List of Neglected Tropical Diseases

Aedes aegypti mosquito

 James Gathany/CDC 

Neglected tropical diseases (NTD) affect one in six people worldwide, and over half live in areas where they could become infected. While most NTDs are concentrated in the tropical areas of Asia and Africa, many can be found all over the world—including wealthy nations like the United States.

The World Health Organization (WHO) formally recognizes at least 21 infections as NTDs, each of which has its own distinct causes, symptoms, and methods of transmission.

The one thing they all have in common, however, is that they overwhelmingly affect the poor.

Buruli Ulcer

This disease is caused by the bacterium Mycobacterium ulcerans. While it’s not clear how people get the bacteria, once inside the body, they make a toxin that attacks and destroys human tissue, resulting in sores typically on the person’s arms or legs.

Antibiotics can help treat the infection, but without them, the disease can cause life-long disability or deformities. While most cases are in Central and West Africa, infections are found in wealthier nations, too, including Australia and Japan.

Chagas Disease

Found mostly in Latin America, this condition is the result of Trypanosoma cruzi, a parasite transmitted by the triatomine bug, or the “kissing bug,” an insect that likes to live in the adobe walls and thatched roofs often found in impoverished areas. Most people with Chagas disease don’t have any symptoms at all, and those who do have symptoms that can be mistaken for other diseases, like the flu.

Unless it’s treated, the parasite can cause a chronic (long-term) infection that can lead to heart issues or death. Roughly 8 million people worldwide have Chagas disease—including more than 300,000 people in the United States and another 25 million people around the globe are at risk for getting it, according to the WHO.

Dengue

Often called “breakbone fever” because of the physical pain it can cause, the dengue virus is spread by Aedes mosquitoes, the same species that can transmit West Nile, yellow fever, and zika. In severe cases, the virus can cause bleeding from the nose or mouth—a condition called dengue hemorrhagic fever.

Dengue is already endemic in more than 100 countries, and while it’s difficult to pin down exactly how many people are impacted directly (underreporting and misclassification of cases is an issue), a 2013 published report estimates there are anywhere from 284 to 528 million dengue infections every year worldwide.

Those numbers could go up. By one estimate, more than half the planet could be at risk for dengue, and some health officials are bracing for more cases as rising global temperatures expand mosquitoes habitats and increases in international travel make it easier for individuals to bring the virus to new places where the Aedes mosquitos are already common.

Chikungunya

Another mosquito-borne illness, chikungunya is often confused for dengue because it can cause many of the same symptoms, such as fever, muscle pain, headache, and rash.

Like dengue, there’s no effective treatment or cure for this virus, but so far the disease doesn’t seem to be quite as widespread.

Cases have popped up in over 60 countries, including in the Caribbean and Europe, but is most often confined to parts of Africa and Asia.

Dracunculiasis

Also known as guinea worm diease, this parasitic worm has a complex life cycle and infects humans through contaminated water. Once inside the body, the worm causes painful, burning sores. Infected individuals often attempt to get relief by going into water sources, where the worms then make their way out of the skin to release more larvae that can lead to even more infections.

Guinea worm disease has affected humans for centuries. There are reportedly documented cases in the Old Testament of the Bible, according to the WHO, but it is now on the brink of eradication, thanks to worldwide, coordinated efforts led by the Carter Center.

The number of infections has plummeted from more than 3 million in 1986 to just 30 cases total in 2017.

To get to this point wasn't easy. Global partners had to secure large amounts of political will in order to get the resources needed to investigate, contain, and report suspected cases; as well as educate and mobilize villages to filter unsafe water.

Echinococcosis

Caused by tiny tapeworms, this disease primarily affects animals, but it can spread to humans when people come into contact with feces from infected creatures, such as domesticated cats and dogs or livestock like sheep or goats. This often happens when food (ex. berries or vegetables) or water become contaminated with the parasite’s eggs, or after touching contaminated fur (such as petting a dog).

Two different classifications of tapeworms can cause this disease, both of which can lead to serious illness or death. The greater threat to people, according to the CDC, is alveolar echinococcosis disease, which can cause tumors in the liver, brain, and other organs. The other classification, cystic echinococcosis, typically doesn’t cause symptoms in people, but when it does, it can lead to cysts on vital organs that can be dangerous yet go unnoticed for years.

Foodborne Trematodiases

Also called foodborne trematode infections, this group of diseases is the consequence of ingesting flatworms (also called “flukes”) in their larval stage. This can happen when food—especially raw fish or crustaceans—isn’t fully cooked. While the WHO reports these infections happen mostly in East and Southeast Asia, and in Central and South America, at least 40 million people are infected worldwide.

