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IntroductionBlastocystis is a protozoan which infectshumans and
some other animals.Knowledge of the life cycle is incomplete.Spread
between humans is most likelyfaecal-oral.
Some reports suggest that there is a corre-lation between the
number of organismidentified in smears and the occurrence of
symptoms, with heavy infections linked todiarrhoea and abdominal
discomfort.No definition of heavy infection, however,is currently
available. The problem with thishypothesis is the variable shedding
of theorganisms and while in oneperiod, there may be low numbers
identi-fied, a week later there may be highnumbers.
Blastocystis hominis should be consideredto be pathogenic if the
patient has persist-ent symptoms for more than three daysand it is
the only organism detected inrepeat faecal specimens, or if it is
the onlyorganism repeatedly identified from animmunocompromised
patient.
DiagnosisThe diagnosis is based on the identificationof the
cyst-like stage in faeces. Permanentstained smears made from fixed
specimensoffer greater sensitivity compared to wetmount
preparations.Like all testing for parasites, the greater thenumber
of collections, the greater the sen-sitivity of the testing. At
least three stoolcollections are recommended, preferablyon
different days. The stool specimensshould be placed immediately
into SAFfixative and kept at room temperature
until tested.
MorphologyThe organism appears as spherical-ovalshaped cyst like
structures. Size varieswidely but is usually 8-10um.
Typically,there is a central body or vacuolesurrounded by a thin
rim of cytoplasmcontaining up to six nuclei.
Referenceswww.cdfound.to.it/HTML/blas.htm
www.ksu.edu/parasitology/625tutorials/Blastocystis.html
www.vfair.com/conditions/blastocystis.htm
www.cdc.gov/epo/mmwr/preview/
mmwrhtml/00016167.htm
Sheorey, H, Walker, J, Biggs, B: ClinicalParasitology, A
Handbook for MedicalPractitioners and Microbiologists,Melbourne
University Press, 2000
K Haresh et al, Trop Med Int Health1999;4:274
Ok UZ et al, Am J Gastroenterol;94:3245
Treatment
Treatment should be considered in allcases, but may not be
warranted inasymptomatic patients.
Metronidazole is the first line of treatment,however resistant
organisms are nowpresent in Australia and these may bedifficult to
clear with treatment.Re-testing after treatment isrecommended.
Standard dosage of Metronidazole400-750mg of Metronidazole three
times a
day for 7-10 days
Alternative treatments include,Iodoquinol 650mg tds x 20d
(Haresh et al)Trimethoprim-sulfamethoxazole (Ok UZet al)
Furazolidine 8mg/kg/day (max400mgs/day) 7-10 daysCombination
treatments may be required.
Symptoms of infectionWhether or not Blastocystis causes
infec-tion in humans is a point of active debate.This is because of
the common occurrenceof the organism in both symptomatic
andasymptomatic patients. At least 20% ofpatients in which the
organism is found are
asymptomatic. When symptomatic, theusual spectrum of symptoms
includes
Watery DiarrhoeaAbdominal PainPerianal Pruritis
Excessive Flatulence
Histopath is an independent NATA accredited Practice providing
specialised Pathology testing
Phone 9764 4300 Fax 9764 [email protected]
Partners: Dr Chris Douglas, Dr Stephen Mann
Blastocystis hominis
