Throughout the UK, there has been a sustained replacement of the native red squirrel by the introduced grey squirrel over the last 100 years. Merseyside is one of the few places with both squirrel species and the presence of a grey squirrel endemic infection, squirrelpox virus, which can transfer to the red squirrels causing a fatal ulcerative dermatitis. Our aim was to directly link the decline in the UK red squirrel populations with outbreaks of squirrelpox which previously hasn't been demonstrated in the wild, on a regional level.

We used a surveying system for both red squirrel population size and disease surveillance before, during and after a squirrelpox epidemic in the Sefton Coastal Woodland reserve 2003–09. For the first time, we showed that by post mortem monitoring of the casualty population and PCR, that the causal factor in red squirrel population decline (by up to 90%) was due to a sustained outbreak of squirrelpox in 2006-08. We demonstrated that the disease rapidly died out in small isolated red squirrel populations (where there were insufficient hosts to infect) but in large relatively dense populations, new cases of squirrelpox rapidly spread through the susceptible red squirrels. In addition, the surrounding grey squirrel population was shown to have a low but sustained level of infection (14%) which was constant despite the epidemic in the reds. Finally, we demonstrated that the surviving red squirrels had a low level of antibody to the disease and that the remaining population increased again in the absence of squirrelpox.