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There has been considerable recent interest in why free-living red squirrels (Sciurus vulgaris) (Fig 1) in the British Isles die. The reason for this is that the red squirrel population in Great Britain has been in noticeable decline since the early 20th century, which Middleton1 reported was almost certainly due to epidemics of disease. Although he was only able to name coccidiosis with any certainty, Middleton felt that other causes, some producing external signs of disease, were also responsible for red squirrel mortalities. He even ventured the suggestion that introduced grey squirrels (Sciurus carolinensis) may carry a disease to which they themselves were immune, but to which the red squirrels were highly susceptible. He did concede, however, that disease epidemics in red squirrels had been reported not only in parts of the country where grey squirrels were found, but also in areas where grey squirrels had yet to be seen.
Although there had been reports of grey squirrels in the Welsh countryside as early as 1830, the best documented introductions of grey squirrels from America occurred between 1876 and 1910.2 By the 1940s it was realised that grey squirrels had expanded considerably from their points of introduction and that in areas of England and Wales where the grey squirrels had been resident for 15 years or more there were no red squirrels.3
As the grey squirrel population …
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