Vikram Tarugu, M.D., AGA, ACG
A tropical disease is an illness that is most common or is limited to tropical or subtropical areas. These areas are characterized by high temperatures and humidity, which are often necessary for the disease-causing organism (vector) or their animal hosts to live. Depending on the disease, it may occur throughout the year, in relation to seasonal climatic changes or at irregular intervals. Outbreaks of these diseases may also occur outside tropical areas but tend to be sporadic.
More than two dozen illnesses may be classified as tropical diseases. They are diverse and linked more by where they occur than any other factor. They are caused by a variety of organisms, including viruses (e.g., dengue fever), bacteria (e.g., cholera, typhoid fever), protozoa (e.g., malaria, African sleeping sickness) and parasitic worms (e.g., schistosomiasis, filariasis). These organisms may be transmitted through various means, including insect bites, contaminated foods and environmental exposure.
While residents of areas where these diseases are endemic tend to be at highest risk, travelers to the region may also be infected. Several methods are available to reduce the chance of developing a tropical disease. Before the trip, the traveler is encouraged to gather information about current disease outbreaks from the U.S. Centers for Disease Control and Prevention (CDC) and World Health Organization(WHO). Patients should also contact their physician for appropriate vaccines or other prophylactic measures, such as medications. Insect and tick bites, which are potential routes of transmission, may be prevented by wearing long sleeves and pants and applying insect repellent to exposed skin. Thoroughly cooking foods and boiling water kills bacteria and may help prevent cholera, typhoid fever and other foodborne or water-related illnesses.
The tropical diseases cause a wide variety of symptoms that can range from mild to life threatening. In the early stages of disease many resemble symptoms of the flu but may quickly become severe. Travelers who develop symptoms, even within a year after traveling, should consult their physician and discuss the areas visited.
A tropical disease may be suspected based on the patient’s symptoms and travel history, but tests may be required for confirmation. These tests often require blood, urine, stool or tissue samples. In some instances, laboratory testing may be required, such as viewing the organism under a microscope. A wide range of treatment options are available for tropical disease. Some may be treated with medications that kill the responsible organism. For others, medications may be prescribed to prevent the disease from progressing. When no medications are available, supportive care is recommended to allow time for the body’s immune system to respond to the infection.
About tropical disease
A tropical disease is any illness that is most common or only occurs (endemic) in tropical or subtropical areas. Beyond location, many have little in common with each other. They are caused by a variety of factors, including viruses, bacteria, protozoa and parasitic worms (nematodes and trematodes). Some frequently cause no symptoms, while others are severe and may become fatal. The routes of transmission also range widely and include insect bites, contaminated water and food, and direct contact with an infected person.
The tropics are defined by geography and climate. They are the areas between the Tropic of Cancer and the Tropic of Capricorn (23.5 degrees of latitude north and south of the equator, respectively). In the Western Hemisphere, this includes much of the Caribbean and Mexico, Central America and much of South America. Also considered tropical are Africa between the Sahara Desert and South Africa, southern Asia, much of India, northern Australia and many of the island nations in the western Pacific Ocean. These areas have high temperatures and humidity throughout the year. Seasons are marked more by changes in rainfall than other conditions. The subtropics are the climatic region between tropical and temperate areas. Unlike the tropics, they do not occur within defined latitudes.
The reasons these diseases are geographically limited vary. Some can not develop outside the environmental conditions (e.g., temperature, humidity) provided by tropical and subtropical climates. Others require specific animal hosts that occur within this limited range. Certain diseases once had broader ranges but have been eliminated. Many of the countries in the tropics are developing nations and may not be able to afford the sanitation and medical programs required to eliminate these diseases.
The tropical diseases are largely absent naturally or as a result of eradication programs in the United States and other industrialized nations. Outbreaks in these countries are often linked to immigrants or travelers returning from areas where the diseases are endemic. They may occur sporadically, but are often limited by seasonal climate changes.
Types and differences of tropical disease
The tropics include portions of nearly every continent and some of the world’s most populous countries. The number of diseases endemic to such a large area with so many people is difficult to estimate. To better understand them, they may be categorized by the organism that causes the disease.
