Introduction to Protozoa

Protozoa Introduction

  • Phylum Protozoa classified into four subdivisions based on locomotion methods:

    • Amoebae (Sarcodina): Move using pseudopodia.

    • Flagellates (Mastigophora): Move using long, whiplike flagella.

    • Ciliates (Ciliata): Propel themselves with synchronized waves of cilia.

    • Sporozoans (Sporozoa): Lack specialized motility organelles.

Protozoa General

  • Approximately 45,000 protozoan species:

    • Around 8,000 are parasitic.

    • About 25 species are significant to humans.

  • Diagnosis: Key to differentiate between harmless and medically important species, often based on organism morphology.

  • Transmission routes:

    • Person-to-person via fecal-oral route.

    • Contaminated food or water.

    • Other means include sexual transmission and insect bites or feces.

Protozoa Lifecycle Stages

  • Trophozoite:

    • The motile vegetative stage.

    • Multiplication occurs via binary fission.

    • Colonizes the host.

  • Cyst:

    • The inactive, non-motile infective stage.

    • Survives environmental damage due to a cyst wall.

    • Cysts do not multiply, though some organisms can divide inside the cyst wall.

Protozoa Diagnostic Features

  • Nuclear structure: Crucial for differentiating species.

  • Size: Important for identifying organisms; requires calibrated microscope objectives for accurate measurement.

  • Cytoplasmic inclusions: Includes chromatoid bars (coalesced RNA), red blood cells, and food vacuoles with bacteria or yeast.

  • Cytoplasm appearance: Can be smooth & clean or vacuolated.

  • Type of motility: Directional vs non-directional; sluggish vs fast.

Protozoa Nuclear Structure

  • Chromatin: Nuclear DNA represented as:

    • Peripheral chromatin: Adheres to the nuclear membrane.

    • Karyosome: Small mass of chromatin within the nuclear space, also called endosome or centrosome.

  • Nuclear membrane: Encloses all nuclear material.

  • Chromatoid body: Coalesced RNA within cyst stage cytoplasm.

Intestinal Protozoa - The Amoebae General Life Cycle

  • Definitive host ingests infective cyst from fecal contamination.

  • Excystation occurs in the small intestine transforming the cyst into trophozoite stage which:

    • Colonizes the host.

    • Multiples asexually via binary fission.

    • Can remain non-pathogenic in the lumen or become pathogenic by invading the intestinal wall.

  • Both cysts and trophozoites are passed in feces of infected hosts.

Intestinal Protozoa - Entamoeba histolytica

  • Epidemiology:

    • Found worldwide; highest incidence in areas with poor sanitation.

  • Pathology and Clinical Manifestations:

    • Most pathogenic protozoan; causes amoebic dysentery and can lead to extra-intestinal complications, including hepatic abscesses.

    • Chronic infections can be misdiagnosed as colitis or cancer, lasting for years.

  • Distribution: Mostly in tropics and subtropics.

Intestinal Protozoa - Entamoeba histolytica Morphology & Laboratory Identification

  • Trophozoites: 12 to 30 microns in diameter.

    • Characterized by an evenly distributed peripheral chromatin and a small, centrally located karyosome.

    • Cytoplasm is smooth and granular, with possible red blood cell inclusions.

  • Cysts: Size ranges from 10 to 20 microns with four nuclei when mature; may contain cigar-shaped chromatoid bars.

Intestinal Protozoa - Entamoeba hartmanni

  • Epidemiology: Similar to E. histolytica; formerly referred to as “small race” of E. histolytica.

  • Distinction: Important for technologists to differentiate E. hartmanni (non-pathogenic) from E. histolytica.

Intestinal Protozoa - Entamoeba hartmanni Morphology & Laboratory Identification

  • Morphologically similar to E. histolytica with size distinction:

    • Trophozoites: Measure less than 12 microns.

    • Cysts: Measure less than 10 microns.

Intestinal Protozoa - Entamoeba coli

  • Significance: Harmless commensal; differentiation from pathogenic species is critical.

