- Email: [email protected]
Blastocystis
Blastocystis
260
260 Blastocystis Christen R. Stensvold
Blastocystis is a protist parasitizing the lower intestine of humans and a vast range of animals. Although Blastocystis has been implicated as a cause of various gastrointestinal tract symptoms, its role as a pathogen, mechanism of transmission, and life cycle have been subject to much debate.1–4 A clinical syndrome of self-limited intestinal symptoms has been described in several series of predominantly adult patients, with infection intensity possibly accounting for differences in clinical presentation.5 Extraintestinal manifestations, such as urticaria, also have been described.6,7 Blastocystis spp. from humans, other mammals, and birds are classified into subtypes, but because non−DNA-based laboratory methods do not enable distinction between subtypes, the organism should be reported as Blastocystis sp. not as Blastocystis hominis.8
ORGANISM Blastocystis is a protist belonging to the Stramenopiles.9 It has been isolated from a wide variety of animals and has been reported in human populations from most parts of the world.10 A total of 9 subtypes (arguably species) have been identified in humans to date, with humans colonized most commonly by subtypes 1, 2, 3, and 4, depending on geographic region.8,10,11 Several morphologic stages have been described, including the vacuolar, avacuolar, multivacuolar, granular, ameboid, and cystic stages. The vacuolar form is observed most frequently in clinical samples and in fecal cultures; it is spherical and usually ranges in size from 4 to 15 µm in diameter12; much larger cells may be seen in cultures. The cyst (2–5 µm) is the transmissible form and is able to survive in water at room temperature for up to 19 days but is fragile at extremes of heat and cold and when exposed to common disinfectants.13–16 After ingestion of an infectious cyst, the organism excysts in the lumen of the cecum and proximal colon, developing into vacuolar forms and undergoing encystation before being excreted as the cyst form, which, if eaten, completes the cycle.
EPIDEMIOLOGY Blastocystis has a global distribution. Surveys of intestinal parasites in population groups in developing countries documented prevalence rates
of 54% in Papua New Guinea and 33% among travelers and foreign residents with diarrhea in Nepal.17,18 In a study of fecal specimens from 48 states and the District of Columbia, Blastocystis was identified in 23% of 5792 stool specimens.19 There are few reports of Blastocystis infection in children. A study surveying 1042 internationally adopted children submitting at least one stool specimen for ova and parasite testing within 120 days after arrival to the United States saw a prevalence of Blastocystis of 10%.20 A community-based survey northeast of Mexico City reported a frequency of 7% in children younger than 14 years.21 A study of the prevalence of enteroparasites in a facility for orphaned and homeless children in Argentina found Blastocystis in 44% of stool samples.22 A similar study of children living in residential institutions and street communities in metropolitan Manila, Philippines found Blastocystis in 41% of children examined.23 A community-based study in Papua New Guinea found that >65% of children younger than 18 years had Blastocystis.17 Meanwhile, the use of DNA-based diagnostic methods recently led to identification of a 100% prevalence in Senegalese children with gastrointestinal symptoms.24 Infection has been associated with recent travel to the tropics and consumption of untreated water.25,26 Blastocystis has been detected in cases of nosocomial diarrhea and in children attending childcare centers.27,28 Sexually active homosexual men have Blastocystis infection rates as high as 52%, a finding consistent with person-to-person transmission.29 Limited data suggest that illness due to Blastocystis can be more severe in patients with AIDS and other immunocompromising disorders.30,31 Working with animals may increase the risk for becoming infected with Blastocystis.32
