TOP SCORES PAPERS AND NOTES
MEDICAL PARASITOLOGY NOTES
INTRODUCTION
Man and other living things on earth live in an entangling relationship with
each other. They don’t exist in an isolated fashion. They are interdependent;
each forms a strand in the web of life. Medical parasitology is the science that
deals with organisms living in the human body (the host) and the medical
significance of this host-parasite relationship.
ASSOCIATION BETWEEN PARASITE AND HOST
A parasite is a living organism, which takes its nourishment and other needs from a host; the
host is an organism which supports the parasite. The parasites included in medical
parasitology are protozoa, helminths, and some arthropods. (See box 1 for broader
classification of parasites). The hosts vary depending on whether they harbor the various
stages in parasitic development. BOX 1.
DIFFERENT KINDS OF PARASITES
• Ectoparasite – a parasitic organism that lives on the outer surface of its host, e.g., lice,
ticks, mites etc.
• Endoparasites – parasites that live inside the body of their host, e.g., Entamoeba
histolytica.
• Obligate Parasite - This parasite is completely dependent on the host
during a segment or all of its life cycle, e.g., Plasmodium spp.
• Facultative parasite – an organism that exhibits both parasitic and non-
parasitic modes of living and hence does not absolutely depend on the
parasitic way of life, but is capable of adapting to it if placed on a host.
E.g., Naegleria fowleri
• Accidental parasite – when a parasite attacks an unnatural host and survives. E.g.
Hymenolepis diminuta (rat tapeworm).
• Erratic parasite - is one that wanders in to an organ in which it is not usually found.
E.g., Entamoeba histolytica in the liver or lung of humans.
Most of the parasites which live in/on the body of the host do not cause
disease (non-pathogenic parasites). In Medical parasitology we will focus on
most of the disease causing (pathogenic) parasites. However, understanding
parasites which do not ordinarily produce disease in healthy
(immunocompetent) individuals but do cause illness in individuals with
1
, impaired defense mechanism (opportunistic parasites) is becoming of
paramount importance because of the increasing prevalence of HIV/AIDS in
our country.
DIFFERENT KINDS OF HOSTS
• Definitive host – a host that harbors a parasite in the adult stage or where the
parasite undergoes a sexual method of reproduction.
• Intermediate host - harbors the larval stages of the parasite or an asexual
cycle of development takes place. In some cases, larval development is
completed in two different intermediate hosts, referred to as first and second
intermediate hosts.
• Paratenic host – a host that serves as a temporary refuge and vehicle for
reaching an obligatory host, usually the definitive host, i.e., it is not necessary
for the completion of the parasite’s life cycle.
Reservoir host – a host that makes the parasite available for the transmission to another host and
is
2
,usually not affected by the infection.
• Natural host – a host that is naturally infected with certain species of parasite.
• Accidental host – a host that is under normal circumstances not infected with the parasite.
There is a dynamic equilibrium which exists in the interaction of organisms. Any organism that
spends a portion or all of its life cycle intimately associated with another organism of a different
species is considered as Symbiont (symbiote) and this relationship is called symbiosis (symbiotic
relationships).
The following are the three common symbiotic relationships between two organisms:
Mutualism - an association in which both partners are metabolically dependent upon each other
and one cannot live without the help of the other; however, none of the partners suffers any harm
from the association. One classic example is the relationship between certain species of
flagellated protozoa living in the gut of termites. The protozoa, which depend entirely on a
carbohydrate diet, acquire their nutrients from termites. In return they are capable of synthesizing
and secreting cellulases; the cellulose digesting enzymes, which are utilized by termites in their
digestion.
Commensalism - an association in which the commensal takes the benefit without causing
injury to the host. E.g., Most of the normal floras of the humans’ body can be considered as
commensals.
Parasitism - an association where one of the partners is harmed and the other lives at the
expense of the other. E.g., Worms like Ascaris lumbricoides reside in the gastrointestinal tract of
man, and feed on important items of intestinal food causing various illnesses.
Once we are clear about the different types of associations between hosts and parasites, we can
see the effect the parasite brings to the host and the reactions which develop in the host’s body
due to parasitic invasion.
EFFECT OF PARASITES ON THE HOST
The damage which pathogenic parasites produce in the tissues of the host may be described in
the following two ways;
(a) Direct effects of the parasite on the host
• Mechanical injury - may be inflicted by a parasite by means of pressure as it grows
larger, e.g., Hydatid cyst causes blockage of ducts such as blood vessels producing
infraction.
• Deleterious effect of toxic substances- in Plasmodium falciparum production of toxic
substances may cause rigors and other symptoms.
• Deprivation of nutrients, fluids and metabolites -parasite may produce disease by
competing with the host for nutrients.
3
, (b) Indirect effects of the parasite on the host:
Immunological reaction: Tissue damage may be caused by immunological response of the
host, e.g., nephritic syndrome following Plasmodium infections. Excessive proliferation of
certain tissues due to invasion by some parasites can also cause tissue damage in man, e.g.,
fibrosis of liver after deposition of the ova of Schistosoma.
