Epidemiological and postmortem findings in 262 red squirrels (Sciurus vulgaris) in Scotland, 2005 to 2009

Molecular and Serological Surveillance for Mycobacterium leprae and Mycobacterium lepromatosis in Wild Red Squirrels (Sciurus vulgaris) from Scotland and Northern England

Leprosy is a poverty-associated infectious disease in humans caused by Mycobacterium leprae or M. lepromatosis, often resulting in skin and peripheral nerve damage, which remains a significant public health concern in isolated areas of low- and middle-income countries. Previous studies reported leprosy in red squirrels in the British Isles, despite the fact that autochthonous human cases have been absent for centuries in this region. To investigate the extent of M. leprae and M. lepromatosis presence in wild red squirrels in the northern UK, we analyzed 220 blood/body cavity fluid samples from opportunistically sampled red squirrels (2004-2023) for specific antibodies against phenolic glycolipid-I, a cell wall component specific for these leprosy bacilli. Additionally, we assessed bacillus-derived DNA by real-time PCR (qPCR) in 250 pinnae from the same cohort. M. lepromatosis and M. leprae DNA were detected by qPCR in 20.4% and 0.8% of the squirrels, respectively. No cases of co-detection were observed. Detectable levels of anti-PGL-I antibodies by UCP-LFA were observed in 52.9% of animals with the presence of M. lepromatosis determined by qPCR, and overall in 15.5% of all animals. In total, 22.6% (n = 296) of this UK cohort had at least some exposure to leprosy bacilli. Our study shows that leprosy bacilli persist in red squirrels in the northern UK, emphasizing the necessity for ongoing molecular and serological monitoring to study leprosy ecology in red squirrels, gain insight into potential zoonotic transmission, and to determine whether the disease has a conservation impact on this endangered species.

Ectopic Pregnancy and T-Cell Lymphoma in a Eurasian Red Squirrel (Sciurus vulgaris): Possible Comorbidity and a Comparative Pathology Perspective

Ectopic pregnancy (EP) is a life-threatening disease that affects humans and other mammals. Tumors causing ruptures of the reproductive tract have been identified as possible predisposing factors in human and veterinary medicine. We here describe a case of concomitant ectopic pregnancy and lymphoma in a Eurasian red squirrel found deceased in Italy and submitted to the public health laboratory Istituto Zooprofilattico Sperimentale del Lazio e della Toscana (IZSLT) for post-mortem examination. A full-term partially mummified ectopic fetus in the abdomen and a large fibrinonecrotic tubal scar adjacent to the right ovary were observed at necropsy. The tubal scar is likely the point of tubal rupture through which the fetus displaced. Histology revealed the presence of neoplastic cells referable to lymphoma infiltrating the ovary, spleen, small intestine, heart and peripancreatic adipose tissue. The lymphoma was further characterized as T-cell-type using immunohistochemistry. We suggest that the lymphoma, by involving the ovary, played a pathogenetic role in the development of a secondary EP by altering the genital tract at the structural and hormonal levels. To the best of our knowledge, this is the first report of concomitant ovarian lymphoma and EP in animals and humans in the literature.

Interactions between native and invasive species: A systematic review of the red squirrel-gray squirrel paradigm

The eastern gray squirrel (Sciurus carolinensis) has been labeled as one of the 100 worst invasive alien species by the IUCN. In Europe, the species has been introduced to Britain, Ireland and Italy, and its subsequent spread has resulted in wide-scale extinction of native Eurasian red squirrels (Sciurus vulgaris) from the areas colonized by the gray squirrel. This replacement of a native by an alien competitor is one of the best documented cases of the devastating effects of biological invasions on native fauna. To understand how this replacement occurs, we present a systematic review of the literature on competition and interactions between red and gray squirrels. We describe the patterns of red and gray squirrel distribution in those parts of Europe where gray squirrels occur and summarize the evidence on the different processes and mechanisms determining the outcome of competition between the native and alien species including the influence of predators and pathogens. Some of the drivers behind the demise of the red squirrel have been intensively studied and documented in the past 30 years, but recent field studies and mathematical models revealed that the mechanisms underlying the red-gray paradigm are more complex than previously thought and affected by landscape-level processes. Therefore, we consider habitat type and multi-species interactions, including host-parasite and predator-prey relationships, to determine the outcome of the interaction between the two species and to better address gray squirrel control efforts.

