Epsilon Archive for Student Projects

Abstract

Infections with nidoviruses can cause various problems in animals and human beings. Bovine coronavirus (BCoV) has been considered as an important pathogen, causing severe diarrhea of newborn calves, winter dysentery of adult cattle, and respiratory disease in calves. Since this virus is repeatedly reported to cause severe disease in the Swedish cattle population, it is important to develop a highly sensitive diagnostic method for the detection of BCoV and to study the genetic characteristics of virus involved in the outbreaks.

A nested RT-PCR method targeting the HE gene was developed, which can be successfully applied for the diagnosis of BCoV infection. A conventional PCR was used to amplify the S gene of BCoV. Sequence analysis of the S gene showed a genetic diversity among Swedish and Danish virus isolates. S gene sequencing did not reveal differences between viruses from nasal and fecal swabs originating from the same animal, suggesting that the same virus can cause respiratory and enteric disease.

Horses are infected with equine arteritis virus (EAV) primarily through the respiratory and venereal routes. The outcome can vary greatly from subclinical infection to systemic disease characterised by fever, nasal discharge, lacrimation, arteritis, abortion, fetal death, and persistent infection in stallions. The LP3A1+ strain, a virus stock that was obtained by one additional cell culture passage of the LP3A virus, a large plaque variant of EAV Bucyrus strain, in equine embryonic cells was shown to be more virulent than the LP3A virus. An experimental infection has been done with a group of 14 horses to further characterise the in vivo biology of this LP3A1+ virus.

Analysis of open reading frames (ORFs) encoding EAV glycoproteins (GP) has shown the occurrence of mutations in the course of the experimental infection. While ORFs 2b and 3 had stabilizing mutations, ORFs 4 and 5 were subject to positive selection. Mutations in ORF5 mainly clustered in variable region 1, resulting in new genetic variants with changed neutralization domain in the GP5 (previously termed GL). The new virus variants, however could not be entirely related to induction of recurrent viraemia in the infected horses, which is presumed to have been caused by other factors. It would be of interest to investigate the occurrence of other forms of persistent infection, rather than carrier stallions, upon infection with EAV.

Samples originating from a group of 14 horses experimentally infected with a variant of equine arteritis virus (EAV) Bucyrus strain, termed large plaque variant (LP3A1+) were analysed. These included 182 nasal swabs collected from day 1 to day 14 post-infection (p.i), and 21 virus isolates obtained from white blood cells of horses that showed a secondary viraemia between day 30 and day 72 p.i. In order to understand the reason for this virus behaviour, genetic stability of the virus was studied by comparison of partial open reading frame 5 (ORF5), specifying glycoprotein 5 (GP5). Viruses with amino acid mutations in GP5 were used for further amplification and sequencing of a fragment encompassing ORFs 2b, 3, and 4. The sequences of the virus obtained from nasal swabs shared complete homology with the inoculated virus showing that the ORF5 gene of LP3A1+ was genetically stable during the first two weeks p.i. However, a number of mutations were found in the ORF5 of virus isolates obtained from day 30 p.i. when compared to the original inoculated virus. These mutations mainly clustered in antigenic neutralization site C within variable region 1 of the GP5 ectodomain. ORFs 2b and 3 had mostly silent substitutions and were more stable, whereas ORF4 showed non-conservative substitutions. Our results show that ORFs 4 and 5 of the inoculated LP3A1+ virus were subject to positive selection, most likely driven by immunological pressure, during the course of infection. However, genetic variation did not appear to be the underlying reason for the onset of a second wave of viraemia. It is plausible that the virus had been “trapped” at immune privilege sites, following initial infection, and was triggered to replicate by an unknown stimulus, therefore causing reappearance of viraemia.