Study of squirrelpox virus in red and grey squirrels and an investigation of possible routes of transmission
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The red squirrel (Sciurus vulgaris) is native to Eurasia, but in the UK its survival is being threatened by the non-native grey squirrel (Sciurus carolinensis). Since its introduction to the UK from the USA the grey squirrel has increased its range at the expense of the red squirrel. Although competition for resources clearly plays a role in this replacement, an infectious viral disease, caused by squirrelpox virus (SQPV) and hosted apparently asymptomatically by the grey squirrels, has now been recognised as a major contributing factor. Little is known about the pathogenesis of infection in grey squirrels in comparison to red squirrels, but understanding this is essential to determining how the virus spreads within and between the red and grey squirrels. The aims of this thesis were to investigate the course of SQPV infection in red and grey squirrels and possible routes of virus transmission. Specifically, for the first time, a novel Real Time PCR (qPCR) assay and immunohistochemistry were used to investigate the presence of SQPV in various tissues from naturally infected red squirrels and experimentally infected grey squirrels. In diseased red squirrels SQPV DNA was found in several tissues with the highest amounts being found in skin samples. This reflects the multiple lesions that were easily visible on the red squirrel carcasses. There was no indication of systemic disease although the viral DNA was detected, at lower levels, in other internal organs. Grey squirrels were experimentally infected with SQPV isolated from naturally-infected red squirrels with fatal clinical disease. In contrast to SQPV-infected red squirrels no clinical lesions, other than mild scab formation at the site of inoculation, were found in the grey squirrels post-infection. No gross pathological changes indicative of systemic infection were observed and these findings were reflected in the qPCR and histopathology results. Viral DNA was only detected by qPCR in samples from the site of inoculation (scarified skin) and at lower concentrations in other skin tissues such as digital and eyelid skin. In addition, histopathology and immunohistochemistry examination revealed evidence of infection characterized by ballooning degeneration of keratinocytes, and acanthosis and spongiosis of the epidermis. These skin lesions were self limiting and minor compared to the infected red squirrel skin samples. The molecular variation in the virus isolated over time from different parts of the UK was also investigated. Seven SQPV isolates (4 from Scotland and 3 from England) were tested and results indicated that there are no significant changes in the amino acid sequence of any of the three genes examined apart from one amino acid change (one base change) in one gene. All Scottish isolates examined showed this change in comparison to English isolates. The results in this thesis show that there is a mild pathology associated with SQPV infection in grey squirrels. Scabs form at the site of infection but are less proliferative than in infected red squirrels, though they may still serve to contaminate the environment with virus leading to further outbreaks of disease. In contrast it seems likely that the proliferative lesions suffered by red squirrels and the greater amounts of virus that this leads to are likely to be more significant to the epidemiology of disease in localised outbreaks.