Blastocystis subtypes in symptomatic and asymptomatic family members and pets and response to therapy

Intern Med J. 2012 Nov;42(11):1187-95. doi: 10.1111/j.1445-5994.2011.02626.x.


Background: Blastocystis is a common, enteric parasite. The pathogenicity of the organism is uncertain, but subtypes (ST) 1 and 3 have been reported more likely to cause irritable bowel-like symptoms.

Aims: We treated symptomatic patients positive for Blastocystis with conventional therapy and analysed 16 small-subunit (SSU) rDNA to assess clearance and carriage rates and ST prevalence of the parasite in the asymptomatic household members.

Methods: In a longitudinal, prospective case study, 11 symptomatic patients positive for Blastocystis underwent outpatient clinical assessment to exclude other diagnoses before 14 days of either metronidazole 400 mg three times daily or trimethoprim/sulfamethoxazole 160/800 mg twice-daily therapy. Faecal specimens were collected from patients at baseline, day 15, 28 and 56 after therapy and from 17 family members and eight pets at day 15. Specimens were analysed using faecal smear, culture and polymerase chain reaction analysis of 16SSU rDNA.

Results: No patient cleared the organism following therapy. ST 1 (45%), 3 (36%), 4 (36%) and 6 (9%) were found in the symptomatic Blastocystis patients, and ST identified before and after therapy were identical in each individual. All household contacts were positive for Blastocystis and 16/17 (94%) contacts showed identical Blastocystis ST to the symptomatic family member. All pets were positive for Blastocystis with polymerase chain reaction testing, 7/8 (88%) demonstrating ST concordance with the symptomatic Blastocystis patients.

Conclusions: Conventional therapy is ineffective for symptomatic Blastocystis infection. The high prevalence of Blastocystis infection within households suggested transmission between humans and their pets. Subtyping analysis of SSU rDNA alone in Blastocystis does not appear to predict pathogenicity.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Aged
  • Animals
  • Antiprotozoal Agents / therapeutic use
  • Asymptomatic Diseases
  • Biopsy
  • Blastocystis / classification*
  • Blastocystis / isolation & purification
  • Blastocystis / pathogenicity
  • Blastocystis Infections / drug therapy
  • Blastocystis Infections / parasitology*
  • Blastocystis Infections / transmission
  • Blastocystis Infections / veterinary
  • Carrier State / drug therapy
  • Carrier State / parasitology
  • Cat Diseases / drug therapy
  • Cat Diseases / parasitology*
  • Cats
  • Disease Reservoirs
  • Dog Diseases / drug therapy
  • Dog Diseases / parasitology*
  • Dogs
  • Family Health
  • Female
  • Humans
  • Intestinal Mucosa / parasitology
  • Intestinal Mucosa / pathology
  • Male
  • Metronidazole / therapeutic use
  • Middle Aged
  • Prospective Studies
  • Ribotyping
  • Treatment Failure
  • Trimethoprim, Sulfamethoxazole Drug Combination / therapeutic use
  • Young Adult


  • Antiprotozoal Agents
  • Metronidazole
  • Trimethoprim, Sulfamethoxazole Drug Combination