African Sleeping Sickness

Many NTDs are transmitted through animals or pests, and African sleeping sickness (also known as human African trypanosomiasis) is no exception. This parasite is spread by the tsetse fly in rural Africa. Once in the bloodstream, the parasite slowly works its way to the central nervous system, sparking sleeping disorders, sensory disturbances, seizures, and a host of other serious physiological and psychological conditions.

Treatments exist, but they’re often complicated, difficult to administer and result in some unpleasant side effects. If the disease is left untreated, however, it’s often fatal.

Leishmaniasis

Caused by the phlebotomine sandfly, leishmaniasis is a condition caused by parasites in the Leishmania species. Most people infected with the parasite don’t have any symptoms, but leishmaniasis specifically refers to those who do.

Infections can manifest in several ways, most commonly skin sores (cutaneous leishmaniasis), or the more serious visceral leishmaniasis, which can cause serious weight loss, fever, anemia, or swelling of the spleen or liver.  The latter of which is highly fatal if not treated.

Often found in densely populated areas, outbreaks of leishmaniasis in recent years have hit conflict zones and refugee populations particularly hard.

Leprosy

Leprosy is a chronic infection caused by the bacterium Mycobacterium leprae, most known for the lesions and discoloration it can cause on a person’s skin. Without treatment, the infection can lead to lifelong disfigurement or nerve damage.

More than 200,000 cases of leprosy are reported each year, including 150-250 in the United States, with most infections occurring in South and Southeast Asia. Despite affecting humans for millennia, health professionals and researchers still aren’t sure how the bacteria spread, though some think it could be transmitted via touch and/or through respiratory droplets.

Lymphatic Filariasis

Known for causing extensively swollen limbs, elephantiasis is a painful infection caused by nematode worms (filariae). The microscopic worms get into the human body through mosquito bites and reproduce in the human lymphatic system. The swelling, pain, and disfigurement can be so intense that people are unable to work, with endemic countries (primarily in Southeast Asia and parts of Africa) losing an estimated $1 billion a year—in some cases hurting economic activity up to 88 percent, according to the WHO.

Global partners are currently trying to eradicate the disease by helping endemic countries treat whole populations at risk. According to the CDC, treating people with the anti-parasitic medications every year for 5+ years could be enough to break the cycle of transmission that keeps infections going, but with more than 120 million people infected, there is still a lot of work to be done before the disease is eliminated completely.

Mycetoma

Mycetoma, chromoblastomycosis, and other deep mycoses—not much is known about how prevalent this group of diseases is, at least in part, because of who it affects: very low-income adults working as laborers, herdsmen, or agriculturists in developing countries. Such workers often walk barefoot, and it’s believed that bacteria or fungi get into the body through open wounds in the skin.

Over time the infection leads to swelling or sores (typically in the foot) that get more and more debilitating over time. Drugs are available to treat the infection, but they aren’t great. They’re costly and lead to numerous side effects. Surgery is frequently necessary to treat the fungal infections.

Onchocerciasis

River Blindness is a leading cause of preventable blindness worldwide. The worms responsible for the disease (called filarial worms or O. volvulus) spread from person to person through biting black flies, and they can live for up to 15 years in the human body. An estimated 37 million people are currently infected, almost exclusively in Africa, with those living in rural agricultural areas being the hardest hit by the disease.

River blindness can be treated successfully with a single dose of a medication, but to eliminate the disease completely, countries must also control the black fly population responsible for its spread—something many endemic countries simply can’t afford to do.

Rabies

Thanks to a vaccine developed over a century ago, death from rabies is nearly 100 percent preventable, so long as the vaccine is administered before an infected person starts to have symptoms—a tall order for areas without good access to medical care.

Without the vaccine, rabies is almost always fatal and is the cause of tens of thousands of deaths every year worldwide. Most of these deaths occur in Asia and Africa, though the disease is found in 150 countries around the world and on every continent but Antartica. 

The key to eliminating deaths from rabies, however, isn’t vaccinating humans, but vaccinating dogs. The overwhelming majority of human rabies cases are the result of bites from rabid dogs, which is why the United States spends more than $300 million a year on rabies prevention, mostly to vaccinate canines.

These efforts have been wildly successful—only 23 cases of rabies in humans in the United States have been reported in the past decade, according to the CDC. But to eradicate the disease from the planet, the same strategy will need to be employed on a global scale.