  • Morphology:

    • Trophozoites: Range from 10 to 35 microns.

    • Cysts: Range from 10 to 30 microns, containing 8 to 16 nuclei when mature; characterized by eccentric karyosome with irregular chromatin.

    • Cytoplasm is heavily vacuolated, with debris and inclusions.

Intestinal Protozoa - Entamoeba gingivalis

  • Infective Site: Thrives in diseased gums, not a causal agent but is destroyed by stomach acid if ingested.

  • Transmission: Via contact with fomites or kissing.

  • Morphology: Similar to E. histolytica but lacks a cyst stage; uniquely ingests leukocytes.

Intestinal Protozoa - Endolimax nana

  • Occurrence: Found in about 14% of the U.S. population and 21% worldwide.

  • Pathogenicity: Non-pathogenic.

  • Morphology:

    • Trophozoites: 5 to 10 microns; nucleus showcases a large blot-like karyosome with little peripheral chromatin.

    • Cysts: Sub-oval, 4 to 6 by 6 to 10 microns.

Intestinal Protozoa - Iodamoeba butschlii

  • Pathogenicity: Non-pathogenic.

  • Morphology:

    • Cyst is colloquially termed the “iodine cyst” due to a large glycogen vacuole that stains brown with iodine.

Tissue Dwelling Amoebae - Naegleria fowleri

  • Classification: An ameboflagellate free-living organism, alternating between amoeboid and flagellated forms; only amoeboid form found in tissues.

  • Life Cycle: Entry through nasal mucosa during swimming; moves along the olfactory nerve to the brain via the cribriform plate. Infections are invariably fatal, with no person-to-person spread.

Tissue Dwelling Amoebae - Naegleria fowleri Symptoms

  • Symptoms develop dramatically and progress quickly within 1-2 days:

    • Headache, fever, nausea, and vomiting.

    • Followed by meningoencephalitis, irrational behavior, coma, and death within 9 days of exposure.

  • Diagnosis: Typically made post-mortem; CSF contains motile amoebae, which can be cultured on non-nutrient agar containing bacteria.

Tissue Dwelling Amoebae - Acanthamoeba spp.

  • Life Cycle: Enters brain through skin wounds or lesions; more commonly associated with eye infections from contaminated cleaning solutions.

  • Symptoms: Slow onset (>10 days) presenting as chronic granulomatous lesions in the brain; eye lesions resemble herpes infections.

  • Acanthamoeba keratitis: Frequently seen in users of extended-wear contact lenses.

Superclass Mastigophora - The Flagellates Morphological Characteristics

  • Flagellum(ae): Organelles of locomotion, extensions of ectoplasm that move with a whip-like motion.

  • Axostyle: Supporting mechanism that is a rod-shaped structure, present in some flagellates.

  • Undulating membrane: Protoplasmic membrane with a flagellar rim extending like a fin along the body edge of some flagellates.

  • Costa: Thin, firm rod-like structure at the base of the undulating membrane.

  • Cytosome: A rudimentary mouth, also known as a gullet.

Superclass Mastigophora - The Flagellates Identification

  • Identification of a flagellate based on:

    • Size

    • Shape

    • Motility

    • Number and morphology of nuclei

    • Number and location of flagellae

    • Location within the host's body.

Intestinal Flagellates - Giardia lamblia

  • General Info: Most common protozoan parasite in the USA.

  • Life Cycle: Human ingests cysts from fecally contaminated sources; organism excysts in the upper intestine, trophozoites multiply and attach to the intestinal mucosa, may also invade the gall bladder. Both trophozoites and cysts are expelled in feces.

  • Diagnosis: Identification of cysts or trophozoites in stool specimens or duodenal contents.

Intestinal Flagellates - Giardia lamblia Morphology

  • Highly distinctive morphology:

    • Cyst: 9 x 12 micrometers, contains 2 to 4 nuclei; parabasal bodies present.

