The Overlooked Tropical Diseases You May Not Know About

This article first appeared on our sister site, Infection Control Today.

For the month of February 2024, Contagion Live and Infection Control Today are collaborating on rare infectious diseases for Rare Disease Awareness Month, examining both old and new infectious diseases that healthcare workers need to know.

It starts with a fly, feeding off bacteria-ridden, purulent discharge from a human eye and carrying it to an uninfected person.¹ Shortly after its next infection, within 5 to 12 days, mild itching and irritation begin in the eye and around the eyelids, with the same purulent discharge developing. Left untreated and with repeated infections due to living within endemic areas of the world, the cornea develops multiple scratches, eventually leading to blurred vision and possible blindness. This communicable disease, trachoma, is the world’s leading cause of infectious blindness. Endemic in 42 countries, it’s responsible for the visual impairment or blindness of 1.9 million people. And yet, it’s likely that you’ve never heard of it.¹³

The World Health Organization (WHO) estimates that the 20 conditions, or neglected tropical diseases (NTDs), affect more than 1 billion people, requiring preventative and curative interventions for 1.6 billion people.¹²

These ancient diseases are known to have profound health, social, and economic consequences, stagnating the development of already impoverished communities. Commonly found in tropical and subtropical regions, these conditions cause significant challenges with school attendance, the ability to earn wages, and the care of families in already economically unstable communities.

The 5 most prevalent of these diseases include Trachoma, Onchocerciasis (River Blindness), Schistosomiasis (Snail fever), Lymphatic Filariasis (elephantiasis), and soil-transmitted helminths (hookworm, roundworm, and whipworm).

Onchocerciasis (River Blindness)

The Simulium blackfly is an aquatic insect breeds exclusively near or in running water. Once flies reach adulthood, they seek a blood meal from humans or animals.² Humans are the only known host of the parasite Onchocerca volvulus, a filarial nematode worm that obtains its nutrients from human blood.³ Black flies are the only known vector of Onchocerca volvulus, transmitting the parasite from person to person in endemic regions.³

Onchocerciasis, also known as river blindness, is a skin and eye disease resulting from infection with the parasite Onchocerca volvulus.¹ Once inside the human body, the parasitic worms live within fibrous nodules underneath the skin. When the parasites mature, they can migrate to other body sites, spreading to the eyes and producing visible nodules under the skin and a pruritic skin rash.³

Onchocerciasis is the second most common cause of infective blindness worldwide. It is thought that in areas where the disease is endemic, in communities near flowing water, up to 4 in every 5 people may be infected with Onchocerca volvulus.¹ In 2017, it was estimated that 220 million people required preventive chemotherapy against Onchocerciasis, with 14.6 million people already with skin disease and 1.15 million with vision loss secondary to Onchocerciasis.³

Current global efforts to prevent and control this disease include annual population-based treatment with Ivermectin for 12 to 15 years in endemic areas and black fly control.³

Schistosomiasis (Snail Fever)

The Schistosoma parasite infects and develops inside certain species of freshwater snails. Once it reaches its larval form of development, the parasite releases back into the water from its snail host. When this larval form encounters human skin, the parasite enters the body and replicates inside the blood vessels where the female produces its eggs.⁴

Symptoms of the resulting NTD Schistosomiasis are caused by the body’s reactions to the parasite's eggs. Although a skin rash or itchy skin may develop within a few days of infection, subsequent symptoms generally take at least 1 to 2 months. In intestinal schistosomiasis, symptoms may include abdominal pain, diarrhea, and bloody stool. In urogenital schistosomiasis, hematuria is a classic tell-tale sign of schistosomiasis.⁶

According to the WHO, 11,792 people die annually from complications related to schistosomiasis. However, the economic toll of the disease is substantial as it leads to reduced ability to earn wages or attend school.⁴ Estimates from 2018 reveal that approximately 207 million individuals in 74 countries are infected with schistosomiasis. Over 779 million people are at risk of infection, with 85% of them being in Africa.⁵ Current interventions to reduce the prevalence of Schistosomiasis include population-based preventative chemotherapy, snail control, and improved access to safe water and sanitation.⁴

Lymphatic Filariasis (Elephantiasis)

Filarial worms are a type of thread-like parasitic roundworm that can cause disease in humans. When a mosquito bites an infected host, the parasite is transmitted to the mosquito, maturing inside it before passing it to another human. Filarial worms can live for 6 to 8 years, reproducing inside the human’s lymphatic system and circulating through the blood. Most people never develop symptoms of infection, but their infection contributes to the continual spread of the disease. However, for others, the parasitic residence inside the lymphatic system disrupts its normal functioning, eventually resulting in the classic symptoms of chronic Lymphatic Filariasis, also known as elephantiasis.⁷

The most well-known symptom of lymphatic filariasis is the swelling and thickening of the limbs (lymph edema), resulting in sustained damage to the lymphatic system and the kidneys by the disease. Men can also develop scrotal swelling from the disease or hydroceles.⁸ The appearance of the disease often results in significant social stigma, while the pain and deformity make it challenging to earn wages.

