As a biological discipline, the scope of parasitology is not determined by the organism or environment in question but by their way of life. Depending on the species, cestodes can complete their life cycle in one or several hosts. This section gives focus to the life cycle of Taenia saginata which requires an intermediate and definitive host to complete its life cycle. saginata can be said to star
t when the eggs (contained in the proglottid) are released, along with f***s of an infected human (the definitive host) into the environment. In the environment (soil, grass, water etc), the eggs can survive for several months due to the covering that protects the eggs within. When the eggs are consumed by an intermediate host (e.g. cow) they hatch to form oncospheres (larvae) which then enter the bloodstream before migrating to the muscle tissue. Here, they encyst to form cysticerci which are essentially encysted larvae. This form allows the organism to remain/survive (and remain infective) for years in the intermediate host.
* An intermediate host refers to the that which serves to support the immature form of a parasite. By properly cooking meat (beef), the tiny cysticerci can be destroyed. However, in the event that the infected meat is poorly cooked and ingested (e.g. rare steak), the cysticerci excysts in the small intestine of the definitive host (human being) and matures into an adult in a period of about 2 months. Using four suckers located on its head, the tapeworm is able to attach itself to the small intestine and starts producing proglottids that make up the body (stronila). By absorbing nutrients through its membrane, the proglottids continue to grow with eggs being increasingly produced. When the proglottids (which contain the eggs) break off and are excreted to the environment, the life cycle continues once the eggs are again ingested by an intermediate host.Cestodes: Classification and General Characteristics
Cestodes or tapeworms are the members of the class Cestoda of the phylum Platyhelminthes. About 6000 species of cestodes are identified as parasites and they are mainly intestinal. They are both marine and terrestrial; vertebrates are the definitive hosts and their intermediate hosts are crustaceans, insects, annelids, mollusks, etc. The species T. saginata (beef tapeworm) can grow up to 65 ft (20m) in length while the largest tapeworm, known as the whale tapeworm (Tetragonoporus calyptocephalus) can grow up to 100 ft (30 m) or more. Besides these, the smallest parasite, known as vole and lemming tapeworms, can grow up to only 13–240 mm in length.Taenia saginata (Beef tapeworm)
Taenia solium (Pork tapeworm)
Hymenolepis nana (Dwarf tapeworm)
Hymenolepis diminuta ( Rat tapeworm)
Diphylobothrium latum (Fish tapeworm)
Diphylidium carinum (Double-pored Dog tapeworm)
B. Tissue Tapeworms-Larval Stage
Hydatid cyst of Echinococcus granulosus (Dog tapeworm), E. multilocularis
Cysticercus cellulose of Taenia solium. Coenurus cerebralis of Multiceps multiceps. Plerocercoid of Sparganum mansoni and Sparganum proliferum. General Characteristics of Cestodes
1. Adult Cestodes: Adult cestodes have tape-like, ribbon shaped and segmented body and the length varies from few mm to several meters. They are flattened dorso-ventrally. An adult worm has three regions:
Head or Scolex
Neck and
Strobila (a trunk or body) consisting of proglottid or segments.
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