Masticatory biomechanics of red and grey squirrels (Sciurus vulgaris and Sciurus carolinensis) modelled with multibody dynamics analysis

The process of feeding in mammals is achieved by moving the mandible relative to the cranium to bring the teeth into and out of occlusion. This process is especially complex in rodents which have a highly specialized configuration of jaw adductor muscles. Here, we used the computational technique of multi-body dynamics analysis (MDA) to model feeding in the red (Sciurus vulgaris) and grey squirrel (Sciurus carolinensis) and determine the relative contribution of each jaw-closing muscle in the generation of bite forces. The MDA model simulated incisor biting at different gapes. A series of 'virtual ablation experiments' were performed at each gape, whereby the activation of each bilateral pair of muscles was set to zero. The maximum bite force was found to increase at wider gapes. As predicted, the superficial and anterior deep masseter were the largest contributors to bite force, but the temporalis had only a small contribution. Further analysis indicated that the temporalis may play a more important role in jaw stabilization than in the generation of bite force. This study demonstrated the ability of MDA to elucidate details of red and grey squirrel feeding biomechanics providing a complement to data gathered via in vivo experimentation.

The Patterns and Causes of Dermatitis in Terrestrial and Semi-Aquatic Mammalian Wildlife

Causative disease and stress agents which manifest as dermatitis in mammals have varying effects on individual animals, from benign irritation and inflammation, to causing morbidity and even mortality. Bacteria, viruses and ectoparasites are all potential causes of dermatitis, and it can be exacerbated by various environmental, genetic and social factors. Furthermore, it is uncertain whether dermatitis is more likely to manifest in already-vulnerable wildlife species. Here, we systematically review the literature for reports of dermatitis in terrestrial and semi-aquatic wild mammalian species, with the goal of determining the biogeographical scale of dermatitis reports, the causes of dermatitis, and whether manifestation of dermatitis is reported more commonly in certain wildlife species or their captivity status (i.e., free-living, in captivity or in a laboratory). We reveal biases in the reporting of dermatitis by a biogeographic realm, with 55% of cases reported in the Nearctic, and towards particular orders of mammals, namely Artiodactyla and Carnivora. Overall, free-living wildlife is almost twice as likely to be reported as having dermatitis than individuals in captivity and six times more likely than individuals in laboratories, which we interpret as owing to exposure to a broader spectrum of parasites in free-ranging individuals, and potential reporting bias in captive individuals. Notably, dermatitis was reported in 23 threatened species, with some species more likely than others to be reported exhibiting clinical signs of dermatitis resulting from underlying health problems. We also find that threatened species are more likely to be reported as having dermatitis in captivity, particularly outside of their endemic home range. This review highlights diverse patterns of dermatological disease causes in captive and free-ranging wildlife, conditions under which they are more likely to be documented, and the need for cross-disciplinary research to ascertain (and so better manage) the varied causes.

Wildlife conservation in a fragmented landscape: the Eurasian red squirrel on the Isle of Wight

Island populations may have a higher extinction risk due to reduced genetic diversity and need to be managed effectively in order to reduce the risk of biodiversity loss. The Eurasian red squirrels (Sciurus vulgaris) in the south of England only survive on three islands (the Isle of Wight, Brownsea and Furzey islands), with the Isle of Wight harbouring the largest population in the region. Fourteen microsatellites were used to determine the genetic structure of red squirrel populations on the Isle of Wight, as well as their relatedness to other populations of the species. Our results demonstrated that squirrels on these islands were less genetically diverse than those in Continental mainland populations, as would be expected. It also confirmed previous results from mitochondrial DNA which indicated that the squirrels on the Isle of Wight were relatively closely related to Brownsea island squirrels in the south of England. Importantly, our findings showed that genetic mixing between squirrels in the east and west of the Isle of Wight was very limited. Given the potential deleterious effects of small population size on genetic health, landscape management to encourage dispersal of squirrels between these populations should be a priority.

