A recent communication from a Swiss lab gives details of SARS-CoV-2 infections in at least two foxes in the Zurich area of Switzerland.
The 1st fox from the Swiss canton of Zurich tested positive for SARS-CoV-2 RNA using RT-qPCR as part of a research project led by the Clinical Laboratory, Vetsuisse Faculty (VSF) at the University of Zurich (UZH), Switzerland. Within the project, routine samples from different animal species and submitted for diagnostic purposes to the laboratory since spring 2020 have been tested for SARS-CoV-2 viral RNA or antibodies in order to improve knowledge on SARS-CoV-2 infections within different animal populations and investigate their potential role in a One Health context.
The RT-qPCR positive fox lived in a zoo in the Canton of Zurich. On 11 Mar 2022, 2 adult foxes from this zoo living in a separate enclosure were presented to the University Animal Hospital in Zurich (Clinic for Zoo Animals, Exotic Pets and Wildlife, VSF, UZH); a conjunctival swab from these 2 foxes tested positive for canine distemper virus (CDV) RNA by RT-qPCR at the Clinical Laboratory (VSF, UZH). The swab samples from these 2 foxes tested negative for SARS-CoV-2 RNA at this time. The 2 foxes were euthanized because of the distemper diagnosis.
On 22 Mar 2022, a 3rd male fox was found dead in another enclosure that he had shared with a 2nd fox and 2 bears. The animal underwent a full post mortem examination at the Institute of Veterinary Pathology, VSF, UZH and was diagnosed with multiple wounds consistent with trauma likely due to post mortem biting by a bear, plus a severe verminous pneumonia due to infestation with the nematode _Angiostrongylus vasorum.
Samples taken from the nose and the 3rd eyelid from this fox tested negative for SARS-CoV-2 RNA by RT-qPCR; the fox tested also negative for CDV RNA. The 4th clinically healthy fox living in the enclosure with the 2 bears was quarantined, vaccinated against CDV and sampled on 29 Mar 2022. Combined material from the conjunctival and nasal swab tested negative for CDV RNA. However, the sample tested positive for SARS-CoV-2 RNA with Ct values of 36.9 (E gene assay) and 35.7 (RdRp gene assay), respectively.
The limited remaining nucleic acid material was submitted in the framework of an ongoing collaboration to the Department of Biosystems Science and Engineering of the Swiss Federal Institute of Technology (ETH) Zurich for next-generation sequencing. The presence of SARS-CoV-2 RNA was confirmed. The fox died within the naturally structured enclosure, but the carcass could not be found. The source of SARS-CoV-2 infection in this fox is unknown. None of the animal caretakers in contact with the fox knowingly had been infected with SARS-CoV-2 at the time. Possible sources of infection other than the animal caretakers include feeding animals (rats, rabbits) or other animals, such as free living rodents, martens or even foxes. Since all 4 foxes from this zoo were euthanized or died, to date no further control measures are planned.
Another fox tested serologically positive for SARS-CoV-2 antibodies. The fox had been identified in the framework of a research project on Swiss wildlife mandated and supported by the Swiss authorities. The project is under the responsibility of the Institute for Fish and Wildlife Health (FIWI), VSF at the University of Bern, and of the Clinical Laboratory (VSF, UZH), Switzerland.
This fox had been shot by a game warden in the Swiss canton of Wallis on 31 Jan 2022. On inspection, the animal was in good nutritional and health status (assessed by the game warden) and was estimated to be an adult animal (>1 year old). The fox had been found in close proximity to a human settlement (approximately 150 meters from the closest houses) and had been in contact with municipal waste.
A blood sample from this animal taken by the warden after the animal was shot tested questionably positive in the SARS-CoV-2 S1 ELISA at the Clinical Laboratory. The sample was sent for confirmation to the Friedrich-Löffler Institut (FLI) in Germany in the framework of an ongoing collaboration. The fox tested positive in both the SARS-CoV-2 RBD ELISA and the indirect immunofluorescence test (iIFT).
The source of infection in this fox is unknown, but the infection could have resulted from an earlier contact with contaminated human waste. The positive fox was one of 176 free-ranging foxes serologically tested so far; apart from the herein reported fox, all foxes tested negative for SARS-CoV-2 antibodies. These preliminary results suggest that exposure of foxes to SARS-CoV-2 is rare in the wild and that there is no reason to see free-ranging foxes as a threat to humans.
These 2 herein reported foxes are the 1st SARS-CoV-2 infected foxes documented in Switzerland. To the best of our knowledge, it is the 1st detection of SARS-CoV-2 RNA in a fox, apart from experimental infections. Both cases are reported to the World Organisation for Animal Health (WOAH).
No control measures are currently planned or deemed necessary. Although we have only limited data so far, the information reported herein indicates that infections of foxes with SARS-CoV-2 are rare in Switzerland. The main source of human infection remains other infected humans.