Medical Parasitology Review Notes

Fundamental Concepts, Host-Parasite Relationships, and Terminology

• Definitions in Medical Parasitology

  • Parasitology: The area of biology concerned with the phenomenon of dependence of one living organism on another.

  • Medical Parasitology: The branch of medical science that deals with parasites causing human infections and the diseases they produce. It is broadly divided into protozoology and helminthology.

  • Medical Protozoology: The study of protozoa, the animal-like, unicellular protists of medical significance to humans.

  • Medical Helminthology: The study of helminths (parasitic worms) capable of causing human disease, encompassing nematodes, trematodes, and cestodes.

  • Medical Entomology: The study of arthropods that serve as vectors, intermediate hosts, or direct causative agents (ectoparasites) of disease.

• Forms of Symbiotic and Trophic Relationships

  • Symbiosis ("living together"): An association of two unlike organisms. Forms of symbiosis differ based on benefit or detriment to the partners.

  • Commensalism ("eating at the same table"): An association beneficial to one partner without causing harm or benefit to the other (e.g., Entamoeba coli feeding on intestinal bacterial flora in the human gut).

  • Mutualism: A specialized relationship in which both organisms derive mutual benefit (e.g., wood-eating termites and the digestive flagellates within their intestinal tract).

  • Parasitism: A relationship wherein one organism (the parasite) lives in or on another (the host) for nourishment and survival at the host's expense. Differentiated from predation by intimate and protracted contact.

  • Predation: An animal (predator) attacks, kills, and consumes part or all of another living animal (prey).

  • Scavenging: An animal obtains nutrition by consuming animals that are already dead from natural causes or left by predators.

• Classification of Parasites

  • According to Habitat:

  • Endoparasites: Live within the interior of the host's body. The invasion of an endoparasite produces an infection.

  • Ectoparasites: Inhabit the external body surface or superficial skin layers (e.g., lice, ticks). Their presence denotes an infestation.

  • Hematozoic: Thrive within red blood cells (e.g., Plasmodium, Babesia).

  • Cytozoic: Live within intracellular cells or tissues (e.g., Trichinella spiralis encysted in muscle cells).

  • Coelozoic: Reside within body cavities (e.g., Mansonella spp.).

  • Enterozoic: Inhabit the lumen of the intestines (e.g., Entamoeba histolytica, adult tapeworms).

  • According to Mode of Living and Development:

  • Obligate Parasites: Dependent entirely on a host to complete their developmental cycle; cannot survive in a free-living state.

  • Facultative Parasites: Capable of existing in a free-living state or as commensals, adopting a parasitic existence when the opportunity arises (e.g., Strongyloides stercoralis).

  • Accidental (Incidental) Parasites: Establish infection in a host species in which they do not ordinarily live.

  • Erratic Parasites: Encountered in organs or tissues outside their normal physiological habitat.

  • Aberrant Parasites: Invade a host in which they are incapable of further development or reaching sexual maturity.

  • Spurious Parasites: Foreign organisms or eggs that traverse the human digestive tract without establishing tissue infection or colonization.

  • Parthenogenetic Parasites: Female organisms capable of laying viable eggs without male fertilization, which hatch immediately into larvae (e.g., parasitic female Strongyloides stercoralis).

  • Coprophilic Parasites: Able to multiply within fecal matter outside the mammalian host.

  • According to Duration on the Host:

  • Permanent Parasites: Remain within or on the body of the host throughout their entire lifespan.

  • Temporary Parasites: Inhabit or feed upon the host only for brief intervals.

  • Intermittent Parasites: Visit the host periodically only during feeding periods (e.g., hematophagous mosquitoes).

  • According to Pathogenicity:

  • Pathogenic Parasites: Induce overt tissue damage, clinical symptoms, and morbidity.

  • Non-Pathogenic Parasites (Commensals): Incapable of inflicting injury or disease upon the host.

• Classification of Hosts

  • Definitive (Final) Host: The organism in which the parasite attains sexual maturity or undergoes the sexual phase of its reproductive cycle (e.g., humans for Wuchereria bancrofti, female Anopheles mosquitoes for Plasmodium spp.).

  • Intermediate Host: Harbors the larval, developmental, or asexual reproductive stages of the parasite.

  • First Intermediate Host ($1^{\text{st}}$ IH): Harbors the early larval stages (e.g., aquatic mollusks/snails for trematodes).

  • Second Intermediate Host ($2^{\text{nd}}$ IH): Harbors the advanced or infective larval stages (e.g., fish, crabs, plants, or second snails).

  • Paratenic (Transport) Host: An animal in which the parasite remains viable in an arrested or non-developmental state. The parasite does not undergo development but remains infective if ingested by a definitive host (e.g., wild boars for Paragonimus westermani).

  • Reservoir Host: An animal species that naturally harbors the infection, maintaining the parasite's life cycle in nature and serving as an additional, ongoing source of human exposure (e.g., pigs for Balantidium coli).

  • Accidental Host: A host in which the parasite is not normally found, often resulting in altered migratory behavior or non-completion of the developmental cycle.

