Tropical disease is any disease that primarily occurs in tropical and subtropical regions. The term is commonly applied to any infectious disease that is endemic in hot, humid climates.
Early explorers from northern climes catalogued the so-called exotic diseases that they encountered in the tropics, and there are hundreds of tropical diseases that are public health concerns. The World Health Organization (WHO) considers tropical diseases to include those diseases that are endemic in Africa, the eastern Mediterranean, Southeast Asia, the Western Pacific, and Latin America. However, tropical diseases are present in—and increasingly spreading to—temperate regions.
Many tropical disease are caused by parasites, bacteria, or viruses that are transmitted by insect bites. These insect vectors thrive in hot climates with heavy rainfall, whereas they tend to hibernate or die out in cold weather. Diverse tropical wildlife provide natural reservoirs for disease, many of which are zoonoses (animal diseases that can be transmitted to humans). Hotter temperatures may promote more efficient reproduction of pathogens, both inside and outside their hosts. Finally, tropical countries are among the poorest in the world, with few resources for preventing the establishment and spread of disease. Thus, tropical disease causes immense suffering throughout the developing world, sickening and killing, interfering with children's physical and cognitive development, keeping people from work and school, and perpetuating a cycle of poverty and disease.
Tropical diseases are usually referred to as neglected tropical diseases (NTDs) because they have been largely eradicated in the developed world. NTDs persist in poor countries, marginalized communities, and conflict zones. Throughout most of the twentieth century, resources were directed to infectious diseases in developed countries, to the neglect of tropical disease. As a result, there are few vaccines against tropical disease, and many NTDs are difficult to treat or incurable.
The primary risk factors for tropical disease are residing in or visiting an endemic area. Poverty, lack of access to clean water and sanitation, natural disasters, political instability, violent conflicts, and human displacement are all risk factors for tropical disease.
At least one NTD is present in 149 countries and territories. At any given time, every low-income country in the world is affected by at least five NTDs, and some NTDs affect the southern United States, especially among the poor. Tropical disease is increasing worldwide and spreading beyond the tropics with international travel, human migration, deforestation, and warming global temperatures that allow disease agents and their vectors to invade higher altitudes and latitudes.
Malaria is the most prevalent NTD caused by protozoans. Five different parasites of the genus Plasmodium cause malaria in different parts of the world. Plasmodium is transmitted by the bite of female Anopheles mosquitoes that feed on human blood. The parasites invade red blood cells, causing repeated cycles of fever, chills, and other symptoms.
Chagas or American trypanosomiasis is caused by the Trypanosoma cruzi. It is transmitted by a bloodsucking “assassin” or “kissing” bug that resembles a cockroach. Chagas infects up to eight million people in the Western Hemisphere, including about 300,000 in the United States, and is a major cause of heart failure and sudden death. Pregnant women can transmit it to their babies. Some experts have called Chagas “the new AIDS of the Americas,” because its spread resembles the early spread of HIV, it has a long incubation period, and it is difficult or impossible to cure. African trypanosomiasis or sleeping sickness is caused by Trypanosoma gambiense and Trypanosoma brucei that are transmitted by tsetse flies.
Leishmaniasis is caused by Leishmania protozoa that are transmitted by tiny sand flies. Depending on the species, symptoms range from skin ulcers to more serious disease and even death. Leishmaniasis affects up to 12 million people worldwide and is increasing due to international travel, global conflicts, alterations in vector habitats, and susceptibility from HIV/AIDS and malnutrition.
Schistosomiasis is caused by Schistosoma trematodes (flatworms) in contaminated water. Freshwater snails are its natural reservoir. Many other kinds of trematodes—liver, lung, and intestinal flukes—cause foodborne trematodiases from consumed fish and other animals and even plants from contaminated waters. Tapeworms are intestinal parasitic flatworms transmitted through undercooked beef or pork. Cysticercosis is caused by pork tapeworm larvae, and neurocysticercosis is the most common parasitic disease of the central nervous system.
Roundworms—such as Ascaris lumbricoides that causes ascariasis, hookworms, and Trichuris trichiura that causes trichuriasis or whipworm disease—are transmitted through feces-contaminated soil. Toxicariasis is caused by roundworms that normally infect dogs and cats. Lymphatic filariasis is caused by thread-like filarial nematodes or roundworms that are spread between humans by mosquitoes. Loa loa is a filarial worm transmitted by deer flies. These worms cause elephantiasis—thickening of the skin and tissues. Lymphatic filariasis infects more than 120 million people. Onchocerciasis or river blindness is caused by a nematode transmitted by black flies. Approximately 18 million people have onchocerciasis infections, and about 300,000 have been permanently blinded by the disease. Dracunculiasis or guinea-worm disease is caused by drinking water contaminated with larvae of the nematode Dracunculus medinensis. The worms emerge through the skin causing painful blisters.
