1. CHAPTER 6
BASIC CONCEPTS
OF MEDICAL
PARASITOLOGY
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Part I
General Microbiology
2. Dr Sonal Saxena, MD
Director Professor and Head of the Department of Microbiology
Maulana Azad Medical College,
New Delhi
and
Dr Amala A Andrews, MD
Maulana Azad Medical College,
New Delhi
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3. GENERAL
CONCEPTS OF
PARASITOLOGY
Parasites are eukaryotic organisms with a true
nucleus
Unicellular: These can be visualised with a
microscope, e.g., protozoa such as Entamoeba
histolytica
Multicellular: These can be seen with the naked eye,
e.g., helminths such as roundworms
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4. LIFE CYCLE OF
PARASITES
Infective forms: Specific stages of the parasite life cycle which can
establish infection
Habitat: A place in which the parasite naturally lives and grows
Host: An entity that provides nutrition and harbours the parasite
◦ Definitive host: This is a host in which the adult stage of the
parasite lives/undergoes sexual reproduction
◦ Intermediate host: A host that harbours the larval stage of the
parasite/in which asexual reproduction takes place
◦ Reservoir host: Harbours the parasite and also acts as a source of
infection
◦ Paratenic host: The parasite is unable to develop further in this host;
it can act as a transport or carrier host
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5. TYPES OF
PARASITES
Ectoparasites: Parasites that live on the external body surfaces
of the host
Endoparasites: These live inside the host’s body
Obligate parasites: These organisms cannot exist without a host
Facultative parasites: These organisms can exist in the free-
living state or as parasites
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6. HOST–PARASITE RELATIONSHIP
Symbiosis: Parasite and host are mutually dependent on each other
Commensalism: Parasite benefits while the host is neither benefitted nor harmed
Parasitism: Parasites derive the benefits of nutrition and shelter from the host they live in; in the
process, the host is harmed
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7. LIFE CYCLE OF
PARASITES
ROUTES OF ENTRY OF PARASITES
Fig. 6.1 Routes of entry of disease-causing parasites
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8. Three kingdoms:
(i) Protozoa
(ii) Stramenopila
(iii) Animalia
Protozoa are unicellular, eukaryotic, simple, microscopic parasites
Stramenopila are unicellular, plant-like animals
Helminths are multicellular worms belonging to kingdom Animalia
CLASSIFICATION OF PARASITES
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9. PROTOZOOLOGY
Table 6.1 Common protozoa associated
with human infections and their
characteristics
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12. GENERAL CHARACTERISTICS, LIFE CYCLE AND MODES OF
TRANSMISSION OF PARASITES
CHARACTERISTICS OF NEMATODES
Long, cylindrical, bilaterally symmetrical and
Unsegmented, with both ends pointed and without
any appendages
Sexes are separate
Alimentary canal well-developed
Fig. 6.2 Internal structure of nematodes (Source: Textbook of Medical
Parasitology, Dr Sumeeta Khurana and Dr Abhishek Mewara)
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13. GENERAL CHARACTERISTICS, LIFE CYCLE AND MODES OF
TRANSMISSION OF PARASITES
Intestinal nematodes complete their life cycle in a single host
Nematodes either lay
◦ Eggs (oviparous, e.g., Ascaris), or
◦ Larvae (viviparous, e.g., Wuchereria) or
◦ Eggs that immediately hatch into larvae (ovoviviparous, e.g., Strongyloides)
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14. GENERAL
CHARACTERISTICS,
LIFE CYCLE AND
MODES OF
TRANSMISSION OF
PARASITES
LIFE CYCLE OF NEMATODES
Fig. 6.3 Schematic representation of the life cycle of
nematodes
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15. GENERAL CHARACTERISTICS,
LIFE CYCLE AND MODES OF
TRANSMISSION OF PARASITES
CHARACTERISTICS OF CESTODES
Segmented, long and flattened dorsoventrally:
‘Tapeworms’
Have a head (scolex), a neck and body (strobila)
Lacks alimentary canal
Sexes are not separate
Gravid proglottids containing hundreds of eggs are
released into the environment
Fig. 6.5 Schematic representation of the life cycle of
cestodes
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16. GENERAL CHARACTERISTICS,
LIFE CYCLE AND MODES OF
TRANSMISSION OF PARASITES
Fig. 6.4 Morphology of cestodes (Source: Textbook of
Medical Parasitology, Dr Sumeeta Khurana and Dr
Abhishek Mewara)
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17. GENERAL CHARACTERISTICS,
LIFE CYCLE AND MODES OF
TRANSMISSIONOF PARASITES
CHARACTERISTICS OF TREMATODES
Leaf-like, flat, unsegmented worms
Eggs are oval to round, operculated and
embryonated when laid
Sexes are not separate except in
schistosomes
Several larval forms are seen
Alimentary tract consists of a blind,
bifurcated intestinal tract without an
anus
Fig. 6.6 Morphology of trematodes (Source: Textbook of Medical
Parasitology, Dr Sumeeta Khurana and Dr Abhishek Mewara
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18. GENERAL CHARACTERISTICS, LIFE CYCLE AND MODES OF
TRANSMISSION OF PARASITES
Fig. 6.7 Schematic diagram of the
life cycle of trematodes
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20. PATHOGENESIS
OF PARASITIC
DISEASES
Physical obstruction, irritation, and trauma
Vascular or lymphatic obstruction: Elephantiasis
Enzymatic damage
Allergic reactions
Inflammation
Neoplastic reactions: Schistosoma are
associated with carcinoma
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21. IMMUNITY IN PARASITIC DISEASES
Cytotoxic T cell response, natural killer cells, activated macrophages and antibodies
Immunological responses against parasites are lower than those against bacteria and viruses due to
the intracellular location of parasites and immunological tolerance
Large and complex antigenic structures and multiple antigenic variations
Premunition immunity: immunity which lasts only while the original infection is active
LABORATORY DIAGNOSIS
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22. LABORATORY DIAGNOSIS
OF PARASITIC DISEASES
Table 6.5 Recommended samples to demonstrate
commonly encountered parasites
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23. LABORATORY DIAGNOSIS OF PARASITIC DISEASES
EXAMINATION OF FECAL SAMPLES
Collection of samples
i) The NIH swab is used to collect samples when there is a suspicion
of Enterobius infection
ii) Cellophane tape is applied to the perianal area of the patient
early in the morning
iii) String test for duodenal samples: examined for trophozoites
Fig. 6.8 NIH swab to collect sample for demonstrating eggs of E.