  • Zoonosis: A parasitic disease primarily adapted to animal hosts that can accidentally be transmitted to humans (e.g., Chagas' disease, African trypanosomiasis, leishmaniasis, echinococcosis, trichinellosis, Schistosoma japonicum).

• Vectors and Vector Classifications

  • Vector: An organism (typically an arthropod) that does not directly cause disease itself, but transmits pathogens from one host to another.

  • Biological Vector: An essential host in the parasite's life cycle; the parasite undergoes multiplication, cellular transformation, or morphological development within the vector before transmission is possible (e.g., Anopheles mosquitoes for Plasmodium, tsetse flies for Trypanosoma brucei).

  • Mechanical (Phoretic) Vector: Serves strictly as a passive transport agent. No parasitic growth or morphogenesis takes place on or inside the vector (e.g., domestic flies and cockroaches transferring Entamoeba histolytica cysts or Ascaris lumbricoides ova on appendages).

• Sources of Infection and Transmission Routes

  • Soil: The most widespread source of infection globally, perpetuated by open defecation, lack of sanitary facilities, and using untreated human excreta (night soil) as fertilizer. Soil-transmitted helminths include the "HATS" cluster: Hookworms, Ascaris lumbricoides, Trichuris trichiura, and Strongyloides stercoralis.

  • Water: Ingestion of protozoan cysts (Entamoeba, Giardia), oocysts (Cryptosporidium, Cyclospora), or active skin penetration by cercariae (Schistosoma).

  • Food: Consuming undercooked meat, freshwater fish, crabs, crayfish, or plants containing encysted larvae (e.g., metacercariae, cysticerci, plerocercoids).

  • Animals: Domestic and wild reservoirs (e.g., felines shedding Toxoplasma gondii oocysts).

  • Arthropods: Direct inoculation of infective stages via blood-sucking insect bites (e.g., Plasmodium, filariae, Leishmania).

  • Another Individual: Direct fecal-oral contact or contaminated food handling from asymptomatic cyst passers (Entamoeba histolytica).

  • Specific Routes:

  • Oral (Fecal-Oral): The most common route; primarily food- and water-borne ingestion of cysts, oocysts, and embryonated eggs.

  • Skin Penetration: Direct cuticular or percutaneous boring by filariform larvae (hookworms, Strongyloides) or cercariae (Schistosoma).

  • Congenital (Transplacental): Trophozoites or tachyzoites cross the placental barrier (Toxoplasma gondii, rarely Plasmodium and Trypanosoma cruzi).

  • Transmammary: Transmission via colostrum or breast milk (Ancylostoma, Strongyloides).

  • Inhalation: Inhaling airborne, lightweight embryonated eggs (Enterobius vermicularis).

  • Sexual Intercourse: Urogenital mucosal transfer (Trichomonas vaginalis).

  • Iatrogenic: Accidental transmission through blood transfusion or organ transplantation.

• Epidemiological Terminology and Metrics

  • Epidemiology: The study of the distribution, patterns, frequencies, and determinants of disease occurrence in human populations.

  • Incidence: The number of new cases of a specific parasitic infection occurring within a defined population over a designated timeframe.

  • Prevalence ($\%$): The estimated proportion or total number of individuals in a population harboring a specific parasitic infection at a given point in time.

  • Cumulative Prevalence: The percentage of individuals in a community infected with at least one parasite species.

  • Intensity of Infection (Worm Burden): The number of worms or burden of parasites harbored per infected person.

  • Morbidity: The clinical, pathological, and functional consequences of parasitic disease that impact individual well-being and productivity.

  • Deworming: The administration of anthelminthic medications to eliminate parasitic worms.

  • Selective Treatment: Individual-level screening and targeted deworming.

  • Targeted Treatment: Group-level deworming administered to high-risk demographic clusters (e.g., school-age children).

  • Universal Treatment: Mass population-level deworming regardless of infection status.

  • Coverage: The proportion of the targeted at-risk population reached by a public health intervention.

  • Efficacy: The measured effect of an antiparasitic drug against an infective agent under ideal, controlled laboratory conditions.

  • Effectiveness: The measured operational impact and performance of a drug against an infection in real-world clinical or community settings.

  • Cure Rate ($\%$): The percentage of previously egg-positive individuals who become egg-negative following treatment.

  • Egg Reduction Rate (ERR): The percentage decline in mean fecal egg counts across a cohort post-deworming.

  • Information-Education-Communication (IEC): Health education approaches designed to promote behavioral changes and sanitation practices.

  • Environmental Management: Modifying or manipulating environmental vectors and conditions to reduce transmission risks.

  • Environmental Sanitation: Physical interventions (e.g., safe disposal of human waste, clean water supplies) aimed at eliminating environmental health hazards.

  • Eradication: Permanent global reduction to zero of the incidence of infection caused by a specific pathogen due to deliberate intervention.

  • Elimination: Reduction to zero of the incidence of a specified disease in a defined geographic locality.

  • Carrier: An asymptomatic individual harboring a pathogenic organism who is capable of transmitting it to others.

