Botulus microporus, a giant digenean parasite from the intestine of a lancetfish.
Fluke is the common name for any member of the class Trematoda, a group of flat, leaf-like endoparasites within the phylum Platyhelminthes ("flatworms"). (For the term "fluke" to denote a type of flatfish, see flounder).
As with other members of Platyhelminthes, flukes or trematodes are relatively simple, soft-bodied, bilaterially symmetrical, invertebrate animals that are acoelomates characterized by having three germ layers (ectoderm, mesoderm, and endoderm) and lacking respiratory and circulatory systems. (Acoelomates are invertebrates that do not have a coelom, or body cavity).
There is a great deal of individual and social responsibility related to preventing the transmission and infection of these parasites. For example, a common cause of infection by liver flukes in Thailand is related to eating undercooked cyprinoid’s fish (Kaewpitoon et al. 2007). Good personal hygiene, better community sanitation, treatment of contaminated water resources, and awareness campaigns are all avenues toward preventing infection.
There are two subclasses within Trematoda—Aspidogastrea and Digenea. However, there are no known cases of human infection with Aspidogastreans. Therefore the use of the term "fluke" in relation to human beings refers solely to digeneans (which require at least two hosts for the completion of their life cycle).
Flukes are laterally expanded to an oval to oblong shape, and are flattened dorso-ventrally into a thin sheet. The fluke body is unsegmented and covered by a tough cuticular tegument for its protection from digestive enzymes of the host. The anterior body end has an oral cone bearing an oral sucker with a centrally placed mouth. Posterior to this is a ventral sucker called acetabulum, used for strong attachment on the surface of the host organs.
The fluke alimentary canal is incomplete, as the intestine never opens out via an anus. A small opening at the posterior end of the body is the common pore for the protonephric excretory system.
Most flukes are hermaphrodites, having the organs of both sexes in a single individual. Although there is a common gonopore, cross fertilization is facilitated whenever possible.
Species of the fluke genus Schistosoma differ from other flukes in two major aspects—they are dioecious (male and female in separate individuals) with well-defined sexual dimorphism, with the male considerably larger than the female. Indeed, the female, whose body is more slender and cylindrical, is permanently enclosed within the gynacophoric canal of a male.
Flukes have a complex life cycle, often involving several intermediate hosts.
Depending on the type of parasite, the eggs are passed from the host in the feces, sputum, or urine. When the eggs reach water, they hatch into free-swimming larvae called miracidia. The miracidia penetrate the specific fresh water snail or other molluscan intermediate host to become second larvae called sporocysts. The cells inside the sporocysts divide by mitosis to form redia larvae. Rediae, in turn, give rise to free-swimming larvae known as cercariae, which escape from the mollusk into the water again.
In general cases, the cercaria larvae burrow through exposed skin using enzymes, penetrate another intermediate host (often an arthropod, but can also be aquatic vegetation), and then encyst as metacercariae. When this host is eaten by the definitive host, the metacercariae excyst and develops into adult to repeat the life cycle.
In case of Schistosoma, cercaria larvae infect the definitive host (humans) by directly penetrating the skin while in water. Some parasitic trematodes castrate their hosts (usually intermediate hosts like snails) either by feeding on their sex organs or by chemical means.
Flukes can be categorized into three groups on the basis of the systems they infect: intestinal flukes, tissue flukes, and blood flukes.
Intestinal flukes occur attached to the mucosa of the intestine. Some of the intestinal flukes infecting human beings are the species of genera Echinostoma, Gastrodiscoides, Heterophyes, and Metagonimus. Some zoonotic flukes like Echinostoma ilocanum and Heterophyes heterophyes can infect human beings because of exposure, however, their definitive hosts are other mammals, like dogs and cats.
Tissue flukes infect tissues of liver, lungs, or bile duct, attaching themselves on their surface. Some of the flukes of this category include the lung fluke Paragonimus westermani, and the liver flukes Clonorchis sinensis, Fasciola hepatica, Fasciolopsis buski, and Dicrocoelium spp..
The third category of flukes is known as ‘‘blood flukes’’, and inhabits the blood in some stages of their life cycle. Blood flukes include various species of the genus Schistosoma (Bilharzia), such as S. japonicum, S. mansoni, and S. haematobium.
Previous classification systems included the Monogenea class of flatworms as within Trematoda, alongside the subclasses Digenea and Aspidogastrea, on the basis that they were all vermiform (worm-like) parasites. The taxonomy of the Platyhelminthes is being subjected to extensive revision and modern sources place the Monogenea in a separate class within the phylum Platyhelminthes.
Pathogenicity of flukes varies with the sites of their infection.
In the intestinal fluke, the patient may show abdominal discomfort (diarrhea, nausea, vomiting, abdominal pain, and ulceration) associated with the inflammation of the gut wall, as well as facial, abdominal, and lower limb edema due to immune sensitization caused by the foreign proteins of the parasites.
Infection of blood flukes causes fever and chills, but also elevates the number of white blood cells (specifically eosinophils) and causes abdominal pain resulting from enlargements of the liver and spleen.
Medication is achieved generally by the use of different types of antimony containing drugs. They are effective against flukes of all kinds.
Prophylaxis (preventive measures) includes personal hygiene, community sanitation, awareness campaigns against toilet and sewage discharge in or near water resources, control of intermediate hosts, treatment of the infected persons and the pets, disinfection of contaminated water reservoirs and swimming pools, and so forth.
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