Fluke
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 Botulus microporus, a giant digenean parasite from the intestine of a lancetfish
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Aspidogastrea |
"Flukes" or "fatworms" is the common name applied to the flat, leaflet like endoparasites of class Trematoda within the phylum Platyhelminthes. This term can be traced back to the Saxon name for flounder, and refers to the flattened, rhomboidal shape of the worms.
All trematodes are parasitic flatworms. Previous classification systems included the Monogenea amongst the trematoda, alongside the Digenea and Aspidogastrea, on the basis that they were all vermiform parasites. The taxonomy of the Platyhelminthes is being subjected to extensive revision, and the modern sources place the Monogenea in a separate class within the phylum. Moreover, 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 (require at least two hosts for the completion of their life cycle).
Types of Flukes
The flukes can be categorized into three groups on the basis of the systems they infect. 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, Metagonimus, and so forth. Some zoonotic flukes like Echinostoma ilocanum and Heterophyes heterophyes can infect human beings because of exposer, though their definitive hosts are other mammals like dog, cats, and others. Tissue flukes infect biological tissues of liver, lungs, or bile duct attaching themselves on their surface. Some of the flukes of this category are lung fluke, for example, Paragonimus westermani, and liver flukes, for examples, Clonorchis sinensis, Fasciola hepatica, Fasciolopsis buski, Dicrocoelium spp., and so forth. The third category of flukes is known as blood flukes, and inhabit the blood in some stages of their life cycle. Blood flukes include various species of the genus Schistosoma (Bilharzia), like S. japonicum, S. mansoni, S. haematobium, and so forth.
General Morphology
Flukes are laterally expanded to oval to oblong shape, and flattened dorso-ventrally into thin sheet. The unsegmented fluke body is covered by a tough cuticular tegument for its protection from digestive enzymes of the host. Anterior body end is produced into an oral cone for bearing an oral sucker with a centrally placed mouth. Posterior to this is a ventral sucker called acetabulum for strong attachment on the surface of the host organs. Alimentary canal is incomplete, as the intestine never open out via an anus. A small opening at the posterior end of the body is the common pore for protonephric excretory system. Most of the flukes are hermaphrodite having the organs of both sexes in a single individual. Although there is a common gonopore, cross fertilization is facilitated whenever possible. Species of Schistosoma differ from other flukes in two major aspects: They are dioecious (male and female in separate individuals) with well-defined sexual dimorphism. Their body is elongated, female being more slender, cylindrical, and permanently lodged in the gynaecophoric canal of a male.
Life Cycles
Flukes being mostly digenetic, have a complex life cycle, often involving several intermediate hosts. Depending on the type of the parasites, 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 water again. In general case, 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 develop into adult to repeat the life cycle. In case of Schistosoma, cercaria larvae infect definitive host (man) by directly penetrating the skin while in water. For more information on life cycles, see the respective pages on Digenea and Aspidogastrea.
Pathogenicity
Pathogenicity of the flukes varies with the sites of their infection. In the intestinal fluke, patient may show abdominal discomfort (diarrhea, nausea, vomiting, abdominal pain, 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. Among the tissue flukes, liver flukes cause liver abscesses, fibrosis, jaundice, and a type of hepatitis, while the Lung flukes produce symptoms similar to pulmonary tuberculosis. Infection of blood flukes causes fever and chills, but also elevates the number of white blood cells (eosinophils), as well as abdominal pain resulting from enlargements of the liver and spleen. Some parasitic trematodes castrate their hosts (usually intermediate hosts like snails) either by feeding on their sex organs or by chemical means.
Treatments
Medication is achieved generally by the use of different types of antimony containing drugs. They are effective against the flukes of all kinds.
Prophylaxis (preventive measures) includes personal hygiene, community sanitation, awareness campaign 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, swimming pools, and so forth.
Selected Bibliography
- Gibson, D.I., R.A. Bray, and A., Jones (Editors). 2002. Keys to the Trematoda. 3 Volumes. Cabi Publishing. New York. ISBN 0851995470
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