Molecular epidemiology of the microsporidian Enterocytozoon bieneusi in food animals

Microsporidia are newly emerging pathogens of humans and animals. The species found most often in human infections is Enterocytozoon bieneusi, first discovered in 1985. It has since been detected in a number of food animals, including pigs and cattle. Most recently, we have discovered E. bieneusi in non-mammalian hosts, broiler chickens, too. However, the potential reservoirs and the mode of transmission of this pathogen are controversially discussed. Traditional epidemiological studies to address the zoonotic potential of this pathogen, for example case control studies to identify risk factors such as contact to certain animals, are hampered by the small number of diagnosed microsporidial infections. Experimental infections of humans are prohibited for ethical reasons. As an alternative, this problem might be solved by the differentiation of strains within this species and a comparison of the strains found in humans with those detected in animals. Unfortunately, because the spores of E.bieneusi strains are morphologically indistinguishable since this species cannot be cultured, traditional morphological, biochemical or immunological methods are unavailable for strain differentiation. Instead, we developed a genotypic method to differentiate characteristic genotypes of the internal transcribed spacer (ITS) of the ribosomal RNA gene (rDNA). To date, 20 genotypes can be differentiated. Phylogenetic analyses using maximum likelihood, distance matrix (neighbor joining) and maximum parsimony algorithms all failed to demonstrate the existence of monophyletic groups consisting only of E. bieneusi genotypes from humans. Although this lack of genetic evidence for a transmission barrier between humans and the investigated animal hosts does not preclude that this parasite may also be transmitted from person to person, possibly even as the prevalent mode, it can not longer be ruled out that infections of humans may be acquired from food animals.