Zoonotic hepatitis E: animal reservoirs and emerging risks

Abstract

Hepatitis E virus (HEV) is responsible for enterically-transmitted acute hepatitis in humans with two distinct epidemiological patterns. In endemic regions, large waterborne epidemics with thousands of people affected have been observed, and, in contrast, in non-endemic regions, sporadic cases have been described. Although contaminated water has been well documented as the source of infection in endemic regions, the modes of transmission in non-endemic regions are much less known. HEV is a single-strand, positive-sense RNA virus which is classified in the Hepeviridae family with at least four known main genotypes (1-4) of mammalian HEV and one avian HEV. HEV is unique among the known hepatitis viruses, in which it has an animal reservoir. In contrast to humans, swine and other mammalian animal species infected by HEV generally remain asymptomatic, whereas chickens infected by avian HEV may develop a disease known as Hepatitis-Splenomegaly syndrome. HEV genotypes 1 and 2 are found exclusively in humans while genotypes 3 and 4 are found both in humans and other mammals. Several lines of evidence indicate that, in some cases involving HEV genotypes 3 and 4, animal to human transmissions occur. Furthermore, individuals with direct contact with animals are at higher risk of HEV infection. Cross-species infections with HEV genotypes 3 and 4 have been demonstrated experimentally. However, not all sources of human infections have been identified thus far and in many cases, the origin of HEV infection in humans remains unknown.

Figure 1.

Phylogenetic tree representing 52 sequences of full length HEV genome retrieved from GenBank (accession numbers are indicated). Alignment was performed using ClustalW in MEGA 3.1. and the tree was built using the neighbor joining method with 1 000 bootstrap replicates. (A color version of this figure is available at www.vetres.org.)

Figure 2.

Approximation of the natural time course of HEV infection in pigs. Representation of the kinetics of seroconversion: maternal anti-HEV antibodies (dark blue), IgM anti-HEV (green), IgG anti-HEV (light blue) and fecal excretion of HEV (red) in pigs infected naturally. Adapted from the results presented in Section 4.1: prevalence of HEV in pigs). (A color version of this figure is available at www.vetres.org.)