Tuberculosis (TB), Buruli ulcer, and leprosy are caused by related species of Mycobacterium. Although TB is now rare in developed countries, more than one-third of the human population has been infected with the bacterium, and the disease is prevalent in the tropics. TB is communicable through aerosols and, if untreated, has a mortality of above 50%. Buruli ulcer causes severe skin ulcerations and can spread to the bone. Leprosy is a chronic infectious disease of the peripheral nerves and upper respiratory tract.
Other bacterial NTDs include:
Dengue, caused by four related flaviviruses, is spread by infected female Aedes mosquitoes, the same mosquito that transmits yellow fever, which killed millions before the development of a vaccine in the 1930s. Dengue causes a range of symptoms from severe muscle and joint pain to hemorrhage, shock, and death.
Viral hemorrhagic fevers are potentially fatal NTDs caused by various RNA viruses. They are highly infectious, with symptoms that include fever, vomiting, gastrointestinal bleeding, and low blood pressure. They are caused by viruses in four different families:
Tropical disease diagnosis may be straightforward where a disease is endemic and known outbreaks are occurring. However, many NTDs have similar symptoms and occur in regions with few health care workers. Migrants, emigrants, and travelers arrive with NTDs that local health care providers are unfamiliar with. Newer diagnostic tools are being developed for some NTDs to facilitate diagnoses in remote areas.
Some tropical countries, such as Brazil, that have been able to invest in public health and sanitation have had dramatic success in reducing or eliminating NTDs. Other countries have effectively interrupted the transmission of some NTDs. However, progress has been slow, and natural disasters and armed conflict increase the tropical disease burden. For example, cholera swept through the population in the aftermath of the 2010 earthquake in Haiti, due to overcrowding, poor sanitation, and limited access to clean food and water.
The World Health Organization (WHO), the U.S. Centers for Disease Control and Prevention (CDC), and other organizations work to reduce illness, disability, and death from NTDs. The CDC began a global campaign to eradicate guinea worm in 1980. At the time, there were about 3.5 million cases annually in 20—mostly African—countries, and an additional 120 million people were at risk through unsafe drinking water. By educating villagers and providing cloth and pipe water filters and larvicides to kill the fleas that transmit the disease, there had been a 99% reduction in guinea-worm infections by 2012. Guinea worm was on the verge of becoming the second human disease, after smallpox, to be completely eradicated. As of 2012, the CDC was focusing on soil-transmitted ascariasis, hookworm, and whipworm, lymphatic filariasis, onchocerciasis, schistosomiasis, and trachoma, since these diseases can be controlled or even eradicated by mass administration of safe and effective drugs. There is also an ongoing effort, led by the WHO and the Bill and Melinda Gates Foundation, to eradicate malaria, with short-term focus on new antimalarial drugs and a long-term aim of vaccine development.
Unfortunately, pharmaceutical companies have had little financial incentive for developing new diagnostic tests, drugs, and vaccines for NTDs that afflict poverty-stricken regions. However, new tropical disease institutes are focusing on research and development and training health care providers to recognize, diagnose, and treat NTDs. In part, this is a response to the increase in NTDs in the United States and other developed countries, both from immigration and from domestic transmission.
It is unlikely that tropical disease will ever be completely eradicated. With population growth, increased human migration, and climate change, NTDs are expected to spread, and new diseases will continue to emerge. However, with prevention and the development of new vaccines, diagnostic tools, and treatments, tropical disease could be controlled.
Prevention is key to tropical disease control, and vector control is a major strategy. Draining wetlands reduces populations of insects and other vectors. Insecticides can be used on bed nets, clothing, exposed skin, and insect habitats, and inside buildings. Wells and water filtration and treatment are necessary for eliminating waterborne parasites. Better sanitation helps control the spread of NTDs through human waste. The distribution of available vaccines, prophylactic drugs for preventing disease before and after exposure, and drugs for treating infections and infestations after they develop are important preventives measures. However, the most important goal is economic development—particularly investment in more productive subsistence farming—so that affected communities can afford to prevent and treat their own tropical disease.
See also Antimicrobial resistance ; Chagas disease ; Cholera ; Climate change ; Dengue fever ; Ebola virus disease ; Guinea worm disease ; Hemorrhagic fevers ; Malaria ; Parasites ; Schistosomiasis ; Traveler's health ; Tuberculosis ; Vector (mosquito) control ; Yellow fever ; Zoonoses .
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Margaret Alic, PhD