1. Do not maintain standing water that serves as a breeding ground for mosquitoes, which lay eggs in the water. Drain or dump all standing water on a weekly basis. This includes water as shallow as 1 inch deep, as may be found in flower pots, planter bases, old tires, child pools, and so on.
2. Be sure that all doors and windows have tight-fitting screens. Repair any holes or rips, and if possible, treat screens and door jambs with mosquito control products.
3. Most bites occur at dawn and dusk, so limit outdoor activities during these times.
4. Use effective insect repellents, such as those containing DEET (N,N-diethyl-m-toluamide) or picaridin (KBR 3023). Use repellents according to the manufacturer’s labeled instructions, and reapply frequently, particularly if you swim or become sweaty.
5. Wear clothing designed to cover your arms and legs, including long sleeves and pants.