- Karin Leder, MBBS, FRACP, PhD, MPH, DTMH
Karin Leder, MBBS, FRACP, PhD, MPH, DTMH
- Section Editor — Travel Medicine
- Head of Infectious Diseases Unit
- Monash University, Australia
- Peter F Weller, MD, FACP
Peter F Weller, MD, FACP
- Editor-in-Chief — Infectious Diseases
- Section Editor — Tropical Medicine
- William Bosworth Castle Professor of Medicine
- Harvard Medical School
- Professor of Immunology and Infectious Diseases
- Harvard T. H. Chan School of Public Health
Blastocystis species (previously referred to as Blastocystis hominis) are anaerobic protozoan parasites found in the human gastrointestinal tract . The organism was initially discovered in 1911 and for many years was considered to be a harmless yeast. Studies in the 1970s demonstrated that Blastocystis spp are protozoans [2,3]. Blastocystis spp are among the most common organisms to be detected in stool specimens, but there is considerable controversy regarding whether they represent a commensal organism or a true pathogen. (See "Nonpathogenic enteric protozoa".)
Blastocystis spp have been observed worldwide . The organism resides in the colon and cecum of children and adults. The mode of transmission is not fully understood; fecal-oral transmission has been postulated . Some authors have suggested that contaminated water may also be a source of infection [6-9]. Blastocystis spp have also been found in animals including pigs, monkeys, rodents, and poultry. There seems to be poor host specificity; transmission occurs from human to human and between humans and animals . Blastocystis infections are more common among individuals with occupational exposure to animals, supporting the potential for zoonotic transmission.
The prevalence of Blastocystis spp varies between countries and between communities . In general, the estimated prevalence of Blastocystis spp is higher in developing countries than developed countries (30 to 50 percent versus 5 to 10 percent, respectively). This may be related to poor hygiene, animal exposure, and consumption of contaminated food or water . In one study from Senegal, 100 percent of 93 fecal samples were positive for Blastocystis spp . In a study from Canada, 8 percent of stool samples submitted to a reference laboratory were positive for Blastocystis spp; when Blastocystis was the sole organism identified, 76 percent of subjects continued to harbor the parasite two months after initial detection .
Blastocystis spp are also commonly found in the stool of returned travelers from developing countries [6,13-15]. In one study of nearly 2000 stool specimens from travelers and expatriates in Nepal, the prevalence of Blastocystis spp was 30 percent . (See "Travelers' diarrhea: Microbiology, epidemiology, and prevention" and "Evaluation of fever in the returning traveler".)
Blastocystis spp demonstrate marked morphologic variability and measure between 5 and 40 mcm (picture 1). The organism lacks a cell wall but contains mitochondria, Golgi apparatus, and smooth and rough endoplasmic reticula typical of protozoa . It reproduces asexually, usually by binary fission. It grows only under anaerobic conditions in culture, and it is highly susceptible to changes in temperature and in environmental tonicity.
- Tan KS. New insights on classification, identification, and clinical relevance of Blastocystis spp. Clin Microbiol Rev 2008; 21:639.
- Zierdt CH, Rude WS, Bull BS. Protozoan characteristics of Blastocystis hominis. Am J Clin Pathol 1967; 48:495.
- Zierdt CH. Studies of Blastocystis hominis. J Protozool 1973; 20:114.
- Stenzel DJ, Boreham PF. Blastocystis hominis revisited. Clin Microbiol Rev 1996; 9:563.
- Blastocystis hominis: commensal or pathogen? Lancet 1991; 337:521.
- Kain KC, Noble MA, Freeman HJ, Barteluk RL. Epidemiology and clinical features associated with Blastocystis hominis infection. Diagn Microbiol Infect Dis 1987; 8:235.
- Nimri LF. Evidence of an epidemic of Blastocystis hominis infections in preschool children in northern Jordan. J Clin Microbiol 1993; 31:2706.
