Viruses are thought to spread across susceptible cells through an iterative process of infection, replication, and release, so that the rate of spread is limited by replication kinetics. Here, we show that vaccinia virus spreads across one cell every 75 minutes, fourfold faster than its replication cycle would permit. To explain this phenomenon, we found that newly infected cells express two surface proteins that mark cells as infected and, via exploitation of cellular machinery, induce the repulsion of superinfecting virions away toward uninfected cells. Mechanistically, early expression of proteins A33 and A36 was critical for virion repulsion and rapid spread, and cells expressing these proteins repelled exogenous virions rapidly. Additional spreading mechanisms may exist for other viruses that also spread faster than predicted by replication kinetics.Mechanisms enhancing the cell-to-cell spread of intracellular pathogens are important for virulence and are targets for development of antimicrobial therapeutics. Vaccinia virus (VACV) is a poxvirus and is the live vaccine used to eradicate smallpox (1). VACV replication is unusual in that it produces both single-and double-enveloped virions (2, 3). The single-enveloped virions, called intracellular mature virus (IMV), remain intracellular until cell lysis and spread slowly from cell to cell. In contrast, the double-enveloped virions, called cell-associated enveloped virus (CEV) and extracellular enveloped virus (EEV), are released rapidly and mediate efficient cell-to-cell spread and long-range dissemination (3, 4). VACV spreading mechanisms include virus-induced cell motility (5) and the formation of actin projections (6-8) that propel VACV particles toward other cells late during infection (9). However, we wondered whether either mechanism could explain how VACV Western Reserve (WR) spreads rapidly to form a plaque of diameter 2.90 ± 0.07 mm (SEM, nine experiments, n = 11 to 12 plaques) in 3 days (Fig. 1A). The distance between nuclei of adjacent BSC-1 cells was 37.26 ± 1.02 μm (SEM, n = 25 single cells and the 5 to 8 cells in Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts contact with it), so that VACV was spreading across each cell in <2 hours. To study this further, live video microscopy was used to measure the spread of VACV-induced cytopathic effect after infection with VACV WR (Fig. 1, B and C) or VACV expressing enhanced green fluorescent protein (EGFP) fused to core protein A5 that is expressed late during infection (vEGFPA5L) (Fig. 1, D to F, and movies S1 to S5) (10, 11). A linear increase in plaque size with time was observed (Fig. 1G), and the mean rate of spread was 32.36 ± 0.74 μm/hour (SEM, n = 9 plaques) for VACV WR. Knowing the distance between nuclei of adjacent cells, this indicated that VACV crossed one cell every 1.2 hours. This rate of spread is inconsistent with VACV replication kinetics, in which new virions are formed only 5 to 6 hours after infection (12), or virus-induced cell motility, in which ...

During the past ten years, several new hepatitis E viruses (HEVs) have been identified in various animal species. In parallel, the number of reports of autochthonous hepatitis E in Western countries has increased as well, raising the question of what role these possible animal reservoirs play in human infections. The aim of this review is to present the recent discoveries of animal HEVs and their classification within the Hepeviridae family, their zoonotic and species barrier crossing potential, and possible use as models to study hepatitis E pathogenesis. Lastly, this review describes the transmission pathways identified from animal sources.