CLINICAL MANIFESTATIONS Whether Blastocystis causes disease is controversial, and potential mechanisms of pathogenicity is unclear. Symptoms described in Blastocystispositive individuals include abdominal pain, diarrhea, nausea, anorexia, bloating, and flatulence. Means to differentiate infection from colonization do not exist.1 Differences in clinical presentation might be due to one or more of the following factors: subtype virulence factors, immune
1305
status of the host, host genotype, parasite load, host response, and dysbiosis.1 Nevertheless, the presence of this organism in stool suggests that the host was exposed to fecally contaminated food or drink. Blastocystis does not appear to be more common in patients with inflammatory or functional bowel diseases than in healthy individuals, although conflicting data exist.33–35 Attempts to infect germ-free guinea pigs did not provide convincing evidence of pathogenicity, except when large numbers of organisms were inoculated with normal enteric flora, with gross pathologic effects being limited to mild intestinal hyperemia.36 Microscopic examination revealed a slight increase in cellularity and intraepithelial penetration by Blastocystis. A case-control study examining the association between acquisition of Blastocystis and presence of mucosal lesions by upper gastrointestinal endoscopy and sigmoidoscopy failed to demonstrate intestinal pathology.37 Studies on Blastocystis in a gut microbiota context are emerging, and it is possible that such studies hold the key to unraveling the clinical significance of Blastocystis.38
DIAGNOSIS Diagnosis most often is made by staining of a fixed fecal specimen with iron hematoxylin or trichrome stain (Fig. 260.1), which is superior to iodine staining of wet mounts and microscopy of fecal concentrates.39,40 Recognition of the organism can be difficult because of large variation in size and form. Blastocystis can be grown readily in culture using Jones medium, with relatively high diagnostic sensitivity.39,41 Polymerase chain reaction testing is both highly sensitive and specific, but is used primarily for research.41,42 Studies using serology are scarce, and it is not known whether serology could be a useful tool to distinguish infection from colonization.
TREATMENT AND PREVENTION Detection of the organism in feces should not lead automatically to administration of antimicrobial therapy because clinical significance is controversial and carriage is common; most children in whom Blastocystis is detected should not be treated. Therapy should be considered in cases of persistent intestinal symptoms and when no other cause is found. Metronidazole alone, which often is used for Blastocystis eradication, appears to have very limited effect, whereas other drugs, such as paromomycin, nitazoxanide, and so-called triple therapy (diloxanide furoate, trimethoprim-sulfamethoxazole, and secnidazole) might be somewhat efficacious.43–45
FIGURE 260.1 Vacuolar forms of Blastocystis in a fixed fecal smear stained with trichrome stain (Courtesy of DPDx image library, Centers for Disease Control and Prevention).
Prevention of Blastocystis infection involves adequate sanitation, careful handwashing, avoidance of untreated water, and use of enteric precautions to interrupt person-to-person spread. All references are available online at www.expertconsult.com.
KEY REFERENCES 1. Scanlan PD, Stensvold CR. Blastocystis: getting to grips with our guileful guest. Trends Parasitol 2013;29:523–529. 2. Clark CG, van der Giezen M, Alfellani MA, Stensvold CR. Recent Developments in Blastocystis Research. In: Rollinson D (ed) Advances in Parasitology, Vol. 82. San Diego, Elsevier Academic Press Inc., 2013, pp 1–32. 3. Poirier P, Wawrzyniak I, Vivarès CP, et al. New insights into Blastocystis spp.: A Potential Link with Irritable Bowel Syndrome. PLoS Pathog 2012;8:e1002545. 4. Tan KS, Mirza H, Teo JD, et al. Current views on the clinical relevance of Blastocystis spp. Curr Infect Dis Rep 2010;12:28–35. 11. Alfellani MA, Stensvold CR, Vidal-Lapiedra A, et al. Variable geographic distribution of Blastocystis subtypes and its potential implications. Acta Trop 2013;126:11–18. 12. Tan KS. New insights on classification, identification, and clinical relevance of Blastocystis spp. Clin Microbiol Rev 2008;21:639–665. 44. Stensvold CR, Smith HV, Nagel R, et al. Eradication of Blastocystis carriage with antimicrobials: reality or delusion? J Clin Gastroenterol 2010;44:85–90.
