Competition between the red and grey squirrels and an epidemic disease caused by a
Squirrel parapoxvirus (SPPV) both play a crucial role in the replacement of red squirrels by
the grey species. Serological evidence suggests that the grey squirrel may act as a reservoir
host for SPPV, however no route of transmission between the two species has been
identified. The aims of this study were to characterise the SPPV genome and to identify
other potential host species. Knowledge gained from this will be invaluable for the future
development of a vaccine against SPPV, which could be used in conjunction with red
squirrel conservation programmes.A preliminary investigation to identify rodent species that may act as a reservoir for SPPV
was carried out. Sera collected from wood mice and bank voles were screened for antibodies
to SPPV using an enzyme-linked immunosorbent assay. No alternative hosts were identified
using this approach.Viral DNA was purified from scab material collected from a red squirrel found dead in the
wild during an epidemic of SPPV in Northumbria. The composition of the SPPV genome is
G+C-rich at approximately 66%, and the genome is estimated to be 158kb in size. The G+C
content is comparable with parapox and molluscipox viruses, but the genome does not
correspond in size to viruses belonging to either of these genera. DNA hybridisation studies
indicate that the central regions of the SPPV and Orf virus (ORFV) genomes are highly
related; however the two viruses are divergent towards the genome ends. Twenty-eight
previously identified poxvirus genes were identified and mapped on to the SPPV genome,
including two genes previously believed to be unique to the molluscipox virus Molluscum
contagiosum (MOCV). Comparison of the organisation of genes in SPPV with Vaccinia
(VACV), MOCV and ORFV revealed an overall conservation in the order of genes, but
differences in the spatial distribution.Further characterisation revealed that three known parapoxvirus-specific genes that are
associated with virus virulence and immuno-modulation (homologues of interleukin-10 and
vascular endothelial growth factor and the granulocyte macrophage colony stimulating factor
inhibitory protein) appear to be absent in SPPV. In addition, sequence analysis of a putative
SPPV homologue of the conserved poxvirus interferon (IFN) inhibitory gene suggests that
this gene is disrupted in SPPV and not involved in the inhibition of IFN-induced anti-viral
pathways.The genomic inconsistencies between SPPV and the other parapoxviruses suggest that the
initial classification of SPPV as a parapoxvirus may be incorrect. Preliminary phylogenetic
analysis suggests that SPPV does not belong in any of the previously described poxvirus
genera
