Blastocystis spp

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Blastocystis hominis cyst-like forms in a wet mount stained in iodine.[1]

Blastocystis spp are a zoonotic stramenopile protozoan parasite closely related to algae and water molds.

Despite relative high rates of prevalence of this parasite in the gastrointestinal microbiota of dogs[2], they are normally an opportunistic parasite associated with concurrent gastrointestinal disease such as exocrine pancreatic insufficiency[3] and are not thought of as primary initiators of disease.

Species which are found in dogs include:

  • Blastocystis hominis (subtypes 1, 2, 3, 5 & 10)[4]

The transmission of B. hominis is by the fecal-oral route between animals or via drinking contaminated water or food containing excreted cysts from a reservoir host.

Blastocystis hominis subtypes 1[5] and 5[6] are commonly detected in dogs.

Concurrent isolation of Giardia spp is common in epidemiological studies[7].

Although mostly nonpathogenic in dogs[8], clinically affected patients usually present with abdominal pain, mucoid diarrhea and occasionally blood in stools associated with ulerative large bowel enteritis[9].

Diagnosis is based on coprological identification of motile trophozoites of direct smears or detection of cysts in floatation devices (e.g. Baermann), axenic culture[10] or via PCR assays[11].

Treatment is usually not required, although broad-spectrum parenteral antimicrobials such as clindamycin or metronidazole (30mg/kg daily for 1 - 2 weeks) may assist recovery.


  1. CDC
  2. Duda A et al (1998) Detection of Blastocystis sp. in domestic dogs and cats. Vet Parasitol 76(1-2):9-17
  3. Chapman S et al (2009) What is your diagnosis? Rectal scraping from a dog with diarrhea. Vet Clin Pathol 38(1):59-62
  4. Stensvold CR et al (2009) Subtype distribution of Blastocystis isolates from synanthropic and zoo animals and identification of a new subtype. Int J Parasitol 39(4):473-479
  5. Leelayoova S et al (2009) Prevalence of intestinal parasitic infections in military personnel and military dogs, Thailand. J Med Assoc Thai 92(1):S53-S59
  6. Parkar U et al (2007) Direct characterization of Blastocystis from faeces by PCR and evidence of zoonotic potential. Parasitology 134(3):359-367
  7. Sagebiel D et al (2009) Giardiasis in kindergartens: prevalence study in Berlin, Germany, 2006. Parasitol Res 105(3):681-687
  8. López J et al (2006) Intestinal parasites in dogs and cats with gastrointestinal symptoms in Santiago, Chile. Rev Med Chil 134(2):193-200
  9. Bowman, DD (2009) Georgis' parasitology for veterinarians. Elsevier Saunders, Missouri. 9th edn. pp:92
  10. König G & Müller HE (1997) Blastocystis hominis in animals: incidence of four serogroups. Zentralbl Bakteriol 286(3):435-440
  11. Eroglu F & Koltas IS (2010) Evaluation of the transmission mode of B. hominis by using PCR method. Parasitol Res 107(4):841-845