Effect of trimethoprim-sulfamethaxazole in Blastocystis hominis infection

Blastocystis hominis (B. hominis) is a common intestinal parasite that has long been considered nonpathogenic. Recently there have been many reports supporting a role for the organism as a potential pathogen. We performed a study to examine the pathogenicity of B. hominis and the effect of trimethoprim-sulfamethaxazole (TMP-SMX) on this organism.

Stool samples of patients, who came to the Department of Parasitology, Faculty of Medicine, Celal Bayar University, were examined by direct wet-mount, trichrome staining, formalin-ethyl acetate concentration, and Kinyoun acid fast techniques for intestinal parasites, and bacteriological stool cultures were performed. Fifty-three symptomatic patients (38 children and 15 adults) with two consequent stool samples positive for abundant B. hominis (five or more organisms per x400 field) and negative for other parasitic and bacterial pathogens were treated with TMP-SMX for 7 days, children 6 mg/kg TMP, 30 mg/kg SMX, and adults 320 mg TMP, 1600 mg SMX, daily. On the seventh day, at the end of treatment, stool samples of all patients were examined by same methods, and clinical symptoms were again evaluated.

B. hominis was eradicated in 36 of 38 (94.7%) children, and 14 of 15 (93.3%) adults. Clinical symptoms disappeared in 39 (73.6%), decreased in 10 (18.9%), and no change was observed in one (1.9%) patient, whereas symptoms persisted in all three (5.7%) patients in whom B. hominis could not be eradicated. Mean number of stools per day was significantly decreased from 4.3 to 1.2 in the 33 children (p < 0.001), and decreased from 3.5 to 1.0 in the four adults (p = 0.06) with diarrhea.

These results suggested that B. hominis may be pathogenic, especially when it is present in large numbers, and TMP-SMX is highly effective against this organism. Although there are some anecdotal reports, to our knowledge this is the first study examining the effect of TMP-SMX on B. hominis in humans.