Medical Parasitology
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.
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.
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, helminthes, and some
arthropods. The hosts vary depending on whether they harbor the various
stages in parasitic development.
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.
• Erratic parasite: is one that wanders in to an organ in which it is not usually
found. E.g. Entamoeba histolytica is a good example. Typically, this parasite
is found in the gastrointestinal tract where they cause amoebiasis (bloody
diarrhea). However, the parasite has been shown to wander and invade such
organs as the liver and lungs in human.
Some of the parasites which live in/on the body of the host do not cause disease
(non-pathogenic parasites). In Medical parasitology, focusing 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 impaired defense
mechanism (opportunistic parasites) is becoming of paramount importance
because of the increasing prevalence of HIV/AIDS worldwide.
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 parasites life cycle.
• Reservoir host: a host that makes the parasite available for the transmission
to another host and is 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.
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:
1-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.
2-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.
CLASSIFICATION OF MEDICAL PARASITOLOGY
Parasites of medical importance come under the kingdom called protista and
animalia. Protozoa is a microscopic single-celled eukaroytes. In contrast,
helminthes are macroscopic, multicellular worms possessing well differentiated
tissues and complex organs belonging to the kingdom animalia. Medical
Parasitology is generally classified into:
• Medical Protozoology - Deals with the study of medically important
protozoa.
• Medical Helminthology - Deals with the study of helminthes (worms) that
affect man.
• Medical Entomology - Deals with the study of arthropods which cause or
transmit disease to man.