Liver Fluke Infections
Liver Fluke Infections
Disease History, Characteristics, and Transmission
Introduction
Liver fluke infections are the result of infestation by parasitic worms known as liver flukes. There are two main types of liver fluke infections, Fascioliasis and Pisthorchiasis. Although each infection is caused by different species of flukes, they share similarities in characteristics and transmission. Humans become infected when they ingest the cysts containing parasitic forms of the flukes. These cysts open in the digestive system and release the parasites. Humans most often ingest the cysts after drinking contaminated water or eating raw or undercooked food that contains the cysts.
Infection can be asymptomatic or can be either acute or chronic. Mild cases of both Fascioliasis and Opisthorchiasis result in tiredness, fever, aches, swollen liver, abdominalpain, and rash. Symptoms of chronic forms include exacerbated versions of the acute symptoms with possible diarrhea, nausea, swelling of the face, blockage of the bile ducts, and sometimes complications such as migration of flukes to other regions in the body. Administration of one of a variety of antihelminthic drugs is usually effective, with recovery likely to occur.
Fascioliasis occurs worldwide, while Opisthorchiasis generally occurs in regions of Asia. Increased cases of liver fluke infection have been reported in China, argued to be a consequence of an increase in the consumption of raw foods.
WORDS TO KNOW
ANTIHELMINTHIC: Antihelminthic drugs are medicines that rid the body of parasitic worms.
ASYMPTOMATIC: A state in which an individual does not exhibit or experience symptoms of a disease.
CYST: Refers to either a closed cavity or sac or the stage of life of some parasites during which they live inside an enclosed area. A stage in a protozoan's life when it is covered by a tough outer shell and has become dormant.
HELMINTH: A representative of various phyla of worm-like animals.
LARVAE: Immature forms (wormlike in insects; fishlike in amphibians) of an organism capable of surviving on its own. Larvae do not resemble the parent and must go through metamorphosis, or change, to reach the adult stage.
PARASITE: An organism that lives in or on a host organism and that gets its nourishment from that host. The parasite usually gains all the benefits of this relationship, while the host may suffer from various diseases and discomforts, or show no signs of the infection. The life cycle of a typical parasite usually includes several developmental stages and morphological changes as the parasite lives and moves through the environment and one or more hosts. Parasites that remain on a host's body surface to feed are called ectoparasites, while those that live inside a host's body are called endoparasites. Parasitism is a highly successful biological adaptation. There are more known parasitic species than nonparasitic ones, and parasites affect just about every form of life, including most all animals, plants, and even bacteria.
TREMATODES: Commonly known as flukes; a class of worms characterized by flat, oval-shaped bodies.
Disease History, Characteristics, and Transmission
Liver fluke infections are caused by liver flukes, which are a type of helminth or parasitic worm. There are two main diseases that affect humans infected with liver flukes: Fascioliasis, or liver fluke infection, and Opisthorchiasis, or Chinese liver fluke infection.
Liver flukes belong to a specific group of parasitic worms known as flukes, or trematodes. Fascioliasis is caused by species of fluke from the genus Fasciola, namely one species, Fasciola hepatica, the sheep liver fluke. The life cycle of this fluke is as follows: eggs released from host's feces into water; larvae from the eggs infect snails; snails release larvae, which forms cysts containing infective parasite stages, onto vegetation; humans ingest cysts when they eat the vegetation (for example, watercress). The cysts then break open in the human digestive tract and the flukes enter the liver and destroy tissue. Opisthorchiasis is caused by a species of fluke from the genus Clonorchis or Opisthrochis, namelyC. sinensis, O. viverrini, or O. felineus. These flukes have a similar lifecycle to the sheep liver fluke, except, rather than being encysted on plants, they are encysted inside fish. Humans then consume the fish and become infected.
Scope and Distribution
Liver fluke infections occur worldwide, though the distribution of Fascioliasis and Opisthorchiasis differs. Fascioliasis is known to occur worldwide, occurring in both temperate and tropical regions. F. hepatica infections have been reported in Europe, the Middle East, and Asia. F. gigantica infections have been reported in Asia, Africa, and Hawaii. In general, Fascioliasis is closely connected to regions where sheep and cattle are raised. Sheep and cattle are natural hosts for liver flukes, and thus are likely to transmit the parasite to humans in close contact with the animals or their water supply.
Opisthorchiasis occurs in areas of Asia and Europe. In particular, O. viverrini infections have been reported from northeast Thailand, Laos, and Kampuchea. O. felineus infections have been reported mostly in Europe and Asia. Almost all of China contains infections of O. sinensis.
In China, health officials reported a 75% increase in the number of cases of liver fluke from 2001 to 2004. The cause of the increase has been attributed to an increased desire to consume raw or undercooked seafood and meat, both of which are a source of liver flukes. As a consequence of this increase in infections, there has also been an increase in the number of cases in which a liver disorder has developed due to the flukes.