Blastocystis
REFERENCES 1. Scanlan PD, Stensvold CR. Blastocystis: getting to grips with our guileful guest. Trends Parasitol 2013;29:523–529. 2. Clark CG, van der Giezen M, Alfellani MA, Stensvold CR. Recent Developments in Blastocystis Research. In: Rollinson D (ed) Advances in Parasitology, Vol. 82. San Diego, Elsevier Academic Press Inc., 2013, pp 1–32. 3. Poirier P, Wawrzyniak I, Vivarès CP, et al. New insights into Blastocystis spp.: A Potential Link with Irritable Bowel Syndrome. PLoS Pathog 2012;8:e1002545. 4. Tan KS, Mirza H, Teo JD, et al. Current views on the clinical relevance of Blastocystis spp. Curr Infect Dis Rep 2010;12:28–35. 5. El Shazly AM, Awad SE, Sultan DM, et al. Intestinal parasites in Dakahlia governorate, with different techniques in diagnosing protozoa. J Egypt Soc Parasitol 2006;36:1023–1034. 6. Vogelberg C, Stensvold CR, Monecke S, et al. Blastocystis sp. subtype 2 detection during recurrence of gastrointestinal and urticarial symptoms. Parasitol Int 2010;59:469–471. 7. Zuel-Fakkar NM, Abdel Hameed DM, Hassanin OM. Study of Blastocystis hominis isolates in urticaria: a case-control study. Clin Exp Dermatol 2011;36:908–910. 8. Stensvold CR, Suresh GK, Tan KS, et al. Terminology for Blastocystis subtypes—a consensus. Trends Parasitol 2007;23:93–96. 9. Silberman JD, Sogin ML, Leipe DD, Clark CG. Human parasite finds taxonomic home. Nature 1996;380:398. 10. Alfellani MA, Taner-Mulla D, Jacob AS, et al. Genetic diversity of Blastocystis in livestock and zoo animals. Protist 2013;164:497–509. 11. Alfellani MA, Stensvold CR, Vidal-Lapiedra A, et al. Variable geographic distribution of Blastocystis subtypes and its potential implications. Acta Trop 2013;126:11–18. 12. Tan KS. New insights on classification, identification, and clinical relevance of Blastocystis spp. Clin Microbiol Rev 2008;21:639–665. 13. Rene BA, Stensvold CR, Badsberg JH, Nielsen HV. Subtype analysis of Blastocystis isolates from Blastocystis cyst excreting patients. Am J Trop Med Hyg 2009;80:588–592. 14. Suresh K, Smith HV, Tan TC. Viable Blastocystis cysts in Scottish and Malaysian sewage samples. Appl Environ Microbiol 2005;71:5619–5620. 15. Chen XQ, Singh M, Howe J, et al. In vitro encystation and excystation of Blastocystis ratti. Parasitology 1999;118(Pt 2):151–160. 16. 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;82:439–444. 17. Ashford RW, Atkinson EA. Epidemiology of Blastocystis hominis infection in Papua New Guinea: age-prevalence and associations with other parasites. Ann Trop Med Parasitol 1992;86:129–136. 18. Taylor DN, Houston R, Shlim DR, et al. Etiology of diarrhea among travelers and foreign residents in Nepal. JAMA 1988;260:1245–1248. 19. Amin OM. Seasonal prevalence of intestinal parasites in the United States during 2000. Am J Trop Med Hyg 2002;66:799–803. 20. Staat MA, Rice M, Donauer S, et al. Intestinal parasite screening in internationally adopted children: importance of multiple stool specimens. Pediatrics 2011;128:e613–e622. 21. Diaz E, Mondragon J, Ramirez E, Bernal R. Epidemiology and control of intestinal parasites with nitazoxanide in children in Mexico. Am J Trop Med Hyg 2003;68:384–385. 22. Guignard S, Arienti H, Freyre L, et al. Prevalence of enteroparasites in a residence for children in the Córdoba Province, Argentina. Eur J Epidemiol 2000;16:287–293. 23. Baldo ET, Belizario VY, De Leon WU, et al. Infection status of intestinal parasites in children living in residential institutions in Metro Manila, the Philippines. Korean J Parasitol 2004;42:67–70. 24. 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.