HEMATOLOGIC AND BIOCHEMICAL VALUES OF THE JUVENILE EASTERN GRAY SQUIRREL (SCIURUS CAROLINENSIS)

Venous blood samples were collected from 64 apparently healthy juvenile Eastern gray squirrels (Sciurus carolinensis) after sedation with midazolam at the Wildlife Center of Texas located in Houston, Texas, during 2012. Blood gas (pH, PCO2, PO2, base excess, bicarbonate, oxygen saturation), electrolyte (sodium, potassium), biochemical (total CO2, ionized calcium, glucose), and hematologic parameters (hematocrit, hemoglobin, complete blood count) were determined using the i-STAT point-of-care analyzer. Sex did not affect any analyte. All squirrels recovered uneventfully and were successfully rehabilitated and released. Most values were as expected based on comparison to other young rodent species. These analyte data for healthy juvenile Eastern gray squirrels may be useful in assessment of Eastern gray squirrel population health and management and treatment of individual squirrels presented in need of medical care.

Bacterial Pathogens and Symbionts Harboured by Ixodes ricinus Ticks Parasitising Red Squirrels in the United Kingdom

Red squirrels (Sciurus vulgaris) are native to most of Eurasia; in much of the United Kingdom, they have been supplanted by the non-native grey squirrel, and are considered an endangered species. Very little is known about the range of tick-borne pathogens to which UK red squirrels are exposed. As part of trap-and-release surveys examining prevalence of Mycobacterium spp. in red squirrel populations on two UK islands, Ixodes ricinus ticks were removed from squirrels and PCR screened for Borrelia spp., intracellular arthropod-borne bacteria and the parasitic wasp Ixodiphagus hookeri. At both sites, the most commonly encountered tick-transmitted bacterium was Borrelia burgdorferi sensu lato (overall minimum prevalence 12.7%), followed by Anaplasma phagocytophilum (overall minimum prevalence 1.6%). Single ticks infected with Spiroplasma were found at both sites, and single ticks infected with Borrelia miyamotoi or an Ehrlichia sp. at one site. Ticks harbouring Wolbachia (overall minimum prevalence 15.2%) were all positive for I. hookeri. Our study shows that UK red squirrels are potentially exposed to a variety of bacterial pathogens via feeding ticks. The effects on the health and survival of this already vulnerable wildlife species are unknown, and further studies are needed to evaluate the threat posed to red squirrels by Borrelia and other tick-borne pathogens.

PROGNOSTIC INDICATORS FOR SURVIVAL OF ORPHANED EASTERN GRAY SQUIRRELS (SCIURUS CAROLINENSIS)

The eastern grey squirrel (EGS), Sciurus carolinensis, is a tree squirrel native to the eastern United States. This species commonly presents to wildlife medical clinics for a variety of human-related injuries including confrontations with road traffic and pet predation. The purpose of this study was to assess initial examination findings as prognostic indicators for survival in EGS. The medical record database of the University of Illinois Wildlife Medical Clinic was searched from January 2012 through December 2018 for records of EGS weighing <300 g. The squirrels were identified as survivors (individuals surviving, released, or transferred to a rehabilitator within 72 hr of intake) or nonsurvivors (individuals euthanized or dying within 72 hr of intake after receiving medical care). Presenting weight, health status, method of feeding, and singleton versus group presentation were categorically recorded for each case. The data were modeled using a series of candidate logistic regression models fitted using the generalized linear model. An information theoretical approach determined the best fit model. A total of 955 EGS were included in this study. Factors that predicted a nonsurvivor status included EGSs that presented with any health system abnormality (odds ratio [OR], 4.81; 95% confidence interval [CI], 3.34-6.72), EGSs that presented between December and May (OR, 1.60; 95% CI, 1.12-2.27) rather than between June and November, and individuals with neurologic signs (OR, 2.61; 95% CI, 1.51-4.51) compared with EGSs without neurologic signs. Despite not being included in the final model, the presence of respiratory signs (OR, 3.43; 95% CI, 2.41-4.89) and diarrhea (OR, 4.01; 95% CI, 1.59-10.09) were significantly associated with a higher likelihood of nonsurvival status. Wildlife medical clinics and rehabilitation centers may use this information by initiating more aggressive therapies or instituting distinct euthanasia protocols for EGS that present with body system abnormalities, particularly neurologic clinical signs, and those that present in the winter months.

