Unit 1: Medical parasitology
Parasitology and Virology notes · PTU syllabus (BMLS604-18)
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Unit summary
Parasites remain a major cause of illness in India, and the laboratory identifies them by their stages in stool and blood. This unit covers the general features of protozoa and helminths, sample handling and concentration techniques, and the morphology, life cycle and diagnosis of Giardia, Entamoeba, roundworms, hookworms, Taenia, malaria parasites, hydatid cyst and cysticercosis.
After this unit you can
- Classify parasites and handle stool samples
- Perform concentration techniques for ova and cysts
- Describe intestinal protozoa and helminths and their diagnosis
- Diagnose malaria, hydatid disease and cysticercosis
PTU syllabus topics
- Introduction and general characteristics of protozoa and helminths
- collection/transport/processing/preservation of samples
- morphology/life cycle/diagnosis of Giardia and Entamoeba
- roundworms and hookworms
- Taenia solium and T. saginata
- malarial parasite (P. vivax and P. falciparum)
- laboratory diagnosis of hydatid cyst and cysticercosis
- concentration techniques for ova
- 1. Mosquito bite: Sporozoites enter blood
- 2. Liver stage: Multiply in liver cells
- 3. Blood stage: Merozoites infect red cells
- 4. Gametocytes form: Taken up by a mosquito
- 5. Mosquito stage: Sexual cycle, new sporozoites
Topic 1
Protozoa and helminths: general characteristics
Structure
Single-celled eukaryotes
Multicellular worms
Multiplication in host
Yes
Usually no (except Strongyloides)
Stages seen
Trophozoites and cysts
Eggs (ova), larvae, adults
Examples
Entamoeba, Giardia, Plasmodium
Ascaris, hookworm, Taenia, Echinococcus
Nematodes (roundworms)
Ascaris, hookworms, Enterobius, Strongyloides, filarial worms
Cestodes (tapeworms)
Taenia solium, T. saginata, Echinococcus
Trematodes (flukes)
Schistosoma, Fasciola
- Terms: definitive host (adult or sexual stage), intermediate host (larval or asexual stage), vector (transmits), reservoir.
Topic 2
Collection, transport and preservation of samples
- Container
- Clean, wide-mouthed, leak-proof; free from urine and water
- Amount
- About 5 g (walnut size) or 5–10 mL liquid
- Timing
- Examine liquid stool within 30 minutes (trophozoites), formed stool within 1–2 hours
- Repeat samples
- Three samples on alternate days increase yield
- Preservatives
- 10% formalin (cysts, ova), PVA (permanent stains), SAF
- Blood
- Thick and thin films at fever peak; EDTA for later films
Topic 3
Concentration techniques
Saline and iodine mounts (direct)
Unconcentrated
Motile trophozoites (saline); cysts (iodine)
Formol-ether (formalin–ethyl acetate) sedimentation
Centrifugation; fats and debris removed by ether
Most cysts, ova and larvae — routine method
Zinc sulphate floatation (specific gravity 1.18)
Light cysts and eggs float
Giardia and hookworm; not operculated or heavy eggs
Saturated salt floatation
Specific gravity about 1.20
Hookworm and Ascaris eggs
Simple centrifugation
Sediment examined
General
Topic 4
Giardia and Entamoeba
Trophozoite
Pear-shaped, two nuclei, four pairs of flagella, sucking disc; falling-leaf motility
Single nucleus with central karyosome; ingested red cells; directional motility
Cyst
Oval, four nuclei
Round, up to four nuclei; chromatoid bars
Disease
Diarrhoea, malabsorption (steatorrhoea)
Amoebic dysentery, amoebic liver abscess
Diagnosis
Cysts or trophozoites in stool; stool antigen ELISA
Trophozoites with red cells in fresh stool; serology for liver abscess; stool antigen or PCR to distinguish E. dispar
- Life cycle: both infect by swallowing cysts in contaminated food and water (faecal–oral).