Treatment and Prevention
Treatment for liver fluke infections is achieved through administration of medications. There are a number of effective drugs, including triclabendazole, praziquantel, bithionol, albendazole, and mebendazole. Treatment of infection caused by Fasciola hepatica usually involves triclabendazole or bithionol. Use of praziquantel has been ineffective in some cases and is thus not recommended by the CDC to be used to treat F. hepatica fluke infections. Opisthorchiasis fluke infections can be effectively treated using praziquantel, which is the preferred treatment as suggested by the Centers for Disease Control and Prevention (CDC). These medications act to eradicate the parasite. Praziquantel works by paralyzing the flukes’ attachment apparatus, which disables them from remaining attached to the host's blood vessels. This leads to the death of the parasites and eventually the infection dissipates.
On occasions, treatment using the above mentioned drugs may cause side effects such as diarrhea, dizziness, or headache. However, full recovery is likely to occur following treatment. In some cases, liver damage resulting from the attachment of flukes to tissue may make patients vulnerable to other infections. Treatment may be required for several days or weeks depending on the type of fluke causing the infection.
There are no vaccines against liver fluke, thus avoidance of these parasites is the best method of prevention. Avoidance can be achieved byboiling or purifying drinking water, ensuring freshwater fish and vegetation is cooked thoroughly prior to consumption, and eradicating or controlling snails as they are an intermediate host for flukes.
IN CONTEXT: DISEASE IN DEVELOPING NATIONS
In taxonomic terms, liver flukes belong to the the class Trematoda, which is then subdivided into two orders of obligate parasites, Digenea and Monogenea. Obligate parasites are those that are unable to live independently of a host. The trematodes of importance to human disease are part of the Digenea order and include blood, tissue, and intestinal flukes. All are mainly found in developing nations located in tropical and subtropical regions. Flukes are rarely a natural problem in temperate climatic zones.
Impacts and Issues
Liver flukes are transmitted to humans via consumption of raw meat, fish, and vegetation. Therefore, the food habits of humans have strong implications for the prevalence of liver fluke infections. In China, an increase in the consumption of raw or undercooked meat and seafood resulted in a large increase in the number of people infected with liver flukes. During the years 2001–2004, Chinese health officials reported a 75% increase in fluke infections, highlighting the possibility that increased consumption of food potentially containing liver flukes is causing increased infection.
The liver fluke F. hepatica, also known as the sheep liver fluke, often infects livestock such as sheep, cattle, and pigs. This creates issues for the beef, lamb, and pork industries, as the flukes can do extensive damage to the animals, and can be a source of infection for human populations. Prevention of liver fluke infection in livestock requires routine worming of animals, as well as the implementation of snail control methods such as exclusion of animals from snail-infested regions, or drainage of water bodies containing snails. The disease is likely to occur in wet areas where snails are present. If snails are absent, so too are these flukes due to their dependence on snails.
Incidence of liver flukes are greatest in areas that lack adequate sanitation and water purification resources. Increased sanitation practices, including proper disposal and treatment of human and livestock wastes, prevention of water source contamination by fecal matter, and safer food storage and preparation practices, could dramatically reduce the occurrence of disease caused by all helminths. However, the World Health Organization (WHO) notes that over one billion people worldwide do not have access to clean, uncontaminated water. Coupled with a diet rich in the foods that most often carry liver flukes, unsanitary conditions make liver flukes difficult to prevent in underdeveloped regions.
See AlsoFood-borne Disease and Food Safety; Helminth Disease; Lung Fluke (Paragonimus) Infection; Opportunistic Infection; Parasitic Diseases.
BIBLIOGRAPHY
Web Sites
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Cambridge University. “Opisthorchis sinensis: The Chinese Liver Fluke.” Oct. 5, 1998 <http://www.path.cam.ac.uk/∼schisto/OtherFlukes/Opisthorchis.egg.html> (accessed February 23, 2007).
Centers for Disease Control (CDC). “Fascioliasis.” May 6, 2004 <http://www.dpd.cdc.gov/dpdx/html/Fascioliasis.htm> (accessed February 23, 2007).
Centers for Disease Control (CDC). “Opisthorchiasis.” May 6, 2004 <http://www.dpd.cdc.gov/dpdx/HTML/Opisthorchiasis.htm> (accessed May 2, 2007).
Meat Promotion Wales. “Liver Fluke.” <http://www.hybucigcymru.org.uk/content.php?nID=206&lID=1> (accessed February 23, 2007).
ProMED-Mail. “Food-borne Parasitic Infections Increase in China.” May 18, 2005 <http://www.promedmail.org/pls/promed/f?p=2400:1202:997653105689672184::NO::F2400_P1202_CHECK_DISPLAY,F2400_P1202_PUB_MAIL_ID:X,28969> (accessed February 23, 2007).