Oh Deer as Phoenix Park inhabitants first in Europe to  test positive for Covid 

by Alex Greaney
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Alex Greaney 

UCD scientists have discovered that deer at the Phoenix Park in Dublin are the first in Europe to be infected with the COVID-19 virus, most likely a result of regular human contact.

The research has been highlighted by Science from the pre-print article “First Eurasian cases of SARS-CoV-2 seropositivity in a free-ranging urban population of wild fallow deer”. 

Doe-eyed and majestic, the deer in Dublin’s Phoenix Park draw thousands of admirers each week, many of whom pet and feed the animals. But these fans may have brought more than snacks to the creatures: They are the first deer in Europe shown to have been infected with the virus that causes COVID-19, likely a result of the regular human contact, according to a bioRxiv preprint posted on July 7th. 

Dr Nicola Fletcher, Assistant Professor and Ad Astra Fellow at UCD School of Veterinary Medicine, has been monitoring the Phoenix Park deer for the presence of SARS-CoV-2 (the virus that causes COVID-19) since 2020.  

Dr Fletcher said: “Our study revealed that in 2022, 57% of the animals we tested had antibodies to SARS-CoV-2. This is the first time European deer have been demonstrated to have anti-COVID antibodies and the first deer species, apart from US white-tailed deer, with demonstrated previous exposure to SARS-CoV-2.” 

The team’s research indicates a change in host tropism as new variants emerged in the human reservoir. This underlines the importance of continued wildlife disease monitoring and of limiting human-wildlife contacts. 

Dr Fletcher warned: “Animals with anti-SARS-CoV-2 antibodies were known to take food from people who visit the Phoenix Park. This highlights the importance of people not feeding these deer, due to the risk of ‘reverse zoonosis’ – transmitting diseases from humans to animals which could then transmit back to people.” 

The discovery of antibodies to SARS-CoV-2 in the blood of the fallow deer (Dama dama) adds to concerns that the virus could begin to circulate in European deer, researchers say. Although there’s not currently any clear risk to humans, “it’s not something we should ignore,” says Eman Anis, a veterinary microbiologist at the University of Pennsylvania School of Veterinary Medicine (Penn Vet), who was not involved in the study. “It is very important to not allow these viruses to make their way into wildlife.” 

Fletcher and her team have been granted access to the culled animals since their project began, allowing them to run more invasive tests than would otherwise be possible. These include lymph node and tonsil biopsies for polymerase chain reaction (PCR) testing that could reveal active SARS-CoV-2 infections and blood draws to test for antibodies that could indicated a cleared infection. 

None of the deer culled in late 2020 were positive for the virus. But in February 2022, more than half of the blood tests on 21 deer showed the presence of SARS-CoV-2 antibodies, suggesting a majority of the overall herd had been exposed to the virus. The PCR tests that the team ran, meanwhile, continued to come back negative: Though these deer had been exposed at some point, none was actively infectious at the time they were culled. 

The team also found that the Omicron variant of SARS-CoV-2—the dominant strain circulating among the Irish public at the time that the deer would have been infected—was significantly more adept at infecting D. dama lung tissue than an ancestral strain of SARS-CoV-2. This could explain why the deer had not been infected until this stage of the pandemic, Fletcher says. It also supports the idea that the deer caught the virus from humans, she adds. 

The presence of SARS-CoV-2 antibodies in the blood of the Phoenix Park deer is “the most compelling evidence” so far that the virus could become established in European deer populations, says Erick Gagne, a wildlife disease ecologist at Penn Vet. But he stresses that such an outcome is not a given. To establish a reservoir, the virus must be able to transmit from deer to deer—and at this point, there’s no proof that that is happening in the Phoenix Park herd. From studying antibodies alone, it is impossible to determine where the virus came from, Fletcher says. Until the team finds an actively infected deer and can trace the infection to its source, it won’t know whether it is transmitting deer-to-deer or individual animals are simply picking up the virus in interactions with the park’s scores of carrot-wielding visitors. 

Still, the findings “underscore how important continued surveillance is, not only for SARS-CoV-2, but for other viruses as well, and the importance of limiting human-wildlife interactions,” Fletcher says. For now, her team plans to keep a close watch on the herd. 

The study is a collaboration between Dr Fletcher’s laboratory at UCD School of Veterinary Medicine, and Dr Simone Ciuti’s team at UCD School of Biology and Environmental Science who have been studying the behaviour of the Phoenix Park deer for many years. Researchers at the UCD National Virus Reference Laboratory, UCD Centre for Experimental Pathogen Host Research (CEPHR) and UCD Conway Institute also contributed to the findings. 

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