Investigating the role of bovine herpesvirus-1 in abortion and systemic disease in cattle
Crook, Tara Catherine
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Bovine herpesvirus-1 (BoHV-1) is a pathogen of cattle, which most commonly affects the upper respiratory tract to cause infectious bovine rhinotracheitis (IBR). It can also spread systemically to cause fatalities in calves and abortion in pregnant cattle. The virally encoded mechanisms of this systemic spread are poorly understood and therefore have been addressed by comparing isolates from the respiratory form of disease with isolates that have previously demonstrated systemic spread. A survey of 400 bovine abortions in Scotland from 2007-2009 demonstrated a BoHV-1 prevalence of 2.5%. It also demonstrated the importance of real-time PCR as a diagnostic technique when analysing samples from natural cases. The study of BoHV-1 distribution in the placenta and foetal tissue provided support for a haematogenous route of viral spread. Whole genome sequencing of 11 BoHV-1 isolates using Illumina Solexa technology was completed and added significantly to the sequencing data of BoHV-1. In terms of identifying genetic variation between isolates causing respiratory infection and those causing systemic infection, no differences were observed by SNP or phylogenetic analysis. However, there were significant differences in the extent of variation between essential and non-essential genes, which may reflect the evolution of BoHV-1. An in vivo challenge of the natural host to compare two isolates representing the respiratory and systemic forms of infection showed differences in clinical presentation, histopathological analysis, viral distribution and viral transcript expression, measured throughout the infection period. In particular, it was noted that a more severe ocular infection, rather than respiratory based infection was caused by infection with the ‘systemic’ isolate. Differences in the tropism of the virus were observed early in the infection with the ‘systemic’ isolate showing more association with the nasal mucosa than the trachea. The tonsils demonstrated different responses to the virus and differences in viral transcript expression. However, this may simply represent different stages of virus infection. Both isolates demonstrated spread to the brain at day 10 post infection. In vitro methods were used to study the differences in transcript expression in more detail. In a bovine turbinate cell infection faster replication of the respiratory isolate was observed by a significantly faster development of cytopathic effect. This was also reflected in the higher gene expression levels of the respiratory isolate in the first 12 hours of infection. More isolates were studied to investigate whether these differences were consistent, or as suggested by the sequencing, random differences between isolates. Six isolates were used to infect bovine lung slices. Differences in transcript expression were minimal between the two isolate groups. Immunofluorescence did not provide the sensitivity to detect virus in all samples where PCR showed replication. This compromised the study of co-localization but did show promise as a model to study the tropism of respiratory viruses. Overall, this work has showed that systemic spread of BoHV-1 does not appear to be controlled by virally encoded mechanisms. The in vivo experimental infection suggested host factors may play a large part. Further work is also needed to consider any differences that may exist between reactivated virus and the original infecting isolate.