4
MEDICAL PARASITOLOGY NOTES
INTRODUCTION
Man and other living things on earth live in an entangling relationship with
each other. They don’t exist in an isolated fashion. They are interdependent;
each forms a strand in the web of life. Medical parasitology is the science that
deals with organisms living in the human body (the host) and the medical
significance of this host-parasite relationship.
ASSOCIATION BETWEEN PARASITE AND HOST
A parasite is a living organism, which takes its nourishment and other needs from a host; the
host is an organism which supports the parasite. The parasites included in medical
parasitology are protozoa, helminths, and some arthropods. (See box 1 for broader
classification of parasites). The hosts vary depending on whether they harbor the various
stages in parasitic development. BOX 1.
DIFFERENT KINDS OF PARASITES
• Ectoparasite – a parasitic organism that lives on the outer surface of its host, e.g., lice,
ticks, mites etc.
• Endoparasites – parasites that live inside the body of their host, e.g., Entamoeba
histolytica.
• Obligate Parasite - This parasite is completely dependent on the host
during a segment or all of its life cycle, e.g., Plasmodium spp.
• Facultative parasite – an organism that exhibits both parasitic and non-
parasitic modes of living and hence does not absolutely depend on the
parasitic way of life, but is capable of adapting to it if placed on a host.
E.g., Naegleria fowleri
• Accidental parasite – when a parasite attacks an unnatural host and survives. E.g.
Hymenolepis diminuta (rat tapeworm).
• Erratic parasite - is one that wanders in to an organ in which it is not usually found.
E.g., Entamoeba histolytica in the liver or lung of humans.
Most of the parasites which live in/on the body of the host do not cause
disease (non-pathogenic parasites). In Medical parasitology we will focus on
most of the disease causing (pathogenic) parasites. However, understanding
parasites which do not ordinarily produce disease in healthy
(immunocompetent) individuals but do cause illness in individuals with
1
, impaired defense mechanism (opportunistic parasites) is becoming of
paramount importance because of the increasing prevalence of HIV/AIDS in
our country.
DIFFERENT KINDS OF HOSTS
• Definitive host – a host that harbors a parasite in the adult stage or where the
parasite undergoes a sexual method of reproduction.
• Intermediate host - harbors the larval stages of the parasite or an asexual
cycle of development takes place. In some cases, larval development is
completed in two different intermediate hosts, referred to as first and second
intermediate hosts.
• Paratenic host – a host that serves as a temporary refuge and vehicle for
reaching an obligatory host, usually the definitive host, i.e., it is not necessary
for the completion of the parasite’s life cycle.
Reservoir host – a host that makes the parasite available for the transmission to another host and
is
2
,usually not affected by the infection.
• Natural host – a host that is naturally infected with certain species of parasite.
• Accidental host – a host that is under normal circumstances not infected with the parasite.
There is a dynamic equilibrium which exists in the interaction of organisms. Any organism that
spends a portion or all of its life cycle intimately associated with another organism of a different
species is considered as Symbiont (symbiote) and this relationship is called symbiosis (symbiotic
relationships).
The following are the three common symbiotic relationships between two organisms:
Mutualism - an association in which both partners are metabolically dependent upon each other
and one cannot live without the help of the other; however, none of the partners suffers any harm
from the association. One classic example is the relationship between certain species of
flagellated protozoa living in the gut of termites. The protozoa, which depend entirely on a
carbohydrate diet, acquire their nutrients from termites. In return they are capable of synthesizing
and secreting cellulases; the cellulose digesting enzymes, which are utilized by termites in their
digestion.
Commensalism - an association in which the commensal takes the benefit without causing
injury to the host. E.g., Most of the normal floras of the humans’ body can be considered as
commensals.
Parasitism - an association where one of the partners is harmed and the other lives at the
expense of the other. E.g., Worms like Ascaris lumbricoides reside in the gastrointestinal tract of
man, and feed on important items of intestinal food causing various illnesses.
Once we are clear about the different types of associations between hosts and parasites, we can
see the effect the parasite brings to the host and the reactions which develop in the host’s body
due to parasitic invasion.
EFFECT OF PARASITES ON THE HOST
The damage which pathogenic parasites produce in the tissues of the host may be described in
the following two ways;
(a) Direct effects of the parasite on the host
• Mechanical injury - may be inflicted by a parasite by means of pressure as it grows
larger, e.g., Hydatid cyst causes blockage of ducts such as blood vessels producing
infraction.
• Deleterious effect of toxic substances- in Plasmodium falciparum production of toxic
substances may cause rigors and other symptoms.
• Deprivation of nutrients, fluids and metabolites -parasite may produce disease by
competing with the host for nutrients.
3
, (b) Indirect effects of the parasite on the host:
Immunological reaction: Tissue damage may be caused by immunological response of the
host, e.g., nephritic syndrome following Plasmodium infections. Excessive proliferation of
certain tissues due to invasion by some parasites can also cause tissue damage in man, e.g.,
fibrosis of liver after deposition of the ova of Schistosoma.
4