Viruses are tiny parasites that often exist as inactive spores outside of a host organism. Once inside a plant, animal or bacteria cell, they are able to replicate and may cause disease. The tropical diseases they cause include the following:
- Dengue fever and dengue hemorrhagic fever. These two diseases are caused by four viruses that typically occur in urban areas in Asia, Africa, the South Pacific, the Caribbean, and Central and South America. The viruses transmit from person to person by mosquito bites and cause 100 million cases annually, according to the U.S. Centers for Disease Control and Prevention (CDC). Patients who have had dengue fever previously are more likely to develop the more serious dengue hemorrhagic fever if infected later in life. Overall, these diseases pose a low risk to travelers unless an epidemic is in progress in the area.
- Hemorrhagic fever. This group of more than a dozen viral diseases causes illness throughout the tropics. In addition to dengue hemorrhagic fever, they include yellow fever, Lassa fever, Ebola hemorrhagic fever and Rift Valley fever. The number of annual infections varies widely and mortality can range from less than 1 percent of those infected with hemorrhagic fever with renal syndrome to nearly 90 percent of people who develop Ebola. Outside of periodic outbreaks, the chance for travelers being infected with most hemorrhagic fevers is low.
Bacteria can have a more complex effect on humans than viruses. These one-celled organisms may live freely in the environment or within a plant or animal. While some are necessary for the human body to function, others may cause disease. Tropical diseases caused by bacteria include:
- Typhoid fever. Salmonella Typhi (also known as Salmonella enterica serovar Typhi) is responsible for typhoid fever. The CDC estimates 12 million to 22 million cases and 200,000 deaths occur internationally each year. Among those, 400 occur in the United States, mostly among recently returned travelers. The bacteria are expelled through the feces of infected people. Without proper sanitation, they can be spread through contaminated food or water. Outside the body, the bacteria can also be found in bodies of water contaminated with sewage.
- Cholera. This disease is widespread and is often found outside the tropics. It occurs in Indonesia, throughout much of Asia, Eastern Europe, Africa, the Iberian Peninsula (Spain and Portugal) and Peru. In 2003, more than 100,000 cases in 45 countries were reported to the World Health Organization (WHO). Much like typhoid fever, it is spread by eating contaminated foods or beverages. After infection, patients may show no or only mild symptoms but may still transmit the disease in their feces. The bacteria can also live in contaminated brackish (slightly salty) water and infect shellfish. In general there is a low risk for travelers who avoid eating raw or undercooked foods.
- Buruli ulcer. Little is known about this disease caused by Mycobacterium ulcerans infection. The number of cases of Buruli ulcer is uncertain, but it occurs in 30 countries in sub-Saharan Africa, North and South America, Australia, and Asia. Scientists are also unsure of how it is transmitted, but it is often associated with swimming in rivers and skin injury and may be carried by aquatic insects, fish or mosquitoes. Most cases are reported in children younger than 15 but it may occur at any age.
- Leprosy or Hansen disease. Mycobacterium leprae is responsible for this rare disease endemic to 15 countries in Africa, Asia and Latin America. In 2002, the CDC estimated more than 750,000 cases in the world, with fewer than 100 in the United States. Transmission typically requires close or long-term contact with an infected person, making it a very low risk to travelers.
Like bacteria, protozoa are also single-celled organisms. They have a more complex structure, including a nucleus (the core of a cell containing chromosomes) and are often classified within the animal kingdom. Some may infect humans and cause disease. In the tropics, these diseases include:
- Malaria. Parasites of the Plasmodium genus spread by mosquitos are responsible for malaria in humans. Four species have long been known to cause variations of the disease, and a fifth has recently been discovered that was once thought to only affect monkeys. They are endemic to more than 100 countries and may infect up to 500 million people each year, according to the WHO. More than 1 million people die from malaria every year. Travelers are at risk of being infected if prophylactic, or preventative, medications are not taken. In addition, preventative medications are recommended for travel to all areas where people may be exposed to malaria. For example, recent travelers to the Caribbean countries of the Bahamas and the Dominican Republic have contracted malaria even though it is not endemic to these areas.
