Environmental Samples Test Negative for Avian Influenza Virus H5N1 Four Months after Mass Mortality at A Seabird Colony

Abstract

High pathogenicity avian influenza (HPAI) profoundly impacted several seabird populations during the summers of 2021 and 2022. Infection spread rapidly across colonies, causing unprecedented mortality. At Foula, Shetland, 1500 breeding adult great skuas Stercorarius skua, totalling about two tonnes of decomposing virus-laden material, died at the colony in May-July 2022. Carcasses were left where they died as Government policy was not to remove dead birds. The factors influencing risk of further spread of infection are uncertain, but evidence suggests that HPAI can persist in water for many months in cool conditions and may be a major transmission factor for birds living in wetlands. We investigated risk of further spread of infection from water samples collected from under 45 decomposing carcasses and in three freshwater lochs/streams by sampling water in October 2022, by which time the great skua carcasses had rotted to bones, skin, and feathers. No viral genetic material was detected four months after the mortality, suggesting a low risk of seabird infection from the local environment when the seabirds would return the next breeding season. These findings, although based on a relatively small number of water samples, suggest that the high rainfall typical at Shetland probably washed away the virus from the decomposing carcasses. However, limitations to our study need to be taken on board in the design of environmental monitoring at seabird colonies during and immediately after future outbreaks of HPAI.

Keywords: H5N1; environmental contamination; highly pathogenic avian influenza; nucleic acid detection; water sample.

Figure 1

Example of a great skua carcass at Foula, Shetland, in October 2022 from immediately under which a water sample was collected for virus testing. The vegetation on which the carcasses were lying is growing in peat, with large amounts of Sphagnum moss under the grass, so that sampling water was possible from almost all of the carcasses.

Figure 2

Example of a great skua carcass at Foula, Shetland, in October 2022 from immediately under which a water sample was collected from Sphagnum moss growing under the grass for virus testing.

Figure 3

Locations of carcasses of adult great skuas found at Foula in May–August 2022, and location of water samples that were sampled on the island in October 2022. Samples from under 45 great skua carcasses were collected from throughout the blue hatched area. Samples from water bodies were from Rossies Loch, Mill Loch, and Ham Burn at points marked with stars. Green rectangles indicate locations of major bathing sites used by adult great skuas during the breeding season.