New pathogens continue to be discovered as the cause of travelers'
diarrhea, and a few case reports have demonstrated that microsporidia
may be the cause of travelers' diarrhea (1, 17, 19, 21).
Human microsporidiosis has been reported predominantly from developed
nations in North America, Europe, and Australia, but cases are now
increasingly identified in the developing countries as well, and human
infections with microsporidia have been reported from several African
nations (2, 4, 6, 7, 11, 12, 14, 22), Southeast Asia
(15), and Central and South America (3, 8, 24,
25). Although reliable estimates of the prevalence of
microsporidiosis in developing countries are not available, some
studies seem to indicate that the prevalence of intestinal microsporidiosis may be high in developing countries (10,
22). Most cases of human microsporidiosis are associated with
immunosuppression, but increasing numbers of cases in non-HIV-infected
immunocompetent patients are reported. These reports include infections
of travelers to developing countries as well as infections of residents
of various tropical countries (1, 5, 6, 11, 13, 14, 17, 19-21,
23). Only a few studies examined the role of microsporidia in
returning travelers. One study examined stool samples of 750 Swedish
travelers with and without diarrhea by light microscopy and identified
only one case of intestinal microsporidiosis due to E. bieneusi (21). Another study examined 40 European
travelers from the tropics with a clinical picture of protracted
diarrhea and identified four cases of imported E. bieneusi
infection: one HIV-infected short-term traveler, a pregnant long-term
traveler, and two immunocompetent short-term travelers. Diarrhea was
self-limited, and the spores cleared from the stools in all
non-HIV-infected travelers but showed a chronic course in the
HIV-infected one. Infections were diagnosed by light microscopy with
confirmation by PCR (13). Spores of microsporidia were
detected by light microscopy in the stools of four French travelers
presenting clinically with chronic diarrhea. Molecular identification
of microsporidian species was based on the PCR amplification of an SSU
rRNA sequence followed by HinfI endonuclease restriction.
E. intestinalis microsporidiosis was thus shown in two of
the four patients examined (17). These three studies
employed only light microscopy for detection of microsporidia in
stools, and molecular-based techniques, such as PCR, were used only for
confirmation and/or species differentiation. None of these studies used
PCR for the examination of all stool samples. In our study, as well as
in a previous study of members of our group (16), some of
the cases were diagnosed only by PCR amplification. In a study that
evaluated the prevalence of intestinal parasites in patients with
diarrhea and AIDS in Zimbabwe, microsporidia were found by light
microscopy in 10 out of 55 (18%), whereas PCR detected microsporidia
in 28 out of 55 patients (51%) (10). Probably the limited
sensitivity of light microscopy is responsible for these results. The
detection limit of light microscopy has been determined to be between
104 and 106 microsporidian spores per g of
stool, whereas PCR is able to detect spore concentrations as low as
102 per g of stool (16, 18). Thus the true
prevalence of microsporidia has to be determined by highly sensitive
techniques, such as PCR. However, diagnostic approaches regarding
diarrhea are usually done late in the course of the disease, at a time
when spore excretion in the stool may have decreased. In all of our
cases except one, only low numbers of spores were seen by light
microscopy. Interestingly, the two patients in our study with confirmed
double infection due to E. bieneusi and E. hellem
were a traveler and his daughter who returned from Singapore. In stool
samples of the mother who accompanied the two patients to Singapore, no
microsporidia were detected by light microscopy or by PCR. To the best
of our knowledge this is the first report of E. hellem in
stool samples and the first report of E. hellem in
non-HIV-infected patients. Our study shows that intestinal
microsporidiosis seems to be an underappreciated cause of
travelers' diarrhea. New interest in these organisms by clinicians
caring for travelers returning from tropical areas and improved
diagnostic approaches should now help to increase our knowledge
regarding the role of microsporidia in travelers' diarrhea.
This work was supported by the Köln Fortune program from the
Faculty of Medicine, University of Cologne.
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