- Leishmaniasis. This disease is caused by more than 20 species of protozoa that are spread by sand flies. According to the CDC, there are approximately 2 million new cases each year in 88 countries. The majority occur in India, Bangladesh, Nepal, Sudan and Brazil. Leishmaniasis is also found throughout much of the Americas, including occasionally in south-central Texas. The disease occurs most often in rural settings and rarely infects travelers.
- African sleeping sickness. The tsetse fly is responsible for spreading the two Trypanosoma parasites that cause both forms of this disease (East African and West African). Each year, up to 500,000 people are infected in 36 sub-Saharan countries in Africa, according to CDC estimates. Of those people, 50,000 die, according to the WHO. Of the two varieties, East African sleeping sickness is more severe and is also responsible for most cases reported in international travelers.
- Chagas disease or American trypanosomiasis. Like African sleeping sickness, Chagas disease is also caused by a parasite of the genus Trypanosoma. According to CDC estimates, it infects 11 million to 18 million people annually in 18 countries. The protozoa are occasionally found in the United States, but normally range from Mexico through Central and South America. The parasites are spread in the fecal matter of triatomine bugs, which are often found in the walls of substandard housing in areas where the disease is endemic. While it rarely directly causes death in adults, the WHO estimates Chagas infection lowers life expectancy on average by nine years.
Parasitic worms may also cause disease in humans. These include many species of roundworms (nematodes) and flatworms (trematodes or flukes). In the tropics, diseases caused by these worms include:
- Schistosomiasis or bilharzia. Five species of parasitic trematodes are responsible for this disease that is endemic to 74 countries. It has infected 200 million people, according to the CDC, with most cases occurring in sub-Saharan Africa. The parasites are also found in South America, the Caribbean, the Middle East, southern China and Southeast Asia. The parasite spends part of its life in snails inhabiting bodies of water contaminated with urine or feces of infected humans. The larval form of the worm leaves its snail host and is free-swimming. If a person contacts the water (e.g., swimming, bathing), the larvae are able to penetrate skin and grow in the blood vessels. The eggs they lay move through the body and may cause severe disease.
- Filariasis or lymphatic filariasis. This disease is caused by three species of nematodes found in South America, the Caribbean, Africa, Asia and the islands of the Western Pacific. According to the WHO, 120 million people in 80 countries have been infected around the world, with approximately 40 percent of cases occurring in India. While rarely fatal, filariasis is a leading global cause of long-term disability. The larval form of the parasite is spread by mosquitoes from person to person. Once inside the body, they move into the patient’s lymphatic system, where they may cause fluid to collect. For the parasites to reproduce and cause disease, the patient must be infected repeatedly over months to years. This limits the risk to short-term travelers.
- River blindness or onchocerciasis. Like filariasis, river blindness is caused by a parasitic nematode. The WHO estimates this disease infects a total of 17.7 million people, causing 270,000 to go blind. Of the people infected, 99 percent are from 28 African countries. The remainder of the cases occur in Yemen, Mexico, Guatemala, Ecuador, Colombia, Venezuela and Brazil. Black flies spread the larval form of the parasite and the severity of the disease is often related to the number of times the patient is bitten.
Risk factors and causes of tropical disease
The greatest risk factor for developing a tropical disease is to live in an area where these diseases are endemic. While travelers to these regions are also at risk, their chance of infection tends to be much lower than residents because of more limited exposure.
Infection with a tropical disease may result from several routes. Some may spread directly from the environment (e.g., swimming in a river). Others require a bite by an animal or insect vector for transmission, and some require close contact with an infected person. Possible routes of tropical disease infection include:
- Bites. Certain species of insects and ticks may carry tropical disease in their saliva without the insect showing signs of illness. They are transmitted when the mosquito, tick or fly bites a person to feed. The severity of some diseases is related to the number of insect bites the person receives.
- Contaminated food or water. Cholera and typhoid fever are two of the infections that may result from consuming raw or undercooked foods, including fruits, vegetables and shellfish. Beverages containing contaminated water or ice are also possible routes of transmission of these diseases.
- Exposure to animal wastes. Some tropical diseases are excreted through animal urine or feces. They may become airborne and inhaled with dust or may infect through contact with a person’s eyes.
- Exposure to contaminated water. Contact with bodies of water where the parasitic worm responsible for schistosomiasis lives can result in infection. Some other diseases may be transmitted through similar means.
