A Brazilian species of Entamoeba dispar (ADO) produces amoebic liver abscess in hamsters

In contrast to Entamoeba histolytica, which is the etiologic agent of human amoebiasis, Entamoeba dispar has been considered as a non-pathogenic species present in asymptomatic carriers. This latter species is currently classified as a different protozoan species of the human gut and unable to produce invasive disease.¹ The distinctions between both species of Entamoeba have been achieved through the combination of biochemical, immunological and genetic data. Although E. dispar was originally regarded as morphologically identical to E. histolytica, recent studies have shown the existence of differences between both species, including the absence of liver lesions produced by E. dispar.² However, all previous comparative studies have been performed using the strain SAW 760 RR of E. dispar isolated in England and very little is known regarding other strains.

In this study, we tested another sample of Entamoeba of human origin, isolated in Minas Gerais, Brazil. The trophozoites of this genus maintained in monoxenic (Figure 1) and polyxenic cultures (associated with Escherichia coli) (Figure 2), showed a Type I (nonpathogenic, according to Sargeunt) isoenzymatic pattern, and was classified in fact as E. dispar species (ADO strain) by using immunological and molecular biology techniques. Surprisingly, when an in vivo test was performed by inoculating the trophozoites of ADO polyxenic cultures into the hamster liver, animals showed at the 7^th day, the presence of large (amoebic type) liver abscesses (Figure 3), which were later confirmed as typical amoebic lesions by light microscopy analysis (Figure 4). Control animals inoculated only with the associated E. coli did not show any liver damage.

Figure 1.

Trophozoites of Entamoeba dispar strain ADO in monoxenic culture. Cells show great pleomorphism with abundant small vacuoles. Nomarski optics. Bar = 10 μm.

Figure 2.

Scanning electron micrograph of trophozoites of E. dispar strain ADO in a polyxenic culture. Elongated amoebae show a rough surface with abundant associated Escherichia coli. Bar =10 μm.

Figure 3.

Hamster inoculated intrahepatically with 2.5 × 10⁵ trophozoites of E. dispar strain ADO. An opened abdomen shows large whitish lesions (arrow) in the left lobe of the liver.

Figure 4.

Light microscopy of lesions produced in hamster liver by trophozoites of E. dispar ADO strain. Necrotic areas (upper right) are associated to multiple trophozoites (arrows). Some inflammatory reactions are seen in the left and lower sides of the picture. H&E stain. x40.

Our data suggest that, while strain SAW 760 RR of E. dispar is practically innocuous to some target cells,² other E. dispar strains may have different behavior and pathogenic capacity. Here we are reporting for the first time an E. dispar strain (ADO), which was capable to produce large liver abscess. Even though the bacteria E. coli was incapable to produce liver damage by itself, their association with E. dispar constitutes important factor to be considered for further studies related to pathogenesis of this protozoan.