helminths

1. General Characteristics
• Helminths are elongated flat or round worm-like parasites
measuring few milimeters to meters.
• They are eukaryotic multicellular and bilaterally symmetrical.
• They belongs to two phyla
1. Phylum Platyhelminths (flat worms)
2. Phylum: Nemathelminths
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2. Classification of helminths
based on habitat
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3. Morphology
• In general, helminths exist in three morphological
forms—
• (1) adult form (or the worm),
• (2) larval form and
• (3) eggs.
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4. Adult Worm
Phylum Platyhelminths
• Shape is tape like (in cestodes) or leaf like (in trematodes)
• They have a definite head end called as suckers
• They lack body cavity
• Alimentary canal is absent in cestodes but incomplete in
trematodes
• They are monoecious or hermaphrodite (i.e. both the
sexes are present in the same worm), except in
Schistosoma (diecious).
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5. Adult Worm
• Phylum Nematoda
• They are evolutionary more developed than Platyhelminths
• They possess a definite body cavity (space between body
wall and alimentary canal)
• Alimentary canal is complete, starting from mouth leading
to esophagus, intestine and ending at anus.
• They are diecious, i.e. male and female worms are separate
• The nervous system and excretory system are rudimentary
and there is no circulatory system.
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6. Larval Form
• There are various larval forms of helminths found in man
and other hosts.
• In cestodes: Cysticercus, hydatid cyst, coenurus,
cysticercoid, coracidium, procercoid and plerocercoid
forms.
• In trematodes: Cercaria, metacercaria, redia, miracidium
• and sporocyst
• In nematodes: Rhabditiform larva, filari form larva and
• microfilaria.
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7. Eggs
Based on their reproduction, helminths can be classified into the following:
• oviparous: Most of the helminths (cestodes, trematodes and many
nematodes) are oviparous, i.e. after fertilization, the adult worm lay eggs
(e.g. all cestodes, trematodes and intestinal nematodes except
Strongyloides)
• Viviparous: Some higher helminths do not have egg stage. After
fertilization, they directly discharge larvae (e.g. tissue nematodes such as
filarial worms, Dracunculus and Trichinella)
• Ovoviviparous: They lay egg containing larva, that immediately hatches
out (e.g. Strongyloides).
• Various helminths have distinct morphology of eggs which can be used to
differentiate the helminths.
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8. Differences between cestodes, trematodes and nematodes
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9. Life Cycle
• Life cycle of helminths gets completed in one or more
hosts.
• Cestodes complete their life cycle in two hosts
(definitive host and intermediate host) except
– Hymenolepis (requires only one host—man) and
– Diphyllobothrium requires three hosts (one
definitive host— man, and two intermediate
hosts—First, cyclops and second, fish)
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10. Life Cycle continued…
• Most trematodes require three hosts (one definitive host—man,
and two inter mediate hosts—first snail and second aquatic plant or
fish) except
– Schistosomes (need two hosts, definitive host—man and
intermediate host—snail)
• Nematodes complete their life cycle in one host (man) except
– filarial worms (need two hosts, definitivehost—man and
intermediate host—mosquito) and Dracunculus (need two
hosts, definitive—host man and intermediate host—cyclops).
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