3. Parasite
A paarsite : is an organism that is entirely dependent on
another organism, referred to as its host, for all or part
of its life cycle and metabolic requirements.
Parasitology : is the area of biology concerned with the
phenomenon of dependence of one living organism on
another.
Medical parasitology : deals with the parasites which infect
man ,the diseases they produce ,the response generated by
him against them and various methods of diagnosis and
prevention.
4. Nature of Parasites
A parasite could be unicellular, worm or an arthropod.
Features of Parasites
1. Smaller than their host
2. Short life span than their host
3. Have greater reproductive potential than their host.
5. Classification of parasite
Parasites can be classified into
1. Micro parasite
2. Macro parasite
Micro parasite it is small, unicellular and multiplies within
its vertebrate host, often inside cells.
Viruses , bacteria , and protozoa are micro parasites.
Macro parasite its large , multicellular , and has direct
reproduction within its vertebrate host.
This category includes Helminthes.
6. According to their habitat :
on the basis of their location parasites may also be divided
into two types.
1. Ectoparasites:
Organisms which live on the surface of the body, example
the human lice, are known as ectoparasite.
NB. The infection by these parasites is known as
infestation.
They are important as vector transmitting pathogenic
microorganisms.
7. 2. Endoparasites:
Organisms that live within the body of the host ( in the
blood, tissue, body cavities, digestive tract and other
organs ) are known as endoparasites.
All protozoan and helminthic parasites on man are
endoparasites.
The invasion by endoparasites is known as infection.
8. According to their dependence on the host:
Obligate parasites: organisms that cannot exist with out a
host example:- e.g:Toxoplasma Gondii
Facultative parasites: organisms that under favorable
circumstances may live either a parasitic or free living
existence example :-
Naegleraia fowleri
Others
9. Accidental parasites : organisms that attack unusual host.
( e.g. Echinococcus granulosus in man )
Aberrant parasites: organisms that attack a host where
they cannot live or develop further
(e.g. Toxocara canis in man )
Free- living: the term free-living describes the non-
parasitic stages of existence which are lived independently
of a host.
e.g. hookworms have active free living stages in the soil.
10. According to their Pathogenicity:
1. Pathogenic parasites:- It causes disease in the host.
E.g.E. histolytica
2. Non-Pathogenic (Commensal) parasite:-The parasite derives
food and protection from the host without causing harm to the
host. E.g. Entamoeba coli
3. Opportunistic parasites:- Parasites which cause mild
disease in immunologically healthy individuals, but they
cause severe disease in immuno-deficient hosts.
Example
Isospora belli
11. Host
It is defined as an organism which harbours the parasite and
provides the nourishment and shelter to the latter .
Types of the host
Definitive host
This is the host in which sexual reproduction of the parasite takes
place or in which the most highly developed form of a parasite
occurs.
Intermediate host
This is the host which alternates with the definitive host and
harbours the larval or asexual stages of a parasite.
Some parasite require two intermediate hosts for completion of
their life cycle.
These are referred to as first and second intermediate hosts
respectively.
12. Paratenic host
It is a host in which larval stage of parasite survives but does not
develop further. It is often not a necessary part of the 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.
Zoonosis
This term is used to describe an animal infection that is naturally
transmissible to humans either directly or indirectly via a
vector.
Example of parasitic diseases that are zoonoses include
leishmaniasis , Trypanosomiasis, japonicum shistosomiasis,
13. Vector
A vector is an agent, usually an insects that transmits and
infection from one human host to another.
Types of vectors
1. Mechanical vector: the term mechanical is used to
describe a vector which assists in the transfer of
pathogens between hosts but is not essential in the life
cycle of the parasite. Example house fly that carries
parasite cysts and eggs , bacteria or virus from infected
faeces to food that is eaten by humans.
2. Biological vector: a vector in which the pathogens
multiple or undergo developmental changes with or
without multiplication.
14. Host parasite relationships
Host parasite relationships are of the following types
Symbiosis
An association in which both host and parasite are so dependent
upon each other that cannot live without help of the other.
Commensalism
An association in which only parasite derives benefit without
causing any injury to the host.
Parasitism
Parasitism is relationship in which a parasite benefits and the host
provides the benefit. The host gets nothing in return and always
suffers from some injury.
The degree of dependence of parasite on its host varies.
15. Sources of infections
Parasite infections originate from following sources
Contaminated Soils and water: Soil polluted with
human excreta acts as a source of infection with :-
Ascaris lumbricoides, S.stercolaris, Trichuris trichuria and
hook worms.
Water polluted with human excreta may contain viable
cysts of entamoeba histolytica , giardia lamblia,
balantidium coli, eggs of taenia solium, hymenolepis nan,
and the infective cercarial stage of schistosoma
haematobium, S.mansoni and S. japonicum.
16. Freshwater fishes constitute the source of diphyllobothrium
latum and others.
Crab and crayfishes are the source of paragonimus
westermani.
Raw or undercooked pork is the source f Trichinella
spiralis, T.solium, T.saginata .
Raw or uncooked beef : is the source of T.sagnita,
Toxoplasma gondi and others..
17. Watercress: is the source faciola hepatica .
Blood-sucking insects transmit plasmodium spp,
wuchereria bancrofti, brugia malayi, onchocerca volvulus,
trypanosome brucie gambience, and babesia spp.
Housefly( mechanical carrier ) is the source of E.histolytica.
Dog: is the source of Echinococcus granulosus and
toxocara canis.
Cat : is the source of t. gondii
Man: is the source E.histolytica, giardia lambia, enterobius
vermicularis and H.nana.
18. Portal of entry into the body
Mouth
The commonest portal of entry of parasites is oral through
contaminated food ,water , soiled finger or fomites.