- Nimri L, Batchoun R. Intestinal colonization of symptomatic and asymptomatic schoolchildren with Blastocystis hominis. J Clin Microbiol 1994; 32:2865.
- Leelayoova S, Rangsin R, Taamasri P, et al. Evidence of waterborne transmission of Blastocystis hominis. Am J Trop Med Hyg 2004; 70:658.
- Parkar U, Traub RJ, Kumar S, et al. Direct characterization of Blastocystis from faeces by PCR and evidence of zoonotic potential. Parasitology 2007; 134:359.
- El Safadi D, Gaayeb L, Meloni D, et al. Children of Senegal River Basin show the highest prevalence of Blastocystis sp. ever observed worldwide. BMC Infect Dis 2014; 14:164.
- Senay H, MacPherson D. Blastocystis hominis: epidemiology and natural history. J Infect Dis 1990; 162:987.
- Sheehan DJ, Raucher BG, McKitrick JC. Association of Blastocystis hominis with signs and symptoms of human disease. J Clin Microbiol 1986; 24:548.
- Doyle PW, Helgason MM, Mathias RG, Proctor EM. Epidemiology and pathogenicity of Blastocystis hominis. J Clin Microbiol 1990; 28:116.
- Jelinek T, Peyerl G, Löscher T, et al. The role of Blastocystis hominis as a possible intestinal pathogen in travellers. J Infect 1997; 35:63.
- Shlim DR, Hoge CW, Rajah R, et al. Is Blastocystis hominis a cause of diarrhea in travelers? A prospective controlled study in Nepal. Clin Infect Dis 1995; 21:97.
- Miller RA, Minshew BH. Blastocystis hominis: an organism in search of a disease. Rev Infect Dis 1988; 10:930.
- Zaman V, Howe J, Ng M. Variation in the cyst morphology of Blastocystis hominis. Parasitol Res 1997; 83:306.
- Moe KT, Singh M, Howe J, et al. Observations on the ultrastructure and viability of the cystic stage of Blastocystis hominis from human feces. Parasitol Res 1996; 82:439.
- Ok UZ, Girginkardeşler N, Balcioğlu C, et al. Effect of trimethoprim-sulfamethaxazole in Blastocystis hominis infection. Am J Gastroenterol 1999; 94:3245.
- Wong KH, Ng GC, Lin RT, et al. Predominance of subtype 3 among Blastocystis isolates from a major hospital in Singapore. Parasitol Res 2008; 102:663.
- Stensvold CR, Alfellani MA, Nørskov-Lauritsen S, et al. Subtype distribution of Blastocystis isolates from synanthropic and zoo animals and identification of a new subtype. Int J Parasitol 2009; 39:473.
- Scanlan PD, Stensvold CR, Cotter PD. Development and Application of a Blastocystis Subtype-Specific PCR Assay Reveals that Mixed-Subtype Infections Are Common in a Healthy Human Population. Appl Environ Microbiol 2015; 81:4071.
- Zierdt CH. Pathogenicity of Blastocystis hominis. J Clin Microbiol 1991; 29:662.
- Zierdt CH. Blastocystis hominis--past and future. Clin Microbiol Rev 1991; 4:61.
- Blastocystis hominis as a human pathogen. Rev Infect Dis 1989; 11:661.
- Qadri SM, al-Okaili GA, al-Dayel F. Clinical significance of Blastocystis hominis. J Clin Microbiol 1989; 27:2407.
- Markell EK, Udkow MP. Blastocystis hominis: pathogen or fellow traveler? Am J Trop Med Hyg 1986; 35:1023.
- Sun T, Katz S, Tanenbaum B, Schenone C. Questionable clinical significance of Blastocystis hominis infection. Am J Gastroenterol 1989; 84:1543.
- Chen TL, Chan CC, Chen HP, et al. Clinical characteristics and endoscopic findings associated with Blastocystis hominis in healthy adults. Am J Trop Med Hyg 2003; 69:213.