260
25. Herwaldt BL, de Arroyave KR, Wahlquist SP, et al. Multiyear prospective study of intestinal parasitism in a cohort of Peace Corps volunteers in Guatemala. J Clin Microbiol 2001;39:34–42. 26. Leelayoova S, Rangsin R, Taamasri P, et al. Evidence of waterborne transmission of Blastocystis hominis. Am J Trop Med Hyg 2004;70:658–662. 27. Miller SA, Rosario CL, Rojas E, Scorza JV. Intestinal parasitic infection and associated symptoms in children attending day care centres in Trujillo, Venezuela. Trop Med Int Health 2003;8:342–347. 28. Aygun G, Yilmaz M, Yasar H, et al. Parasites in nosocomial diarrhoea: are they underestimated? J Hosp Infect 2005;60:283–285. 29. May G, McLeod C, Whiteside M Intestinal colonization of Blastocystis hominis in a homosexual male community. 32nd Annual Meeting of the American Society of Tropical Medicine and Hygiene; 1983; Baltimore. 1983. 30. Gassama A, Sow PS, Fall F, et al. Ordinary and opportunistic enteropathogens associated with diarrhea in Senegalese adults in relation to human immunodeficiency virus serostatus. Int J Infect Dis 2001;5:192–198. 31. Cirioni O, Giacometti A, Drenaggi D, et al. Prevalence and clinical relevance of Blastocystis hominis in diverse patient cohorts. Eur J Epidemiol 1999;15:389–393. 32. Wang W, Owen H, Traub RJ, et al. Molecular epidemiology of Blastocystis in pigs and their in-contact humans in Southeast Queensland, Australia, and Cambodia. Vet Parasitol 2014;203:264–269. 33. Petersen AM, Nielsen HV, Stensvold CR, et al. Blastocystis and Dientamoeba fragilis in active and inactive inflammatory bowel disease. Gastroenterology 2011;140:S329–S330. 34. Cekin AH, Cekin Y, Adakan Y, et al. Blastocystosis in patients with gastrointestinal symptoms: a case-control study. BMC Gastroenterol 2012;12:122. 35. Krogsgaard LR, Engsbro AL, Stensvold CR, et al. The prevalence of intestinal parasites is not greater among individuals with irritable bowel syndrome: a population-based case-control study. Clin Gastroenterol Hepatol 2015;13:507–513.e502. 36. Phillips BP, Zierdt CH. Blastocystis hominis: pathogenic potential in human patients and in gnotobiotes. Exp Parasitol 1976;39:358–364. 37. 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;69:213–216. 38. 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. 39. Stensvold CR, Arendrup MC, Jespersgaard C, et al. Detecting Blastocystis using parasitologic and DNA-based methods: a comparative study. Diagn Microbiol Infect Dis 2007;59:303–307. 40. 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;84:308–312. 41. Stensvold CR, Ahmed UN, Andersen LO, Nielsen HV. Development and evaluation of a genus-specific, probe-based, internal process controlled real-time PCR assay for sensitive and specific detection of Blastocystis. J Clin Microbiol 2012. 42. Poirier P, Wawrzyniak I, Albert A, et al. Development and evaluation of a real-time PCR assay for detection and quantification of blastocystis parasites in human stool samples: prospective study of patients with hematological malignancies. J Clin Microbiol 2011;49:975–983. 43. Nagel R, Bielefeldt-Ohmann H, Traub R. Clinical pilot study: efficacy of triple antibiotic therapy in Blastocystis positive irritable bowel syndrome patients. Gut Pathog 2014;6:34. 44. Stensvold CR, Smith HV, Nagel R, et al. Eradication of Blastocystis carriage with antimicrobials: reality or delusion? J Clin Gastroenterol 2010;44:85–90. 45. Stensvold CR, Arendrup MC, Nielsen HV, et al. Symptomatic infection with Blastocystis sp. subtype 8 successfully treated with trimethoprim-sulfamethoxazole. Ann Trop Med Parasitol 2008;102:271–274.
