The Molecular Biology of Pestiviruses

Norbert Tautz*; Birke Andrea Tews†; Gregor Meyers†,1    * Institute for Virology and Cell Biology, University of Lübeck, Lübeck, Germany
† Institut für Immunologie, Friedrich-Loeffler-Institut, Federal Research Institute for Animal Health, Greifswald-Insel Riems, Germany
1 Corresponding author: email address: gregor.meyers@fli.bund.de

1 Introduction

The first description of a pestivirus-induced disease was a report on classical swine fever (CSF). It dates from 1833, long before viruses were recognized as pathogens of sub-bacterial size. Much later, reports on bovine viral diarrhea (BVD) and mucosal disease (MD) (1940s) as well as border disease of sheep (BD) (1950s) were published. The observation of serological cross-reactivity between the agents causing CSF and BVD initiated the idea of pestiviruses as a group of related viruses (Darbyshire, 1969). Nucleotide sequencing allowed consolidation of the classification resulting in the genus Pestivirus that nowadays comprises the four species classical swine fever virus (CSFV), bovine viral diarrhea virus types 1 and 2 (BVDV-1 and BVDV-2), and border disease virus (BDV) of sheep as well as several more exotic viruses (Simmonds et al., 2012). More recently, further molecular characterization of new genus members has led to the proposal of a classification with eight pestivirus species which is under discussion by the responsible ITCV study group.

Initially, a significant lack of data on the molecular characteristics of pestiviruses led to a wrong classification into the family Togaviridae. However, the determination of the first genomic sequences and the assignment of viral proteins to defined coding regions of the viral RNA revealed fundamental differences between togaviruses and pestiviruses with regard to genome organization and strategy of gene expression so that the genus Pestivirus was moved into the family Flaviviridae, whose members share basic features like formation of small enveloped virus particles, a single-stranded RNA genome of positive polarity with a similar organization of the coding regions, and the strategy of gene expression via one long open reading frame (Lindenbach, Murray, Thiel, & Rice, 2013). These basic features of the pestiviral genome together with a diagram of the virus particle and electron micrographs are shown in Fig. 1.

Apart from the above-described similarities, many features of the Flaviviridae family members are quite diverse and reflect their adaptation to different propagation strategies and hosts. However, the molecular biology of hepaciviruses, pegiviruses, and pestiviruses is strikingly similar so that pestiviruses have been widely used as surrogate system for human HCV, a virus that could not be propagated in tissue culture until 2005 (Lindenbach et al., 2005; Wakita et al., 2005; Zhong et al., 2005).

Progress in pestivirus analysis was quite slow during the early years soon after recognition of pestiviruses as animal pathogens. This was due to the absence of cell culture systems allowing high titer propagation and difficulties to establish efficient purification protocols for pestiviruses. Moreover, pestivirus particles do not exhibit a very characteristic morphology and their size of 40–60 nm is in the range of many enveloped cellular structures, rendering electron microscopic studies of pestiviruses and pestivirus-infected cells difficult. Most of the problems in pestivirus research are inherent to these viruses. Modern molecular biology has fueled detailed investigation of many aspects of pestivirus biology, but working with these viruses is still demanding.

Pestiviruses are among the economically most important pathogens of livestock (Lindenbach et al., 2013; Thiel, Plagemann, & Moennig, 1996). Stamping-out strategies, vaccination, as well as strict biosafety measures have been employed for control of these pathogens but, despite considerable efforts, pestiviruses continue to cause severe losses in livestock farming and the risk of reintroduction represents a constant threat for countries having managed to eradicate a given pestivirus. The difficulties to control pestiviruses are not only due to the high frequency of transport and trade of animals susceptible to pestivirus infection but also originate from specific survival strategies these pathogens gained during evolution. Among them, the most important factor is their ability to establish long-lasting persistent infection in their host animals (Thiel et al., 1996). Small numbers of virus carriers are sufficient to ensure continuous virus spread since they live for considerable periods of time while shedding high amounts of virus. Intensive research on the molecular biology of these interesting pathogens during the last three decades has elucidated many of their basic features providing a key to understanding the molecular basis of pestivirus survival strategies. This review summarizes recent data with the aim to provide a look into the fascinating world of pestiviruses and their interplay with their host's biology. There will be a clear focus on the molecular biology of these viruses restricting description of pathogenesis and epidemiology of pestivirus-induced diseases and the details of exotic pestivirus classification to a necessary minimum. For further information, the reader is referred to other excellent reports (Baker, 1987; Bauermann, Ridpath, Weiblen, & Flores, 2013; Brackenbury, Carr, & Charleston, 2003; Chase, 2013; Hamers et al., 2001; Lindberg, 2003; Luo, Li, Sun, & Qiu, 2014; Moennig, 2000; Moennig, Floegel-Niesmann, & Greiser-Wilke, 2003; Moennig, Houe, & Lindberg, 2005; Moennig & Plagemann, 1992; Passler & Walz, 2010; Ridpath, 2003, 2010; Simmonds et al., 2012; Tao et al., 2013; Thiel et al., 1996; Vilcek & Nettleton, 2006).

2 Taxonomy, Hosts, and Diseases

2.1 Taxonomy: Approved and Tentative Species

The genus Pestivirus belongs to the virus family Flaviviridae that also comprises the genera Flavivirus, Hepacivirus, and Pegivirus (Simmonds et al., 2012). In addition to the typical members of the four recognized species, CSFV, BVDV-1, BVDV-2, and BDV, a considerable number of novel pestiviruses have been isolated from different animal species (Arnal et al., 2004; Becher, Orlich, Kosmidou, et al., 1999; Becher, Schmeiser, Oguzoglu, & Postel, 2012; Becher et al., 1997, 2003; Kirkland et al., 2007; Schirrmeier, Strebelow, Depner, Hoffmann, & Beer, 2004; Stahl et al., 2007; Stalder et al., 2005; Thabti et al., 2005; Vilcek, Ridpath, Van Campen, Cavender, & Warg, 2005). These viruses have been proposed to represent additional pestivirus species but have not been approved as such yet (Fig. 2). The Ninth report of the International Committee on Taxonomy of Viruses (ICTV) lists four tentative species, namely Giraffe-1 pestivirus, Pronghorn antelope pestivirus, Atypical pestivirus, and Bungowannah virus. Among them, the Giraffe-1...