Pathogenicity of Blastocystis hominis

The letter of J. E. Rosenblatt (6) refutes some published work affirming Blastocystis hominis to be an important agent of intestinal disease in humans. The superb and well-controlled study by Kain et al. is one example of a plethora of sound studies (2). The proportion of affirmative literature on B. hominis pathogenicity to the "anti" literature is approximately 20:1. The two most quoted "anti" articles are basically flawed. A complete bibliography can be found in an upcoming review by me (11). Unfortunately, the scant criticism in the literature of work on the pathogenicity of this protozoan is based on speculation rather than credible data. I submit that ". speculation concerning its pathogenicity," as stated by Dr. Rosenblatt, might more accurately be stated ". speculation concerning its nonpathogenicity. " Dr. Rosenblatt states: "Some patients with diarrhea have the organism in their stool and some do not; some asymptomatic patients have it in their stool and some do not. " What a beautiful description of the presence of Entamoeba histolytica! In response to Dr. Rosenblatt's numbered critiques: (i) "Koch's postulates have never been satisfied. " To invoke Koch's postulates in this area is unnecessary. At least a dozen articles describing B. hominis epidemics in soldiers and institutions and familial outbreaks, in which transmission followed by symptomatic infection is described, are available. (ii) "No pathogenic evidence of or immunologic response to 'infection' has been demonstrated in humans. There are many descriptions of gastrointestinal pathology (1-3, 7-10). Immunological aspects are essentially unstudied. Eosinocytosis is frequent. (iii) "No mechanisms of pathogenicity, such as toxin elaboration, attachment to intestinal mucosa, or invasiveness, in humans have been described. " Here, again, purified-toxin studies have not been reported to date. Employing the isolated ileal segment technique in the rabbit, injection of purified fractions from B. hominis cultures elicits strongly positive fluid response (12). Attachment to or invasion of intestinal mucosa is not a requirement for pathogenicity (as, for example, with Cryptosporidium parvum and Giardia lamblia). However, mucosal invasion in B. hominis infections in gnotobiotic guinea pigs has been recorded (5). As for the requirement for bacterial cohorts for pathogenesis, this is true also of E. histolytica, and this has been in the literature for 35 years (4). (iv) Dr. Rosenblatt states that "no antimicrobial agent which is uniquely active against B. hominis has been shown to reliably eradicate both the organism and the diarrhea. Emetine is very effective against blastocystosis but must be used in the hospital because of possible toxicity. The arsenicals Stovarsol (acetarsone) and Narsonal are also very effective and are approved for use. References to use of the various drugs are too numerous to list here. In recent years, metronidazole has been used most and is usually effective, although there is indication of resistant strains of B. hominis. Trimethoprim-sulfamethoxazole is effective, and Floraquin (diiodohydroxyquinoline) is moderately effective. (v) "There has never been a point-source outbreak of gastroenteritis in which epidemiologic evidence suggested that B. hominis was the cause. " Dr. Rosenblatt seems to believe that all intestinal protozoa must fit the mold of some archetypical pathogen. Unfortunately, there is no such model against which all others are measured. Diversity is the hallmark of pathogenic protozoa. But there will probably be "point-source" outbreaks reported as more studies are done. The many military and institutional outbreaks reported had a source, even though it was too convoluted to discern. One might comment that the Comte de Buffon and John Needham have been dead for over 200 years, and Felix Pouchet for over 100 years, but their spirit lives on.