The platyhelminths’ group of helminths are commonly known as flatworms because of their flat body shape. They are divided into two subgroups, the trematodes, commonly known as flukes, and the cestodes, commonly known as tapeworms. Below are the most common or well-known types of flatworms, but it does not constitute an exhaustive list.
Infections with trematodes are more frequent in Africa, Asia, Latin America and the Middle East. There are two types of trematodes infecting humans. The tissue flukes infect generally biological tissues, such as the bile ducts or the lungs. This group includes some well-known parasites, such as the liver flukes Clonorchis sinensis and Fasciola hepatica, as well as the lung fluke Paragonimus westermani. The blood flukes are only represented by Schistosoma species. Trematodes species infecting humans almost always have snails as their intermediate hosts. Diagnosis of trematodes’ infection is done by visualisation of parasite’s eggs in faeces and confirmed by the presence of antibodies against the parasite in the patient’s blood.
The different tissue flukes have specific characteristics and usually cause symptoms in the tissue that they infect or remain asymptomatic. Clonorchis sinensis, also known as the Chinese liver fluke, is the most prevalent trematode in Asia infecting more than 30 million people and is known to eventually cause cancer of the liver and the bile duct. As its second intermediate host is freshwater fish, eating raw or undercooked fish contributes to the infection of humans. Some infection cases related to the ingestion of raw or undercooked imported fish have also been reported in non-Asiatic countries. Fasciola hepatica, commonly known as the sheep liver fluke, is distributed worldwide and though to infect more than 2.4 million people. It is mainly transmitted by the ingestion of several aquatic or semi-aquatic plants, such as watercress. Paragonimus westermani, also known as the Oriental lung fluke, is mostly present in South America and eastern Asia. This parasite is mainly transmitted by the ingestion of raw or undercooked seafood, such as freshwater crab. Tissue flukes are usually treatable using different anti-helminthics.
The blood flukes caused by Schistosoma species infect more than 260 million people worldwide. It is considered as the second parasitic disease with the greatest economic impacts behind malaria. This disease is transmitted by contact with parasites released in the water by the intermediate host, the freshwater snails. Parasites are then able to penetrate directly within the skin. As such, controlling the population of snails in endemic areas is view as a method of prevention. It is interesting to note that symptoms are not caused by the parasite itself, but by the body’s reaction to the presence of parasite’s eggs. The first step of infection includes fever, chills, skin rash and muscle pain. After that initial phase, some organs can be affected, such as the liver, intestine, bladder or lungs, depending where the eggs are localised within the body. These parasites were also link to bladder cancer. Infected children can also develop symptoms of malnutrition and anemia leading to learning difficulties. As this parasitic disease is considered to be chronic, it is usually controlled (but not definitively treated) with one dose a year of an anti-helminthic.
The most common types of cestodes are distributed worldwide, but are mainly present in underdeveloped countries with poor sanitation systems, such as Africa and Southeast Asia, and infect usually humans by ingestion of raw or undercooked meat from an infected animal. These parasitic diseases are usually diagnosed by visualisation of parasite’s eggs in the faeces and are treatable with anti-helminthics (with the exception of Echinococcus species). It is very difficult to estimate the worldwide total burden of these parasitic diseases, because infected people are mostly asymptomatic.
Different species of Taenia are present in the pork and in the beef. Most of these infections remain asymptomatic, but gastrointestinal symptoms can occur. Furthermore, cysts of Taenia solium, the so-called pork tapeworm, could eventually relocate elsewhere in the body and cause a disease called cysticercosis that can affect the skin, the muscles, the nervous system or the eyes. This disease is more complicated to treat and often require surgical procedures to remove the cysts.
Diphyllobothrium species are present in fish and are usually transmitted by the ingestion of sushi, ceviche and other raw fish’s dishes. It is noteworthy that freezing the fish for 24 to 48 hours before making this type of dishes is sufficient to kill this parasite. About 80% of the cases of Diphyllobothrium infections remain asymptomatic, but in a small number of cases, a severe vitamin B12 deficiency occurs because the parasite absorbs the majority of the vitamin B12 ingested by the host. In the long term, this vitamin B12 depletion can lead to other neurological symptoms.
Hymenolepiasis species are the most common cestode infecting humans and are present in grain beetles that can be eaten by humans in contaminated cereals or floor in areas with poor hygiene. It can also be transmitted by contact with soil contaminated with faeces from infected people. This infection is mostly asymptomatic, but can also lead to gastrointestinal symptoms.
Echinococcus species can infect humans in contact with contaminated soil or eating viscera from infected animals. About one million people are thought to be infected worldwide, but human remain an accidental intermediate host for this parasite, meaning that he cannot fulfill its life cycle anymore after infecting a human. Infection can remain asymptomatic for many years or develop in a wide range of symptoms. Images of cysts from scans or biopsies are needed for the diagnosis. This parasitic disease is very difficult to treat and can required a very long-term drug therapy and surgical interventions.
Sources: Centers for Disease Control and Prevention; World Health Organization