Parasitic Protozoa Chapter 6

Introduction to Parasitic Protozoa

• Protozoa are single-celled organisms that exist as structurally and functionally independent individual cells.
• They come in many different shapes and sizes, ranging from amoeba to paramecium with complex structures.
• Protozoans have developed complex subcellular features (membranes & organelles) to survive in their environments.

Characteristics of Protozoa

• A single cell performs all functions: reproduction, digestion, respiration, and excretion.
• They do not have a cell wall, but some possess a flexible layer or a rigid shell of inorganic materials outside the cell membrane.
• They have the ability to move from one place to another during their entire life cycle using their locomotor organelles or by a gliding mechanism.
• They are heterotrophic, deriving nutrients from the body fluids of their hosts or ingesting particulates like bacteria, yeast, and algae.
• They reproduce by asexual means, although some groups also have sexual modes of reproduction.

Structure of a Protozoan Cell

• A typical protozoan cell is bounded by a trilaminar unit membrane supported by a sheet of contractile fibrils.
• The cytoplasm consists of two portions: ectoplasm (outer homogeneous part) and endoplasm (inner granular part containing the nucleus).
• Ectoplasm serves as the organ for locomotion, engulfment of food, and respiration, while endoplasm contains structures like Golgi bodies, endoplasmic reticulum, food vacuoles, and contractile vacuoles.

Reproduction in Protozoa

• Asexual reproduction occurs by binary fission, multiple fission, or endodyogeny.
• Sexual reproduction occurs by conjugation (in ciliates) or gametogony (in Sporozoa).

Life Cycle of a Parasitic Protozoan

• The life cycle can involve a single host (e.g., intestinal flagellates and ciliates) or two hosts (e.g., Plasmodium).
• Asexual reproduction occurs in one host, and sexual reproduction occurs in another host.

Classification of Protozoa

• Medical important parasitic protozoa are classified into the kingdom Protista, subkingdom Protozoa, and further divided into four phyla: Sarcomastigophora, Apicomplexa, Microspora, and Ciliophora.

Phylum Sarcomastigophora

• Includes many unicellular or colonial, autotrophic, or heterotrophic organisms that use flagellae, pseudopodia, or both for locomotion.
• Subdivided into two subphyla: Sarcodina (amibe) and Mastigophora (flagellates).

Entamoeba histolytica

• A single-celled protozoan that constantly changes its shape.
• Classification based on habitat: intestinal amoebae and free-living amoebae.
• Entamoeba histolytica is a parasitic amoeba that can cause amoebic dysentery and amoebic liver abscess.
• Life cycle involves three stages: trophozoite, precyst, and cyst.
• Trophozoite is the invasive form, precyst is the intermediate stage, and cyst is the infective form.
• Transmission occurs through the feco-oral route, sexual contact, or vectors like flies and cockroaches.

Clinical Manifestation of Amoebiasis

• Asymptomatic amoebiasis: About 90% of infected persons are asymptomatic carriers.
• Intestinal amoebiasis: Characterized by four clinical forms - amoebic dysentery, amoebic appendicitis, ameboma, and fulminant colitis.
• Amoebic liver abscess: Presents with tender hepatomegaly, fever, weight loss, sweating, and weakness.### Diagnosis of Intestinal Amoebiasis
• Stool microscopy detects cysts and trophozoites
• Stool culture and polyxenic and axenic culture are also used
• Stool antigen detection methods include:
  • Counter Current Immune Electrophoresis (CIEP)
  • Enzyme Linked Immunosorbent Assay (ELISA)
  • Immunochromatographic Test (ICT)
• Serology tests for amoebic antigen and amoebic antibody
• Isoenzyme (zymodene) analysis and molecular diagnosis using PCR

Diagnosis of Amoebic Liver Abscess

• Microscopy detects trophozoites
• Stool culture and antigen detection using ELISA
• Antibody detection using:
  • Indirect hemagglutination Test (IHA)
  • ELISA
  • Indirect fluorescent antibody Test (IFA)
• Histopathology using PAS stain
• Molecular diagnosis using PCR
• Ultrasonography

