Survivorship is dependent on habitat and, in particular, mast crop. Consequently, mortality varies considerably, with higher survival during good mast years. As with most mammals, survival is lowest during the first year of life, increasing dramatically if the individual survives their first winter. In some Red squirrel populations, first year mortality can be as high as 70%, decreasing to about 26% during the second year. Early mortality appears weather dependent and a study of Reds in western Finland, published in the Journal of Mammalogy during 2016, observed smaller litter sizes in cold springs that is probably a result of kittens being susceptible to the cold before their fur has developed. Mortality among Grey squirrels is higher, up to 85% perish by the end of their first winter, reducing to between 30 and 50% during the second year. In particularly good mast years, almost all adults and half of the juveniles may survive the year.
Work by the Red Squirrel Survival Trust in northern England suggests that Red squirrel kitten survival is lower in the presence of Greys; from about 50% surviving to adulthood in Red-only populations, to around 15% in Red-Grey populations. An interesting excerpt relating to this difference was published in the Spring 2016 issue of the Westmoorland Red Squirrel Newsletter. In his short note, Bob Bradley described how a friend had seen a Grey squirrel emerge from a hole in an ash tree with a half-grown Red kitten in its mouth. This incident apparently happened in Dallam Park near Milnthorpe in Cumbria in about 2001, shortly after the Grey spread to the county. Mr Bradley suggests:
“This account is a rare illustration of what must be a fairly common occurrence and accords with our widespread belief that breeding in the red squirrel is reduced or halted altogether where the grey population is moderate or high.”
Squirrels fall prey to a number of predators including birds of prey (particularly buzzards and goshawks), foxes, badgers, pine martens, domestic dogs and cats (see Predators). Many die of starvation during poor mast years or are killed on Britain's road network. Indeed, cars are probably the most significant source of mortality for most squirrel populations. Data from the Isle of Wight show that, of the 215 animals found dead between July 2014 and June 2016, 132 (61%) had been hit by cars. Supplemental food can be a significant draw for squirrels and, where food is provided in gardens separated from adjacent woodland by a road, traffic mortality can be considerable and the practice is actively discouraged in some Red squirrel conservation areas, including on the Isle of Wight.
Diseases can also be a significant source of mortality. The squirrelpox virus (SQPV), sometimes colloquially referred to as “parapox” or simply “pox”, is of particular concern to Red squirrel conservationists because it is almost invariably lethal to Reds and can significantly impact populations in a relatively short time frame—infected animals typically die within two weeks of the clinical symptoms developing (see the SQPV QA). Infected animals develop deep sores and ulcers (particularly around the eyes, on the face, feet and thighs) that often lead to secondary infection. In November 2007, an outbreak killed an estimated 85% of the Reds at Formby in Merseyside, but about 8% of the population contracted the disease and recovered.
There is one record of the squirrelpox virus, or a variant of, in a Grey squirrel from Hampshire in the south of England, but in most cases the virus is not fatal to Greys and they show no indication of infection. This contrasts with the situation in the USA where a very similar viral agent is widespread in Greys from the east of the country and results in tumour/wart growth that can be fatal. In Britain the prevalence varies by region. In 2013 the National Trust estimated that 40-60% of the UK Grey squirrel population were carriers for squirrelpox and a study led by Anthony Sainsbury at the Zoological Society of London found that 61% of seemingly healthy Grey squirrels they tested from England were positive for SQPV infection. In their chapter in The Grey Squirrel: Ecology & Management of an Invasive Species in Europe, however, Tim Dale and Julian Chantry report that 41 (25%) of the 166 squirrel carcasses they tested from the Sefton Coast on the west coast of the UK were positive for SQPV.
There is no cure, or even treatment for the SQPV, but there are an increasing number of reports suggesting that some Reds survive even significant infection with the virus and Natasha McGowan and colleagues at Queen's University Belfast recently found antibodies in two otherwise healthy Reds caught in Tollymore Forest, suggesting a small percentage of infected animals may survive the virus. Work is currently underway at Moredun Research Institute in Scotland to develop a vaccine after the team, led by Colin McInnes, was the first to characterise the virus; interim results are positive, with both vaccine candidates seeming to provide protection against the wild-type virus.
The first clinically-confirmed case of SQPV in England was in an animal found dead in Norfolk during November 1980; the first Scottish infection was in a Red found in Lockerbie in May 2007; and the first Irish case was from two Reds found dead in Tollymore Forest Park, County Down during March 2011. (See SQPV QA for more details on squirrel pox and its arrival in Britain.)
