# Introduction in tropical diseases

Around the Rio Dulce area, the occurrence of tropical diseases is quite rare but isolated cases of intestinal problems, malaria and dengue fever will occasionally crop up. Some years ago there was a minor outbreak of cholera in the Fronteras area. The solution is simply this: 1) Under no circumstances should you drink the river water. 2) Don't bathe in the river near or downstream of heavily populated areas, ie. Fronteras.
Some have expressed a general interest in tropical diseases and have complained of the lack of information about such diseases. To satisfy this need the following general overview of tropical diseases worldwide is included here. Some of the diseases covered below do not occur in Central America but are included to satisfy the curious. Readers with a taste for the macabre may be especially interested in the news-clip about the Ebola virus.
Tropical diseases are illnesses that either occur uniquely in tropical and subtropical regions (which is rare) or, more commonly, are either more widespread in the tropics or more difficult to prevent or control. The tropics are more problematic for certain diseases for two reasons: 1) Tropical climates are more conducive to certain diseases. 2) Areas of poverty and primitive sanitation conditions are more common in the tropics. The agencies most concerned with improving health in tropical countries include the World Health Organization (WHO) and the Edna McConnell Clark Foundation.
Diseases Most Prevalent in the Tropics
The most important diseases in the tropical regions of Southeast Asia, Africa, and South America are malaria, schistosomiasis, leprosy, filariasis, trypanosomiasis, and leishmaniasis. Although effective chemotherapy and insecticides have reduced or eliminated malaria in most of the western hemisphere, these measures have been less successful in Asia. Both the infecting parasite and its mosquito carrier have become resistant to current drugs and 200 million persons are estimated to have malaria in tropical areas. In sub-Saharan Africa some 1 million children under five die of the disease each year.
Schistosomiasis has never been common in temperate climates, but it affects 125 million persons worldwide, of whom approximately 20 percent are at least partly disabled by the disease. Praziquantel, a highly effective new drug, is now available for treatment of schistosomiasis. Leprosy has also always been more common in tropical than in moderate climates, and about 11 million persons in the world have this illness. In endemic areas many severe cases of leprosy are now resistant to the drug first used against it, and newer, more expensive therapy must be employed. Filariasis is a common tropical debilitating illness caused by infection with roundworm larvae. Trypanosomiasis, which results from infection with a protozoan parasite, has caused 10 million cases of human sickness in Africa alone. A related protozoan in South America causes a less deadly form of trypanosomiasis called Chagas' disease. Leishmaniasis is also a result of worm infection, and in its Asian and African forms the disease can damage the internal organs.
Additional Health Problems
Although tuberculosis is largely under control in developed countries, it is still a considerable public health problem in much of the world and is responsible for about half a million deaths annually, 75 percent of them in Asia. Other diseases for which treatment is available but which are still common in developing countries include cholera, yellow fever, yaws, and amoebic dysentery (see Dysentery).
Two forms of cancer, Burkitt's lymphoma and liver cancer, are very common in Africa and Asia, respectively, although rare in temperate zones. Burkitt's lymphoma is thought to be due to a combination of massive infection with a virus early in life and malaria in adulthood. Liver cancer may be caused by infection with the hepatitis B virus.
As many as 25 million persons have become blind from preventable diseases in tropical countries. These diseases include xerophthalmia, due to lack of vitamin A in the diet; onchocerciasis, or river blindness, an infection of the skin by filarial larvae that may also affect the conjunctiva of the eye; and trachoma, a chronic conjunctival infection caused by the parasitic bacterium Chlamydia trachomatis, which is transmitted by flies or through close personal contact.
Finally, a number of severe virus-caused fevers that were identified during the 1970s are found predominantly in tropical regions. These diseases include Lassa, Ebola, Marburg, Bunya, and Chikungunya fevers, some of which cause death by hemorrhage (hemorrhagic fever). One member of this family, dengue virus (see Dengue Fever), was known for many years but has recently spread to the Caribbean and Mexico. All these diseases are rare.
Factors Which Aggravate Tropical Diseases
The severity of diseases in tropical areas is due to widespread poverty and poor sanitation as well as climatic influences. That is, because of low national incomes, most developing countries cannot afford to buy vaccines to prevent poliomyelitis, measles, and yellow fever. Only about 10 percent of the 80 million children in poor countries have been immunized against diphtheria, whooping cough, and tetanus, and such countries cannot afford to distribute drugs against tuberculosis or leprosy. Poverty is a condition that also leads to malnutrition, which makes people more susceptible to disease.
Poor sanitation is especially to blame for the spread of cholera, in which the infecting agent is transmitted through contaminated sewage; and schistosomiasis, in which the intermediate vector, a snail, lives in contaminated water.
Climate indirectly makes disease in tropical regions more severe by reducing agricultural production, which increases the risk of malnutrition. In a more direct way, hot weather and humid forests favor growth of the flies and mosquitoes that transmit malaria, yellow fever, dengue fever, trachoma, trypanosomiasis, and onchocerciasis.