  • Pre-Patent Period (Biological Incubation): The duration between initial host infection and the earliest laboratory demonstration of parasite presence.

  • Clinical Incubation Period: The elapsed time between exposure/infection and the onset of clinical signs and symptoms.

  • Auto-Reinfection: An individual reinfecting themselves with their own progeny without external exposure (e.g., Capillaria philippinensis, Enterobius vermicularis, Strongyloides stercoralis, Taenia solium, Hymenolepis nana, Cryptosporidium).

  • Superinfection (Hyperinfection): Massive, uncontrolled proliferation and dissemination of parasites within a previously infected host (e.g., Strongyloides stercoralis in immunocompromised hosts).

• Host Adaptations, Resistance, and Pathology

  • Morphological and Physiological Adaptations: Parasites frequently demonstrate secondary simplification—loss of nonessential structures (such as digestive systems in cestodes and thorny-headed worms, or locomotor organelles in Apicomplexa) coupled with massive hypertrophy of reproductive structures (ovaries, uteri, and high daily fecundity).

  • Biochemical Streamlining: Complete loss of specific de novo metabolic pathways, shifting dependence onto the host for purines, amino acids, and lipids (e.g., Kinetoplastida, Giardia, Entamoeba).

  • Immune Evasion Mechanisms: Antigenic variation (differential variant surface glycoprotein [VSG] expression in African trypanosomes), acquisition of host-like surface molecules, anatomical sequestration within privileged sites, and secretion of proteases degrading host immunoglobulins and complement.

  • Premunition: An acquired, non-sterilizing state of immunity stimulated by the persistent presence of low-level parasite burdens, protecting the host against overwhelming reinfection.

  • Host Genetic Influences:

  • Duffy Null Phenotype ($\text{Fy(a-b-)}$): Complete absence of the Duffy antigen receptor for chemokines on erythrocytes, conferring total resistance against Plasmodium vivax and Plasmodium knowlesi merozoite invasion.

  • Sickle Cell Trait ($\text{HbAS}$): Confers significant protection against lethal complications of Plasmodium falciparum malaria.

  • Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency: Impairs erythrocyte parasite replication, limiting falciparum severity.

• Common Names of Medically Important Parasites

Diagnostic Methodologies and Laboratory Procedures in Parasitology

• Fecal Specimen Collection and Handling

  • Specimen Quantity and Timelines: Unpreserved liquid/watery stools must be examined within 30 minutes of passage to view motile trophozoites. Semi-formed stools must be evaluated within 1 hour. Formed stools can be processed within 3 to 4 hours (up to 24 hours if refrigerated at 3^\n\circ\text{C} to $5^\circ\text{C}$). Stool samples should never be incubated or frozen.

  • Collection Protocol: A standard series requires three normally passed fecal specimens collected every other day (or across a span of 10 days) into clean, wide-mouthed containers. For Entamoeba histolytica, up to six examinations yield greater than $90\%$ diagnostic detection.

  • Interfering Compounds: Mineral oil, castor oil, bismuth, magnesium, barium sulfate (withhold specimen collection for 7 to 10 days post-barium), antacids, and antimicrobials/antimalarials (delay collection for 2 to 4 weeks).

• Stool Preservation and Fixative Chemistry

  • Formalin: All-purpose preservative. $5\%$ concentration preserves delicate protozoan cysts; $10\%$ concentration fixes helminth eggs and larvae. Compatible with FECT concentration.

  • Schaudinn's Fluid: Mercuric chloride and $95\%$ ethyl alcohol with glacial acetic acid added prior to use. Historical gold standard for fixing fresh fecal smears for permanent iron hematoxylin and trichrome stains. Highly toxic.

  • Polyvinyl Alcohol (PVA): Plastic resin incorporated into Schaudinn's fixative; acts as an adhesive to bind liquid/loose stools to glass slides. Excellent for permanent trichrome stains.

  • Sodium Acetate-Acetic Acid-Formalin (SAF): Mercury-free, long-shelf-life fixative. Suitable for concentration techniques and permanent smears; requires albumin coating on glass slides to facilitate adhesion.

  • Merthiolate-Iodine-Formalin (MIF): Dual-purpose fixative-stain containing thimerosal, formaldehyde, and freshly added Lugol's iodine. Good for field surveys, but yields inferior permanent staining compared to PVA/Schaudinn's.

• Fecal Smear and Concentration Techniques

  • Direct Fecal Smear (DFS): $2\,\text{mg}$ of stool emulsified in one drop of $0.85\%$ physiological saline ($\text{NaCl}$) under a $22 \times 22\,\text{mm}$ coverslip. Examined under low power ($100\times$) and high dry ($400\times$). Iodine staining (Lugol's or D'Antoni's) highlights nuclear chromatin but lyses motile trophozoites.

  • Formalin-Ether / Ethyl Acetate Concentration Technique (FECT/FEACT): Sedimentation procedure. $10\%$ formalin preserves organisms; ethyl acetate/ether dissolves neutral fats and fecal lipids. Centrifugation at $500 \times g$ for 10 minutes produces four distinct layers:

  • Top layer: Ethyl acetate/ether.