Squirrelpox was considered absent from Europe until a new variant was isolated from a juvenile Red squirrel submitted for post-mortem at the Catalan wildlife rehabilitation centre in north-east Spain during July 2010 after being runover. In a note to The Veterinary Record, Elena Obon and colleagues described lesions to the ear tips, paws and tail of the squirrel that they “considered suspicious of a poxvirus infection”. It is noteworthy that there are no records of Grey squirrels in Spain and, unlike the SQPV described from the UK, the disease didn't produce lesions on the eyelids or lips. Despite an expanding Grey squirrel population in Italy, I am not aware of any cases of SQPV from the country. In a recent paper to Emerging Infectious Diseases, Gudrun Wibbelt and colleagues described a new variant of SQPV, that they name “Berlin Squirrelpox Virus”, genetically distinct from that found in the UK but that has apparently been circulating in the Berlin area for the past decade.
More recently, Red squirrels have been found infected with the bacteria Mycobacterium lepromatosis or M. leprae, the species that cause leprosy. In a paper to the journal Science in November 2016 a team of 18 microbiologists from eight institutes from Mexico and across Britain and Ireland report that leprosy has been found in Reds from Scotland, the Isle of Wight and Brownsea Island. In all they analysed tissue samples from 114 animals from Britain and Ireland and found evidence of leprosy from all the locations, although prevalence varied considerably—all 25 squirrels tested from Brownsea Island were positive for leprosy, while only one of 92 samples from the Isle of Wight yielded the bacteria (a male runover in Freshwater during 2016). In the case of the Isle of Wight, a retrospective analysis of records and photographs by Helen Butler published in 2019 suggests at least two other probable cases in the island's reds, both from Quarr Woods, but that leprosy is unlikely to be a significant cause of mortality among this population. The researchers of the Science paper concluded that 21% of the squirrels without clinical symptoms and all 13 with clinical symptoms harboured leprosy bacilli. In their paper, they also describe the clinical symptoms of the disease, which include hair loss, extensive swelling of the snout, lips, eyelids, ears and feet.
Leprosy was endemic in the British human population during the Middle Ages, until is disappeared during the 1400 and 1500s; the genetic profile of the M. leprae isolated from squirrels on Brownsea suggests it was essentially the same as the one that circulated in medieval England and Denmark. More recently, work by Cambridge University archaeologist Sarah Inskip and her colleagues identified the same lineage of leprosy in the skull of a woman who lived in Suffolk during the 10th century AD. In a 2017 paper to the Journal of Medical Microbiology, Inskip and her co-workers note that similar strains having been found in Red squirrels, medieval human skeletal remains from Winchester and their skull implies that leprosy may be partially a zoonotic infection. They suggest the Viking trade in squirrel meat and fur may have contributed to the spread of leprosy in Britain, writing:
“Historically, this route of transmission is made viable by the common usage of squirrel for fur and meat in the medieval period and it is known that squirrel fur was imported into East Anglia from Scandinavia and the Baltic region.”
While the researchers caution that we don't know how long the live bacteria could survive in meat or on fur, they do note that squirrels were occasionally kept as pets, which might've offered greater potential for transmission. To the best of my knowledge, no Grey squirrels have tested positive for leprosy bacilli in Britain to date.
In early 2022, Hugh Rowlands photographed a Red squirrel in Anglesey exhibiting deformed ears and left arm, and with wart-like growths on its face. The disease did not appear to be squirrelpox or leprosy, the latter of which is known from the population, and Craig Shuttleworth suggested a possible causal infection of paramyxovirus. During an epidemiological survey of 22 squirrels, published in Veterinary Microbiology in 2014, Fiona Brooks and her colleagues found 15 animals, five Greys and ten of the 15 Reds from Jersey, were positive for paramyxovirus. Interestingly, none of the squirrels showed outwardly indications of the disease.
The protozoan parasite Toxoplasma gondii has been documented in British Greys, which can cause the brain disease toxoplasmosis. Several protozoan parasites have been found in the guts of Red squirrels, including Eimeria sciurorum, which can cause the fatal disease coccidiosis, particularly in malnourished or stressed animals. Adenovirus damages the villi in the Red squirrel's intestine retarding nutrient uptake and causing diarrhoea in infected animals; it has been known to kill squirrels in Britain and appears to be widespread in the UK and may have a role in the Red population decline.