Treatment of Intestinal Amoebiasis and Amoebic Liver Abscess

• Metronidazole or tinidazole is the drug of choice
• Other measures include:
  • Fluid and electrolyte replacement
  • Symptomatic treatment

Prevention of Intestinal Amoebiasis

• Avoid ingestion of food and water contaminated with human feces
• Treatment of asymptomatic persons who pass E.histolytica cysts in the stool

Subphylum Mastigophora: Class Zoomastigophora (Flagellates)

• Flagellates are protozoans that use flagella for locomotion
• Flagella are slender, long, and thread-like extensions of cytoplasm
• Flagella arise from kinetoplast (made up of copies of mitochondrial DNA)

Classification of Flagellates based on Habitat

• Lumen-dwelling flagellates: found in the alimentary tract and urogenital tract
• Hemoflagellates: found in blood and tissues

Lumen-dwelling Flagellates

• Examples:
  • Giardia lamblia
  • Enteromonas hominis
  • Retortamonas intestinalis
  • Chilomastix mesnili
  • Dientamoeba fragilis
  • Trichomonas tenax
  • Pentatrichomonas hominis

Hemoflagellates

• Examples:
  • Leishmania
  • Trypanosoma

Giardia lambia

• First observed by A.V.Leeuwenhoek in 1681
• Different species of Giardia can be differentiated based on the origin of the host
• Giardia lamblia is the most common protozoan pathogen and is worldwide distributed
• Infection is endemic in places with poor sanitation

Morphology of Giardia lamblia

• Trophozoite has a falling leaf-like motility
• Shape:
  • Pear shaped (or tear drop or tennis racket shaped) in front view
  • Sickle shaped in lateral view
• Bilaterally symmetrical with:
  • One pair of nuclei
  • Pair of median bodies
  • Four pairs of basal bodies or blepharoplasty
  • Four pairs of flagella
  • Pair of parabasal bodies
  • Pair of axoneme or axostyle

Life Cycle of Giardia lamblia

• Host: completes its life cycle in one host
• Infective form: mature cyst
• Mode of transmission: ingestion of food and water contaminated with mature cysts
• Excystation: two trophozoites are released from each cyst in the duodenum
• Multiplication: trophozoites multiply by longitudinal binary fission
• Adhesion: trophozoites adhere to the duodenal mucosa by the bilobed adhesive ventral disc
• Encystation: begins in the large intestine

Pathogenicity of Giardia lamblia

• Infective dose: as few as 10-25 cysts can initiate the infection
• Risk factors: children, elderly, debilitated persons, and patients with cystic fibrosis
• Pathogenic mechanisms:
  • Adhesion to duodenal mucosa
  • Disruption of the intestinal epithelial brush border
  • Malabsorption
  • Elaboration of enterotoxin
• Malabsorption: types include:
  • Malabsorption of fat
  • Disaccharidase deficiencies
  • Malabsorption of vitamin B12 and folic acid
  • Protein losing enteropathy

Clinical Features of Giardiasis

• Asymptomatic carriers
• Acute giardiasis:
  • Incubation period: 1-3 weeks
  • Symptoms: diarrhea, abdominal pain, bloating, belching, and vomiting
  • Foul smelling diarrhea with fat and mucus
• Chronic giardiasis:
  • Intermittent and recurring symptoms
  • Foul smelling diarrhea, foul flatus, and weight loss
  • Uncommon symptoms: fever, presence of blood and/or mucus in the stools

Diagnosis of Giardiasis

• Stool examination: detects cysts and trophozoites
• Entero-test: antigen detection in stool
• ELISA, ICT, and other serological tests
• Culture
• Molecular method: PCR
• Radiological findings: barium meal, X-ray

