Argentine strain of equine herpesvirus 1 isolated from an aborted foetus shows low virulence in mouse respiratory and abortion models

Abstract

The equine herpesvirus 1 (EHV-1) was isolated in Argentina from an aborted equine foetus in 1979. This virus (SPv) has special restriction patterns (RP) in comparison with other Argentine isolates. In addition, SPv could be distinguished on the basis of its pathogenicity in baby mice inoculated intracerebrally. We studied the growth properties of the SPv in cell culture and its effects in a mouse respiratory and abortion model. We observed that SPv did not modify its capacity to grow in cell culture with respect to reference HH1 strain. Nevertheless, we found significant differences between the titres of the two strains at 8–14 h post-infection (PI). In this work we demonstrated that SPv showed low virulence in female at different stages of gestation, consistently, with results found in the mouse respiratory model. We considered that this low virulence of SPv could be related to its RP because the RP of HH1 strain are similar to those of the HVS25A strain and both showed effect on pregnant mice. More specific studies about genomic alterations to the SPv are necessary for identifying, more clearly, if the intra-strain variations have relation with the low virulence in the mouse respiratory and abortion model.

Introduction

Equine herpesvirus 1 (EHV-1) is a major cause worldwide of epidemic abortion, perinatal mortality, respiratory disease and neurological disorders in horses. Although primary infection generates a humoral response and the production of neutralizing antibodies, the infected animals do not develop a long lasting protection against EHV-1 and remain susceptible to re-infection throughout life, although, the severity of secondary infection is reduced (Allen and Bryans, 1986).

The efficacy of available vaccines is questionable. Both attenuated and killed virus vaccines are used for immunization of horses but they do not provide complete protection against abortion and all of them require frequent booster immunizations to achieve protection (Gilkerson et al., 1997). New EHV-1 vaccines based on recombinant DNA techniques are now being developed but the protective efficacy in horses may need some years to establish (van Woensel et al., 1995, Colle et al., 1996, Skinner and Davies, 2000, Zhang et al., 2000). Only the use of inactivated vaccines has been permitted in Argentina since 1982 and no earlier information about EHV vaccines is available.

A mouse model for EHV-1 disease was established some years ago (Awan et al., 1990). The comparative analysis of acute EHV-1 infection in the horse and the mouse model suggests that the mouse model provides a valid method for investigation into virological and histophatological aspects of EHV-1 induced disease (Walker et al., 1999a). It has many features similar to the natural infection. These include respiratory clinical signs, local virus replication in the respiratory mucosa and viremia. Inoculation of pregnant mice results in premature birth of dead or normal offspring (Awan et al., 1991, Csellner et al., 1995). The model has been used to investigate the vaccine potential of various immunogens (Ruitemberg et al., 1999, Walker et al., 1999a, Zhang et al., 2000) and the effect of antiviral agents on EHV-1 infection (de la Fuente et al., 1992), it has also lead to a considerable number of studies about the pathogenesis, immune responses to EHV-1 infection (Awan et al., 1990, Inazu et al., 1993, Csellner et al., 1995) and the pathogenic characteristics of EHV-1 strain variants (Awan et al., 1995, van Woensel et al., 1995, Fitzmaurice et al., 1997, Osterrieder, 1999, Walker et al., 1999b).

The first isolation of EHV-1 in Argentina (designated SPv) occurred in 1979 from an aborted equine foetus (Etcheverrigaray et al., 1982) and since then, many other viral isolates have been obtained. The SPv has special DNA characteristic in comparison with other Argentine EHV-1 strains. It can be differentiated by lack of fragment BamHI ‘e’ and the different mobility of the group of fragments BamHI ‘h-i-j-k’. These restriction patterns (RP) were obtained either from the original viral isolation in equine primary tissue culture (EFK cells) or from the strain passaged for 62 times through different non-equine cells (Galosi et al., 1998b). In addition, the baby mice inoculated intracerebrally revealed that SPv could be distinguished from other Argentinean strains and HH1 Japanese strain on the basis of its pathogenicity (Patel and Edington, 1983, Galosi et al., 1998a). This study details, for the first time, the growth properties in the cell culture of the SPv in comparison with HH1 strain. At the second time, it describes clinical signs, pathological and virological response to infection with both strain in mouse respiratory model and on pregnant mice inoculated with SPv at different stages of gestation. The description of infectivity titration and the presence of histopatological herpesvirus typical lesions (eosinophilic intranuclear inclusion bodies, bronquiolar epithelial necrosis and focal alveolar necrosis) as well as cellular infiltrate into the lung parenchyma in adult and foetuses mice were evaluated. The presence of vasculitis, endothelial inclusion bodies, thrombosis, necrosis and inflammatory infiltrate (Walker et al., 1998, Walker et al., 1999b) were evaluated in pregnant mice. The aim of this work was to investigate the possibility that these RP affected the virulence of SPv in mouse respiratory and abortion models.