Parasitology
Master protozoa, helminths, arthropods, and laboratory diagnosis for the MedTech board exam.
Table of Contents
1. Introduction to Parasitology
Classification of Parasites
- • Protozoa: Single-celled organisms (amoebae, flagellates, ciliates, sporozoa)
- • Helminths: Multicellular worms (nematodes, cestodes, trematodes)
- • Arthropods: Insects and arachnids (vectors, ectoparasites)
Key Terminology
Host Types
- • Definitive host: Sexual reproduction occurs
- • Intermediate host: Asexual/larval stages
- • Reservoir host: Maintains infection in nature
- • Paratenic host: Carries parasite, no development
Life Cycle Terms
- • Trophozoite: Active, feeding stage
- • Cyst: Dormant, resistant stage
- • Oocyst: Sporozoans' environmental stage
- • Vector: Transmits parasite between hosts
Routes of Transmission
- • Fecal-oral: Ingestion of cysts/eggs (Entamoeba, Giardia, Ascaris)
- • Vector-borne: Mosquito, sandfly, tsetse fly (Malaria, Leishmania)
- • Skin penetration: Direct larval invasion (hookworm, Strongyloides)
- • Ingestion of larvae: Raw meat/fish (Taenia, Clonorchis)
- • Inhalation: Rare (Ascaris eggs)
- • Sexual contact: Trichomonas vaginalis
- • Transplacental: Toxoplasma, Plasmodium
2. Intestinal & Urogenital Protozoa
Intestinal Amoebae
| Species | Trophozoite | Cyst | Pathogenic? |
|---|---|---|---|
| Entamoeba histolytica | Ingested RBCs, directional motility | 4 nuclei, chromatoid bars (rounded ends) | YES - Amoebic dysentery, liver abscess |
| E. dispar | Identical to E. histolytica | Identical to E. histolytica | NO - Non-pathogenic |
| E. coli | Sluggish motility, bacteria in cytoplasm | 8 nuclei, splinter chromatoid bars | NO - Commensal |
| Endolimax nana | Smallest amoeba, large karyosome | 4 nuclei, oval | NO - Commensal |
| Iodamoeba bütschlii | Large glycogen vacuole | 1 nucleus, large glycogen vacuole | NO - Commensal |
Free-Living Amoebae (CNS Infections)
- • Naegleria fowleri: Primary amoebic meningoencephalitis (PAM), warm freshwater
- • Acanthamoeba: Granulomatous amoebic encephalitis (GAE), keratitis (contact lens)
- • Balamuthia mandrillaris: GAE, soil exposure
Intestinal Flagellates
Giardia lamblia (intestinalis)
- • Trophozoite: Pear/tennis racket shape, 2 nuclei ("owl face"), 4 flagella pairs
- • Cyst: Oval, 4 nuclei, axonemes
- • Site: Duodenum, upper jejunum
- • Transmission: Fecal-oral, contaminated water
- • Disease: Giardiasis - fatty, foul-smelling diarrhea, malabsorption
- • Treatment: Metronidazole
Trichomonas vaginalis
- • No cyst stage - only trophozoite
- • Features: 4 anterior flagella, 1 recurrent flagellum, undulating membrane
- • Site: Vagina, prostate, urethra
- • Transmission: Sexual contact
- • Disease: Trichomoniasis - frothy, green-yellow discharge
- • Treatment: Metronidazole (treat partner)
Dientamoeba fragilis
- • Amoeba-like but actually flagellate
- • No cyst stage (transmitted with Enterobius eggs)
- • Binucleate trophozoite
- • Causes GI symptoms
Non-Pathogenic Flagellates
- • Chilomastix mesnili: Lemon-shaped cyst
- • Trichomonas hominis: Intestinal, non-pathogenic
- • Retortamonas intestinalis
Intestinal Ciliates & Coccidia
Balantidium coli
- • Only ciliate pathogenic to humans
- • Largest intestinal protozoan
- • Cilia on surface, macronucleus (kidney-shaped)
- • Reservoir: Pigs
