Parasitic diseases remain a major cause of human morbidity and mortality worldwide. According to the World Health Organization (WHO), parasitic diseases account for more than 300,000 deaths every year globally.
What are parasites?
Parasites are organisms that live on or in a host organism and get their food from or at the expense of the host. There are three main classes of parasites that can cause disease in humans:
- Protozoa – Single-celled microscopic organisms such as Plasmodium, which causes malaria
- Helminths – Worms such as tapeworms, roundworms, and flukes
- Ectoparasites – Organisms that live on the skin or hair, such as lice and mites
Many parasites are species-specific, meaning they only infect certain types of hosts. But some have the ability to infect several different species. Parasitic infections are more common in tropical regions where sanitation is poor and access to clean food and water is limited.
Parasitic diseases in humans
There are a multitude of different parasitic infections that afflict humans globally. The major ones include:
- Malaria – Caused by Plasmodium parasites transmitted through the bite of infected Anopheles mosquitoes. There were an estimated 241 million cases and 627,000 malaria deaths worldwide in 2020.
- Schistosomiasis – Caused by blood flukes (Schistosoma worms). More than 200 million people infected worldwide, with about 200,000 deaths annually.
- Leishmaniasis – Caused by Leishmania parasites and transmitted by sandflies. Estimated 700,000 to 1 million new cases and 26,000 to 65,000 deaths annually.
- Chagas disease – Caused by Trypanosoma cruzi and transmitted by triatomine bugs. Around 6 million people infected, with 10,000 deaths per year.
- African sleeping sickness – Caused by Trypanosoma brucei and transmitted by tsetse flies. Around 65 million people at risk, with about 10,000 new cases reported annually.
- Ascariasis – Infection by Ascaris roundworms, which infects over 800 million people worldwide.
- Hookworm – Infection by hookworms such as Necator americanus, infecting nearly 600 million people globally.
- Foodborne trematodiases – Infections by liver, lung and intestinal fluke species through contaminated food. At least 56 million people infected worldwide.
Which parasite causes the most deaths globally?
According to WHO, the parasite that causes the most deaths globally is Plasmodium, which causes malaria. The WHO estimates there were 627,000 malaria deaths worldwide in 2020, mostly among children under 5 years old in sub-Saharan Africa.
Malaria remains one of the world’s deadliest infectious diseases. It is transmitted to humans through the bite of an infected Anopheles mosquito. Once in the bloodstream, the Plasmodium parasite infects and destroys red blood cells, leading to fever, anemia, organ failure and potentially death if untreated.
The five Plasmodium species that infect humans are:
- Plasmodium falciparum – Most prevalent and lethal species in Africa
- Plasmodium vivax – Most common outside Africa
- Plasmodium malariae – Causes “quartan” malaria with fevers every 3 days
- Plasmodium ovale – Found mostly in West Africa
- Plasmodium knowlesi – Primarily infects monkeys but can also infect humans
P. falciparum accounts for the majority of malaria cases and deaths worldwide. Infection with this species can progress rapidly to severe illness and death if not treated within 24 hours of symptom onset.
Other major parasitic killers
While malaria tops the list, several other parasitic diseases result in a high mortality burden globally:
Schistosomiasis
Schistosomiasis, also known as bilharzia or snail fever, is a chronic disease caused by parasitic Schistosoma worms. An estimated 200 million people are infected worldwide, with the vast majority occurring in sub-Saharan Africa. It is estimated to cause about 200,000 deaths per year, due to organ damage and cancer caused by the worms.
Leishmaniasis
This disease is caused by Leishmania parasites and transmitted by sandflies. WHO estimates it causes between 26,000 – 65,000 deaths annually worldwide. The most lethal form is visceral leishmaniasis, which infects internal organs and has a mortality rate over 95% if untreated.
Chagas disease
Chagas disease is endemic to Latin America, where it is transmitted by triatomine bugs. Chronic Chagas disease, caused by Trypanosoma cruzi infection, leads to around 10,000 deaths annually, often due to heart failure or malfunction of the digestive system.
African sleeping sickness
This fatal disease is caused by a parasite called Trypanosoma brucei and transmitted by the tsetse fly in sub-Saharan Africa. Without treatment, it is almost always fatal. Around 10,000 new cases are reported to WHO annually.
Geographic trends in parasitic diseases
The regions most plagued by parasitic diseases are sub-Saharan Africa, Southeast Asia, and Latin America. Poverty, inadequate sanitation, and tropical climates contribute to higher prevalence in these areas.
Malaria accounts for over 90% of parasite-related deaths worldwide. Sub-Saharan Africa shoulders the highest malaria burden, with 95% of all malaria cases and 96% of deaths. Children under 5 account for 80% of malaria deaths in this region.
| Region | Estimated malaria cases in 2020 | Estimated malaria deaths in 2020 |
|---|---|---|
| South-East Asia | 9 million | 44,000 |
| Eastern Mediterranean | 4 million | 40,000 |
| Western Pacific | 4 million | 5,000 |
| Americas | 1 million | 5,000 |
| Africa | 224 million | 606,000 |
Schistosomiasis is focused in sub-Saharan Africa, which accounts for over 90% of cases worldwide. Leishmaniasis is prevalent in tropical regions including east Africa, Brazil, India, and Bangladesh. Chagas disease is confined to endemic tropical areas of Latin America. Therefore, parasitic disease mortality disproportionately impacts poor, tropical regions of the world.
Global efforts against parasites
International organizations like the WHO, UNICEF, and the Global Fund have prioritized fighting parasitic diseases like malaria and neglected tropical diseases (NTDs) in developing countries. Key interventions include:
- Insecticide-treated bed nets – Help protect people from mosquito and tsetse fly bites while sleeping
- Indoor residual spraying – Spraying insecticide on house walls to kill malaria vectors and prevent transmission
- Antiparasitic drugs – Mass administration of deworming drugs helps reduce worm infections
- Rapid diagnostic tests – Allow quick detection of malaria parasites for fast treatment
- Artemisinin combination therapies – Highly effective malaria treatments that help reduce mortality
- Snail control – Destruction of freshwater snails that transmit schistosomiasis
- Vector control – Reduction of vectors like mosquitoes, sandflies, and triatomine bugs through environmental modification
- Health education – Teaching communities how to prevent parasite transmission
Thanks to expanded prevention and control efforts, malaria mortality rates fell by 60% globally between 2000-2015. However, parasites continue to thrive in tropical areas with weak health systems. Sustained political commitment and funding is critical to accelerate progress against parasitic diseases.
Conclusion
Plasmodium parasites, which cause malaria, lead to the most parasite-related deaths worldwide, estimated at over 600,000 annually. The vast majority of malaria cases and deaths occur in sub-Saharan Africa, especially among children under 5. Schistosomiasis, leishmaniasis, Chagas disease, and African sleeping sickness are other deadly parasites impacting tropical developing regions.
Global programs are working to expand access to diagnostic testing, quality medications, and preventative measures like insecticide-treated nets and deworming. But continued high burden of parasitic diseases highlights the need for improved sanitation, living conditions, and health systems in endemic developing countries.