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260
260 Blastocystis Christen R. Stensvold
Blastocystis is a protist parasitizing the lower intestine of humans and a vast range of animals. Although Blastocystis has been implicated as a cause of various gastrointestinal tract symptoms, its role as a pathogen, mechanism of transmission, and life cycle have been subject to much debate.1–4 A clinical syndrome of self-limited intestinal symptoms has been described in several series of predominantly adult patients, with infection intensity possibly accounting for differences in clinical presentation.5 Extraintestinal manifestations, such as urticaria, also have been described.6,7 Blastocystis spp. from humans, other mammals, and birds are classified into subtypes, but because non−DNA-based laboratory methods do not enable distinction between subtypes, the organism should be reported as Blastocystis sp. not as Blastocystis hominis.8
ORGANISM Blastocystis is a protist belonging to the Stramenopiles.9 It has been isolated from a wide variety of animals and has been reported in human populations from most parts of the world.10 A total of 9 subtypes (arguably species) have been identified in humans to date, with humans colonized most commonly by subtypes 1, 2, 3, and 4, depending on geographic region.8,10,11 Several morphologic stages have been described, including the vacuolar, avacuolar, multivacuolar, granular, ameboid, and cystic stages. The vacuolar form is observed most frequently in clinical samples and in fecal cultures; it is spherical and usually ranges in size from 4 to 15 µm in diameter12; much larger cells may be seen in cultures. The cyst (2–5 µm) is the transmissible form and is able to survive in water at room temperature for up to 19 days but is fragile at extremes of heat and cold and when exposed to common disinfectants.13–16 After ingestion of an infectious cyst, the organism excysts in the lumen of the cecum and proximal colon, developing into vacuolar forms and undergoing encystation before being excreted as the cyst form, which, if eaten, completes the cycle.
EPIDEMIOLOGY Blastocystis has a global distribution. Surveys of intestinal parasites in population groups in developing countries documented prevalence rates
of 54% in Papua New Guinea and 33% among travelers and foreign residents with diarrhea in Nepal.17,18 In a study of fecal specimens from 48 states and the District of Columbia, Blastocystis was identified in 23% of 5792 stool specimens.19 There are few reports of Blastocystis infection in children. A study surveying 1042 internationally adopted children submitting at least one stool specimen for ova and parasite testing within 120 days after arrival to the United States saw a prevalence of Blastocystis of 10%.20 A community-based survey northeast of Mexico City reported a frequency of 7% in children younger than 14 years.21 A study of the prevalence of enteroparasites in a facility for orphaned and homeless children in Argentina found Blastocystis in 44% of stool samples.22 A similar study of children living in residential institutions and street communities in metropolitan Manila, Philippines found Blastocystis in 41% of children examined.23 A community-based study in Papua New Guinea found that >65% of children younger than 18 years had Blastocystis.17 Meanwhile, the use of DNA-based diagnostic methods recently led to identification of a 100% prevalence in Senegalese children with gastrointestinal symptoms.24 Infection has been associated with recent travel to the tropics and consumption of untreated water.25,26 Blastocystis has been detected in cases of nosocomial diarrhea and in children attending childcare centers.27,28 Sexually active homosexual men have Blastocystis infection rates as high as 52%, a finding consistent with person-to-person transmission.29 Limited data suggest that illness due to Blastocystis can be more severe in patients with AIDS and other immunocompromising disorders.30,31 Working with animals may increase the risk for becoming infected with Blastocystis.32