- • Causes: Balantidiasis (dysentery-like)
Intestinal Coccidia
- • Cryptosporidium parvum: 4 sporozoites, no sporocysts, acid-fast oocysts, causes watery diarrhea in AIDS
- • Cyclospora cayetanensis: 2 sporocysts × 2 sporozoites, autofluorescent, raspberries
- • Cystoisospora (Isospora) belli: 2 sporocysts × 4 sporozoites
3. Blood & Tissue Protozoa
Plasmodium Species (Malaria)
Vector: Female Anopheles mosquito | Endemic: Philippines (especially Palawan, Mindanao)
| Species | Fever Pattern | RBC Preference | Distinctive Features |
|---|---|---|---|
| P. falciparum | Malignant tertian (36-48h) | All RBCs (high parasitemia) | Ring forms, banana-shaped gametocytes, appliqué forms, Maurer's clefts |
| P. vivax | Benign tertian (48h) | Reticulocytes (young RBCs) | Enlarged RBCs, Schüffner's dots, amoeboid trophozoites, hypnozoites |
| P. ovale | Benign tertian (48h) | Reticulocytes | Oval RBCs, fimbriated edges, Schüffner's dots, hypnozoites |
| P. malariae | Quartan (72h) | Older RBCs | Band forms, rosette schizonts (8 merozoites), Ziemann's dots |
| P. knowlesi | Quotidian (24h) | All RBCs | Monkey malaria, band forms like P. malariae |
Malaria Life Cycle
- 1. Sporozoites (mosquito) → Liver (exoerythrocytic)
- 2. Merozoites → RBCs (erythrocytic cycle)
- 3. Ring → Trophozoite → Schizont → Merozoites
- 4. Some become gametocytes → mosquito
- 5. Sexual reproduction in mosquito (sporogony)
Diagnosis
- • Thick smear: More sensitive (detection)
- • Thin smear: Species identification
- • RDT: HRP2 (P. falciparum), pLDH
- • QBC (Quantitative Buffy Coat)
- • PCR: Most sensitive
Other Blood & Tissue Protozoa
Toxoplasma gondii
- • Definitive host: Cats (sexual cycle)
- • Transmission: Oocysts in cat feces, undercooked meat (cysts), transplacental
- • Disease: Toxoplasmosis - asymptomatic in healthy; encephalitis in AIDS; congenital (triad: hydrocephalus, intracranial calcifications, chorioretinitis)
- • Diagnosis: Serology (IgG/IgM), tissue biopsy
Leishmania species
- • Vector: Sandfly (Phlebotomus, Lutzomyia)
- • Forms: Amastigotes (in tissue), promastigotes (in sandfly)
- • L. donovani: Visceral (kala-azar), hepatosplenomegaly
- • L. tropica: Cutaneous (oriental sore)
- • L. braziliensis: Mucocutaneous
Trypanosoma species
- • T. brucei gambiense: West African sleeping sickness (tsetse fly)
- • T. brucei rhodesiense: East African sleeping sickness (acute)
- • T. cruzi: Chagas disease (reduviid bug), Romana's sign, megacolon, cardiomyopathy
- • Trypomastigotes: C/S-shaped, undulating membrane
Babesia microti
- • Vector: Ixodes tick (same as Lyme)
- • Maltese cross (tetrad) in RBCs
- • No hemozoin pigment
- • Hemolytic anemia, asplenic patients at risk
4. Nematodes (Roundworms)
Intestinal Nematodes
| Species | Common Name | Transmission | Diagnostic Stage | Key Features |
|---|---|---|---|---|
| Ascaris lumbricoides | Giant roundworm | Ingestion of eggs | Fertilized egg (mammillated) | Largest intestinal nematode, Loeffler's syndrome (lung migration) |
| Trichuris trichiura | Whipworm | Ingestion of eggs | Barrel-shaped egg, bipolar plugs | Rectal prolapse in heavy infections |
| Enterobius vermicularis | Pinworm | Ingestion/inhalation of eggs | D-shaped egg (scotch tape test) | Perianal itching, most common in children |