- Scanlan PD, Stensvold CR, Rajilić-Stojanović M, et al. The microbial eukaryote Blastocystis is a prevalent and diverse member of the healthy human gut microbiota. FEMS Microbiol Ecol 2014; 90:326.
- Clark CG. Extensive genetic diversity in Blastocystis hominis. Mol Biochem Parasitol 1997; 87:79.
- Yoshikawa H, Nagano I, Wu Z, et al. Genomic polymorphism among Blastocystis hominis strains and development of subtype-specific diagnostic primers. Mol Cell Probes 1998; 12:153.
- Kukoschke KG, Müller HE. SDS-PAGE and immunological analysis of different axenic Blastocystis hominis strains. J Med Microbiol 1991; 35:35.
- Boreham PF, Upcroft JA, Dunn LA. Protein and DNA evidence for two demes of Blastocystis hominis from humans. Int J Parasitol 1992; 22:49.
- Elwakil HS, Talaat RM. Genetic analysis of Blastocystis hominis isolated from symptomatic and asymptomatic human hosts in Egypt. J Egypt Soc Parasitol 2009; 39:99.
- Souppart L, Sanciu G, Cian A, et al. Molecular epidemiology of human Blastocystis isolates in France. Parasitol Res 2009; 105:413.
- Tan TC, Suresh KG, Smith HV. Phenotypic and genotypic characterisation of Blastocystis hominis isolates implicates subtype 3 as a subtype with pathogenic potential. Parasitol Res 2008; 104:85.
- Vennila GD, Suresh Kumar G, Khairul Anuar A, et al. Irregular shedding of Blastocystis hominis. Parasitol Res 1999; 85:162.
- Keystone JS. Blastocystis hominis and traveler's diarrhea. Clin Infect Dis 1995; 21:102.
- Grossman I, Weiss LM, Simon D, et al. Blastocystis hominis in hospital employees. Am J Gastroenterol 1992; 87:729.
- Markell EK. Is there any reason to continue treating Blastocystis infections? Clin Infect Dis 1995; 21:104.
- Nourrisson C, Scanzi J, Pereira B, et al. Blastocystis is associated with decrease of fecal microbiota protective bacteria: comparative analysis between patients with irritable bowel syndrome and control subjects. PLoS One 2014; 9:e111868.
- Cirioni O, Giacometti A, Drenaggi D, et al. Prevalence and clinical relevance of Blastocystis hominis in diverse patient cohorts. Eur J Epidemiol 1999; 15:389.
- Albrecht H, Stellbrink HJ, Koperski K, Greten H. Blastocystis hominis in human immunodeficiency virus-related diarrhea. Scand J Gastroenterol 1995; 30:909.
- Ok UZ, Cirit M, Uner A, et al. Cryptosporidiosis and blastocystosis in renal transplant recipients. Nephron 1997; 75:171.
- Zuckerman MJ, Watts MT, Ho H, Meriano FV. Blastocystis hominis infection and intestinal injury. Am J Med Sci 1994; 308:96.
- Llibre JM, Tor J, Manterola JM, et al. Blastocystis hominis chronic diarrhoea in AIDS patients. Lancet 1989; 1:221.
- Garavelli PL, Orsi P, Scaglione L. Blastocystis hominis infection during AIDS. Lancet 1988; 2:1364.
- Hussain R, Jaferi W, Zuberi S, et al. Significantly increased IgG2 subclass antibody levels to Blastocystis hominis in patients with irritable bowel syndrome. Am J Trop Med Hyg 1997; 56:301.
- Giacometti A, Cirioni O, Fiorentini A, et al. Irritable bowel syndrome in patients with Blastocystis hominis infection. Eur J Clin Microbiol Infect Dis 1999; 18:436.
- Yakoob J, Jafri W, Jafri N, et al. Irritable bowel syndrome: in search of an etiology: role of Blastocystis hominis. Am J Trop Med Hyg 2004; 70:383.