This mode of transmission is transferred as Faecal- oral
route. Many intestinal parasites.
examples: E.histolytica, giardia lambia, balantidium coli,
E.vermicularis, T.trichiura, ascaris lambricoides and E.T.C
19. Skin
Entry through skin is another important portal of entry of
parasites. Infection with A.duodenale, N.americanus and
s.stercoralis acquired when filari-form larvae of these
nematodes penetrate the unbroken skin of an individual
walking over faecally contaminated soil.
Schistosomiasis caused by S.haematobium S.manoni and
S.japanicum is acquired when the cercarial larvae, in water
penetrate the skin.
20. Sexual contact
Trichomonas vaginalis is transmitted by sexual contact.
E. histolytica and g.lamblia may also be transmitted by anal
–oral sextual practices among male homosexuals.
Kissing
E.gingivalis is transmitted from person to person by kissing
or from contaminated drinking utensils.
21. Congenital
Infection with T. gondii and plasmodium spp may be
transmitted from mother to foetus transplacentally.
Inhalation
Airborne eggs of E.vermicularis may be inhaled into
posterior pharynx leading to infection.
Latrogenic infections
Malaria parasite may be transmitted by transfusion of blood
from the infected donor.
Malaria may also be transmitted by the use of contaminated
syringes and needles this may occur in drug addicts.
22. Mode of Transmission
Direct mode of Transmission:-
The parasite dose not require biological vectors and/or
intermediate hosts and require only a single host to complete its
life cycle. It may use mechanical vectors for transmission.
Direct Mode of Transmission can be classified as:
I. Horizontal Direct Mode of Transmission: Transmission is
mainly effected through:- Feco-oral route:
Most intestinal parasites transmitted in this way.
- Sexual intercourse
- Blood transfusion
- Direct skin penetration (Soil transmitted helminthes)
23. II. Vertical Direct Mode of Transmission:
Transmission of the parasite is from the mother to child
through:
- Congenital / transplacental
- Transmammary (breast milk)
Indirect Mode of Transmission
The parasite has complex life cycle and requires biological
vectors and/or one or more intermediate hosts for
transmission.
24. General Life Cycles of Parasites
Direct Life Cycle
A parasite that can complete its life cycle in a single host.
Examples:-
G. lamblia,
E. histolytica, etc.
Indirect Life Cycle:
When a parasite requires an intermediate host or vector to
complete its development.
Example :-
Plasmodium species,
Leishmania species ,
Taenia species. etc.
25. Pathogenecity
A parasite may live in or on the tissue of its host without
causing evident harm.
How ever , in majority of cases the parasite has capacity to
produce damage like
Traumatic damage e.g A.duodenale
Lytic necrosis e.g E.Histolytica
Competitive for specific nutrient
26. Laboratory diagnosis
Genital specimens
Trophozoite of T.vaginalis may be demonstrated in the vaginal
and urethral discharge and in the prostatic secretions.
Cerebrospinal fluid ( CSF)
Trophozoite of T.brucei gambiense and T.rhodesiense and
trophozoites of Naegleria fowlori, acanthamoeba spp and
balamuthia mandrillars may be demonstrated in CSF.
Sputum – examination of the sputum is useful in the
following:
In cases where the habitat of the parasite is in the respiratory
tract, as in Paragonimiasis, the eggs of Paragonimus
westermani are found.
In amoebic abscess of lung or in the case of amoebic liver
abscess bursting into the lungs, the trophozoites of E.
histolytica are detected in the sputum.
27. Urine
When the parasite localize in the urinary tract, the examination of
urine is useful in establishing parasitological diagnosis, e.g. eggs
of S.haematobium and trophozoites of T.vaginalis may be
demonstrated in the urine.
In case of chyluria caused by w.bancrofti microfilairae are often
demonstrated in chylous urine.
Blood
In those parasitic infection, where the parasite itself or in stage of its
development, circulates in the blood stream, the examination of
blood film forms the main procedure for specific diagnosis
example demonstration of the following parasites:
Plasmodium spp, and babesia spp, inside the RBC, L.donovani
inside monocytes ,trypomastigotes of T.b.gambiensse,
T.b.rhodesiense and T.cruzi, and microfilariae of W.bancrofti and
B.malayi in the blood.
28. Stool
Examination of stool is important for the diagnosis of
intestinal parasitic infections and helminthic infections of
the biliary tract in which eggs are discharged in the
intestine.
In protozoal infections, the trophozoites and eggs can be
demonstrated wet mount stool in normal saline and logol’s
iodine .
in helminthic infections eggs, larvae and adult worms may
be demonstrated .
Demonstration of parasites in the stools confirms the
diagnosis and golden standard in diagnonsis of intestinal
parasitic infections.
29. Tissue biopsy and aspiration
Amastigote forms of l.donovani may be demonstrated inside
the reticuloedothelial cells in the aspirtation of spleen,
bone marrow, liver and lymph nodes.
Larvae of T. spiralis T.solium and T.multiceps may be
demonstrated in the muscle biopsy.
Trophozoite of G.lamblia may be demonstrated in the bile
aspirated from duodenum by intubation.
Trophozoite of E.histolytica may be demonstrated in pus
aspirated from amoebic liver abscess and in the necrotic
tissue obtained from the base of the ulcer in the large
intestine
30. Immunodiagnosis
1. Skin test
These tests are performed by intradermal injection of
parasitic antigens and are read as under:
2. Serological diagnosis
These tests detect antibodies or antigens in patient’s
serum and other clinical specimens.
3. Molecular biological methods
These include DNA probes and polymerase chain
reaction (PCR).
