Helminths

Around 100000 different worm species exist, many of which are free-living; some are parasitic, but fewer of them parasitize man. Helminthic infections are, nonetheless, common on

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a global scale with an estimated 4-5 billion individuals infected. A total of 319 different species infect man (Ashford and Crewe 1998), for example, pinworm and ascaris infections, both of which are endemic to the North and to the rest of the world. For further details of helminths indigenous to the North, see Chapters 3-7.

The word helminth is derived from Greek where 'helmins' means parasitic worm. Helminths belong to sub-kingdom Metazoa of the animal kingdom. They are divided into nematodes (roundworms) and platyhelminths (flatworms). The flatworms can be further divided into cestodes (tapeworms) and trematodes, the latter of which comprises flukes of different kinds. Most flatworms are hermaphrodites, except the group of schistosomes. A few human infections have been documented to be caused by helminths belonging to the phylum Acantocephala.

Helminths are complex multicellular organisms similar to Homo sapiens in many aspects as they posses, for example, neuronal tissues, muscle fibres, digestive canals, and sexual organs. They can be very small worms that can hardly be seen with the naked eye (e.g. schistosome or pinworm) or very large ones, like some of the cestodes. Taenia saginata or Diphyllobtrium latum can reach of length of several meters.

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