DR.

Samshad jahan shumu

Associate Professor.Microbiology
Shsmc
Objectives
 At the end of the lecture class ,students will able to
explain-
1. Definition of terms
2. Host
3. Types of parasite
4. Host parasite relationship
5. Source of parasite
Definition of terms
 Medical Parasitology: That branch of
medical science which deals with the
study of parasites of man and their
medical consequences.
 The term "parasite" came from the Greek
"parasitos" (para-, along side of + sitos,
food) meaning "eating at the side of, or at
the same table."
Parasite- Definition
 Parasites are a living organism which
lives in or upon another living
organism (host) and derives nutrients
and shelter from it,without giving any
benifets and causing some degree of
real damage to its host.
Types of parasites
 Parasite may be classified as:
1.Ectoparasite
2.Endoparasite
-Obligate parasite
-Facultative parasite
-Accidental parasite
-Aberrant parasite or wandering parasite
classification
1.Ectoparasite-Inhabbit the surface of the
body of the host without penetrating into
the tissues.
The infection by these parasites is called as
infestation.e.g.-Sarcoptes scabiej causing
scabies
classification
2.Endoparasite-live within the body of the host.
Invasion by the endoparasite is called infection
The endoparasites are of following types-
a) Obligate parasites-cannot exists without a
parasitic life in the host.(plasmodioum sp.)
b) Facultative parasite-they can live a parasitic life or
a free living life,when opportunity arises. E.g.-
Acanthamoeba
classification
c) Accidental parasite-They infect an
unusual host.(Echinococcus granulosus
infect humans accidentally
d)Abberant parasite or wandering
parasite-they infect a host where they
cannot live or develop further.
Hosts
Definition-Hosts are organisms which gives
shelters and nourishment to the parasite
Types of Hosts
1.Definitive host
2.Intermediate host
3.Accidental host
 Permissive host
 Non permissive host
4.Paratenic Hosts
5.Reservoir Hosts
Hosts
1.Definitive host: which harbors the adult stage
(Helminthes) or sexual stage (Protozoa) of the
parasite.
 Man is the definitive host of all nematodes.

2.Intermediate Hosts: which harbors the larval stage

(Helminthes) or asexual stage (Protozoa) of the
parasite.
e.g.-In malaria parasite life cycle,humans are the
intermediate host.
Host
3.Accidental Hosts-are those hosts in which the
parasite do not normally develop but when
infections occur accidentally, the parasite is able to
complete its lifecycle.
 Permissive Hosts
 Non-Permissive Host
Hosts
 Permissive Hosts are those hosts where the
parasite can complete its life cycle. They include
true definitive and intermediate hosts as well as
many accidental hosts.
 Non-Permissive Host are those accidental hosts
where the parasite can not complete its lifecycle
and eventually die within the host. These are the
dead end host. e.g. Man is the accidental non-
permissive host for Echinococcus granulosus
Hosts
4.Paratenic Hosts are those hosts where the
parasites undergo an arrested development on
infection, larval forms accumulating in these hosts
until they have a chance of infecting the definitive
host. These hosts are therefore not essential for
completion of the parasites lifecycle
5.Reservoir Hosts : a reservoir host is a vertebrate
animal species that harbors a particular pathogen
and acts as a long-term source of infection for
other vertebrates or vectors. e.g. Dog in case of
Leishmania donovani
Host- Parasite Relationships
 relationship between the parasite and the host,
may be divived into the following types-
 1. Parasitism
  a relationship between 2 living
organisms of different species
  one of the members lives at the
expense of the other and at the
same time causes it harm.

Host- Parasite Relationships
2.Commensalism: it is an association in which the parasite
only derives the benefit without causing any injury to the
host. (e.g. non pathogenic intestinal protozoa)
a commensal is capable of living an independent life.
3. Mutualism- beneficial to both but life without the other is
still possible

4. Symbiosis
 close association of 2 organism
 both members are so dependent upon each other
that life apart is impossible.
SOURCES OF PARASITIC INFECTION
A. Contaminated Soil or water
- Soil Ascaris lumbricoide, Trichuris trichura
Hookworms, Strongyloides
- Water Amoeba, Flagellates, Taenia solium
Blood Flukes
B. Food containing the Immature Infective Stage of the
Parasite
--Fish, crab, snail  tapeworms, capillaria
-- Beef, pork  Trichinella, Taenia
-- Intestinal, liver and lung flukes
SOURCES OF PARASITIC INFECTION

C. Blood sucking Insect

-- Malaria, Leishmaniasis, Trypanosoma

D. Domestic or Wild Animal Harboring the

Parasite
-- Dogs: D. caninum, E. granulosus, T. canis
A. Caninum
--Cats: Toxocara cati
-- Herbivorous animals: Trichostrongylus
SOURCES OF PARASITIC INFECTION

