Medical Parasitology

1. Introduction to Helminthology
Helminth means worm. Their motile activity is
accomplished by wriggling movement.
The helminth of medical importance belong to
3 classes:
Nematoda,
Trematoda
Cestoda.

2. TERMINOLOGY
• Adults: sexually reproductive life cycle stage
• Larvae: developmental or asexually reproductive life cycle stage
• Eggs: stage protective of zygote and /or embryo
• Cysts: usually a larval stage encapsulated in tissues of an intermediate host
• Hypobiosis: worms in temporary developmental arrest
• Monoecious/hermaphroditic: both sexes 1 body
• Dioecious: sexes separate; males & females
• Oviparous: production of eggs, discharged from uterus of female
• Ovoviviparous: production of eggs which hatch prior to discharge from uterus of
female
• Viviparous: production of embryos/L1 larvae, no rigid encapsulation of embryo

3. HELMINTHES (WORMS) - Characteristics
• Eukaryotic, multicellular
animals that usually have
digestive, circulatory,
nervous, excretory, and
reproductive systems.
• Worms with bilateral
symmetry, head and tail,
and tissue differentiation
(endoderm, mesoderm, and
ectoderm).
Flatworm
Roundworm
Earthworm

4. Helminthes
Two main groups (phyla)
• Platyhelminths (Flatworms)
• Nematoda (Roundworms)
Life Cycle
• Extremely complex (egg → larva → adult)
• Intermediate hosts harbor larval (developmental) stage.
• Definitive host harbors adult stage.

5. Sexual reproduction strategies
Male and female reproductive organs are found in separate individuals.
One animal has both male and female sex organs (most hermaphrodites
copulate with other animals, a few copulate with themselves).
Female helminths:
Oviparous – lay eggs without embryonic development.
Ovoviviparous – embryos develop inside eggs.
Viviparous – the larva develops inside the body of the mother.
Eggs: unsegmented (no larva inside the egg) & segmented (larva inside
the egg)
1
2

6. Way of transmission
• Fecal-oral
• Contaminated food, water
• Ingestion of meat (larvae)
• Through skin
• Through vector
Localization
• Intestinal helminthes
• Blood and tissue helminthes

7. Nematodes (Roundworms)
• Cylindrical body tapered at each end.
• Have a complete digestive system:
mouth, intestine, and anus.
• Body is covered by tough cuticle
that resists drying and crushing.
• Separate males and females (males are
smaller than females
and have one or two spicules on
posterior end).
• Infections can be caused by eggs or
larvae.
• Reproduction and development:
egg → egg fertilization → embryo in egg →
larva 4 molts → adult

8. Nematodes - classification
Intestinal nematodes - with adults in bowel
• Ascaris lumbricoides
• Trichuris trichiura
• Enterobius vermicularis
• Ancylostoma duodenale and Necator
americanus
• Strongyloides stercoralis
Tissue nematodes - adults or larval stage in tissue
• Trichinella spiralis, native etc
• Toxocara canis (visceral larva migrans)
• Filaria - Wuchereria bancrofti
Brugia malayi
Onchocerca
volvulus Loa loa,
etc.

9. Cestodes (Tapeworm) - structure
• Scolex - attachment organ, contains
suckers and hooks used to attach to a host
organism.
• Zone of proliferation - undifferentiated
area behind the scolex (neck region)
• Strobilia - chain of segments (proglottids) -
square body segments used for
reproduction.
Immature proglottids - developing
reproductive
Mature proglottids: mature
reproductive organs.
Gravid proglottids: contain eggs in the
uterus.

10. Immature Segment
• note that the
reproductive organs
are just beginning to
differentiate.
Developing reproductive organs
(Carmine stained)

11. Mature Segments (Proglottids)
• Tapeworms are
Hermaphroditic

12. Cestodes (Tapeworm)
Intestinal cestodes
• Taenia solium (pork tapeworm)
• Taenia saginata (beef tapeworm)
• Diphyllobothrium latum (fish tapeworm)
• Hymenolepis nana (dwarf tapeworm)
• Hymenolepis diminutia
• Dipylidium caninum
Tissue cestodes
• Taenia solium - cysticercosis
• Echinococcus granulosus (unilocular hydatid)
• Echinococcus multilocularis (alveolar hydatid)

13. TREMATODES
Classification of Platyhelminthes:
Trematoda
(Fluks: Flatworms)
. Intestinal fluks
*Fasciolopsis buski
*Heterophyes heterophyes
*Metagonimus yokogawai
. Liver fluks
*Fasciola hepatica
* Fasciola gigantic
* Clonorchis sinensis
*Opisthorchis felineus
*Dicrocoelium dendriticum
.Pancreatic Fluks
*Eurytrema pancreaticum
. Lung Fluks
*Paragonimus wastermani
.Blood species
*Schistosoma haematobium
*Schistosoma mansoni
*Shsistosoma japonicum

14. Principle of stool sampling collection, handling
and processing
for parasites examination
Collection and handling:
• Minimum 3 samples
• Clean, water-tight container with a screw-cap lid
• The smallest acceptable amount of stool is 2-5g
• Urine should not be allowed to contaminate the specimen
• The specimen container should be labeled correctly (patients’
name, date and time of sample collection, test/tests requested,
suspected diagnosis, clinical findings, travel history)

15. Preservation (fixation)
• The ideal specimen is a freshly collected stool sample
• 5-10% formalin