Phylogenetic approach to the variability of the microsporidian Enterocytozoon bieneusi and its implications for inter- and intrahost transmission

Abstract

Enterocytozoon bieneusi is a microsporidian parasite that infects many vertebrate animals, including humans. The rDNA internal transcribed spacer (ITS) shows a hypervariable sequence; however, so far no clear information has been inferred about strain evolution in this species. We reviewed all the sequences described and performed a phylogenetic study. Four groups of sequences strongly differentiated from each other were detected, although most of the isolates (94%) corresponded to group I. The highly diverse sequences of this group were analyzed using median-joining networks. The host species (humans, pets, swine, cattle, birds, and wild animals) and the continents of origin of the isolates were considered. Central haplotypes in the network were obtained from very diverse hosts and geographical origins. The results show that although E. bieneusi has a broad host specificity, transmission is not completely free: some strains were able to circulate within a given host species and were only occasionally transmitted to another host. Additionally, while not relevant for swine or cattle hosts, geography seems to be a relevant factor for human infection by E. bieneusi.

FIG. 1.

Maximum-likelihood tree showing the four main groups of sequences, identified by the corresponding GenBank accession numbers. Group I comprises 168 sequences, all of which are in the enclosed area (bottom-left). The ML probability, followed by a slash and the posterior probability, is given on each branch. The bar represents 0.1 substitution per site.

FIG. 2.

Median-joining network for the 84 haplotypes of the group I sequences. Circles are proportional to the frequency of each haplotype. The color of each sector represents the host species: humans (red), swine (yellow), cattle (orange), pets (green), birds (light blue), and wild animals (dark blue). Each step on the branches represents a single mutation.

FIG. 3.

Median-joining networks for group I haplotypes found in humans (a), swine (b), and cattle (c). Circles are proportional to the frequency of each haplotype. The colors refer to the continents where the isolates were found: Europe (blue), North America (dark brown), South America (orange), Africa (green), and Asia (yellow). The positions of the haplotypes on the graphs are approximately the same as in Fig. 2; for reference, hap-1, hap-11, and hap-13 are indicated. The lengths of the segments are proportional to the number of mutations involved between two haplotypes.