Road traffic accident deaths were responsible for 41.7% of mortality overall and there was no significant difference between locations. This result is consistent that obtained LaRose et al.  in Scotland where 42.9% of mortality was caused by traffic. A slightly lower figure of 36% was recorded in 36 squirrels found dead on the island of Jersey . The impact of road traffic mortality on red squirrel populations is uncertain. On the IoW at least, squirrel numbers appear to have been stable in recent years, although there was a suspected decrease during 2012 (Butler H., unpublished data). Trauma due to attack by predators, principally cats and dogs, was 9.2%. Concern has been expressed by Duff et al.  at the level of red squirrel predation by pets and on the island of Jersey 36% of squirrel mortality was attributed to attack by domestic cats . This problem is in large part due to squirrels being attracted into gardens by people providing supplementary food. Although this may be beneficial from a nutritional point of view  it also increases the risk of predation.
Gross and histopathological examination identified pathological lesions associated with disease in 57 (35%) squirrels. The most important of these was toxoplasmosis. The susceptibility of red squirrels to this condition was first reported by Coles  in 1914 but in recent years it has become increasingly recognised as a cause of mortality [5, 17, 18]. In this study it was notable that all the toxoplasmosis cases were confined to the IoW where it was estimated to have caused almost one tenth of all deaths. The practice of providing supplementary food for squirrels is common on IoW and squirrels foraging in gardens are at increased risk of ingesting food contaminated by cat faeces. The human population density in the IoW is relatively high whereas at the other locations in the study, particularly in north east Scotland, the environment is much more rural with a low human, and presumably cat, population density. A second point of note is that three quarters of the toxoplasma-affected squirrels were females. This apparent sex bias does not appear to have been recorded previously and, whilst the reasons for it are not apparent, it could well be important as regards population recruitment.
In the study of red squirrels in Scotland  squirrelpox was the second most predominant cause of mortality (14.3%). However, squirrelpox does not occur on the IoW and was not seen in any of the squirrels submitted from other locations in this study. This was not unexpected in the case of squirrels submitted from Scotland as the majority came from an area believed to be free of squirrelpox; in addition, any Scottish red squirrels showing pox-like lesions are normally submitted direct to Edinburgh University for their pox screening program (http://www.red-squirrels.org.uk/surveillance.asp).
Thirty two squirrels were in poor or emaciated condition and half of these were juveniles or subadults. However, in no case was starvation diagnosed as the sole or primary cause of death. In some cases a heavy ectoparasite burden probably contributed to mortality but this was considered secondary. This result is in contrast to that obtained by LaRose et al.  in Scotland where starvation was the most common cause of mortality in juveniles and, at 9.8%, the fourth most common cause of death overall. A possible explanation for this is the additional laboratory procedures carried out in the present study. Anaemia associated with severe louse infestations has been reported as a likely cause of death in juvenile squirrels [9, 15]. In this study, although heavy louse infestations were seen in six cases, in only one, a juvenile, was there obvious anaemia. It should be noted that whilst N. sciuri was the predominant louse species, some squirrels were also infested with Enderleinellus nitzschi. Both species have been recorded previously in red and grey squirrels in Great Britain  but in most publications it would appear that the identification of N. sciuri is presumptive [8, 9, 11, 20]. Neohaematopinus sciuri has a Holarctic distribution and Enderleinellus species are found worldwide ; it is therefore surprising that in this study lice were only found on squirrels from Scotland. The apparent absence of lice on squirrels submitted from Cumbria and Anglesey may be because so few were examined from these locations but this does not explain the apparent absence of sucking lice from the IoW. The authors accept that light burdens might have been missed during these post-mortem examinations but it is highly unlikely that this would have happened with heavy infestations. Similarly, ticks were only observed in squirrels from Scotland although they have been observed historically on IoW (H. Butler, unpublished data). In view of the apparent absence of both parasites from squirrels on IoW it is of interest that lice were not recorded, and ticks rarely so, in a post-mortem study on red squirrels from the Island of Jersey [T. Blackett, personal communication].
Capillaria hepatica is capable of infecting a wide range of species but is primarily a parasite of rodents. The life cycle is dependent on the death of the host and digestion of the liver to release the eggs. Typically this occurs through predation, cannibalism or necrophagy. A new host only becomes infected by ingesting embryonated eggs that have been excreted in the faeces of the predator or scavenger. As the highest prevalence of the parasite is normally in commensal populations of brown rats (Rattus norvegicus)  it is likely that rats are the main source of infection in squirrels. Rat infestations are a common problem in gardens where people feed squirrels and birds. In this study it was difficult to assess the pathological significance of the lesions caused by C. hepatica. Four of the five infected squirrels were in good condition and were killed by road traffic whilst the fifth was suffering from a severe facial abscess and mandibular osteomyelitis. However, in other studies [17, Simpson, V. unpublished data] mortality has been associated with severe parasite-induced liver lesions in both wild and captive red squirrels.