  • Second layer: Plug of debris.

  • Third layer: Formalin.

  • Bottom layer: Parasitic sediment.

  • Acid Ether Concentration Technique (AECT): Uses $40\%\,\text{HCl}$ to dissolve albuminous materials and ether for lipids. Ideal for Trichuris, Capillaria, and heavy operculated trematode eggs (Schistosoma). Destroys protozoan cysts.

  • Zinc Sulfate Flotation Technique: Relies on high-density centrifugal flotation. Working specific gravity: $1.18$ for fresh specimens, $1.20$ for formalin-preserved specimens. Operculated eggs, dense unfertilized Ascaris ova, and schistosomes do not float; zinc exposure causes osmotic collapse and distortion of delicate walls if left too long.

  • Sheather's Sugar Flotation: Boiled sucrose solution with phenol (specific gravity $1.20$). The gold standard for recovering coccidian oocysts (Cryptosporidium, Cyclospora, Cystoisospora).

• Specialized Coprological Cultures and Recovery Methods

  • Harada-Mori Filter Paper Strip Method: Stool ($0.5$ to $1.0\,\text{g}$) is smeared along the middle third of a filter paper strip and placed in a conical tube containing 3 to $4\,\text{mL}$ of water. Capillary moisture maintains larval viability. Incubated at room temperature for 7 to 10 days. Hookworm larvae migrate downward into bottom water; Strongyloides larvae may migrate upward.

  • Baermann Funnel Apparatus: Uses a glass funnel fitted with rubber tubing, pinchcock, and wire gauze holding fecal matter submerged in warm water. Relies on hydrotropism and thermotropism; living Strongyloides larvae migrate out of stool into water and settle to the bottom.

  • Agar Plate Culture for Strongyloides: $2\,\text{g}$ of fresh stool placed at the center of nutrient agar. Plates are taped sealed and incubated at room temperature for 48 hours. Moving larvae carry bacteria across the agar, creating visible sinusoidal tracks.

  • Cellulose Tape (Scotch Tape) Technique: Clear adhesive tape pressed against perianal folds early in the morning prior to bathing or defecation. Examined for Enterobius vermicularis ova and Taenia proglottids. Negative confirmation requires consecutive negative tapes across 4 to 6 mornings.

• Quantitative Egg Counting Protocols

  • Kato-Katz Thick Smear: Standard quantitative method recommended by WHO. Uses a calibrated template delivering $41.7\,\text{mg}$ of sieved stool, overlaid with cellophane soaked in glycerin (clearing agent) and malachite green (counterstain). Egg counts are multiplied by a factor of 24 to compute eggs per gram (EPG). Hookworm eggs overclear and disappear within 30 to 60 minutes.

  • Stoll Dilution Egg Count: $4\,\text{g}$ of stool mixed with $56\,\text{mL}$ of $0.1\,\text{N}\;\text{NaOH}$ ($60\,\text{mL}$ total volume). $0.15\,\text{mL}$ is drawn and counted under a coverslip; multiplied by 100 to yield EPG.

• WHO Intensity Classifications for Soil-Transmitted Helminths and Schistosomes

• Permanent Staining Protocols

  • Wheatley's Modification of Gomori Trichrome: Rapid staining technique using Chromotrope 2R, Light Green SF, and phosphotungstic acid in glacial acetic acid. Cytoplasm stains blue-green to purple; nuclear chromatin, chromatoidals, and ingested erythrocytes stain bright cherry-red to purple-red; background debris stains pale green.

  • Iron Hematoxylin Stain: Highest cytological resolution for fine nuclear chromatin, karyosomes, and linin fibrils. Chromatin structures stain intense blue-black; cytoplasm stains pale grayish-blue.

  • Modified Kinyoun Acid-Fast Stain: Carbolfuchsin with cold or gentle heating, decolorized with $1\%\text{--}2\%\,\text{H}_2\text{SO}_4$, counterstained with methylene blue. Coccidian oocysts stain bright pinkish-red to magenta; non-acid-fast yeasts and fecal debris stain blue or green.

  • Ryan's Trichrome Blue (for Microsporidia): Incorporates aniline blue and Chromotrope 2R at pH 2.5. Microsporidian spore walls ($1\text{--}2.5\,\mu\text{m}$) stain bright pinkish-red, often revealing an equatorial or diagonal pink stripe representing the polar tube; background stains blue.

• Blood Examination Protocols and Calculations

  • Thick Blood Film: Unfixed film containing 3 drops of stirred blood laked in water/buffer. Lyses all erythrocytes. Examined under oil immersion ($1000\times$) across at least 300 fields. Approximately 30 times more sensitive than a thin film; detects as few as 20 parasites/$\mu\text{L}$.

  • Thin Blood Film: Fixed in absolute methanol for 1 minute before staining (Giemsa at pH 7.2). Preserves red blood cell morphology and intracellular inclusions. Examined across a minimum of 300 oil immersion fields before reporting as negative.