    • Trophozoite: Dorsal-ventrally flattened, bilaterally symmetrical with 4 pairs of flagella distributed as:

    • 1 pair anterior

    • 2 pairs ventral

    • 1 pair posterior.

    • Also possesses an axostyle and parabasal bodies.

Intestinal Flagellates - Giardia lamblia Epidemiology

  • Prevalence: Ranges from 1% to 30%, common in children’s day care centers; also transmitted via contaminated water and sexual contact.

  • Pathology and Clinical Manifestations: Symptoms include severe diarrhea (foul-smelling, greasy, mucus-laden), flatulence, nausea, and cramps, though most infections are asymptomatic; chronic cases result in weight loss and malabsorption issues of fat, protein, folic acid, and fat-soluble vitamins.

Intestinal Flagellates - Dientamoeba fragilis

  • General Info: Formerly classified as an amoeba; evidence supports its flagellate nature via electron microscopy and immunological studies.

  • Laboratory Diagnosis: Detection of binucleated trophozoites with fragmented karyosomes containing 4 to 8 chromatin granules.

  • Diagnostic Stage: The trophozoite is passed in feces; no cyst stage present.

  • Morphology: Exhibits 1 or 2 nuclei with minimal peripheral chromatin; karyosome split into 4 to 8 distinct granules.

  • Pathology: Usually asymptomatic; can occasionally cause diarrhea, anorexia, abdominal pain. May associate with pinworm infection transmission.

  • Distribution: Global presence with prevalence rates between 1% to 20%.

Intestinal Flagellates - Chilomastix mesnili

  • General Info: Classified as a non-pathogen; must differentiate from Giardia.

  • Location: Found in cecum and colon.

  • Transmission: Occurs through ingestion of mature cysts.

Intestinal Flagellates - Chilomastix mesnili Morphology

  • Trophozoite: Displays 4 flagella (3 anterior, 1 at the cytostome); single nucleus located anteriorly.

  • Cyst: Lemon-shaped, contains 1 nucleus with a visible cytostome.

The Trichomonads Characteristics

  • **Undulating membr…

  • Flagella: Clustered for locomotion.

  • Axostyle: Provides structural support.

  • Costa: Firm rod-like structure along the base of the undulating membrane.

  • Cytostome: Batched rudimentary mouth for nutrient uptake.

The Trichomonads - Trichomonas hominis

  • Classification: A commensal organism; must differentiate from pathogens.

  • Transmission: Direct fecal transmission, no cyst stage present.

  • Morphology: Exhibits an arc-shaped appearance with a jerky motility; differs from T. vaginalis, especially in cases of fecal contamination with urine.

The Trichomonads - Trichomonas vaginalis

  • Life Cycle: Trophozoites inhabit the vagina, urethra, epididymis, and prostate; multiply through longitudinal fission, lacking a cyst stage.

  • Mode of Infection: Transmitted via sexual intercourse or via fomites.

  • Diagnosis: Identification of trophozoites in body fluids (wet mount preparations of discharges) or via PAP smears.

The Trichomonads - Trichomonas vaginalis Pathology

  • Symptoms in Females: Vaginal discharge, burning, itching, chafing, and increased urination or dysuria.

  • Symptoms in Males: Often asymptomatic; if prostate involved, symptoms include discharge, dysuria, and prostate tenderness/enlargement.

  • Morphology: Characterized by an axostyle and limited-length undulating membrane; possesses 4 flagella.

Class Ciliophora - The Ciliates - Balantidium coli

  • Epidemiology: Rarely seen in the USA; recognized as the only ciliate human parasite.

  • Morphology: Largest parasitic protozoan; trophozoite dimensions 30-120 by 25-125 microns, with cysts averaging 50-70 microns.

  • Life Cycle: Initiated via ingestion of cysts through fecal contamination; cysts excyst in the small intestine, with trophozoites migrating to the large intestine.

Class Ciliophora - The Ciliates - Balantidium coli Pathology

  • Symptoms: Many infections asymptomatic; severe infections lead to mucosal ulcers and may present as dysentery, abdominal pain, nausea, vomiting, fever, and headache.

