Hepatitis E Virus

First indications of hepatitis E as a distinct disease, which differs from hepatitis A, were found in 1980 when sensitive and specific detection systems for hepatitis A virus (HAV) first became available and hepatitis epidemics were examined in India. Epidemiologic studies up to that time pointed to the fact that these hepatitis infections with clinically very similar course were caused by HAV pathogens transmitted by the faecal-oral route. The experimental proof that a second virus transmitted by the faecal-oral route, which was later called hepatitis E virus (HEV), was provided in 1983 by the transmission studies of Balayan et al. [1]. In these experiments, HAVimmune voluntary individuals were infected with stool suspensions from patients with a hepatitis A-like disease which occurred in Tashkent, Uzbekistan. Spherical virus-like particles, 27–30 nm in diameter, could be isolated from the stools of the experimentally infected persons, which formed a band in the CsCl gradient at a density of 1.35 g/cm3. These particles could be detected by immuno-electron microscopy (IEM) both in the pre-clinical (as from the 27th day post infection (dpi)) and in the post-clinical phase (45 dpi, onset of the disease approximately 36 dpi) (fig. 1). Up to 1990, IEM and the inoculation of monkeys were the only diagnostic methods for the detection of HEV infections and their differentiation from HAV. In 1991, Reyes et al. [2] succeeded in cloning the genome. This opened new paths for the development of diagnostic methods for serological and molecular detection of HEV infections. HEV was first categorised as part of the Caliciviridae family, based on its morphological properties. Sequence analysis, however, revealed that the genome structure of HEV is significantly distinct from this virus family. Therefore, HEV is today classified as the so far only representative of the hepevirus genus in the new family of Hepeviridae. HEV is a small non-enveloped, single-stranded icosaedric virus with a diameter of approximately 32–34 nm. The capsid of the virus probably consists of one single protein. The genome is single-stranded positive-sense with a size of 7.2 kbp. It is flanked by non-encoding regions, polyadenylated at the 3' end, and bears a m7G-cap at the 5' end. The genome itself encodes for three overlapping open reading frames (ORF 1–3). Hepeviruses have the feature that the ORF 2 with a length of 2 kbp located at the 3' end encodes for the capsid protein. It is especially this characteristic that makes the HEV virus distinct from the calicivirus, in which the sequence encoding for the capsid is located at the 5' end. ORF 1 with a length of approximately 5 kbp is located at the 5' end and encodes for the nonstructural proteins involved in RNA replication: RNA-dependent RNA polymerase, guanylyl and methyl transferases, helicase and a papain-like protease. ORF 3 with a length of 372 bp overlaps at the 3' end with the first 331 bp from ORF 2 and at the 5' end with ORF 1. ORF 3 encodes for a small immunogeneic phosphoprotein with a size of not more than 123 amino acids. Based on phylogenetic analyses, the isolates examined up to now are classified into four different genotypes which have different world-wide distributions [3]. For genotype 1 the Burma isolate represents the prototype, for genotype 2 the Mexican isolate, for genotype 3 the USA isolate, and for genotype 4 the Chinese isolate. The different genotypes are then further grouped into different genetic subtypes. Serologically, HEV seems to behave in a uniform manner, as neutralisation studies in the cell culture and protection studies with different genotypes in infection studies have shown [4]. Various studies provide evidence that HEV replicates in hepatocyte culture of humans and macaques. For this purpose, cell cultures were infected with HEV from stool suspensions. In such in vitro studies, Emerson et al. [5] were able to show that approximately half the HEV infectiveness was inactivated after heat treatment at 45–50 °C for 1 h and almost the entire HEV infectiveness at 56 °C. In addition, the authors were able