  • Malaria Parasite Density Calculations:

  • Thick Film Computation:       $\text{Parasites}/\mu\text{L blood} = \frac{\text{Number of Asexual Parasites Counted} \times 8,000\;\text{WBCs}/\mu\text{L}}{\text{Number of WBCs Counted}}$       (Count stops when $\ge 100$ parasites are tallied per 200 WBCs, or $\le 99$ parasites per 500 WBCs).

  • Thin Film Computation:       $\text{Parasites}/\mu\text{L blood} = \frac{\text{Number of Parasitized RBCs} \times 5,000,000\;\text{RBCs}/\mu\text{L}}{\text{Total RBCs Counted (typically } 20\;\text{fields} \times 250\;\text{RBCs} = 5,000\;\text{RBCs)}}$

  • Knott's Concentration: Mixes $1\,\text{mL}$ of venous blood with $10\,\text{mL}$ of $2\%$ formalin. Centrifuged at $500 \times g$ for 1 minute. Formalin lyses red cells while preserving microfilarial cuticles.

  • Quantitative Buffy Coat (QBC): High-precision microhematocrit capillary tube precoated with acridine orange and potassium oxalate, containing a specialized plastic float. Centrifuged at high speeds; under ultraviolet light, parasite DNA/RNA fluoresces bright green and red.

• Pseudoparasites and Artifactual Confusers

  • Oil Droplets and Fat Globules: Refractile, perfectly circular spheres lacking internal chromatin or nuclei; often confused with amebic cysts.

  • Air Bubbles: Prominent, thick, double-contoured dark margins; confused with Taenia eggs or protozoan cysts.

  • Plant and Vegetable Cells: Sculpted, thick cellulose cell walls resembling helminth ova.

  • Plant Hairs: Thick-walled, refractile fibers containing an internal longitudinal canal; frequently misidentified as nematode larvae.

  • Starch Granules: Concentric rings of homogeneous material. Turn deep blue-black with iodine if undigested, or pinkish-red if partially digested.

  • Pollen Grains: Symmetrical, thick-walled structures with variable surface sculpturing, mimicking Taenia or Ascaris ova.

  • Polymorphonuclear Leukocytes (PMNs): Lobulated nucleus may resemble a 4-nucleated amebic cyst, but nuclear-to-cytoplasmic ratio is much higher and karyosomes are irregular.

  • Platelets over Red Blood Cells: A purplish-blue platelet overlying an erythrocyte mimics a malaria ring form, but lacks true red chromatin and blue cytoplasm, presenting fuzzy margins.

  • Charcot-Leyden Crystals: Slender, diamond-shaped, pointed crystals composed of eosinophil lysophospholipase. Stained bright red in trichrome; diagnostic markers of hypersensitivity, amebic colitis, and helminthic invasion.

  • Psorospermium haeckelii (Beaver Bodies): Ovoid protistan parasites of freshwater crayfish passed in human feces after eating crawfish; mimic schistosome eggs.

Protozoa: Lumen-Dwelling and Free-Living Amoebae (Subphylum Sarcodina)

• Entamoeba histolytica

  • Taxonomy and Generalities: Pathogenic invasive ameba inhabiting the large intestine. Differentiated genetically and biochemically from nonpathogenic Entamoeba dispar and sewage-associated Entamoeba moshkovskii.

  • Trophozoite Morphology: Measures 12 to $60\,\mu\text{m}$ (invasive tissue forms generally $>20\,\mu\text{m}$). Rapid, progressive, unidirectional motility via clear, finger-like hyaline pseudopodia. Nucleus has a delicate, uniform peripheral chromatin ring and a minute, centrally positioned karyosome. Endoplasm is finely granular; presence of phagocytosed red blood cells (hematophagous trophozoites) is pathognomonic.

  • Cyst Morphology: Spherical, 10 to $20\,\mu\text{m}$. Immature stages contain one to two nuclei, a diffuse glycogen vacuole, and distinct, elongated chromatoidal bars with smooth, rounded, cigar-shaped ends. The mature infective cyst contains four nuclei and lacks glycogen masses.

• Pathogenesis and Virulence Factors:

  • Gal/GalNAc Lectin: Mediates adherence to the colonic mucosal epithelium and provides complement resistance.

  • Amebapores: Small peptides that insert into eukaryotic target cell membranes, forming ion channels and causing osmotic cytolysis.

  • Cysteine Proteinases: Degrade extracellular matrix, digest host intestinal mucus, and activate host caspase cascades leading to host cell apoptosis.

  • Ulceration: Trophozoites breach the muscularis mucosae, expanding laterally in the submucosa to produce classical flask-shaped ulcers with undermined margins. Interconnecting sinuses create extensive necrotic sloughing.

  • Amoeboma: A chronic localized granulomatous mass lesion developing in the cecal or rectosigmoid wall, frequently mistaken radiologically or clinically for annular adenocarcinoma.

• Extraintestinal Manifestations:

  • Amoebic Liver Abscess (ALA): The most common extraintestinal complication, arising via hematogenous spread through the portal venules. Features right-upper-quadrant pain, hepatomegaly, spiking fever, and liquefactive liver necrosis producing odorless, thick, reddish-brown "anchovy sauce" pus.