Topic 5
Roundworms and hookworms
Infection
Swallowing embryonated eggs
Filariform larvae penetrate skin (barefoot)
Life cycle
Larvae migrate through lungs (Loeffler's syndrome) to intestine
Larvae via lungs to small intestine; worms suck blood
Egg
Fertilised: round, thick mammillated coat, bile-stained; unfertilised: elongated
Oval, thin shell, 4–8 cell stage, not bile-stained
Disease
Intestinal obstruction, malnutrition
Iron deficiency anaemia, eosinophilia
Topic 6
Taenia solium and T. saginata
Intermediate host
Pig (also humans — cysticercosis)
Cattle
Scolex
Four suckers and hooks (armed)
Four suckers, no hooks
Gravid segment
7–13 lateral uterine branches
15–20 branches
Egg
Identical — round, radially striated, hexacanth embryo
Identical
- Humans acquire intestinal taeniasis by eating undercooked infected meat. Eggs cannot be distinguished — identify by segments or scolex.
Topic 7
Malaria parasites
- 1
Infected female Anopheles injects sporozoites
- 2
Liver stage (exo-erythrocytic schizogony); P. vivax forms dormant hypnozoites
- 3
Merozoites invade red cells — ring, trophozoite, schizont (erythrocytic schizogony)
- 4
Red cells burst — fever every 48 hours
- 5
Some form gametocytes
- 6
Mosquito ingests gametocytes — sexual cycle (sporogony)
Infected red cells
Enlarged, with Schüffner's dots
Normal size; Maurer's clefts
Rings
Large, single
Small, delicate, double chromatin dots, multiple rings, appliqué forms
Stages in peripheral blood
All stages
Mainly rings and gametocytes
Gametocyte
Round
Crescent (banana) shaped
Severity
Relapses from hypnozoites
Severe and cerebral malaria
- Diagnosis: thick films (detection) and thin films (species); rapid diagnostic tests detect HRP-2 (P. falciparum) and pLDH (pan-Plasmodium); QBC; PCR.
Topic 8
Hydatid cyst and cysticercosis
- Hydatid disease: larval stage of Echinococcus granulosus (dog tapeworm); humans are accidental intermediate hosts infected by eggs from dog faeces; cysts mainly in liver and lungs. Diagnosis: ultrasound and CT, serology (ELISA, IHA), hydatid sand (scolices and hooklets) in cyst fluid after surgery. Casoni's test is obsolete.
- Cysticercosis: larval stage of T. solium in humans after swallowing eggs (faecal–oral or autoinfection); neurocysticercosis is a leading cause of epilepsy in India. Diagnosis: CT or MRI (cysts with scolex), serology (EITB western blot, ELISA), biopsy of subcutaneous nodules.
Key terms
- Definitive host
- Host harbouring the adult or sexual stage
- Cyst
- Resistant infective stage of protozoa
- Formol-ether concentration
- Sedimentation method for ova and cysts
- Hypnozoite
- Dormant liver stage of P. vivax causing relapse
- Neurocysticercosis
- Larval T. solium infection of the brain
Quick revision
- Protozoa vs helminths; nematodes, cestodes, trematodes; host terms.
- Stool collection, timing, preservatives; three samples.
- Saline, iodine, formol-ether, zinc sulphate, salt floatation.
- Giardia, Entamoeba; Ascaris, hookworm; T. solium, T. saginata.
- Malaria life cycle and species features; RDTs; hydatid cyst and cysticercosis diagnosis.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Distinguish a definitive and an intermediate host.
- Q2.Why should liquid stool be examined within 30 minutes?
- Q3.Which concentration method is used routinely?
- Q4.Describe the cyst of Giardia.
- Q5.How does hookworm cause anaemia?
- Q6.Name two features of P. falciparum in a blood film.
Long-answer questions
- Q1.Describe concentration techniques for stool examination.
- Q2.Describe the morphology, life cycle and diagnosis of Entamoeba histolytica.
- Q3.Describe the life cycle and laboratory diagnosis of malaria.
- Q4.Describe hydatid disease and cysticercosis and their diagnosis.
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