Coccidians

• Coccidians

  • Microscopic, spore-forming, single-celled obligate intracellular protozoan
  • Capable of asexual and sexual reproduction
  • Zoonotic infection is common
  • Coccidian are mostly self-limiting
  • Opportunistic in immunocompromised and immunodeficient individuals
  • Found in: Intestine, muscles and nucleated cells (WBCs)
• Coccidian species

  • Cryptosporodium hominis/parvum
  • Cyclospora cayatanensis
  • Cystoisospora belli
  • Sarcocystis hominis
  • Toxoplasma gondii
• General life cycle of coccidians


  1. Oocyst ingested

  2. Sporozoites released

  3. Merozoites formed in intestine

  4. Merozoites separate into sex cells (microgamonts and macrogamonts)

  5. Microgametes undergo sexual reproduction to form zygotes

  6. Zygotes become oocysts

  7. Sporozoites invade tissues and WBCs

  8. Tachyzoites and bradyzoites formed
• Cryptosporidium hominis

  • Common in humans, acid-fast organism
  • Size: 4 to 6 um
  • Does not contain sporocyst
  • Usually contains 4 sporozoites
  • 2 types of oocysts: thick-walled and thin-walled
  • Stains red-pink doughnut-shaped circular in Kinyoun's modified acid fast stain
• Cryptosporidium hominis infection

  • In immunocompetent host: Self-limiting diarrhea lasting 2-3 weeks
  • In immunocompromised: Severe, progressive, life-threatening
  • Can cause acute and gangrenous cholecystitis
• Cyclospora cayatanensis

  • Can contaminate lettuce and raspberries
  • Size: 7 to 10 um
  • Consists of 2 sporocysts each containing 2 sporozoites
  • No animal reservoir exists
  • Spurious parasite
  • Fluoresces blue or green in microscopy
• Cyclospora cayatanensis infection

  • Diarrhea may continue for 6-7 weeks
  • D-xylose malabsorption
  • Usually self-limiting with repeated infection
• Cystoisospora belli

  • Synonym: Isospora belli
  • Size: 25 to 35 um x 10 to 15 um
  • Contains sporoblast which matures into sporocyst
• Cystoisospora belli infection

  • In immunocompetent: Generally asymptomatic or self-limiting gastroenteritis
  • In immunocompromised: Self-limiting enteritis to severe diarrheal illness, with villous flattening
• Sarcocystis hominis


  • Size: 10 to 18 um
  • Thin cell wall
  • Has oocyst and sporocyst from sexual reproduction
  • Has sarcocyst with bradyzoites from asexual reproduction
• Sarcocystis hominis infection

  • Intestinal sarcocystosis (ingestion of infected meat): Often asymptomatic, may have mild fever, chills, vomiting, respiratory symptoms
  • Muscle invasion (ingestion of stool with oocyst): Myalgia, muscle weakness, transitory edema
• Toxoplasma gondii


  • Size: 10 to 15 um x 8 to 12 um
  • Definitive host: Cats
  • Intermediate host: Humans, birds, mice
  • Has bradyzoites and tachyzoites
  • Can infect WBCs throughout life cycle
  • Can cross placenta
  • Resistant to lysozymes
• Toxoplasma gondii infection

  • Asymptomatic in immunocompetent
  • Mimics infectious mononucleosis: Lymphadenitis, chills, fever, headache, myalgia, maculopapular rash
  • Congenital toxoplasmosis: Severe, often fatal, can cause hydrocephaly, microcephaly, calcifications, convulsions, psychomotor disturbances, ocular infections
  • In immunocompromised: Opportunistic infection, can cause toxoplasmic encephalitis
• Toxoplasma gondii treatment


  • Primary: Trisulfapyrimidines + Pyrimethamine
  • Alternative: Spiramycin, Azithromycin, Clarithromycin
  • For toxoplasmic encephalitis: Atovaquone