  • Pleuropulmonary Amoebiasis: Directly extends from a ruptured liver abscess through the diaphragm, causing hepatobronchial fistulas with expectoration of anchovy-paste sputum.

  • Secondary Amoebic Meningoencephalitis: Hematogenous dissemination to the brain; rapidly destructive and fatal.

  • Cutaneous Amoebiasis: Extensive, painful gangrenous ulcerations of perianal tissues or adjacent to surgical colostomy/drainage sites; penile amoebiasis acquired through anal intercourse.

  • Non-Pathogenic Commensal Amoebae

• Entamoeba coli: Nonpathogenic lumen dweller. Trophozoite (15 to $50\,\mu\text{m}$) exhibits sluggish, nonprogressive motility using blunt, granular pseudopodia; cytoplasm is heavily vacuolated with ingested bacteria and debris; nucleus contains coarse, irregular, eccentric karyosomes and clumpy peripheral chromatin. Mature cyst (10 to $35\,\mu\text{m}$) contains eight nuclei (hypernucleate forms may display 16 or 32) and slender, sharp, splinter-like chromatoidal bodies with pointed ends.

• Entamoeba hartmanni ("Small Race" E. histolytica): Morphologically identical to E. histolytica in fine nuclear detail and cigar-shaped chromatoidals, but nonpathogenic and distinguished by size: trophozoites measure $</p><ul><li><p><strong>Entamoeba gingivalis:</strong> Inhabits the oral cavity in pyorrheal pockets and tonsillar crypts; also recovered from uterine secretions in women using IUDs. <strong>Has no cyst stage</strong>; transmitted directly via saliva, kissing, or droplets. The only amoeba that actively ingests white blood cells (leukocytes).</p></li><li><p><strong>Entamoeba polecki:</strong> Parasite of pigs and monkeys, occasionally establishing human infection. Trophozoite resembles <em>E. coli</em>; cysts are predominantly <strong>uninucleated</strong>, characterized by pointed or angular chromatoidals and a dense, non-glycogen <strong>inclusion mass</strong> in approximately half the cysts.</p></li><li><p><strong>Endolimax nana:</strong> The most prevalent small intestinal amoeba. Trophozoite (5 to$12\,\mu\text{m}$) displays sluggish motility with blunt hyaline pseudopodia; nucleus has a large, blot-like, irregular karyosome with <strong>no peripheral chromatin</strong>. Cyst (5 to$12\,\mu\text{m}$) is typically ovoid and contains four minute nuclei.</p></li><li><p><strong>Iodamoeba bütschlii:</strong> Trophozoite (4 to$20\,\mu\text{m}$) has a large, central karyosome surrounded by a halo of refractive granules. Cyst (6 to$16\,\mu\text{m}$) is irregular in contour, consistently <strong>uninucleate</strong>, and possesses a single eccentric "basket nucleus" alongside a large, sharply demarcated <strong>glycogen vacuole</strong> that stains dark mahogany brown with iodine.</p></li></ul></li></ul><p xmlns="http://www.w3.org/1999/xhtml"></p><img xmlns="http://www.w3.org/1999/xhtml" src="https://assets.knowt.com/pdf-flow-prod/a95b8782-c69b-4715-a172-011af038952c-figures/23.png" data-width="50$30\,\mu\text{m}$, producing blunt lobopodia and amebostomes), transient biflagellate form (pear-shaped with two anterior flagella, induced by hypotonic conditions), and thick-walled cyst (single nucleus, environmentally resistant). Thermotolerant (survives up to$45^\circ\text{C}$).</p></li><li><p><strong>Clinical Disease:</strong> Causative agent of <strong>Primary Amoebic Meningoencephalitis (PAM)</strong>. Typically acquired during hot summer months by healthy children/young adults diving or swimming in warm fresh water. Trophozoites penetrate the nasal mucosa, migrate along the olfactory nerve fibers through the cribriform plate into the olfactory bulbs and frontal cortex.</p></li><li><p><strong>Pathology:</strong> Acute, fulminant, necrotizing hemorrhagic meningoencephalitis. Onset of severe frontal headache, fever, olfactory hallucinations, nausea, rapid neck stiffness, Kernig's sign, and coma, culminating in death within 3 to 6 days. Purulent CSF contains abundant neutrophils, elevated protein, low glucose, and motile ameboid trophozoites (cysts are never found in tissue).</p></li><li><p><strong>Acanthamoeba Species (<em>A. castellanii</em>, <em>A. culbertsoni</em>, <em>A. polyphaga</em>):</strong></p></li><li><p><strong>Morphology:</strong> Trophozoites (15 to$45\,\mu\text{m}$) have distinctive spine-like cytoplasmic projections (<strong>acanthopodia</strong>). Cysts (8 to$25\,\mu\text{m}$) are double-walled, with a wrinkled outer ectocyst and a polygonal or stellate endocyst. Possesses no flagellate stage.