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 coronavirus that causes COVID-19, likely a result of the regular human contact, according to a bioRxiv preprint posted on 7 July.
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.”
In North America, SARS-CoV-2 has spread among white-tailed deer (Odocoileus virginianus) since at least 2021. Researchers have documented numerous instances of both human-to-deer and deer-to-deer transmission, as well as three cases of COVID-19 in humans that may have originated in wild deer.
This has troubled scientists, because populations of wild animals that spread a virus among themselves—known as reservoirs—allow viruses like SARS-CoV-2 to persist and mutate. Eventually, they could transmit a more dangerous version of the pathogen back into humans or to other species.
In light of this risk, Nicola Fletcher, a veterinary virologist at University College Dublin, has monitored the Phoenix Park deer for the presence of SARS-CoV-2 since 2020. Their home, in central Dublin, is the largest urban park in Europe, welcoming about 10 million visitors each year. The herd, now about 600 deer, dates back several centuries and—despite park staff’s best efforts to keep them at least 45 meters away from visitors at all times—have developed a tendency to beg for food. This contributes to their burgeoning numbers, which staff keep in check with a yearly cull.
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.