vermicularis (Source: Textbook of Medical Parasitology, Dr
Sumeeta Khurana and Dr Abhishek Mewara)
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24. LABORATORY
DIAGNOSIS OF
PARASITIC
DISEASES
Stool concentration techniques
Used to separate parasites from fecal matter
i) Flotation method: Stool sample mixed with solutions
with a higher specific gravity so the ova rise to the top
ii) Saturated salt technique: Saturated salt solution added
iii) Zinc sulfate floatation: 33% zinc sulfate solution is used
iv) Formol ether sedimentation method: Ether and 10%
solution of formalin are used
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25. LABORATORY DIAGNOSIS OF PARASITIC DISEASES
Wet mount
To identify protozoan trophozoites, cysts, oocysts, helminth eggs and larval forms
Other staining techniques used to demonstrate ova and cysts
1. Modified acid-fast staining
Identification of oocysts of Cryptosporidium, Cystoisospora, Cyclospora
Chromotrope staining: Used for microsporidial spores
Table 6.6 Stool findings commonly seen in infected patients
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26. LABORATORY DIAGNOSIS OF PARASITIC DISEASES
2. Permanent staining
To visualise coccidian parasites, trophozoites and ova of protozoa and eggs and larvae of the
helminth
3. Trichrome staining
Simple and rapid method
Used to visualise intestinal protozoa, human cells, yeasts and artifact material in the stool
4. Iron hematoxylin staining
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27. LABORATORY DIAGNOSIS OF PARASITIC DISEASES
Preservation of stool samples
If the samples cannot be examined within a few hours
If the specimen for molecular diagnosis must be shipped to a referral laboratory
•Formalin 10% aqueous
•PVA (polyvinyl alcohol)
•Other fixatives: Merthiolate iodine-formalin (MIF); sodium acetate-acetic acid formalin (SAF)
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28. LABORATORY
DIAGNOSIS
OF PARASITIC
DISEASES
EXAMINATION OF BLOOD SAMPLES
Peripheral blood smear (PBS)
Plasmodium in red blood cells
Amastigote forms of hemoflagellates (e.g., Leishmania
donovani) inside monocytes
Microfilariae of W. bancrofti and B. malayi
DEC/hetrazan provocation test: to stimulate nocturnal
microfilariae
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29. LABORATORY DIAGNOSIS OF PARASITIC DISEASES
Blood smears
Finger-prick method or from blood collected in EDTA tubes
Thick and thin films made
Thick film
Screening tool in malaria
Used to visualise Leishman–Donovan (LD) bodies in leishmaniasis
To detect microfilariae in filariasis
Smear from a big drop of blood, dehemoglobinisation done
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30. LABORATORY DIAGNOSIS OF PARASITIC DISEASES
Thin film
Smear should be one cell thick, fixed with methanol before staining
Staining of blood smear
Leishman stain
Giemsa stain
Wright stain
Fig. 6.9 Thick and thin films (Source: PHIL, Image ID 5905/
CDC)
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31. LABORATORY DIAGNOSIS OF PARASITIC
DISEASES
Culturing
Requires special media
There are three types of culture systems
i) Xenic cultivation: The parasite is grown in the presence of an undefined flora
ii) Monoxenic cultivation: The parasite is grown in the presence of a single species of associate
flora
iii) Axenic cultivation: The parasite is grown in the absence of any other metabolising cells
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32. LABORATORY DIAGNOSIS OF PARASITIC DISEASES
Diamond medium—for the cultivation of Trichomonas vaginalis
NNN (Novey, Nicoll and McNeal) medium—for Leishmania species
RPMI 1640 with human RBC in 7% carbon dioxide and 1–5% oxygen—used to
cultivate P. falciparum
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33. LABORATORY DIAGNOSIS OF PARASITIC DISEASES
Antigen detection
ELISA-based antigen detection kits: Giardia lamblia,
Cryptosporidium species in stool samples and T. vaginalis in
urogenital specimens
Serological diagnosis: ELISA, immunochromatographic tests
Skin hypersensitivity tests: E.g., performed to diagnose
hydatid disease (Casoni test)
Molecular diagnostic method: To diagnose malaria, filariasis
and trypanosomiasis
Fig. 6.10 Rapid antigen detection test for
malaria parasite: (a) positive for Plasmodium
vivax and (b) negative test (Source: Dept. of
Microbiology, ([a] MAMMC and [b] PIMS)
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