</p></li><li><p><strong>Granulomatous Amoebic Encephalitis (GAE):</strong> A chronic, indolent, space-occupying focal necrosis of the CNS developing over weeks to months in immunocompromised, debilitated, or HIV/AIDS patients. Infection originates in skin ulcers or the lower respiratory tract, disseminating hematogenously to the brain. <strong>Both trophozoites and cysts are present in tissue lesions</strong>.</p></li><li><p><strong>Acanthamoeba Keratitis:</strong> Chronic, vision-threatening corneal ulceration occurring primarily in contact lens wearers using contaminated homemade saline solutions or wearing lenses while swimming. Presents with excruciating ocular pain disproportionate to clinical signs, corneal ring infiltrates, epithelial defects, and potential perforation.</p></li><li><p><strong>Balamuthia mandrillaris:</strong></p></li><li><p><strong>Morphology:</strong> Large pleomorphic trophozoites (12 to$60\,\mu\text{m}$) with broad pseudopodia; nucleus often contains multiple nucleoli. Cysts (13 to$30\,\mu\text{m}$) have a triple-layered wall: outer wrinkled ectocyst, amorphous mesocyst, and thin endocyst.</p></li><li><p><strong>Pathology:</strong> Causes GAE in both immunocompetent and immunocompromised hosts, with skin lesions and hematogenous dissemination; transmitted via soil contact, respiratory tracts, or transplanted solid organs.</p></li></ul></li></ul><h3 xmlns="http://www.w3.org/1999/xhtml" collapsed="false" seolevelmigrated="true">Protozoa: Flagellates (Subphylum Mastigophora / Phylum Euglenozoa)</h3><ul xmlns="http://www.w3.org/1999/xhtml"><li><p><strong>Intestinal and Urogenital Flagellates</strong></p><ul><li><p><strong>Giardia duodenalis (<em>Giardia lamblia</em>, <em>Giardia intestinalis</em>):</strong></p></li><li><p><strong>Trophozoite Morphology:</strong> Bilaterally symmetrical, pear- or teardrop-shaped organism (9 to$21\,\mu\text{m}$long by 5 to$15\,\mu\text{m}$wide). Dorsal surface is convex; anterior ventral surface possesses a large, concaved <strong>sucking (adhesive) disk</strong> used for attachment to the duodenal/jejunal microvilli. Features two spherical nuclei, two slender central axonemes within an axostyle, two transverse curved <strong>median bodies</strong>, and four pairs of flagella (anterior, lateral, ventral, posterior). Demonstrates characteristic <strong>"falling-leaf" motility</strong>.</p></li><li><p><strong>Cyst Morphology:</strong> Ovoid, 8 to$14\,\mu\text{m}$long by 7 to$10\,\mu\text{m}$wide. Possesses a smooth, refractile cyst wall separated from the shrunken cytoplasm. Immature cysts have two nuclei; mature infective cysts contain <strong>four nuclei</strong>, four median bodies, and a tangled longitudinal cluster of axonemes.</p></li></ul></li></ul><p xmlns="http://www.w3.org/1999/xhtml"></p><img xmlns="http://www.w3.org/1999/xhtml" src="https://assets.knowt.com/pdf-flow-prod/a95b8782-c69b-4715-a172-011af038952c-figures/142.jpg" data-width="50$12\,\mu\text{m}$) exhibits hyaline, leaf-like pseudopodia with serrated margins. Characteristically <strong>binucleate ($80\%$of specimens)</strong>; each nucleus contains four to eight separated chromatin granules embedded in a delicate nuclear envelope with no peripheral chromatin.</p></li><li><p><strong>Clinical Picture:</strong> Colonizes mucosal crypts of the large intestine. May produce low-grade chronic diarrhea, flatulence, abdominal pain, fatigue, and peripheral eosinophilia. Swells in water and then returns to original size (<strong>Hakansson phenomenon</strong>).</p></li></ul></li><li><p><strong>Trichomonas vaginalis:</strong></p><ul><li><p><strong>Morphology:</strong> Trophozoites only (7 to$23\,\mu\text{m}$, occasionally reaching$30\,\mu\text{m}$); <strong>no cyst stage</strong>. Pear-shaped, with four anterior free flagella and a fifth recurrent flagellum forming the border of an <strong>undulating membrane that extends only half the body length</strong> (lacks a free trailing flagellum). A rigid, pointed <strong>axostyle</strong> projects through the posterior end. Cytoplasm contains prominent siderophile granules.</p></li><li><p><strong>Pathology:</strong> Sexually transmitted flagellate that colonizes the squamous epithelium of the vagina, urethra, and prostate. Thrives at alkaline pH ($>4.5$). Causes severe vaginitis with intense pruritus, burning dysuria, and a copious, frothy, malodorous, greenish-yellow vaginal discharge. Speculum examination reveals petechial cervical hemorrhages (<strong>"strawberry cervix"</strong>). In men, causes persistent urethritis, prostatitis, and epididymitis. Amplifies HIV transmission risk twofold.</p></li></ul></li><li><p><strong>Trichomonas hominis (<em>Pentatrichomonas hominis</em>):</strong> Commensal intestinal flagellate. Possesses five anterior flagella and a full-length undulating membrane with a trailing posterior flagellum and a distinct longitudinal costa.