Food-borne trematode infections, or foodborne trematodiases, are a group of parasitic infections caused by trematodes (flatworms or “flukes”) that are acquired through ingestion of food contaminated with the larval stages of the parasite. People are infected by food-borne trematodes after eating raw freshwater fish, crustaceans or vegetables containing the parasite larvae that cause fever and abdominal pains.
Infections with these parasites have been been reported in over 70 countries to date. These diseases are estimated to cause 200 000 illnesses and more than 7 000 deaths per year.
What causes Trematodiases
Trematodiases, also known as trematode infections, are a group of diseases caused by the parasite trematodes. Trematodiases can be transmitted through food, such as fish, molluscs, and other aquatic animals and plants, or water that contains larval forms of the parasite.
Four species of food-borne trematodes are found to be important causes of disability. In all species, the first intermediate host is a freshwater snail. The second host differs according to species: for Clonorchis and Opisthorchis it is a freshwater fish and for Paragonimus it is a crustacean. Infection with Paragonimus spp can also result from the consumption of animals that feed on crustaceans e.g. raw wild boar meat. Fasciola spp. do not require a second intermediate host and can infect humans via the consumption of contaminated freshwater plants.
Where are Trematodiases found
Food-borne trematodiases are most prevalent in East Asia and South America.
The transmission of food-borne trematodiases is restricted to areas where the first and second intermediate hosts coexist (which requires suitable climatic and environmental factors) and where humans have the habit of eating raw, pickled, or under-cooked fish and other aquatic products.
C. sinensis is endemic in China, the Republic of Korea, Taiwan, and Vietnam. O. viverrini is prevalent in Cambodia, the Lao PDR, Thailand, and Vietnam, whereas O. felineus is endemic to the former Soviet Union, Kazakhstan, and the Ukraine . At present, F. hepatica is endemic on all continents but is of particular public health importance in the Andean countries, Cuba, the Islamic Republic of Iran, Egypt, and western Europe (e.g., France, Portugal, and Spain). Infections with F. gigantica are restricted to Africa and Asia. Paragonimus infections occur mainly in tropical and subtropical areas of East and South Asia and sub-Saharan Africa. They are also found in the Americas, ranging from southern Canada to Peru. With regard to intestinal flukes, infections with Echinostoma spp. in China, India, Indonesia, Japan, Malaysia, Russia, the Republic of Korea, the Philippines, and Thailand have been reported. F. buski is endemic to Bangladesh, China, India, Indonesia, the Lao PDR, Malaysia, Taiwan, Thailand, and Vietnam . Heterophyes heterophyes infections in Egypt, Greece, the Islamic Republic of Iran, Italy, Japan, the Republic of Korea, Sudan, Tunisia, and Turkey have been reported . The most commonly found intestinal fluke infection in the Far East (China, the Republic of Korea, and Taiwan) is Metagonimus yokogawai. It also occurs in the Balkan states, Israel, Russia, Siberia, and Spain.
What are the symptoms of Trematodiases
As food-borne trematodiases tend to be asymptomatic, early or light infections often go unnoticed. On the other hand, severe pain can occur, especially in the abdomen, if a large number of worms is present. Abdominal pain is most frequent in cases of infection by fascioliasis. Severe morbidity invariably accompanies chronic infections. The symptoms are mainly specific to the affected organ and indicate where the adult worms are located in the body.
In clonorchiasis and opisthorchiasis, adult worms live in the smaller liver bile ducts, resulting in inflammation and fibrosis of nearby tissue. This can eventually result in bile duct cancer.
In fascioliasis, adult worms live in the larger bile ducts and the gall bladder causing inflammation, fibrosis, blockages, colic pain, and jaundice. Liver fibrosis and anemia are also common.
In paragonimiasis, lung tissue is the final location of the worms. This presents symptoms that can be mistaken for tuberculosis: chronic cough with blood in the sputum, chest pain, dyspnoea (shortness of breath), and fever. Migration of the worms is possible and cerebral locations are the most severe, causing serious symptoms that resemble brain tumors, including headaches, spasms, and paralysis.
How can Trematodiases be prevented
Most trematodiases is transmitted through eating raw aquatic plants and animals such as fish, crustaceans, crabs, watercress, frogs and snails.
Health education programs aim to outline how transmission of disease can be prevented by raising awareness of food safety practices. Food safety, including thoroughly cooking fish and crustaceans before eating them, is essential. Thorough cleaning and washing of raw vegetables and aquatic fruits to prevent infection with intestinal flukes. Adequate cleaning utensils, cutting boards and other equipment to prevent cross-contamination is also essential.
Avoidance of consumption of contaminated water, water plants, fruits, fish, crab, and raw liver in areas where the parasites are prevalent.
Fish which will be consumed undercooked should be placed in low temperatures and frozen under -20 degrees Celsius for at least a week, or under -35 degrees Celsius for a minimum of 15 hours, as freezing removes any chance of transmitting parasites to consumers.
There is no vaccine for foodborne trematodiases that can be used to prevent infections. Anthelmintics can be used preventively in areas with a large number of patients, where individuals are treated yearly to prevent disease.
How are Trematodiases diagnosed
Trematodiases can be diagnosed through a variety of methods. One of these is known as parasitological diagnosis, which relies on lab tests that detect the presence of trematode eggs where samples are taken from faeces or sputum. Different techniques have a different degree in which they can accurately detect eggs of trematodes, and some of these may not be able to detect low amounts.
Another method in which trematodiases can be diagnosed is through antibodies that are produced by the host’s immune response when infected.
Polymerase chain reaction (PCR) tests, such as the ones available from the WoIDMo are also used to detect trematodiases.
For liver fluke infections, ultrasounds are commonly used to search for evidence of the infection.
How are Trematodiases treated
Praziquantel remains the drug of choice for all trematode infections except fascioliasis, for which triclabendazole is the drug of choice.
Praziquantel can act on a broad range of trematodes by disrupting the homeostasis of calcium ions, and is used to treat liver, intestinal and lung fluke infections.
Triclabendazole is a narrow-spectrum treatment of trematodiases and is commonly used to treat fascioliasis.
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