Unit 3: Helminthes (Introduction)
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Lecture 1 – Introduction to
Helminthes:
The term Helminths means Worms
It comprises of phyla
The two classes are parasitic in all or most of their life
cycle stages.
General characters of helminthes
Helminthes are macroscopic & multicellular.
Bilaterally symmetrical.
Have special adaptations for the parasitic mode of life &
for survival.
Like the complete absence of the Digestive tract or
greatly reduced consist of mouth & blind sac only this is
because of their location in the host intestine or tissue
where predigested nutrient are abundant.
In Trematodes the Digestive system is partially loss.
In Cestodes there is no
Digestive system.
While in Nematodes the Digestive system is complete.
There is special adaptation in Trematode & Cestode,
presence of a coat of microvilli on the outer surface of the
tegument for nutrient absorption.
Internal parasite possesses all sorts of adaptation like
hooks, suckers, boring apparatus, etc……….
Reproductive system is very well developed.
(Self-fertilization, cross fertilization may take place. They
are monicious or diecious).
Eggs are produced in large numbers or few of them
survive & manage to infect a suitable host.
Helminthes
do not multiply in human body so the
number of individuals in a worm population living within
a given host does not exceed the number of infective eggs
or larvae that enter from the external environment.
However reproduction by autoinfection to increase
parasite population occur e.g. pin worm by ingestion
eggs through contaminated fingers .
Helminthes (adult worm, their eggs, larvae) can be
distributed in various organs & tissues of the body.
Most of the infected people with helminthes are
symptomatic carriers, and the diseased individual
among the infected groups are those with heaviest worm
burden.
The terms Light, moderate, heavy applied to worm
burden are relative & differ for various species of
helminthes and in people of different age & physical
status.
The number of eggs or larvae eliminated in feces, urine,
sputum is roughly proportional to the number of worms
generating them.
In heavy infection (crowded worms) the collective egg
output is great but egg output\worm is low.
The coexistence of several species of helminthes in the
same individual is widely prevalent, we called it
Polyhelminthism.
Many worms may be quite restricted in distribution. Since
they require certain environmental conditions (temp,
humidity, nature of soil, etc…) also presence of
intermediate host.
(e.g. Schistosomes) require certain species of snails as
intermediate host which in turn require specific
environments for survival. Hook worms as e.g. during
their phase in soil require proper conditions of temp.,
moisture & soil texture in order to survive & continue
their life cycle.
The life cycle of the helminths is either Direct or simple
involving only one host species, or indirect or complex
involving more than one host.
Certain helminthes include in their life cycle special kind
of transmission called Paratenesis involving paratenic
host which provide the parasite with protection, support &
availability to its final host.
Helminths are transmitted through contaminated water,
soil, food. And the modes of infection are:-
a) Oral ingestion:
Most of the worm transmitted through ingestion of
infective stage (either egg, larva through intermediate
infective host)
b) Skin penetration
:
(e.g. Schistosome)
c) Inhalation
:
Very rare (pin worm)
d) Insect bites: (e.g. in Filariasis)
The portal of exit depend on the habitat or localization of
the adult worm, if it is in the intestine (eggs in feces), in
the urinary tract (eggs in urine), in the Respiratory
system (eggs in sputum).
Unit 3: Helminthes (Introduction)
52
Pathogenesis of parasitic infection
The effects produced by parasitic worm depend
on:
1) The organs or tissues occupied or unusual site for
their occurrence
2) Number of adult worm harbored (severity or intensity
of infection).
3) Species of helminth parasite.
(some are potentially dangerous)
4) Re infection (repeated exposure)
5) Immunity of the host
Immunological responses are of great importance in
parasitic diseases. Since they present a large number of
antigens to their hosts. So in this case the host may
elaborate specific antibodies to counteract the antigen
produced as may wall off the pathogen or its products
cellular infiltration, proliferation & differentiation.
Some helminths parasite have special mechanism for
evasion the immune response (e.g. Schistosomes adsorb
host proteins on their surfaces).
Pathogenic effects of parasite
Physical trauma,or destruction of cells, tissues, organs by
mechanical or chemical means.
The parasite cause damage externally on the skin at the
site of invasion (e.g. hook worm, Schistosome cercaria)
or internally (attachment of hook worm, taeniasis).
Irritation of tissues by extruding their eggs into them
(e.g. Schistosome) or by movement & migration of adult
& larvae cause initiation of intestinal wall (e.g. Taenia
solium).
Mechanical pressure by the growing & developing of
cestodes larvae (e.g. hydatid cyst) causing atrophy of the
neighboring organ.
Intestinal obstruction:
Due the size & number (e.g. Ascaris “worm ball” )
Intestinal perforation (e.g. Ascaris, Strongyloides)
Production of anemia. Due to sucking blood (e.g. Hook
worm) or due to vit. B
12
depletion (e.g. Fish tape worm).
Reaction
All worms that live or migrate in blood or tissue sensitize
the body to their section or execution or to parasite surface
glycoprotein & polysaccharides causing allergic reaction
A characteristic feature of an allergic reaction is an
increase in the number of eosinophils so eosinophilia is a
general sign of helminthic infections.
Besides eosinophilia, common signal of occult helminthic
infection are hepatomegal y, pneumonitis, brouchial
asthma, urticaria, sub cutanous cysts, neurologic
disturbance & deviations in behaviour.
So must of the pathological features of helminthic
infection not due to direct action of the parasitic on
tissue but on host’s response to the product of parasite
eggs, larvae & soluble antigens.