| Ancylostoma duodenale | Old World hookworm | Skin penetration (filariform larvae) | Egg (4-8 cell stage) | 2 pairs teeth, iron deficiency anemia |
| Necator americanus | New World hookworm | Skin penetration | Egg (4-8 cell stage) | Cutting plates (no teeth), ground itch |
| Strongyloides stercoralis | Threadworm | Skin penetration, autoinfection | Rhabditiform larva (short buccal cavity) | Autoinfection, hyperinfection in immunocompromised |
Tissue Nematodes
Trichinella spiralis
- • Transmission: Raw/undercooked pork
- • Life cycle: Adults in intestine → Larvae encyst in striated muscle
- • Disease: Trichinosis - periorbital edema, myalgia, eosinophilia
- • Diagnosis: Muscle biopsy (nurse cells), serology
Filarial Worms
- • Wuchereria bancrofti: Lymphatic filariasis, elephantiasis, nocturnal periodicity (Culex/Aedes)
- • Brugia malayi: Lymphatic filariasis, nocturnal
- • Loa loa: Eye worm, Calabar swellings, diurnal (Chrysops deer fly)
- • Onchocerca volvulus: River blindness, skin nodules (Simulium blackfly)
Larva Migrans
- • Visceral (VLM): Toxocara canis/cati (dog/cat roundworm)
- • Cutaneous (CLM): Ancylostoma braziliense (creeping eruption)
- • Ocular: Toxocara in retina
Capillaria philippinensis
- • Endemic: Philippines, Thailand
- • Transmission: Raw freshwater fish
- • Autoinfection: Possible
- • Disease: Malabsorption, protein-losing enteropathy
5. Cestodes (Tapeworms)
Cestode Anatomy
- • Scolex: Head with attachment organs (suckers, hooks, bothria)
- • Neck: Germinal region, produces proglottids
- • Strobila: Chain of proglottids (immature → mature → gravid)
- • No GI tract: Absorb nutrients through tegument
- • Hermaphroditic: Each proglottid has male and female organs
Intestinal Tapeworms
| Species | Common Name | Intermediate Host | Diagnostic Features |
|---|---|---|---|
| Taenia solium | Pork tapeworm | Pig (cysticercus) | Rostellum with 2 rows hooks, 7-13 uterine branches, humans can be IH (cysticercosis) |
| Taenia saginata | Beef tapeworm | Cattle | No rostellum/hooks, 15-20 uterine branches, largest tapeworm (up to 25m) |
| Diphyllobothrium latum | Fish tapeworm | Copepod → Fish | Bothria (grooves), rosette uterus, operculated egg, B12 deficiency |
| Hymenolepis nana | Dwarf tapeworm | None (direct life cycle) | Smallest, most common tapeworm, polar filaments, autoinfection |
| Hymenolepis diminuta | Rat tapeworm | Insects (beetles, fleas) | No hooks, no polar filaments |
| Dipylidium caninum | Dog tapeworm | Flea | Cucumber seed proglottids, double genital pores, children (ingest flea) |
Larval (Tissue) Cestodes
Cysticercosis (T. solium)
- • Humans as intermediate host
- • Neurocysticercosis: Seizures, most common parasitic CNS infection
- • Transmission: Ingestion of T. solium eggs (autoinfection or fecal-oral)
- • Diagnosis: CT/MRI, serology (EITB)
Echinococcosis (Hydatid Disease)
- • E. granulosus: Cystic hydatid disease (dog tapeworm)
- • E. multilocularis: Alveolar hydatid (more invasive)
- • Hydatid cyst in liver, lung
- • Hydatid sand: Brood capsules, protoscolices
- • Anaphylaxis risk if cyst ruptures
6. Trematodes (Flukes)
General Characteristics
- • Flat, leaf-shaped worms (except schistosomes)
- • Oral and ventral suckers
- • Complex life cycle: Snail (first intermediate host) required
- • Most are hermaphroditic (except Schistosoma - separate sexes)