E. Another person, his clothing, bedding or

immediate environment that he has contaminated.
-- E. histolytica, pinworm, tapeworm
-- scabies

F. One’s self (auto-infection)

-- Strongyloides stercoralis
-- E. vermicularis (pinworm)
 Mode of Transamission
A.PORTAL OF ENTRY
1. Mouth- most common portal of invasion -ingestion
2. Skin
a. Skin penetration - Filaria, cercaria
b. Blood sucking arthropods--- mosquitos, flies,
mites
3. Inhalation--- Enterobius vermicularis
4. Transplacental– Ascaris lumbricoides
--Toxoplasma, Malaria (Plasmodia)
5. Transmammary--- strongyloides, Ancyclostoma,
Plasmodia,Schistosomes
6. Sexual Intercourse– Trichomonas vaginalis
Mode of Transamission
B. PORTAL OF EXIT
1. Feces --- mostly all kinds of parasite
2. Urine --- Trichomonas vaginalis
Strongyloides stercoralis,
Schistosoma haematobium
3. Sputum --- Paragonimus westermani
Ascaris lumbricoides
4. Discharges--- Trichomonas vaginalis
Dracunculus medinensis
 MECHANISM OF PATHOGENESIS
A. General Life Cycle
Parasite Host Migrate to location
for
maturation

Reproduce
B. Factors that determine intensity of parasitic infection
a. Topography of Locality
b. Social condition
c. Age
d. Hygienic Measure
e. Sewage Disposal
f. Water supply
 Pathogeneses
C. Ways of damage to Host
1. Inapparent infection
-- causes no symptoms and produces no harm
2. Traumatic damage
a. External --- invades the skin
b. Internal --- Direct damage to organs
3. Lytic necrosis -- Elaborate enzymes
4. Stimulate Host-tissue reaction
-- Cellular proliferation and infiltration
5. toxic and allergic phenomena --- introduce toxins
D. Immunity in Parasitic infections
E. Infective vs. Pathogenic stage
Life cycle of the parasites
1. Direct /Simple life cycle-when the parasite requires
only one host to complete its development.
2. Indirect /complex life cycle-when a parasite
requires two hosts (one definitive host and another
intermediate host)to complete its development.
 Infective form:
 Morphological form of the parasite which is transmitted
to man.
Autoinfection:
 Few intestinal parasites may infect the same person by
contaminated hand (external autoinfection) or by
reverse peristalsis (internal autoinfection). It is

 Observed in: Cryptosporidium parvum, Taenia solium,

Enterobius vermicularis, Strongyloides stercoralis and
Morphological form of Parasites
 In case of Protozoa-
1.Cycst
2.Trophozoite
In case of Metazoa-
1.Egg
2.Larva
3.Adult warm
Vector and carrier
 Vector: an arthropod or other living carrier that
transports a pathogenic organism from an infected to a
non-infected host.
 This can be passive transport (mechanical vector)
 An essential host in the life cycle of the pathogenic
organism (i.e. a biological vector).
 Carrier: a host that harbors a parasite but exhibits no
clinical signs or symptoms
Other terms
Zoonosis : A disease occurring primarily in wild and
domestic animals that can be transmitted to
humans. The word is derived from Greek words
‘zoon’ (animal) and ‘nosos’ (disease).

Disease-the disease is the clinical manifestation of

infection,which shows the active presence and
replication of the parasite causing damage to the
host.
CLASSIFICATION OF PARASITES
ANIMAL KINGDOM

Subkingdom Protozoa Subkingdom Metazoa

-Class Rhizopoda or Sarcodinia - Phylum Nematoda or
(Amoeba) Nemathelminthes
- Class Mastigophora or Flagellata - Phylum Platyhelminthes
(Flagellates) (Flatworms)
- Class Ciliata or Infusoria --- Class Trematoda
(Ciliates) (Flukes)
- Class Sporozoa (Sporozoas) --- Class Cestoda
(Tapeworms)
-Phylum Arthropoda
Protozoa-classification
On the basis of motility
 Non-motile: no motility organs, generally termed as
sporozoa, e.g- Plasmodium, Toxoplasma, Cryptosporidium,
Isopora, Cyclospora, Microsporidia, Sarcocystis

 Motile: different types of motility organs

 Pseudopodia-Rhizopoda-amoebida-Entamoeba
(histolytica, dispar, coli), Naegleria (fowleri, gruberi),
Acanthemoeba (castenelli)
 Flagella- Mastigophora- intestinal flagellates- Giardia
(intestinalis or lamblia), genital-Trichomonas (vaginalis,
homonis, tenax), haemoflagellates- Leishmania
(donovani, tropica, braziliensis), Trypanosoma (bruci,
cruzi)
 Cilia- Cilliate- Balantidium coli
LABORATORY DIAGNOSIS OF PARASITIC
DISEASES
 Examination of Feces
 As many parasites inhabit in the intestinal tract,
stool examination is the most common diagnostic
technique used for the diagnosis of parasitic
infections.
Specimen Collection
 Stool specimens should be
collected in a wide-mouthed, clean,
leak-proof, screw capped
containers and should be handled
carefully to avoid acquiring
infection from organisms present in
stool. 33

Essentials of Medical Microbiology

Specimen Collection
Timing:

 Collected before starting anti-parasitic drugs and

closer to the onset of symptoms.

