Tumour‐like disease in wild red squirrels

IN 2017, Smith and colleagues reported cases of atypical histiocytosis (AH), a tumour-like condition of free-living red squirrels (Sciurus vulgaris), in Scotland.1 Since 2019 the APHA Diseases of Wildlife Scheme has diagnosed AH in nine wild red squirrels from Cumbria2 using similar criteria and with examinations to exclude squirrelpox virus and leprosylike infections. Three of these cases were from two spatial and temporal clusters of suspected and confirmed cases (Table 1). AH causes skin nodules, usually of the face and forelimbs, producing a grossly abnormal appearance (Fig 1). Lesions have also been found in the lung and spleen. From video surveillance it seems that the lesions are initially tolerated but as the size increases, they can cover the eyes and affected animals increasingly rely on supplemental food and will likely be more susceptible to predation. Nevertheless, we believe that at least one animal with AH has survived three months. “These case clusters suggest there may be an environmental or an infectious cause These case clusters suggest there may be an environmental or an infectious cause. Although all sites were visited, no obvious common cause has been uncovered. With contraction of the red squirrel range in Cumbria into small, isolated populations, inbreeding is a possible predisposing factor associated with the condition. The aetiopathogenesis of AH is not fully understood. Neoplastic conditions, per se, in wildlife are reported infrequently and clusters of cases do occur but are less frequent again; where clusters occur, causes have occasionally been identified and we will attempt to find a cause for the Cumbrian cases. While this condition is killing a wild species at risk of extirpation in mainland Britain, at present the threat is relatively small compared with the losses due to squirrelpox which are demanding intense conservation methods; continued surveillance will be essential.