Table of Contents 1.

During 2020, when COVID-19 first took hold in humans, another virus appeared in the UK – this time in birds. First discovered in South Africa in 1959, Usutu virus is transmitted via mosquito bites. This virus was identified in the UK in August 2020, when it was linked to the deaths of five Blackbirds and a House Sparrow in Greater London. Blackbirds are known to be particularly susceptible to the virus, but it also affects other songbirds and some owls. It can also infect mammals, such as bats and (very rarely) humans.

The arrival of the virus was not unexpected. Usutu has spread across multiple countries in mainland Europe since 1996, likely because of the movement of migratory birds or expanding mosquito populations. For this reason, routine virus screening of wild bird samples has been conducted in the UK for some time now.

Since the detection of the virus in London, a team of experts from different disciplines has been working to identify the extent of its impact on our wild bird populations. In June 2022, scientists from the Zoological Society of London, British Trust for Ornithology (BTO), the Natural History Museum, Animal and Plant Health Agency and the UK Health Security Agency, outlined their initial findings. They first examined mosquitoes by trapping these at the site where the virus was first detected. Some of the trapped mosquitoes were found to be carrying the virus, indicating that it was being transmitted locally.

The Team then looked at reports of dead and diseased birds reported to the Garden Wildlife Health survey (www.gardenwildlifehealth.org). This revealed a cluster of reports around Greater London, suggestive of a wider impact. By using data from BTO’s weekly Garden BirdWatch survey and the BTO/JNCC/RSPB Breeding Bird Survey, it has since been possible to show that Blackbird populations in London and the south-east of England have declined because of the virus.

Work is now being carried out to understand whether other Blackbird populations are at risk, and to identify the factors that may increase the risk of virus transmission. Because the virus is passed to birds by mosquitoes, it is likely that conditions favourable to the mosquitoes (warmer temperatures, standing water) will increase the risk of transmission. The team leading on this work is also exploring the risks posed by other viruses, notably those most likely to reach our shores because of climate change (www.vb-radar.com).