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Classification of Helminths - Principles of Epidemiology and Microbiology

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2-24. GENERAL
Helminths are multicellular (Metazoa) worms or wormlike animals. They may be
parasitic or free living. Since they are multicellular, most helminths may be easily seen
with the naked eye in the adult form and are not truly within the scope of microbiology.
Because of their medical importance, however, helminths are usually studied, along
with protozoa, as part of the science of medical parasitology.
2-25. CLASSIFICATION OF HELMINTHS
Helminths are found in two phyla of the subkingdom Metazoa (multicellular
animals):
a. Phylum: Platyhelm1nthes (flatworms)
(1)
Class I: Cestoda (tapeworms)
(2)
Class II: Trematoda (flukes)
b. Phylum: Aschelminthes
Class I: Nematoda (roundworms, threadworms)
2-26. CESTODES (TAPEWORMS)
a. The tapeworm has two stages: larva and adult. The length of an adult worm
varies, according to the species, from 3-8 mm to 25 or 30 feet. The adult form has a
small head (scolex) and varying numbers of segments (proglottides) (figure 2-15 A&B).
The head attaches itself to the intestinal wall of the host by means of suckers.
Segments form from the head, receiving their nourishment through absorption, since the
worm has no alimentary tract. Each mature segment is a sexually complete
hermaphrodite (possessing both male and female sex organs), capable of producing
thousands of eggs. Treatment that fails to dislodge the head is ineffective, since the
head will immediately begin to replace the lost segments. In most instances a cestode
requires both an intermediate host (where certain stages of development occur) and a
definitive host (where the adult worm lives and produces ova) to complete its life cycle.
Table 2-8 lists the principal cestodes affecting man.
b. When eggs or segments of most tapeworms are passed from the definitive
host (man), they are ingested by the intermediate hosts (ruminants, swine, and so forth)
while grazing or rummaging for food. The larval stage, a hexacamth oncosphere, hatches
and encysts as a cysticercus or cysticercoid larva in the muscles or various organs of
the intermediate host. Therefore, the eggs are infective to man and may encyst in
tissue as a cysticercus larva resulting in severe complications. In the life cycle of the
large fish tapeworm of humans, the eggs must reach water where a larval stage
(coracidium) emerges and is swallowed by a small fresh-water crustacean called a
MD0151
2-34



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