- Person-to-person contact. Some tropical diseases are transmitted by person-to-person contact. For example, fluid exchange, such as occurs during sexual contact, is a possible route of transmission for several types of hemorrhagic fever (e.g., Lassa fever). In addition, transmission of leprosy is believed to require very close or long-term contact with an infected person during certain stages of the disease.
Signs and symptoms of tropical disease
The signs and symptoms of tropical diseases vary greatly. Some people may show no sign of infection while others can become ill quickly and develop severe symptoms. Many tropical diseases are characterized by fever, nausea and other symptoms similar to the flu. Some diseases begin with flu-like symptoms but later progress to more severe symptoms, such as schistosomiasis. Patients with these symptoms who have traveled to areas where the diseases are endemic should consult their physician.
Possible major signs and symptoms of some tropical disease include:
|Disease||Signs and Symptoms|
|Malaria||Cycles of fever (which first develops at least one week after exposure), chills, headache, nausea, vomiting, diarrhea|
|Typhoid fever||High fever, headache, anorexia, slow heart rate, enlarged spleen|
|Cholera||Diarrhea, vomiting, leg cramps|
|Dengue fever||Fever, headache, muscle and joint pain, nausea, vomiting, rash|
|Buruli ulcer disease||Bumps under skin, swelling in legs and arms, open sores in skin, disfiguring scarring from the skin sores, chronic infection may invade bone|
|Leishmaniasis||Skin sores, fever, weight loss, anemia|
|African sleeping sickness||Skins sores, fever, severe headaches, irritability, rash, swelling in extremities, enlarged lymph glands and spleen, joint pain, extreme fatigue|
|Chagas disease||Swelling at infection site, fatigue, fever, enlarged liver, swollen lymph glands, rash, diarrhea|
|Schistosomiasis||Rash, itchy skin, fever, chills, muscle aches, seizures, liver disease|
|Filariasis||Recurrent fever, swelling (typically in arms, breasts, legs and genital areas), skin becomes hard and thick|
|River blindness||Rash, eye lesions, bumps under skin|
|Leprosy||White patches of skin, loss of sensitivity to touch, skin thickening and folding, muscle pain|
|Chikungunya feverLeprosy||Severe headache, chills, fever, joint pain, nausea and vomiting. Joints of the extremities become swollen and painful to the touch|
Diagnosis methods for tropical disease
Diagnosis of tropical disease may involve a medical history and physical examination. The medical history will likely include questions on symptoms, including their duration, severity and progression. Patients should also tell the physician about recent travel to tropical or subtropical areas. The physical examination will check for signs and symptoms that may not have been apparent to the patient.
If a tropical disease is suspected based on the patient’s symptoms and recent travels, the physician may perform one or more tests to confirm the illness or determine the infectious agent responsible. Possible diagnosis methods include:
- Blood test. Samples may be viewed under a microscope to identify the organisms responsible for malaria, African sleeping sickness and Chagas disease. The blood may also be tested for antibodies to dengue fever, dengue hemorrhagic fever, schistosomiasis and typhoid fever.
- Urine or stool sample. The body may excrete the agents responsible for some diseases in urine or feces. These waste materials can be examined for eggs to confirm schistosomiasis and the bacteria that cause typhoid fever.
- Biopsy. The parasite responsible for river blindness, leishmaniasis and African sleeping sickness can be viewed in skin samples under a microscope. Buruli ulcer can also be confirmed by viewing the responsible bacteria in samples from affected tissues.
- Spinal tap. The parasites spread by the tsetse flies that cause both West and East African sleeping sickness can be identified with a sample of cerebrospinal fluid from the spinal cord.
Treatment options for tropical disease
The diverse causes of the many types of tropical disease mean there are a variety of treatment methods. Many diseases respond to specific medications. Classes of drugs used to treat tropical disease include:
- Antibiotics. Medication may help shorten and reduce the severity of cholera and typhoid fever. They are also used to treat leprosy and the earliest stages of Buruli ulcer disease.
- Antihelminthics. These drugs reduce the number of parasitic worms in the body, such as those that cause river blindness, schistosomiasis and filariasis. They often do not completely cure the disease, but may slow or stop its progression.
