We have heard much about the inevitable “next pandemic” as viruses move and mutate through animals until they become infectious in humans.

An example of this was published recently, revealing how an avian flu virus caused fatal infections in swans, seals, and a fox, a “terrestrial mammal” not usually vulnerable to such a virus.

Was it a near miss? The thoroughness of the investigation and subsequent paper suggest that the outbreak was taken very seriously.

“An episode of unusual disease resulting in deaths in different species at a wildlife rehabilitation center in the United Kingdom during late 2020 led to the retrospective detection of influenza A virus subtype H5N8 of avian origin,” the investigators reported.1 “The infection of a fox represents an unusual detection of this virus in association with inflammation of the central nervous system in terrestrial mammal species.”

Live virus was isolated from the animals, and researchers detected a single genetic change — “a potential adaptive mutation” in the mammals (meaning the seals and the fox).

“We determined that avian-origin influenza A(H5N8) virus was the cause of death in a red fox and the cause of seizures in a gray seal and several common seals housed at a wildlife rehabilitation center,” investigators found. “These events occurred roughly one week after five swans housed in the same quarantine unit died from infections with highly pathogenic avian influenza (HPAI) H5N8 virus. Genetic and epidemiologic investigations suggest that the swans were most likely the source of infection for the fox and seals. Virus transmission likely occurred by fomite transfer or aerosol spread. The severity of disease and pathologic findings in the seals and fox was unexpected.”

Given that, the investigators questioned whether some combination of underlying conditions, malnutrition, and stress might have contributed to disease onset. The study was necessarily retrospective, since “many weeks passed” before the fox and seal samples were linked to the cluster of swans.

“The retrospective detection of influenza A virus of avian origin in these mammalian species meant that evidence of human exposure was not evaluated,” the authors reported. “However, the disease event occurred during a nationwide coronavirus disease lockdown in the United Kingdom, during which the population was required to self-monitor for signs of coronavirus disease and be tested whenever clinical disease consistent with an influenza-like illness occurred.”

No staff reported any illness of this type during the six-week period after the disease event.

“Although genetic analyses indicated no increased risk for human infection with the H5N8 viruses in this outbreak, the investigation shows how these viruses may have unexpected and severe health risks for mammalian species,” the authors concluded.


  1. Floyd T, Banyard AC, Lean FZ, et al. Encephalitis and death in wild mammals at a rehabilitation center after systemic infection with highly pathogenic avian influenza A(H5N8) virus, United Kingdom. Emerg Infect Dis 2021. https://wwwnc.cdc.gov/eid/article/27/11/21-1225_article