In a paper to Mammal Review in 2014, a team led by David Everest report that 42% of the Red squirrels found dead on Anglesey they studied tested positive for adenovirus, noting that the first report of the virus in the UK came from Suffolk in 2001 and within a decade it had been found in Red populations in Cumbria, Wales, Scotland and Northern Ireland. Preliminary data suggest that wood mice (Apodemus sylvaticus) and other small rodents may have a role in transmitting adenovirus. Serological analysis of Greys in north Wales have found adenovirus, reovirus, rotavirus and Sendai virus, while 18 (37%) of 49 Isle of Wight Red squirrels autopsied by Vic Simpson and colleagues had Hepatozoon protozoa in their lungs. In 1974, Ian Keymer described how post-mortem analysis of a sickly Red squirrel found at Stanford, Norfolk in 1971 isolated the parainfluenza RS6 virus from its lungs; the same infection was suspected in second dead squirrel from the same county.
Various other parasites, including mites (which may lead to mange), ticks, lice, fungus (including ringworm) and bot flies have been documented, but such accounts appear rare in Britain. Both species appear susceptible to infection by yeast fungi (e.g. Candida), although I'm only aware of one record of a Red having died from such an infection - a yearling Red squirrel presented with a fatal infection of Candida albicans near Dundeer during July 2007. The flea Orchopaes howardi can be found on Grey squirrels, while Monopsyllus sciurorum and Tarapsylla octodecimdentata are found on Reds and very occasionally on Greys. In his 1974 veterinary pathology report to the Zoological Society of London, Ian Keymer described finding the tick Ixodes ricinus on wild Red and Grey squirrels, as well as intestinal coccidia most likely caused by the protozoa Eimeria in four Grey and one Red squirrel between 1971 and '72.
During their study of squirrelpox in Reds and Greys from Ireland a team, led by Lisa Collins at Queen's University Belfast, noted that fleas were the most abundant parasite, found on 69% of Grey squirrels and 18% of Red squirrels sampled. Ticks (Ixodes ricinus) were present on 2% of Greys and 13% of Reds sampled, with mites observed only on Greys (3%). In The Eurasian Red Squirrel, Stefan Bosch and Peter Lurz note that various bacteria have been isolated from Red squirrels, listing Streptococcus, Staphylococcus, Corynebakterium, Veillonella, Propionibakterium and Francisella. They also make reference to malnutrition and various mineral deficiencies that can lead to metabolic bone disease and osteodystrophy (see: Food & Feeding: Bone-eating).
Developmental disorders and evidence of substantial healing are also sometimes observed. In a 2004 paper to the Journal of Wildlife Diseases, Anthony Sainsbury and colleagues describe oral disease in seven (8%) of 91 free-living Reds found dead in the UK between January 1994 and August 1998; three had very worn cheek teeth (attrition), believed to be a consequence of old age, while four suffered overgrowth of incisors. Healing long bones have been reported from squirrels in some populations, with fatal falls apparently quite rare. I have seen a squirrel fall out of a tree on to a concrete drive with a wince-worthy 'splat' and then get up and run off as though nothing happened.
Finally, some squirrels may be susceptible to injuries from certain types of wildlife feeders. On the Isle of Wight, Helen Butler reported a case where a Red squirrel apparently suffered a broken neck when the lid of a wooden feeder fell shut. I have heard similar discussion online suggesting that many popular squirrel feeders sold in garden centres are designed for Greys and may be too heavy for Reds, although I know of no formal investigation into this. Nonetheless, Susan Warn, a naturalist based on Jersey in the Channel Island, has reported a small group of squirrels with deformed paws that may be a result of having become trapped these kinds of peanut feeder, which are widely used on the island.
In October 2018, Susan contacted me with some photos of four individuals she had been watching that all exhibited disfigurement of one or other front paw—typically the loss of digits. Initially the concern was leprosy, but no other symptoms were apparent and further observation suggested that these amputations may have been caused by feeders. It is too early to draw any firm conclusions on this, but it is perhaps noteworthy that Susan has observed these missing digits in only one of the three populations she monitors on the island and that this population is the one accessing supplementary food in nearby gardens. Similarly, Susan has observed squirrels getting their paws caught in hinge-lidded peanut feeders:
“... after witnessing on two occasions this year  a red trapping its foot when the lid slams down onto it. Both times the red was in obvious pain, shaking its head and foot frantically and licking at its foot; then running off, unable to put weight onto the foot, stopping to lick at it and running off again limping all the while.'
The view of Elspeth Milne, Senior Veterinary Clinical Pathologist based at Edinburgh University, and Anna-Katarina Schilling, a Veterinary Surgeon at Old Stone Vets also in Edinburgh, who did her Ph.D. on leprosy in Red squirrels, was that these injuries were consistent with trauma and possibly a secondary bacterial infection. Further research is required into this phenomenon and I would be very interested to hear from anyone who has observed similar injuries/deformities.