- Antiprotozoals. Many of the diseases caused by single-celled parasites can be treated with these medications. They include both types of African sleeping sickness, some forms of leishmaniasis and the acute stage of Chagas disease.
Supportive therapy for patients may be recommended while tropical diseases are being treated with medication, and for diseases that can not be cured with medications such as dengue fever. This often helps reduce patient stress and gives time for the body’s immune system to cope with the infection. Therapies for many of the tropical diseases include:
- Cholera. Dehydration caused by fluids and salts lost through diarrhea are replaced through drinking large volumes of liquid. In severe cases, intravenous (I.V.) fluid replacement may be necessary.
- Hemorrhagic fever. Treatment typically requires intravenous fluids to prevent dehydration and maintain blood volume and electrolyte levels. Bed rest may be recommended to reduce the risk of trauma and potentially serious bleeding. Pain relievers with acetaminophen may also be used to reduce the patient’s fever.
- Filariasis. Patients with this disease should wash swollen areas regularly with soap and water to reduce the risk of bacterial infection. Elevating and exercising affected limbs will often help reduce swelling.
- River blindness. Surgery may be necessary to remove nodules caused by parasites living under the patient’s skin.
- Buruli ulcer. Surgery may also be necessary for patients with Buruli ulcer to remove dead, or necrotic, tissue.
Prevention methods for tropical disease
There are many global, regional and local efforts to reduce the occurrence of tropical diseases. These often focus on reducing the populations of insects or animals that transmit the diseases. Efforts to improve sanitation are ongoing to reduce contamination of food and water with disease-causing organisms. These efforts are likely to eventually reduce the occurrence of some tropical diseases. International efforts through organizations such as the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC) are also involved in monitoring populations at risk for tropical diseases.
Vaccines are also used to prevent travelers or residents of the areas where the diseases are endemic from being infected. There are vaccines available for typhoid fever and cholera. However, the CDC has assessed U.S. travelers as being at low risk for cholera. Therefore, the cholera vaccine is no longer available in the United States. The tuberculosis vaccine can be used to prevent occurrences of leprosy.
A number of preventative, or prophylactic, medications are available for malaria. The type of antimalarial medication prescribed depends on the region, because some types of malaria have become resistant to certain medications.
Visitors to tropical areas can also act before, during and after their travels to limit their risk of being sickened with one of these diseases. Ways to reduce the chance of being infected while traveling include:
- Gather information. Before traveling to a tropical country, it is advisable to learn about potential disease risks in the region. The CDC and WHO provide information on current outbreaks in countries around the world. Patients should discuss their travel plans with their physician at least four weeks before the trip. The physician may recommend vaccines or other preventative measures. To be most effective, medications should be taken according to the instructions, even after returning from the trip.
- Reduce bites. Long pants and sleeves reduce the amount of skin exposed to insect and tick bites. Khaki or olive colors do not attract as many insects as bright or dark clothes. Treating clothes with an insecticide will also reduce bites. In addition, effective insect repellents (such as ones that contain DEET) should be applied to exposed skin. Sleeping under a bed net treated with an insecticide helps prevent many types of insect bites, including those from mosquitoes.
- Eat and drink safely. Before drinking, water should be boiled or treated with chlorine or iodine to kill potential disease-causing organisms. Coffee, tea and other beverages made with boiled water are often safe. It is important to remember that ice may be made from contaminated water, and may be contaminated as well. Bottled carbonated beverages without ice and bottled water are also usually safe to drink. Foods should be thoroughly cooked and served hot. Raw fruits and vegetables should be avoided unless they were washed with bottled or boiled water and then peeled by the traveler. Eating salads is not recommended while traveling in tropical or subtropical regions, due to difficulty ensuring that leafy greens are washed appropriately.
Questions for your doctor on tropical disease
Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to tropical disease:
- Am I at risk for a tropical disease?
- What are the types of tropical diseases?
- What regions of the world have the highest prevalence of tropical disease?
- What can I do to prevent tropical disease while traveling?
- Are vaccines available to prevent the tropical diseases?
- How are tropical diseases spread?
- What are some signs and symptoms of tropical disease?
- How long after returning from my travels should I be concerned about tropical disease?
- What treatment options do I have?
- What are my chances of survival?