The drivers of squirrelpox virus dynamics in its grey squirrel reservoir host

Many pathogens of conservation concern circulate endemically within natural wildlife reservoir hosts and it is imperative to understand the individual and ecological drivers of natural transmission dynamics, if any threat to a related endangered species is to be assessed. Our study highlights the key drivers of infection and shedding dynamics of squirrelpox virus (SQPV) in its reservoir grey squirrel (Sciurus carolinensis) population. To clarify SQPV dynamics in this population, longitudinal data from a 16-month mark-recapture study were analysed, combining serology with real-time quantitative PCR to identify periods of acute viraemia and chronic viral shedding. At the population level, we found SQPV infection prevalence, viral load and shedding varied seasonally, peaking in autumn and early spring. Individually, SQPV was shown to be a chronic infection in >80% of grey squirrels, with viral loads persisting over time and bouts of potential recrudescence or reinfection occurring. A key recurring factor significantly associated with SQPV infection risk was the presence of co-infecting squirrel adenovirus (ADV). In dual infected squirrels, longitudinal analysis showed that prior ADV viraemia increased the subsequent SQPV load in the blood. However, there was a strong, negative association between prior ADV viraemia and subsequent SQPV shedding from the forearm, probably caused by ADV prolonging the SQPV acute viraemic phase, so delaying onset of the chronic shedding phase, and thereby altering viral shedding patterns over the time scales examined here. Hence, co-circulating ADV infection may be involved in mediating both the quantitative levels of SQPV infection and the timing and degree of subsequent infectiousness of grey squirrels.

Mortalities, amyloidosis and other diseases in free-living red squirrels (Sciurus vulgaris) on Jersey, Channel Islands

Between 2007 and 2014, 337 free-living red squirrels (Sciurus vulgaris) on Jersey, Channel Islands, were examined post mortem as part of a mortality and disease surveillance scheme. Road traffic accidents (RTAs) were attributable for 50.7 per cent (171/337) of the casualties, 34.4 per cent (116/337) succumbed to diseases including fatal exudative dermatitis (FED), 7.1 per cent (24/337) to predation, 6.5 per cent (22/337) to other trauma and 1.2 per cent (4/337) to suspected poisoning. Cat predation accounted for 5 per cent (17/337) of mortalities. Pathologies were diverse and individual animals were often identified with more than one disease process. Squirrelpox virus (SQPV) particles were not detected in selected cases examined by transmission electron microscopy (TEM). Amyloid was identified in 19.3 per cent (65/337) of squirrels, often in conjunction with inflammatory lesions like hepatic capillariasis. A consistent cause of amyloid accumulation was not identified, although there was a significant association of amyloidosis with hepatic capillariasis and FED. In addition to RTAs, amyloidosis and FED have been identified as important causes of squirrel morbidity and mortality on Jersey, while the underlying aetiology and predisposing factors for these two disease complexes are presently unclear. Disease, fragmented woodlands, an increasingly suburban habitat, along with various anthropogenic factors, may jeopardise the long-term viability of this island red squirrel population.

