PROTOZOA

• The remaining protozoa of human clinical significance are described in this chapter
• This group of organisms is similar in that each of its members is unicellular
• The specific morphologic forms, methods of laboratory diagnosis, life cycle notes, epidemiology, clinical symptoms treatment protocols, and prevention and control measures vary among the organisms in this group
• In addition to the laboratory diagnosis information in this chapter, representative diagnostic methodologies are discussed in Chapter 2
• Two relatively new genera, Cyclospora and Microsporidia, are briefly mentioned in this chapter
• These organisms are known to produce human intestinal disease
• Because of their relatively recent discovery and the fact that much is still to be learned about these genera of protozoan organisms, their exact classification has not been well described
• The remaining members of the protozoa are classified in four groups

  • Ciliates
  • Select sporozoa
  • Blastocystis hominis
  • Pneumocystis jiroveci
• Ciliates


  Parasites that move by means of hairlike cytoplasmic extensions called cilia
• Sporozoa

  Intestinal and tissue-dwelling parasites, belonging to the subclass Coccidia, in which asexual replication occurs outside a human host and sexual replication occurs inside a human host
• Blastocystis hominis

  Initially considered as a yeast, now classified as a Protozoa and the sole member of the class Blastocystea
• Pneumocystis jiroveci


  Formerly known as Pneumocystis carinii, recently reclassified as a fungus
• Balantidium coli is the only human pathogen known in the ciliate group
• Isospora belli, Sarcocystis species, Cryptosporidium parvum, and Toxoplasma gondii are the intestinal and tissue species in the sporozoa group
• Balantidium coli


  Considered as the largest protozoan known to humans
• Balantidium coli trophozoite

  • Ovoid to sac-shaped, tapers at the anterior end
  • Exhibits rotary, boring motility
  • Contains two nuclei (a kidney-shaped macronucleus and a small spherical micronucleus)
  • Has one or two contractile vacuoles
  • Cytoplasm may contain food vacuoles and/or bacteria
  • Has a small cytostome
  • Surrounded by a layer of cilia
• Balantidium coli cyst

  • Subspherical to oval in shape
  • Measures 43-66 μm
  • Contains a kidney-shaped macronucleus (micronucleus may not be observable)
  • May have one or two contractile vacuoles (particularly in young unstained cysts)
  • Has a double-protective cyst wall
  • May have a row of cilia visible between the two cyst wall layers in unstained young cysts
• Laboratory diagnosis of Balantidium coli

  1. Examine stool specimens for presence of trophozoites and cysts

  2. Trophozoites more likely found in diarrheal stools

  3. Cysts may be found in formed stools

  4. Sigmoidoscopy material may reveal organisms in sigmoidorectal infection

  5. Thorough screening of wet preparations and permanent stains is crucial

  6. Multiple samples may be required
• Balantidium coli life cycle

  Initiated by ingestion of infective cysts in contaminated food or water<|>Trophozoites multiply by transverse binary fission in the cecal region, terminal ileum, and large intestine<|>Encystation occurs in the lumen, resulting cysts are the infective form
• Balantidium coli is distributed worldwide, but typical incidence of human infection is very low
• Transmission is via the oral-fecal route and person-to-person spread, possibly from contaminated water from pigs
• Asymptomatic carrier state

  Some patients infected with Balantidium coli remain asymptomatic
• Balantidiasis


  Symptomatic patients may experience mild colitis and diarrhea to full-blown clinical balantidiasis resembling amebic dysentery, with abscesses and ulcers in the large intestine
• Personal hygiene and proper sanitary conditions are effective measures for Balantidium coli prevention and control
• Pigs are a known reservoir host for Balantidium coli, but also carry a morphologically identical parasite Balantidium suis that does not infect humans
• Humans appear to have a relatively high natural resistance to Balantidium coli
• Isospora belli oocysts range in size from 25-35 μm long by 10-15 μm wide, with an average of 30 by 12 μm
• Isospora belli

  Also known as Isosporiasis
• Isospora belli oocyst

  • Surrounded by a smooth, colorless, two-layered cell wall
• Sarcocystis species


  Also known as Sarcocystis infection
• In many cases, only single or double sporocysts cemented together may be visible in stool samples
• Sarcocystis


  Protozoan parasite that can infect humans as an intermediate host, taking up residence in striated muscle
• Cryptosporidium parvum


  Protozoan parasite that can infect humans, causing cryptosporidiosis
• Cryptosporidium first associated with poultry and cattle, now recognized as cause of neonatal diarrhea in calves and lambs
• First human Cryptosporidium cases reported in 1976, initially in immunocompromised
• Cryptosporidium outbreaks linked to contaminated public water supplies
• Blastocystis hominis


  Protozoan parasite that can infect humans
• water


  Usually lyses this organism and may lead to a false-negative result
• It is important to screen suspicious samples with an iodine wet preparation and to use a permanent stain to confirm the presence of the parasite
• B. hominis reproduces by

  1. Sporulation

  2. Binary fission