Treatment of Giardiasis

• Metronidazole or tinidazole
• Nitazoxanide
• Furazolidone
• Paromomycin

Prevention of Giardiasis

• Improved food and personal hygiene
• Boiling or filtering of potentially contaminated water
• Treatment of asymptomatic carriers
• No vaccine is currently available### Trichomoniasis
• Caused by the parasite Trichomonas vaginalis
• Can cause low birth weight, neonatal pneumonia, and conjunctivitis in infants born to infected mothers
• Diagnosis methods:
  • Direct microscopy (wet saline mounting, permanent stain, acridine orange fluorescent stain, direct fluorescent antibody test)
  • Culture (gold standard method)
  • Antigen detection in vaginal secretion (ELISA, ICT, etc.)
  • Antibody detection (ELISA)
  • Molecular method (PCR)
  • Other supportive tests (raised vaginal pH, positive whiff test)
• Treatment:
  • Metronidazole or tinidazole (2g, single dose is usually effective)
  • Both sexual partners must be treated simultaneously to prevent reinfection
  • Resistance to metronidazole is rare but has been reported (2.5-10% to metronidazole, <1% to tinidazole)
  • Mechanism of resistance development: controlled by hydrogenosome
• Prevention:
  • Treatment of both partners
  • Safe sex practices (use of condoms)
  • Avoidance of sex with infected person
  • Vaccine (no effective vaccine licensed so far, but trials are ongoing)

Hemoflagellates

• Flagellated protozoa that complete their life cycle in two hosts (vertebrate host and insect vector)
• Morphology:
  • Oval to elongated body
  • Nucleus
  • Single flagellum arising from kinetoplast (consists of blepharoplasty and parabasal body connected by a delicate fibril, represents multiple copies of mitochondrial DNA)
  • Axoneme (or axostyle) extending from blepharoplast to the cell wall (represents the intracellular portion of flagellum)
• Based on arrangement of flagellum, they exist in four morphological stages:
  • Amastigote form (round to oval, lacks flagellum, found in reticuloendothelial cells of infected humans)
  • Promastigote form (lanceolate shaped, kinetoplast is anterior to nucleus, found in the mid-gut of insect vector)
  • Epimastigote form (elongated, kinetoplast is placed close to the nucleus, found in the insect vector)
  • Trypomastigote form (elongated and spindle shaped, kinetoplast lies near the posterior end, found in the insect vector and peripheral blood of humans)

Trypanosoma

• Classification:
  • Human Trypanosomes (T. cruzi, T. brucei)
  • Animal Trypanosomes (T. brucei brucei, T. congolense, T. vivax, T. rangeli, T. evansi, T. lewisi, T. equiperdum)
• T. cruzi:
  • Causes South American trypanosomiasis (Chagas' disease)
  • Transmitted by reduviid bug (triatomine bug)
  • Found in peripheral blood and tissues of humans
  • Morphology:
    • Amastigote form (intracellular parasite)
    • Trypomastigote form (extracellular, found in peripheral blood)
  • Life cycle:
    • In vertebrate host (human): trypomastigote form transforms into amastigote form
    • In insect vector (reduviid bug): trypomastigote form transforms into epimastigote form, and finally into metacyclic trypomastigote form
  • Pathogenicity and clinical features:
    • Acute Chagas' disease: characterized by chagoma, Romana's sign, generalized lymphadenopathy, hepatosplenomegaly, severe myocarditis, and neurologic signs
    • Chronic Chagas' disease: manifests years or decades after initial infection, characterized by cardiac form, gastrointestinal form, and mixed forms

T. brucei

• Causes African trypanosomiasis (sleeping sickness)
• Transmitted by tsetse fly
• Found in peripheral blood and tissues of humans
• Morphology:
  • Long slender trypomastigote form (found in peripheral blood)
  • Short stumpy trypomastigote form (found in tissues)
• Life cycle:
  • In vertebrate host (human): trypomastigote form transforms into amastigote form
  • In insect vector (tsetse fly): trypomastigote form transforms into epimastigote form, and finally into metacyclic trypomastigote form
• Pathogenicity and clinical features:
  • Trypanosomal chancre (self-limited inflammatory lesion)
  • Stage I disease: characterized by systemic febrile illness, lymphadenopathy, pruritus, maculopapular rashes, and transient edema
  • Stage II disease: characterized by invasion of the CNS, progressive daytime somnolence, restlessness, and insomnia at night, and abnormal speech with few extrapyramidal signs

This quiz covers the basics of parasitic protozoa, including their classification and characteristics. Learn about these 'first animals' and their role in genetics.