The implications of significant adenovirus infection in UK captive red squirrel (Sciurus vulgaris) collections: How histological screening can aid applied conservation management

Opportunistic viral surveillance confirms the ongoing disease threat grey squirrels pose to sympatric red squirrel populations in the UK

BACKGROUND

Red Squirrels United was a UK landscape-scale grey squirrel management programme undertaken between 2016 and 2020.

METHODS

A total of 11034 grey squirrels were removed by culling, with 1506 necropsied and 1405 suitable for adenovirus (AdV) or squirrelpox virus (SQPV) quantitative PCR (qPCR) analysis. Spleen, lip or hair were extracted, and DNA was isolated, with samples tested in duplicate by qPCR.

RESULTS

Of 1378 tissue samples, 43% were positive for AdV and 10% for SQPV. Of 1031 hair samples, 11% were positive for AdV and 10% for SQPV. Overall, 762 of 1405 (54%) animals were positive for one or both viruses.

LIMITATIONS

Ad hoc sampling was undertaken from limited geographical areas but provided the only dataset from that period, instead of extrapolating from historical data.

CONCLUSIONS

The grey squirrel is an asymptomatic reservoir host for AdV and SQPV. Interspecific infection transmission potential is demonstrated. Grey squirrel management by culling is essential for mainland red squirrel viability until other suitable management tools are available.

A scoping review of rodent-borne pathogen presence, exposure, and transmission at zoological institutions

BACKGROUND

Rodents are one of the major taxa most likely to carry zoonotic diseases, harboring more than 85 unique zoonotic pathogens. While the significance of rodents' capacity to carry and transmit disease has been characterized in urban settings, the zoo environment is particularly unique given the overlap of collection, free-living, and feeder rodents as well as non-rodent collection animals, staff, and visitors.

ELIGIBILITY CRITERIA

This scoping review examines reports of rodent-borne pathogen detection or transmission in zoo settings extracted from the literature. Papers were included in the final analysis if there was evidence of presence or exposure to a pathogen in a rodent at a zoological institution.

SOURCES OF EVIDENCE

Publications were included from PubMed, CAB Abstracts and Biological Abstracts searched in August 2019.

CHARTING METHODS

Data extracted from publications on pathogen presence/exposure included publication identifiers, study identifiers, infectious agent identifiers, rodent identifiers, and non-rodent collection animal identifiers. Extraction from papers with evidence of disease transmission included number of rodents involved in transmission, non-rodent collection animal species and numbers, and job title of humans involved, diagnostic tests performed, and clinical outcomes.

RESULTS

Aggregate literature examined included 207 publications presenting evidence of pathogen presence and/or exposure in rodents across 43 countries in over 140 zoological institutions. A total of 143 infectious agent genera were identified, comprising 14 viral genera, 31 bacterial genera, 83 parasitic genera, and 15 fungal genera. Of these infectious agents, over 75 % were potentially zoonotic. The most common disease-causing agent genera identified were Leptospira, Toxoplasma, Salmonella, and Yersinia. Additional screening for evidence of pathogen transmission across species yielded 30 publications, indicating an area for future investigation to better inform surveillance and management priorities in order to reduce exposure, infection, and transmission.

CONCLUSIONS

Analyzing the breadth of rodent species and pathogens identified at zoos highlights the unique opportunity zoos have to be at the forefront of the early detection and identification of novel hosts and geographic ranges of rodent-borne pathogens with high impact on both endangered species and people. The overlap of these populations at zoos exemplifies the importance of considering One Health when prioritizing surveillance and risk mitigation of rodent reservoirs at zoos.

Adenovirus emergence in a red squirrel (Sciurus vulgaris) in Iberian Peninsula

Adenoviruses (AdV) are pathogens capable of infecting animals and humans leading to a wide spectrum of diseases. They have a widespread geographical dissemination and infect several species, including red squirrels (Sciurus vulgaris). In the Iberian Peninsula, there are no reports of infection of red squirrels by AdV. In this study, we report for the first time the detection of squirrel AdV in a red squirrel trapped in central Portugal. The phylogenetic analysis showed that the sequences obtained for the hexon and the penton base genes were similar to the Korean squirrel AdV strain and strongly divergent from the strains present in red squirrels in Europe. These findings indicate the emergence of a new strain circulating in Europe and raise concerns regarding the conservation of the red squirrel.

Widespread occurrence of squirrel adenovirus 1 in red and grey squirrels in Scotland detected by a novel real-time PCR assay

The Eurasian Red Squirrel (Sciurus vulgaris) is distributed throughout large parts of Europe and Asia. However, its distribution in certain regions of Europe is endangered by the invasive, non-native Grey Squirrel (Sciurus carolinensis). Adenoviruses were already described in squirrels in Great Britain almost two decades ago. In 2013, a squirrel adenovirus (SqAdV-1) was additionally found in a red squirrel from Germany, which suffered from acute diffuse catarrhal enteritis, and the complete genome sequence was determined. Here, samples from dead red (n = 25) and grey (n = 12) squirrels collected in Scotland, UK, were analysed for the presence of this squirrel-associated virus. By using a newly developed real-time PCR targeting the adenoviral polymerase gene, viral DNA was detected in at least one of four tissue samples tested per animal in 64.0% of the red squirrels and 41.7% of the grey squirrels. Exceptionally high viral genome loads were detected in the intestine and liver, but SqAdV-1 DNA was also present in lung and kidney samples of affected animals. Almost complete genome sequence determination of a red squirrel-derived SqAdV-1 strain from Scotland indicated a very high degree of identity to the first German strain. Sequence analysis of the hexon gene, which encodes one of the major antigens of the virion, revealed an identity of 100% between viruses found in red and grey squirrels from Scotland. In conclusion, SqAdV-1 appears to be widespread in the Scottish red and grey squirrel population, which highlights the necessity for continuous wildlife surveillance. The novel real-time PCR assay offers a highly sensitive and robust method for SqAdV-1 surveillance.

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.

Herpesvirus skin disease in free-living common frogs Rana temporaria in Great Britain

Infectious disease is a significant driver of global amphibian declines, yet despite this, relatively little is known about the range of pathogens that affect free-living amphibians. Recent detection of the tentatively named Ranid herpesvirus 3 (RHV3), associated with skin disease in free-living common frogs Rana temporaria in Switzerland, helps to address this paucity in knowledge, but the geographic distribution and epidemiology of the pathogen remains unclear. Syndromic surveillance for ranid herpesvirus skin disease was undertaken throughout Great Britain (GB), January 2014 to December 2016. Reports of common frogs with macroscopic skin lesions with a characteristic grey appearance were solicited from members of the public. Post-mortem examination was conducted on one affected frog found dead in 2015 at a site in England. In addition, archived samples from an incident involving common frogs in England in 1997 with similar macroscopic lesions were further investigated. Transmission electron microscopy identified herpes-like virions in skin lesions from both the 1997 and 2015 incidents. RHV3, or RHV3-like virus, was detected in skin lesions from the 2015 case by PCR and sequencing. Our findings indicate that herpesvirus skin disease is endemic in common frogs in GB, with widespread distribution at apparently low prevalence. Further research into the role of host immunity, virus latency and the significance of infection to host survival is required to better understand the epidemiology and impact of cutaneous herpesvirus infections in amphibian populations.