Enhancement of wildlife disease surveillance using multiplex quantitative PCR: development of qPCR assays for major pathogens in UK squirrel populations

Rapid development in polymerase chain reaction (PCR) technology has revolutionised the speed and accuracy of many diagnostic assays. However, comparatively few wildlife epidemiological studies use quantitative PCR (qPCR) for pathogen detection, even fewer employ an internal control, to ensure confidence in negative results, and PCR's ability to multiplex and therefore detect several targets in a single reaction is underutilised. Here, we describe the development of two multiplex qPCR assays for the red and grey squirrel that detect the pathogens squirrelpox virus (SQPV) and adenovirus in squirrels (SADV), both of which cause mortality in the red squirrel. Both assays use a section of the squirrel phosphoglycerate kinase gene as an endogenous internal control that identifies and compensates for both, inadequate sampling or PCR inhibition. Tests on infected squirrel tissue demonstrate that simple swab samples (particularly from distal antebrachial skin) are sufficient to detect and identify the relative quantity of SQPV DNA in both squirrel species, while rectal swabs and blood cell pellets can be used to reliably indicate SADV infection. These assays are sensitive and specific with an endogenous internal control providing confidence in negative results and allowing comparison across laboratories. Using such assays should prove advantageous in wildlife studies with limited resources while allowing the maximum data yield.

European red squirrel population dynamics driven by squirrelpox at a gray squirrel invasion interface

Infectious disease introduced by non-native species is increasingly cited as a facilitator of native population declines, but direct evidence may be lacking due to inadequate population and disease prevalence data surrounding an outbreak. Previous indirect evidence and theoretical models support squirrelpox virus (SQPV) as being potentially involved in the decline of red squirrels (Sciurus vulgaris) following the introduction of the non-native gray squirrel (Sciurus carolinensis) to the United Kingdom. The red squirrel is a major UK conservation concern and understanding its continuing decline is important for any attempt to mitigate the decline. The red squirrel–gray squirrel system is also exemplary of the interplay between infectious disease (apparent competition) and direct competition in driving the replacement of a native by an invasive species. Time series data from Merseyside are presented on squirrel abundance and squirrelpox disease (SQPx) incidence, to determine the effect of the pathogen and the non-native species on the native red squirrel populations. Analysis indicates that SQPx in red squirrels has a significant negative impact on squirrel densities and their population growth rate (PGR). There is little evidence for a direct gray squirrel impact; only gray squirrel presence (but not density) proved to influence red squirrel density, but not red squirrel PGR. The dynamics of red SQPx cases are largely determined by previous red SQPx cases, although previous infection of local gray squirrels also feature, and thus, SQPV-infected gray squirrels are identified as potentially initiating outbreaks of SQPx in red squirrels. Retrospective serology indicates that approximately 8% of red squirrels exposed to SQPV may survive infection during an epidemic. This study further highlights the UK red squirrel – gray squirrel system as a classic example of a native species population decline strongly facilitated by infectious disease introduced by a non-native species. It is therefore paramount that disease prevention and control measures are integral in attempts to conserve red squirrels in the United Kingdom.

A viable population of the European red squirrel in an urban park

Whether urban parks can maintain viable and self-sustaining populations over the long term is questionable. In highly urbanized landscapes, urban parks could play a role in biodiversity conservation by providing habitat and resources to native species. However, populations inhabiting urban parks are usually small and isolated, leading to increased demographic stochasticity and genetic drift, with expected negative consequences on their viability. Here, we investigated a European red squirrel population located in an urban park close to Paris, France (Parc de Sceaux; 184 ha) to assess its viability. Using mitochondrial D-loop sequences and 13 microsatellite loci, we showed that the population presented high levels of genetic variation and no evidence of inbreeding. The size of the population was estimated at 100–120 individuals based on the comparison of two census techniques, Distance Sampling and Capture-Mark-Recapture. The estimated heterozygosity level and population size were integrated in a Population Viability Analysis to project the likelihood of the population's persistence over time. Results indicate that the red squirrel population of this urban park can be viable on the long term (i.e. 20 years) for a range of realistic demographic parameters (juvenile survival at least >40%) and immigration rates (at least one immigration event every two years). This study highlights that urban parks can be potential suitable refuges for the red squirrel, a locally threatened species across western European countries, provided that ecological corridors are maintained.