CLINICAL MANIFESTATIONS Whether Blastocystis causes disease is controversial, and potential mechanisms of pathogenicity is unclear. Symptoms described in Blastocystispositive individuals include abdominal pain, diarrhea, nausea, anorexia, bloating, and flatulence. Means to differentiate infection from colonization do not exist.1 Differences in clinical presentation might be due to one or more of the following factors: subtype virulence factors, immune
1305
status of the host, host genotype, parasite load, host response, and dysbiosis.1 Nevertheless, the presence of this organism in stool suggests that the host was exposed to fecally contaminated food or drink. Blastocystis does not appear to be more common in patients with inflammatory or functional bowel diseases than in healthy individuals, although conflicting data exist.33–35 Attempts to infect germ-free guinea pigs did not provide convincing evidence of pathogenicity, except when large numbers of organisms were inoculated with normal enteric flora, with gross pathologic effects being limited to mild intestinal hyperemia.36 Microscopic examination revealed a slight increase in cellularity and intraepithelial penetration by Blastocystis. A case-control study examining the association between acquisition of Blastocystis and presence of mucosal lesions by upper gastrointestinal endoscopy and sigmoidoscopy failed to demonstrate intestinal pathology.37 Studies on Blastocystis in a gut microbiota context are emerging, and it is possible that such studies hold the key to unraveling the clinical significance of Blastocystis.38
DIAGNOSIS Diagnosis most often is made by staining of a fixed fecal specimen with iron hematoxylin or trichrome stain (Fig. 260.1), which is superior to iodine staining of wet mounts and microscopy of fecal concentrates.39,40 Recognition of the organism can be difficult because of large variation in size and form. Blastocystis can be grown readily in culture using Jones medium, with relatively high diagnostic sensitivity.39,41 Polymerase chain reaction testing is both highly sensitive and specific, but is used primarily for research.41,42 Studies using serology are scarce, and it is not known whether serology could be a useful tool to distinguish infection from colonization.
TREATMENT AND PREVENTION Detection of the organism in feces should not lead automatically to administration of antimicrobial therapy because clinical significance is controversial and carriage is common; most children in whom Blastocystis is detected should not be treated. Therapy should be considered in cases of persistent intestinal symptoms and when no other cause is found. Metronidazole alone, which often is used for Blastocystis eradication, appears to have very limited effect, whereas other drugs, such as paromomycin, nitazoxanide, and so-called triple therapy (diloxanide furoate, trimethoprim-sulfamethoxazole, and secnidazole) might be somewhat efficacious.43–45
FIGURE 260.1 Vacuolar forms of Blastocystis in a fixed fecal smear stained with trichrome stain (Courtesy of DPDx image library, Centers for Disease Control and Prevention).
Prevention of Blastocystis infection involves adequate sanitation, careful handwashing, avoidance of untreated water, and use of enteric precautions to interrupt person-to-person spread. All references are available online at www.expertconsult.com.
KEY REFERENCES 1. Scanlan PD, Stensvold CR. Blastocystis: getting to grips with our guileful guest. Trends Parasitol 2013;29:523–529. 2. Clark CG, van der Giezen M, Alfellani MA, Stensvold CR. Recent Developments in Blastocystis Research. In: Rollinson D (ed) Advances in Parasitology, Vol. 82. San Diego, Elsevier Academic Press Inc., 2013, pp 1–32. 3. Poirier P, Wawrzyniak I, Vivarès CP, et al. New insights into Blastocystis spp.: A Potential Link with Irritable Bowel Syndrome. PLoS Pathog 2012;8:e1002545. 4. Tan KS, Mirza H, Teo JD, et al. Current views on the clinical relevance of Blastocystis spp. Curr Infect Dis Rep 2010;12:28–35. 11. Alfellani MA, Stensvold CR, Vidal-Lapiedra A, et al. Variable geographic distribution of Blastocystis subtypes and its potential implications. Acta Trop 2013;126:11–18. 12. Tan KS. New insights on classification, identification, and clinical relevance of Blastocystis spp. Clin Microbiol Rev 2008;21:639–665. 44. Stensvold CR, Smith HV, Nagel R, et al. Eradication of Blastocystis carriage with antimicrobials: reality or delusion? J Clin Gastroenterol 2010;44:85–90.