  • Diagnosis: Confirmed via observation of cysts and trophozoites in fecal samples.

Class Ciliophora - The Ciliates - Balantidium coli Morphology

  • Shape: Large and oval with:

    • Two nuclei (one large kidney-shaped macronucleus and small micronucleus).

    • Body covered by longitudinal cilia and a cytostome.

  • Primary Reservoir: Pigs and monkeys.

Intestinal Coccidia General

  • Represented Organisms: Includes Isospora, Sarcocystis, Cryptosporidium, Cyclospora, and Toxoplasma.

  • Life Cycle: Some have a two-host life cycle involving sexual and asexual reproduction:

    • Schizogony: Asexual binary fission.

    • Sporogony: Sexual reproduction.

  • Diagnostic Stages: Often challenging to locate; Acid-fast stains help visualization as oocysts do not stain with iodine or permanent stains like trichrome.

Intestinal Coccidia - Sarcocystis spp.

  • Pathology:

    • Sarcocystis bovihominis & Sarcocystis suihominis: Intestinal infections.

    • Sarcocystis lindemanni: Muscle infection.

  • Hosts: Definitive hosts for S. suihominis & S. bovihominis are humans. Intermediate hosts are pigs (sui-) and cows (bovi-), while humans serve as intermediate hosts for S. lindemanni.

  • Infective stages: Sarcocysts in meat for intestinal infections; oocyst ingestion from animal feces for muscle infections.

Intestinal Coccidia - Isospora belli

  • Definitive host: Humans.

  • Reproduction: Schizogony occurs when sporozoites invade the intestinal epithelium and multiply. Sporogony produces gametes which develop into oocysts passed in feces.

Intestinal Coccidia - Isospora belli Pathology

  • Diagnostic/Infective Stage: Mature oocyst containing two sporocysts, each with four sporozoites; does not stain with iodine.

  • Intermediate Hosts: None.

  • Pathology: Can range from asymptomatic to acute severe dysentery.

Intestinal Coccidia - Cryptosporidium parvum

  • Site of Infection: Primarily intestinal but can become systemic, especially in AIDS patients.

  • Hosts: Initially thought to be an animal parasite in rodents, cattle, and sheep; some species in fish, fowl, and reptiles do not infect humans.

  • Immunocompromised Patients: Infections can be severe and even fatal.

  • Identification: Oocysts measure 2-5 microns in diameter, do not stain with iodine, and are acid-fast.

Intestinal Coccidia - Cryptosporidium parvum Transmission

  • Transmission Routes: Endemic cycle maintained through person-to-person, fecal-oral route; commonly found in childhood diarrhea; also sexually transmitted and exhibits waterborne potential due to resistance to disinfectants.

  • Pathology: Most infections cause severe diarrhea; in immunosuppressed patients, conditions can worsen significantly with high mortality.

  • Treatment: No effective drug currently available against this parasite.

Intestinal Coccidia - Cyclospora cayetanensis

  • General Info: First identified outbreak in the USA in 1980; notable outbreak in 1996 associated with contaminated raspberries with over 100 cases in Texas.

  • Transmission: Caused by fecally contaminated food or water.

Intestinal Coccidia - Cyclospora cayetanensis Animal Reservoirs

  • Animal Reservoirs: Unknown; some Cyclospora species infect animals, but C. cayetanensis is the only one identified to infect humans.

  • Pathology: Symptoms resemble those seen in Cryptosporidium infections, resulting in diarrhea.

  • Identification: Oocysts measure 10 microns in diameter and have variable acid-fast staining.

Microsporidium spp.

  • General Info: Currently thought to infect primarily AIDS patients.

  • Transmission: Occurs via fecal-oral route; sexual transmission is also possible.

  • Identification: Very small spores averaging 1.0 by 1.7 microns; they stain reddish-pink with Chromotrope stain, with some spores exhibiting a dark staining “belt” across their midsection.