- • Eggs are operculated (except Schistosoma)
Liver Flukes
Clonorchis sinensis (Chinese liver fluke)
- • Endemic: Philippines, East Asia
- • Transmission: Raw freshwater fish
- • Site: Bile ducts
- • Egg: Operculated, small, shouldered, knob at posterior
- • Disease: Cholangiocarcinoma, biliary obstruction
Fasciola hepatica (Sheep liver fluke)
- • Largest liver fluke
- • Transmission: Watercress, aquatic plants
- • Egg: Large, operculated, yellow-brown
- • Disease: Hepatic fascioliasis
- • Cone-shaped anterior end
Opisthorchis viverrini
- • Southeast Asian liver fluke
- • Similar to Clonorchis
- • Lobed testes (vs branched in Clonorchis)
- • Associated with cholangiocarcinoma
Intestinal Flukes
Fasciolopsis buski (Giant intestinal fluke)
- • Largest intestinal fluke
- • Transmission: Water chestnuts, aquatic plants
- • Egg: Large, operculated (like Fasciola)
- • Disease: Intestinal obstruction, malabsorption
Other Intestinal Flukes
- • Heterophyes heterophyes: Raw fish, Egypt/Asia
- • Metagonimus yokogawai: Raw fish, smallest human fluke
- • Echinostoma: Snails, freshwater fish
Lung Fluke
Paragonimus westermani
- • Endemic: Philippines, East Asia
- • Transmission: Raw/undercooked crabs, crayfish
- • Site: Lungs (paired cysts)
- • Egg: Operculated, shouldered, thick shell
- • Disease: Paragonimiasis - hemoptysis, chest pain, eosinophilia (mimics TB)
- • Diagnosis: Eggs in sputum or stool
Blood Flukes (Schistosomes)
Unique features: Separate sexes, non-operculated eggs, no second IH, eggs have spine
| Species | Location | Egg Spine | Disease |
|---|---|---|---|
| S. mansoni | Africa, South America | Lateral spine | Intestinal/hepatic schistosomiasis |
| S. japonicum | Philippines, China, Japan | Small lateral spine/knob | Oriental/Asiatic schistosomiasis, pipe-stem fibrosis |
| S. haematobium | Africa, Middle East | Terminal spine | Urinary schistosomiasis, bladder cancer |
Schistosomiasis in the Philippines
- • S. japonicum is endemic in certain areas
- • Snail host: Oncomelania quadrasi
- • Endemic areas: Leyte, Samar, Mindanao, parts of Luzon
- • Katayama fever (acute phase)
7. Medically Important Arthropods
Mosquitoes (Diptera)
| Genus | Resting Position | Diseases Transmitted |
|---|---|---|
| Anopheles | 45° angle to surface | Malaria, Wuchereria bancrofti |
| Aedes | Parallel to surface, "salt and pepper" legs | Dengue, Chikungunya, Zika, Yellow fever, Filariasis |
| Culex | Parallel to surface, plain brown | Japanese encephalitis, West Nile, Wuchereria bancrofti |
| Mansonia | Speckled wings | Brugia malayi |
Other Flies
Sandflies (Phlebotomus/Lutzomyia)
- • Vector: Leishmania, Bartonella, sandfly fever virus
- • Small, hairy, moth-like
- • Hold wings at 45° angle
Tsetse Fly (Glossina)
- • Vector: African trypanosomiasis
- • Hatchet cell (folded wings)
- • Africa only
Blackfly (Simulium)
- • Vector: Onchocerca volvulus (river blindness)
- • Small, humpbacked
- • Breed in fast-flowing rivers
Chrysops (Deer fly)
- • Vector: Loa loa
- • Colorful eyes, patterned wings
- • Painful bite
Ticks and Mites (Arachnida)
Hard Ticks (Ixodidae)
- • Ixodes: Lyme disease, babesiosis, anaplasmosis
- • Dermacentor: Rocky Mountain spotted fever, tularemia
- • Amblyomma: Ehrlichiosis
- • Dorsal shield (scutum)