Squirrelpox virus: assessing prevalence, transmission and environmental degradation

Red squirrels (Sciurus vulgaris) declined in Great Britain and Ireland during the last century, due to habitat loss and the introduction of grey squirrels (Sciurus carolinensis), which competitively exclude the red squirrel and act as a reservoir for squirrelpox virus (SQPV). The disease is generally fatal to red squirrels and their ecological replacement by grey squirrels is up to 25 times faster where the virus is present. We aimed to determine: (1) the seropositivity and prevalence of SQPV DNA in the invasive and native species at a regional scale; (2) possible SQPV transmission routes; and, (3) virus degradation rates under differing environmental conditions. Grey (n = 208) and red (n = 40) squirrel blood and tissues were sampled. Enzyme-linked immunosorbent assay (ELISA) and quantitative real-time polymerase chain reaction (qPCR) techniques established seropositivity and viral DNA presence, respectively. Overall 8% of squirrels sampled (both species combined) had evidence of SQPV DNA in their tissues and 22% were in possession of antibodies. SQPV prevalence in sampled red squirrels was 2.5%. Viral loads were typically low in grey squirrels by comparison to red squirrels. There was a trend for a greater number of positive samples in spring and summer than in winter. Possible transmission routes were identified through the presence of viral DNA in faeces (red squirrels only), urine and ectoparasites (both species). Virus degradation analyses suggested that, after 30 days of exposure to six combinations of environments, there were more intact virus particles in scabs kept in warm (25°C) and dry conditions than in cooler (5 and 15°C) or wet conditions. We conclude that SQPV is present at low prevalence in invasive grey squirrel populations with a lower prevalence in native red squirrels. Virus transmission could occur through urine especially during warm dry summer conditions but, more notably, via ectoparasites, which are shared by both species.

Causes of mortality and pathological lesions observed post-mortem in red squirrels (Sciurus vulgaris) in Great Britain

BACKGROUND

The red squirrel population in Great Britain has declined dramatically in recent decades, principally due to squirrelpox. Concern exists that red squirrels may become extinct nationally and, as there has been limited research in to diseases other than squirrelpox, this study aimed to identify additional causes of mortality.

RESULTS

Post-mortem examinations on 163 red squirrels found dead on Isle of Wight (IoW) England, in Scotland and at other locations in Great Britain showed that 41.7% (n = 68) were killed by road traffic and 9.2% (n = 15) by predators, principally domestic cats and dogs. The overall male/female ratio was 1.08/1. Fleas were recorded on 34.9% of IoW squirrels and on 43.8% of Scottish squirrels but sucking lice and ixodid ticks were only seen on Scottish squirrels. Bacterial infections were significant, particularly in association with respiratory disease (n = 16); two squirrels died of Bordetella bronchiseptica bronchopneumonia. Cases of fatal exudative dermatitis (n = 5) associated with a lukM-positive clone of Staphylococcus aureus occurred only on the IoW. Toxoplasmosis (n = 12) was also confined to IoW where it was responsible for almost one tenth (9.5%) of all deaths. Hepatozoonosis was common, especially in IoW squirrels, but was not considered a primary cause of mortality. Hepatic capillariasis affected four IoW squirrels and one from Scotland. Fungal infections included oral candidiasis, adiaspiromycosis and pulmonary phaeohyphomycosis. Neoplastic conditions diagnosed were: pulmonary carcinoma, gastric spindle cell tumour, renal papillary adenoma and trichoepithelioma. Epidermal hyperplasia of unknown aetiology was seen in squirrels showing crusty lesions of the ear pinnae on IoW (n = 3) and Brownsea Island (n = 1), associated in two cases with cutaneous wart-like growths. Miscellaneous diagnoses included chylothorax, electrocution, intussusception, suspected cholecalciferol rodenticide poisoning and foetal death and mummification. No cases of squirrelpox were diagnosed.

CONCLUSIONS

Red squirrels in Britain suffer premature or unnatural mortality due to a number of conditions in addition to squirrelpox, many of which result, directly or indirectly, from human activities: road traffic trauma, pet predation, toxoplasmosis, trap injuries, rodenticide poisoning and electrocution accounted for 61% of all recorded mortality in this study. Red squirrels are also affected by several diseases of unknown aetiology which merit further research.