Frequency:

 At least three stool specimens collected on

alternate days (within 10 days) (except for
intestinal amoebiasis - six specimens may be
recommended).
Specimen Collection
If delay in transport:
 Fecal specimens should be kept at room temperature;
 Preservatives (e.g. 10% formalin) - used to maintain the
morphology of the parasitic cysts and eggs.
Specimens other than stool:
 Perianal Swabs (cellophane tape or NIH swab)
 Duodenal Contents
 Macroscopic Examination
 Mucoid bloody stool: acute amoebic dysentery,
intestinal schistosomiasis, and invasive balantidiasis.

 Color: Dark red stool - upper gastrointestinal tract

(GIT) bleeding and a bright red stool - bleeding from
lower GIT.

 Frothy pale offensive stool (containing fat) - found in

giardiasis.
Microscopic Examination
 Includes direct wet mount examination

 Permanent staining methods.

Direct Wet Mount (Saline and Iodine
Mount)

 Drops of saline and Lugol’s iodine

are placed on left and right halves
of the slide respectively.

 A small amount of feces (~2 mg) -

mixed with a stick to form a
uniform smooth suspension.
38

Essentials of Medical Microbiology

 Cover slip - placed on the mount and examined
under low power objective (10X) for detection of
helminths eggs and larvae; followed by high power
objective (40X) for protozoan cysts and
trophozoites.
Saline Mount
 Useful in the detection of trophozoites and cysts of protozoa,
and eggs and larvae of helminths.

 Advantages than iodine mount:

 Motility of trophozoites and larvae in acute infection.
 Bile staining property can be appreciated - bile stained eggs
appear golden brown and non-bile stained eggs appear
colorless.
Iodine Mount
Advantages:
 Nuclear details of protozoan cysts, helminthic eggs
and larvae are better visualized, compared to
saline mount.
Disadvantages:
 Iodine immobilizes and kills the parasites, hence
motility of the trophozoites and helminthic larvae
cannot be appreciated
 Bile staining property cannot be appreciated.
Non-bile stained eggs
 Eggs of most of the intestinal parasites when they
pass through intestine are stained by bile.

 Exceptions:
Enterobius
Hookworm
Hymenolepis nana
Permanent Stained Smear
 Required for accurate detection of protozoan cysts
and trophozoites by staining their internal
structures.
 Commonly used methods are:
Iron-hematoxylin stain
Trichrome stain
Modified acid-fast stain - useful for coccidian
parasites such as Cryptosporidium, Cyclospora
and Cystoisospora
Examination of Blood
 Useful in diagnosis of infection caused by blood
parasites like Plasmodium, Trypanosoma,
Leishmania, Wuchereria bancrofti, Brugia malayi,
Loa loa
 Various methods of examination of blood include:

 Direct wet mount examination

 Examination of blood smears (thin smear and thick smear)

 Quantitative buffy coat (QBC)

 Concentration of blood
Immunodiagnostic
Methods
 Detection of parasite specific antibodies in serum,
and detection of circulating parasitic antigen in
the serum.
 These methods are useful when:
Parasites are detected only during the early
stages of the disease
Parasites occur in very small numbers
Parasites reside in internal organs and
morphological identification is not possible
When other techniques like culture are time
consuming
 Detection of antigen
 Detection of antibodies
 Detection of parasitic DNA or RNA
 Indirect haematological and biochemical changes
 Antibody Detection Tests
 Amoebic liver abscess: ELISA, detecting antibodies
against 170 kDa of lectin antigen.

 Visceral leishmaniasis: Detecting antibodies to rK-39

antigen by immunochromatographic test (ICT)

 Toxoplasmosis: (i) Sabin-Feldman dye test- a

complement mediated neutralization test, which detects
antibodies, (ii) Detection of specific IgM or IgA or IgG
antibodies by ELISA
Antigen Detection Tests
 Malaria: ICT format available detecting:
Histidine rich protein-2 (Pf. HRP 2)— P.
falciparum specific
Parasite lactate dehydrogenase (pLDH) and
aldolase - common to all species
 Lymphatic filariasis: ELISA and ICT formats are
available detecting filarial antigens by using
monoclonal Ab against