Apart from T. gondii, the importance of protozoal infections in red squirrels is uncertain. Infection with Hepatozoon sp. was common, especially on the IoW where a third of all squirrels were infected. Heavy Hepatozoon burdens were most commonly seen in squirrels suffering other concurrent respiratory infections but it is unclear whether a high Hepatozoon burden predisposes squirrels to these infections or whether high burdens are a consequence of other infections or stress factors. There was no clear evidence that the parasite on its own was causing mortality. A similar situation possibly exists with coccidiosis. Keymer  suggested that this is a common cause of death in red squirrels but in the present study it was apparent that, whilst Eimeria spp infection was common, there was no evidence that this was associated with disease. Sainsbury and Gurnell  came to the same conclusion in a study of red squirrels in Norfolk and Sainsbury  suggests that, as in other mammalian species, disease in red squirrels due to coccidia is precipitated by stressors or concomitant disease. Severe coccidiosis has been described in squirrels dying of colonic intussusception  but there was no evidence of coccidiosis in the two cases of intussusception in the present study.
An enteric adenovirus was first reported as a suspected cause of enteritis and mortality in red squirrels translocated from Cumbria to Norfolk . Since then there is increasing evidence that the virus is associated with fatal enteritis in wild and, particularly, in captive red squirrels in England, Wales, Scotland and Northern Ireland [4, 11, 24, 25]. However, although PCR analysis on spleens demonstrated adenoviral DNA in three squirrels in this study, and in four of seven squirrels from IoW examined as part of another study (Everest and Butler, unpublished data), in no case was there evidence of associated pathology. This suggests that in many cases adenovirus infection in wild red squirrels is asymptomatic.
Keymer  suggested that bacterial infections in red squirrels are rare but in this study they were quite frequent, particularly those affecting the respiratory system where histological examination showed lesions associated with bacteria in approximately 11% of cases. However, in many cases the infection appeared to secondary, for example in squirrels dying of inhalation pneumonia. In the study by LaRose et al.  of red squirrel mortality in Scotland 7.3% of the deaths were attributed to pneumonia but as this diagnosis was based on gross pathology only it is not possible to make a direct comparison with the present study.
The most important bacterial infection in this study was Staphylococcus aureus associated with fatal exudative dermatitis in IoW squirrels. This condition is also a major cause of red squirrel mortality on the island of Jersey  and the S. aureus isolates from affected squirrels on both islands are of the same lineage and all encode the lukM gene . The pathogenesis of the condition is not understood but it is invariably fatal. Further studies into factors that may predispose squirrels to Staphylococcus-associated fatal exudative dermatitis are in progress.
Bordetella bronchiseptica is a common cause of respiratory disease in dogs and cats but is only occasionally recorded in wildlife. In most cases bordetellosis is seen as a secondary or opportunistic infection in stressed or compromised animals, for example dogs and cats in boarding kennels or seals affected with phocine distemper [26–28]. In this study, acute, fatal bronchopneumonia due to B. bronchiseptica was diagnosed in two squirrels and was suspected in a third. There was no evidence to suggest that these were secondary infections although they did have heavy Hepatozoon infections. The source of infection was not apparent.
Neoplastic disease is uncommon in most British free-living wild species but red squirrels appear to be unusually susceptible. Cases of soft tissue sarcoma and lymphoma have been recorded in squirrels in Scotland , two cases of lymphosarcoma in Wales  and pulmonary adenomatosis and lymphoma in Jersey . Neoplasms seen this study were pulmonary carcinoma, trichoepithelioma, gastric spindle cell tumour and renal papillary adenoma. In addition, four squirrels, three from IoW and one from Brownsea, had unusual lesions of epidermal hyperplasia of the ear pinnae associated in two cases with cutaneous wart-like growths. None of the neoplastic lesions observed in this study appear to have been recorded previously in red squirrels and it was notable that all occurred in cases from IoW. A preliminary investigation in to the possibility that red squirrels, in common with other Rodentia carry Gammaretroviruses that are capable of acting as etiological agents for transmissible neoplasia indicated the presence of one or more potentially infectious retroviruses [Tarlinton and Lucassen personal communication].