</p></li><li><p><strong>Trichomonas tenax:</strong> Commensal flagellate inhabiting tartar, pyorrheal pockets, and gingival margins of the oral cavity; lacks a cyst stage.</p></li><li><p><strong>Chilomastix mesnili:</strong> Nonpathogenic intestinal commensal. Trophozoite is pear-shaped with a spiral longitudinal groove and a cytostome bordered by a <strong>shepherd's crook fibril</strong>. Cyst is diagnostic: 6 to$10\,\mu\text{m}$, distinctly <strong>lemon-shaped with an anterior nipple-like hyaline protuberance</strong>.</p><ul><li><p><strong>Hemoflagellates (Family Trypanosomatidae)</strong></p></li></ul></li><li><p><strong>Morphological Stages:</strong></p><ul><li><p><strong>Amastigote (Leishman-Donovan body):</strong> Intracellular, non-flagellated, round to ovoid ($2\text{--}5\,\mu\text{m}$), containing a nucleus, a rod-shaped kinetoplast (consisting of a blepharoplast and parabasal body), and a rudimentary internal axoneme.</p></li><li><p><strong>Promastigote (Leptomonad):</strong> Elongated spindle shape with a central nucleus and a single free flagellum emerging from the anterior kinetoplast.</p></li><li><p><strong>Epimastigote (Crithidial):</strong> Kinetoplast is located anterior to the nucleus; undulating membrane extends along half the body length before the flagellum projects freely.</p></li><li><p><strong>Trypomastigote (Trypanosomal):</strong> Kinetoplast is located at the posterior end; undulating membrane spans the entire body length, ending in a free anterior flagellum.</p></li></ul></li><li><p><strong>Trypanosoma cruzi (American Trypanosomiasis / Chagas' Disease):</strong></p><ul><li><p><strong>Vectors and Transmission:</strong> Triatomine bugs (<em>Triatoma</em>, <em>Panstrongylus</em>, <em>Rhodnius</em>—"kissing" or assassin bugs). Stercorarian transmission: the bug defecates during its blood meal; infectious <strong>metacyclic trypomastigotes</strong> in bug feces are scratched into the bite wound or cross mucosal membranes (conjunctiva).</p></li><li><p><strong>Pathology:</strong> Acute phase begins with a furuncle-like lesion (<strong>chagoma</strong>) or unilateral palpebral edema and conjunctivitis (<strong>Romaña's sign</strong>). Parasites enter reticuloendothelial, glial, and cardiac muscle cells, transforming into proliferating amastigote pseudocysts. Chronic phase manifests years later with extensive autonomic denervation (destruction of Auerbach's plexus), resulting in <strong>cardiomegaly</strong>, arrhythmias, apical aneurysms, <strong>megacolon</strong>, and <strong>megaesophagus</strong>.</p></li></ul></li></ul><p xmlns="http://www.w3.org/1999/xhtml"></p><img xmlns="http://www.w3.org/1999/xhtml" src="https://assets.knowt.com/pdf-flow-prod/a95b8782-c69b-4715-a172-011af038952c-figures/31.jpg" data-width="50$75\%\text{--}95\%$mortality rate. May be followed by <strong>Post-Kala-Azar Dermal Leishmanoid (PKDL)</strong>.</p></li></ul><p xmlns="http://www.w3.org/1999/xhtml"></p><img xmlns="http://www.w3.org/1999/xhtml" src="https://assets.knowt.com/pdf-flow-prod/a95b8782-c69b-4715-a172-011af038952c-figures/3.jpg" data-width="50$200\,\mu\text{m}$in length. Surface is entirely covered by rows of uniform cilia that beat synchronously, imparting a <strong>"thrown-ball" or rotary motility</strong>. The anterior flattened end contains a funnel-shaped mouth (<strong>cytostome</strong>); waste is expelled through an anal pore (<strong>cytopyge</strong>). The interior contains contractile and food vacuoles, along with two distinct nuclei: a massive, dense, kidney- or bean-shaped <strong>macronucleus</strong> (directs metabolic functions) and an adjacent spherical <strong>micronucleus</strong> (governs sexual conjugation).</p></li><li><p><strong>Cyst Morphology:</strong> Spherical to subspherical, 50 to$75\,\mu\text{m}. Surrounded by a thick, transparent, refractile double cyst wall. Cilia are visible beneath the wall in newly formed cysts but disappear over time. The large kidney-shaped macronucleus remains visible in both unstained and stained preparations.

• Pathogenesis and Clinical Features: Infection occurs via ingestion of cysts from water/food contaminated with pig feces. Excystation occurs in the small intestine, and trophozoites establish in the cecum and terminal ileum. Trophozoites secrete hyaluronidase, which facilitates penetration of the mucosa to generate rounded-base, wide-neck ulcers. Clinical symptoms range from asymptomatic carriage to severe balantidial dysentery with up to 15 bloody, mucoid stools daily, tenesmus, intestinal perforation, and rare extraintestinal abscesses.