Blastocystis
REFERENCES 1. Scanlan PD, Stensvold CR. Blastocystis: getting to grips with our guileful guest. Trends Parasitol 2013;29:523–529. 2. Clark CG, van der Giezen M, Alfellani MA, Stensvold CR. Recent Developments in Blastocystis Research. In: Rollinson D (ed) Advances in Parasitology, Vol. 82. San Diego, Elsevier Academic Press Inc., 2013, pp 1–32. 3. Poirier P, Wawrzyniak I, Vivarès CP, et al. New insights into Blastocystis spp.: A Potential Link with Irritable Bowel Syndrome. PLoS Pathog 2012;8:e1002545. 4. Tan KS, Mirza H, Teo JD, et al. Current views on the clinical relevance of Blastocystis spp. Curr Infect Dis Rep 2010;12:28–35. 5. El Shazly AM, Awad SE, Sultan DM, et al. Intestinal parasites in Dakahlia governorate, with different techniques in diagnosing protozoa. J Egypt Soc Parasitol 2006;36:1023–1034. 6. Vogelberg C, Stensvold CR, Monecke S, et al. Blastocystis sp. subtype 2 detection during recurrence of gastrointestinal and urticarial symptoms. Parasitol Int 2010;59:469–471. 7. Zuel-Fakkar NM, Abdel Hameed DM, Hassanin OM. Study of Blastocystis hominis isolates in urticaria: a case-control study. Clin Exp Dermatol 2011;36:908–910. 8. Stensvold CR, Suresh GK, Tan KS, et al. Terminology for Blastocystis subtypes—a consensus. Trends Parasitol 2007;23:93–96. 9. Silberman JD, Sogin ML, Leipe DD, Clark CG. Human parasite finds taxonomic home. Nature 1996;380:398. 10. Alfellani MA, Taner-Mulla D, Jacob AS, et al. Genetic diversity of Blastocystis in livestock and zoo animals. Protist 2013;164:497–509. 11. Alfellani MA, Stensvold CR, Vidal-Lapiedra A, et al. Variable geographic distribution of Blastocystis subtypes and its potential implications. Acta Trop 2013;126:11–18. 12. Tan KS. New insights on classification, identification, and clinical relevance of Blastocystis spp. Clin Microbiol Rev 2008;21:639–665. 13. Rene BA, Stensvold CR, Badsberg JH, Nielsen HV. Subtype analysis of Blastocystis isolates from Blastocystis cyst excreting patients. Am J Trop Med Hyg 2009;80:588–592. 14. Suresh K, Smith HV, Tan TC. Viable Blastocystis cysts in Scottish and Malaysian sewage samples. Appl Environ Microbiol 2005;71:5619–5620. 15. Chen XQ, Singh M, Howe J, et al. In vitro encystation and excystation of Blastocystis ratti. Parasitology 1999;118(Pt 2):151–160. 16. 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;82:439–444. 17. Ashford RW, Atkinson EA. Epidemiology of Blastocystis hominis infection in Papua New Guinea: age-prevalence and associations with other parasites. Ann Trop Med Parasitol 1992;86:129–136. 18. Taylor DN, Houston R, Shlim DR, et al. Etiology of diarrhea among travelers and foreign residents in Nepal. JAMA 1988;260:1245–1248. 19. Amin OM. Seasonal prevalence of intestinal parasites in the United States during 2000. Am J Trop Med Hyg 2002;66:799–803. 20. Staat MA, Rice M, Donauer S, et al. Intestinal parasite screening in internationally adopted children: importance of multiple stool specimens. Pediatrics 2011;128:e613–e622. 21. Diaz E, Mondragon J, Ramirez E, Bernal R. Epidemiology and control of intestinal parasites with nitazoxanide in children in Mexico. Am J Trop Med Hyg 2003;68:384–385. 22. Guignard S, Arienti H, Freyre L, et al. Prevalence of enteroparasites in a residence for children in the Córdoba Province, Argentina. Eur J Epidemiol 2000;16:287–293. 23. Baldo ET, Belizario VY, De Leon WU, et al. Infection status of intestinal parasites in children living in residential institutions in Metro Manila, the Philippines. Korean J Parasitol 2004;42:67–70. 24. 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.
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