Soft Ticks (Argasidae)
- • Ornithodoros: Relapsing fever (Borrelia)
- • No dorsal shield
- • Feed quickly (minutes)
Mites
- • Sarcoptes scabiei: Scabies, burrows in skin
- • Demodex: Follicular mite (demodicosis)
- • Leptotrombidium: Scrub typhus (chigger mite)
- • Dust mites: Allergy
Lice and Fleas
Lice (Pediculosis)
- • Pediculus humanus capitis: Head louse
- • Pediculus humanus corporis: Body louse (vector: epidemic typhus, relapsing fever, trench fever)
- • Phthirus pubis: Pubic/crab louse
- • Wingless, flattened dorsoventrally
Fleas
- • Xenopsylla cheopis: Rat flea (plague, murine typhus)
- • Ctenocephalides: Cat/dog flea (Dipylidium)
- • Tunga penetrans: Chigoe flea (tungiasis)
- • Flattened laterally, jumping legs
Bugs (Hemiptera)
- • Triatominae (kissing bugs/reduviid bugs): Chagas disease (T. cruzi), bite near mouth/eyes
- • Cimex lectularius (bed bug): No disease transmission, causes itching, allergic reactions
8. Laboratory Diagnosis
Stool Examination
Direct Wet Mount
- • Saline mount: Motile trophozoites, larvae
- • Iodine mount: Internal structures, nuclei
- • Fresh stool required (<30 minutes)
Concentration Techniques
- • Sedimentation: Formalin-ethyl acetate (most common)
- • Flotation: Zinc sulfate (protozoan cysts)
- • Increases sensitivity for light infections
Permanent Stains
- • Trichrome: Protozoa (standard)
- • Iron hematoxylin: More detail
- • Modified acid-fast: Cryptosporidium, Cyclospora
- • Weber's chromotrope: Microsporidia
Special Tests
- • Kato-Katz: Quantitative egg count (40-50 mg stool)
- • Harada-Mori: Filter paper, larvae recovery
- • Baermann: Larval recovery (Strongyloides)
- • Scotch tape test: Enterobius eggs
Blood Examination
- • Thick smear: Concentration (more sensitive), parasitemia
- • Thin smear: Species identification, morphology
- • Giemsa stain: Gold standard for blood parasites
- • Wright stain: Routine blood smear
- • Microfilariae: Timing important (nocturnal/diurnal periodicity)
- • Buffy coat examination: Trypanosomes, microfilariae
Other Diagnostic Methods
| Method | Specimen | Parasites Detected |
|---|---|---|
| String test (Enterotest) | Duodenal contents | Giardia, Strongyloides |
| Sputum examination | Sputum | Paragonimus, Ascaris (during migration) |
| Urine examination | Urine | S. haematobium, Trichomonas |
| Skin snip | Skin | Onchocerca (microfilariae) |
| Serology | Serum | Toxoplasma, Echinococcus, tissue parasites |
| Molecular (PCR) | Various | Most sensitive, species identification |
Quality Control & Preservation
- • Fresh stool: Within 30 minutes for trophozoites
- • Preservatives: 10% formalin (eggs, larvae), PVA (permanent stains), SAF
- • 3 stool specimens: Collected on different days increases sensitivity
- • Calibrated ocular micrometer: Essential for accurate identification
- • Positive controls: Use preserved positive specimens
Key Takeaways
- ✓E. histolytica: 4 nuclei cyst, ingested RBCs (pathogenic)
- ✓P. falciparum: Most lethal, banana gametocytes, all RBC ages
- ✓Scotch tape test for Enterobius (pinworm)
- ✓T. solium: Human can be intermediate host (neurocysticercosis)
- ✓Schistosoma: Non-operculated eggs, separate sexes
- ✓Anopheles at 45° angle - malaria vector
- ✓Aedes: Day-biting, dengue and Zika vector
- ✓Thick smear detects, thin smear identifies (malaria)