- Tungtrongchitr A, Manatsathit S, Kositchaiwat C, et al. Blastocystis hominis infection in irritable bowel syndrome patients. Southeast Asian J Trop Med Public Health 2004; 35:705.
- Stensvold CR, Lewis HC, Hammerum AM, et al. Blastocystis: unravelling potential risk factors and clinical significance of a common but neglected parasite. Epidemiol Infect 2009; 137:1655.
- Yakoob J, Jafri W, Beg MA, et al. Blastocystis hominis and Dientamoeba fragilis in patients fulfilling irritable bowel syndrome criteria. Parasitol Res 2010; 107:679.
- Hameed DM, Hassanin OM, Zuel-Fakkar NM. Association of Blastocystis hominis genetic subtypes with urticaria. Parasitol Res 2011; 108:553.
- Casero RD, Mongi F, Sánchez A, Ramírez JD. Blastocystis and urticaria: Examination of subtypes and morphotypes in an unusual clinical manifestation. Acta Trop 2015; 148:156.
- Lee MG, Rawlins SC, Didier M, DeCeulaer K. Infective arthritis due to Blastocystis hominis. Ann Rheum Dis 1990; 49:192.
- Patino WD, Cavuoti D, Banerjee SK, et al. Cytologic diagnosis of blastocystis hominis in peritoneal fluid: a case report. Acta Cytol 2008; 52:718.
- Fréalle E, El Safadi D, Cian A, et al. Acute blastocystis-associated appendicular peritonitis in a child, Casablanca, Morocco. Emerg Infect Dis 2015; 21:91.
- Horiki N, Maruyama M, Fujita Y, et al. Epidemiologic survey of Blastocystis hominis infection in Japan. Am J Trop Med Hyg 1997; 56:370.
- Zuckerman MJ, Ho H, Hooper L, et al. Frequency of recovery of Blastocystis hominis in clinical practice. J Clin Gastroenterol 1990; 12:525.
- Suresh K, Smith H. Comparison of methods for detecting Blastocystis hominis. Eur J Clin Microbiol Infect Dis 2004; 23:509.
- Nigro L, Larocca L, Massarelli L, et al. A placebo-controlled treatment trial of Blastocystis hominis infection with metronidazole. J Travel Med 2003; 10:128.
- Zierdt CH, Zierdt WS, Nagy B. Enzyme-linked immunosorbent assay for detection of serum antibody to Blastocystis hominis in symptomatic infections. J Parasitol 1995; 81:127.
- Jones MS 2nd, Ganac RD, Hiser G, et al. Detection of Blastocystis from stool samples using real-time PCR. Parasitol Res 2008; 103:551.
- Roberts T, Barratt J, Harkness J, et al. Comparison of microscopy, culture, and conventional polymerase chain reaction for detection of blastocystis sp. in clinical stool samples. Am J Trop Med Hyg 2011; 84:308.
- Wolfe MS. The treatment of intestinal protozoan infections. Med Clin North Am 1982; 66:707.
- Zaman V, Zaki M. Resistance of Blastocystis hominis cysts to metronidazole. Trop Med Int Health 1996; 1:677.
- Haresh K, Suresh K, Khairul Anus A, Saminathan S. Isolate resistance of Blastocystis hominis to metronidazole. Trop Med Int Health 1999; 4:274.
- Roberts T, Ellis J, Harkness J, et al. Treatment failure in patients with chronic Blastocystis infection. J Med Microbiol 2014; 63:252.
- Armentia A, Méndez J, Gómez A, et al. Urticaria by Blastocystis hominis. Successful treatment with paromomycin. Allergol Immunopathol (Madr) 1993; 21:149.
- van Hellemond JJ, Molhoek N, Koelewijn R, et al. Is paromomycin the drug of choice for eradication of Blastocystis in adults? J Infect Chemother 2013; 19:545.
- Rossignol JF, Kabil SM, Said M, et al. Effect of nitazoxanide in persistent diarrhea and enteritis associated with Blastocystis hominis. Clin Gastroenterol Hepatol 2005; 3:987.