Parasitology

• 23-12-2018

Medical Parasitology

3rd class

Helminths (Worms)
• Dr. Abdul Rahman Dahham, PhD
• Department of Microbiology
• College of Medicine
• Nineveh University
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Introduction to Helminths (Metazoa)

The term helminth has been derived from a Greek word meaning worm. It was originally meant to refer to only intestinal worms, but now includes tissue parasites as well as many free living species. These are called metazoa. The helminthes or worms unlike the protozoa are macroscopic and multi-cellular, having digestive, excretory, reproductive and nervous systems, through these may be lacking or highly modified in some species.

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Worms can be categorized according to their habitat in the host, there are intestinal round worms, liver flukes, blood parasite as well as many free living species. Helminthes have an outer protective covering "cuticle" the month may be provided with teeth or cutting plates. Helminthes may possess suckers or hook for attachment to host tissues they don't possess organ locomotion, but in some species the suckers assist in movement (locomotion is generally by muscular contraction and relaxation.
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Classification of helminths
The helminths are invertebrates characterized by elongated, flat or round bodies. The metazoa are classified into two phyla: Platyhelminthes and Nemathelminthes.

Platyhelminths (platy from the Greek root meaning "flat“). Platyhelminthesis divided into two classes: Cestodea (tapeworms) & Trematodea (flukes).
Nemathelminthes has only one class Nematodea (roundworms).

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Roundworms are nematodes (nemato from the Greek root meaning "thread"). These groups are subdivided for convenience according to the host organ in which they reside, e.g., lung flukes, extra-intestinal tapeworms, and intestinal roundworms.

Features of helminthic groups:

Trematodes (flukes)– flat, leaf-shaped, un-segmented, separate sexes.
Cestodes (tapeworms)– flat, segmented, hermaphroditic.
Nematodes (roundworms)– un-segmented, separate sexes.
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• For example

• Fasciola hepatica
• Schistosoma spp

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Fascioloa hepatica

Adult worm of trematode (fluke)

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Adult worm of nematode (roundworm)

• Examples

• Ascaris lumbricoides,
• Ankylostoma duodenale,
• Enterobius vermiculoaris



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A Nematode worm,
Ascaris lumbricoides




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A Cestode worm, Echinococcus granulosus

Taenia saginata

• Example
• Taenia solium, E.granulosus,

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They do not have organs of locomotion, so locomotion is by muscular contraction & relaxation.
Covering system known as cuticle or integument, situated on its outer surface & may be armed with spines or hooks (resistant to intestinal digestion).
Nervous system and excretory system are primitive.
Reproductive system is well developed.
Hermaphrodites or separate sexes.


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The common morphological features of helminthes

The alimentary tract has completely disappeared from all stages of the tapeworms (cestodes); it is nearly absent in many of the trematodes, but its present and complete in most nematodes.

The factors that determine helminth population, are those associated with the host-parasite relationship. Massive infection depends initially on massive inoculation of infective larvae & eggs. The co-existence of many species in the same individual (poly-helminthism) is widely common.

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When worms are crowded the collective egg output is great, but the output per worm is relatively low, depending on the degree of crowding.

with respect to helminthes life cycle:

In some helminths, the life cycle is direct & relatively simple; involving only one host species and a brief period of development of an infective stage, ex: pin worm (Enterobius vermicularis).

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In a group of soil transmitted helminths, the life cycle involves only one host (man) but the infective stage (larvae) remaining in the egg, as in Ascaris lumbricords & Trichuris trichiura; or free in soil as in hookworm species which requires a period of development in soil, i.e. the soil functions as an intermediate host.
In other, the man-to-man cycle involves essential development in one intermediate host as in the filarial worms & most tapeworms, or two intermediate hosts, as in most trematodes; the first being a snail or other mollusk, the 2nd is an animal or plant that is eaten by people.

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