Flukes
Flukes are leaf-shaped parasitic flatworms, known scientifically as trematodes, that attach to the internal organs of their hosts using suckers. With over six thousand species identified globally, flukes require at least two hosts to complete their life cycle, which can include various species of vertebrates and intermediate hosts like mollusks. They are categorized based on the organs they infest, such as liver, lung, and intestines. Flukes affecting humans, particularly from families like Schistosomatidae and Fasciolidae, can lead to notable health issues, including food-borne trematodiases and schistosomiasis—one of the most common parasitic infections after malaria.
Infection can occur through ingestion of contaminated food or water, or skin penetration by larvae in freshwater environments. Symptoms may include rash, abdominal pain, and more severe complications like organ damage and immune responses, which can be misdiagnosed. Diagnosis often involves identifying eggs in feces or tissue samples. Preventative measures include improved sanitation and dietary precautions, while treatment primarily involves specific antiparasitic medications. However, drug resistance remains a growing concern, prompting research into potential vaccines against flukes.
Flukes
- TRANSMISSION ROUTE: Ingestion
Definition
Flukes are leaf-shaped parasitic flatworms (trematodes) that adhere with their suckers to the internal organs of a host. More than six thousand species of flukes are found worldwide; they have a life cycle that is dependent on at least two or more hosts. Flukes are classified by the organ location they infest (such as blood, liver, lung, and intestines). Flukes in the families of Schistosmatidae, Echinostomatidae, Fasciolidae, Opisthorchioidea, Heterophyidae, and Paragonimidae are known to infect humans.
Natural Habitat and Features
Flukes are obligate parasites because they are dependent on their hosts for nourishment. They are also considered endoparasites because their life cycle is dependent on the internal environment of a minimum of two different hosts. The host animal of adult flukes tends to be a predatory vertebrate. Flukes that infect humans are found in all countries, but their geographic distribution is dependent on the availability of needed intermediate hosts, such as mollusks found in freshwater environments. Their natural habitat is therefore dependent on the various stages of their complex life cycle and on the environment of the required host.
Flukes range in size from microscopic to large, depending on the species. Liver, lung, and intestinal flukes may enter the final host through ingestion, and blood flukes enter the host through penetration of skin. Flukes in human hosts will reside in various internal organs and will feed off those organs. Eggs of the various species are produced by adult worms and passed through feces or sometimes through sputum. Feces-containing eggs that end up in freshwater supplies and the presence of the necessary intermediate host will restart the life cycle. Once in freshwater, the eggs hatch into a larval state (miracidum) and seek the necessary intermediate host, typically a snail. Once inside the intermediate host, the fluke morphs into a second larval state known as cercariae. The cercariae exit the intermediate host to find secondary hosts, such as aquatic plants, crustaceans, or fish, where they will form a cyst (mesocercaria) until ingested by the final host; here they can mature to adults in the desired internal organ. Blood flukes will enter a human host through skin penetration as cercariae in freshwater. Once the adult stage is reached, production of eggs begins. Many species of flukes have the ability to reproduce both sexually and asexually.
Pathogenicity and Clinical Significance
Flukes are responsible for two neglected tropical diseases (NDT): food-borne trematodiases (FBT), from eating certain types of raw or undercooked food (aquatic vegetation, crustaceans, and fish), and schistosomiasis. Liver flukes, lung flukes, and intestinal flukes are linked to FBT, whereas blood flukes cause schistosomiasis. Schistosomiasis ranks second to malaria as one of the most prevalent parasitic infections in the world.
Symptoms of fluke infestation include allergic responses, rash (swimmer’s itch), fever, weakness, inflammation, abdominal pain, nausea, and diarrhea. Movement of flukes and their eggs through internal organs results in blockage, swelling, and lasting organ damage that may lead to additional immune responses, inflammation, cirrhosis, anemia, hepatomegaly, or cancer. Fluke infections are often misdiagnosed, resulting in irreparable internal organ damage. Diagnosis is confirmed by examination of eggs in a stool sample or from a tissue sample through biopsy. Prevention of fluke parasitism is possible through improved water sanitation and the avoidance of eating raw or undercooked foods.
Drug Susceptibility
Drug therapy for most flukes includes praziquantel, triclabendazole, oxamniquine, and bithionol. Drug resistance is an increasing concern, and scientists have made progress on vaccines against flukes.
Bibliography
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