Approximately 863 million people in 47 countries risk acquiring lymphatic filariasis. Approximately 15 million people are currently living with limb edema secondary to lymphatic filariasis. Elimination of lymphatic filariasis is possible by stopping the spread of the disease. Currently, efforts are in place to provide population-based chemotherapy to prevent the spread of the disease to mosquitos, while mosquito control aids in preventing transmission. Mass drug administration (MDA) for chemoprophylaxis is necessary in 44 countries, with 9 countries yet to receive MDA.⁷

Soil-transmitted Helminths

Helminths, parasitic worms, are some of the world’s most common parasites. Transmitted through the soil, they cause intestinal infections in human hosts. The parasites mature in dirt when their human hosts defecate in the soil. Human infection results from subsequent ingestion of several species of these parasitic worms. This occurs when hands contaminated with the soil are used to cook or eat.⁹ Hookworms are primarily transmitted through the skin when individuals walk barefoot through the contaminated soil. Although these infections are easily controlled by proper sanitation and hygiene, they are prevalent in warm and moist climates where hygiene and sanitation systems are underdeveloped. Because of the pain and disability they cause, soil-transmitted helminths make earning wages and attending school difficult, perpetuating the cycles of poverty in endemic communities. One of the most common worldwide infections, it is estimated that approximately 1.5 billion people live with soil-transmitted helminth infections, 24% of the world’s population.¹⁰

What’s next?

Although these diseases, and the other 15 diseases known as NTDs, continue to be a public health problem for a large part of the developing world, government agencies and nongovernmental organizations worldwide have successfully worked to reduce the prevalence of these conditions.

According to data from the CDC, 43 countries have eliminated at least 1 NTD. At least 1.38 billion people no longer require treatment for trachoma. An astoundingly low number of only 13 human guinea worm cases were reported in 2022, down from the 3.5 million reported annually in the 1980s, and 740 million people no longer require lymphatic filariasis (LF) treatment.¹¹

Efforts in disease prevention and reducing NTDs have proven successful in recent decades. Still, more work must be done to prevent further and reduce the occurrence of these neglected tropical diseases. We must continue working together to encourage the development of impoverished communities, tackling issues such as sanitation, access to clean water, and improved hygiene.

We need population-based mass drug administration and vector control for preventative treatment made available to areas endemic to these neglected tropical diseases. We need access to vaccines, antiserum, and education for communities on preventing, controlling, and managing these neglected diseases.¹²

When faced with problems of this magnitude, it’s easy to feel they are beyond our control or outside the need for an individualized response. However, we can join the fight against neglected tropical diseases. We can do so by educating ourselves on neglected tropical diseases, participating in research, prevention, and control efforts, spreading awareness of these diseases, contributing to charities tackling these issues, and holding our leaders and governments accountable for prioritizing these issues. Together, we end the suffering from neglected tropical diseases.

References

1. Onchocerciasis. CDC. Updated June 2, 2023. https://www.cdc.gov/parasites/onchocerciasis/
2. Livestock Pests - Face Flies. University of Florida Entomology and Nematology Department. Updated: December 2013. Accessed February 27, 2024. https://entnemdept.ufl.edu/creatures/livestock/bfly.htm
3. Onchocerciasis. World Health Organization. Accessed February 27, 2024. https://www.who.int/news-room/fact-sheets/detail/onchocerciasis
4. Schistosomiasis. World Health Organization. Accessed February 27, 2024. https://www.who.int/news-room/fact-sheets/detail/schistosomiasis
5. Recent estimates indicate that 779 million individuals are infected with the nematode National Center for Biotechnology Information. Accessed February 27, 2024. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6211415/#:~:text=Recent%20estimates%20indicate%20that%20779,disease%20%5B3%2C%204%5D
6. Schistosomiasis. CDC. Updated April 11, 2018. Accessed February 27, 2024. https://www.cdc.gov/parasites/schistosomiasis/
7. Lymphatic Filariasis. World Health Organization. Accessed February 27, 2024. https://www.who.int/news-room/fact-sheets/detail/lymphatic-filariasis
8. Lymphatic Filariasis. CDC. Updated June 22, 2023. Accessed February 27, 2024. https://www.cdc.gov/parasites/lymphaticfilariasis/
9. Soil-transmitted Helminths. CDC. Updated February 2, 2022. Accessed February 27, 2024. https://www.cdc.gov/parasites/sth/index.html#:~:text=Ascaris%2C%20hookworm%2C%20and%20whipworm%20are,uncommon%20in%20the%20United%20States.&text=An%20estimated%20604%2D795%20million,world%20are%20infected%20with%20whipworm
10. Soil-transmitted Helminth Infections. World Health Organization. Accessed February 27, 2024. https://www.who.int/news-room/fact-sheets/detail/soil-transmitted-helminth-infections
11. Neglected Tropical Diseases (NTDs). CDC. Accessed February 27, 2024.https://www.cdc.gov/globalhealth/ntd/resources/ntd_factsheet_2023.html
12. World Health Organization - Western Pacific Region. Controlling and Eliminating Neglected Tropical Diseases. World Health Organization. Accessed February 27, 2024. https://www.who.int/westernpacific/activities/controlling-and-eliminating-neglected-tropical-diseases#:~:text=This%20includes%20enhancing%20the%20water,affected%20populations%20in%20the%20Region
13. Trachoma. World Health Organization. Accessed February 27, 2024. https://www.who.int/news-room/fact-sheets/detail/trachoma#:~:text=Trachoma%20is%20a%20disease%20of,Blindness%20from%20trachoma%20is%20irreversible.