Scabies

Scabies and other ectoparasites were added to the WHO’s list of NTDs in 2017, scabies is extremely common in developing countries, affecting more than 200 million people worldwide on any given day, mostly in low-income tropical areas.

It’s caused by parasitic mites that burrow into the skin to lay their eggs. This sets off an immune response by the body that leads to itching and a rash. Scratching at the affected areas can also lead to bacterial infections in the skin, which can complicate the condition or lead to more serious issues inside the body.

The most vulnerable groups impacted by scabies are young children and older adults, living in overcrowded and impoverished areas, where the mites can spread from person to person. 

Schistosomiasis

Schistosomiasis ​(snail fever) kills an estimated 280,000 people annually in Africa alone, making it the world’s deadliest NTD. Millions more live with serious physical and cognitive disabilities as a result of the infection.

The disease is caused by parasitic flukes (worms) that live in freshwater where certain snails are also infected. When kids go swimming or fishing in the water, or people come into contact with the water through everyday activities, the larval forms of the worm burrow through the skin and make their way to blood vessels where the adult worms will eventually release their eggs.

Schistosomiasis is overwhelmingly a disease of poverty. Without access to clean water or adequate bathrooms, people continue to take in the parasite and spread its eggs through their feces, perpetuating the spread of the disease.

Kids with long-term or repeated infections often experience anemia or malnutrition, which can cause lifelong learning challenges and contribute to the cycle of poverty.

Soil-Transmitted Helminths

Like schistosomiasis, soil-transmitted helminths (like hookworm, roundworm, or whipworm) disproportionately impact the poor. These worms live in the intestines of infected people, who then excrete the worms’ eggs in their feces.

In areas without toilets or latrines, it’s not uncommon for people to defecate on the ground or in shallow holes, where the eggs can contaminate water sources or food—leading to new or repeated infections. Hookworms in the larval stage can also burrow through the foot when people walk barefoot in contaminated soil. If these communities were to have access to basic sanitation, much of the transmission cycle would be interrupted.

Over a billion people currently are infected with these intestinal worms, and more than 4 billion (or over half the global population) are at high risk for infection, nearly all of whom live in impoverished areas. The WHO estimates more than 880 million children need medications for these worms, in order to prevent potentially serious consequences as a result of the infections, including delayed development, vitamin A deficiency, or impaired growth.

Snakebite Envenoming

The WHO added snakebite envenoming to its list of NTDs in June 2017. Of the more than 3000 species of snakes that exist in the world, about 250 of them are considered dangerous by the WHO. These snakes live in 160 countries, and their venomous bites cause 81,000-138,000 deaths every year worldwide, along with hundreds of thousands of amputations and disabilities.

These statistics could grossly under-represent the true scope of the problem because many at-risk communities—especially those in rural and agricultural areas—either don’t have access to medical care or choose to get their treatment from non-medical sources due to cultural beliefs.

Taeniasis and Cysticercosis

These two conditions are caused by the same tapeworm: T. solium. An infection by adult tapeworm in the human intestines (taeniasis) is significantly less serious than an infection by the worm in its larval stage (cysticercosis).

When people inadvertently ingest the worm’s eggs (typically through contaminated food or water or as a consequence of poor hygiene), the worms develop into larvae, which can form potentially devastating cysts throughout the body, including in the eyes, muscles, and central nervous system.

Trachoma

The leading cause of infectious blindness in the world, trachoma currently impacts around 84 million people globally, many of whom are children. This painful and debilitating disease is the result of repeated infections by the bacteria Chlamydia trachomatis, which, if left untreated, can cause the eyelids to turn inward. Over time, as eyelashes scrape the eyeball, it causes permanent damage and, in some cases, irreversible blindness.

The bacteria can spread quickly in overcrowded areas that don’t have access to safe water or bathrooms, especially among young children and their caregivers, who live in extreme poverty. According to the CDC, some rural communities report 60-90 percent of their children being infected with the bacteria.

Yaws

Another disease that can lead to permanent disability in children is yaws, a chronic skin disease that is closely linked with poverty. Caused by the bacterium Treponema pallidum (a close relative of syphilis), the disease causes bumps and ulcers on the skin that are highly infectious.

Yaws is extremely easy to treat. It takes only a single dose of an inexpensive antibiotic. But in the absence of treatment, it can affect bones and cartilage and lead to permanent disfigurement or disability. The vast majority (75-80 percent) of those infected are children under 15 years old, most of whom live in rural and/or impoverished areas without health services.