Female Blackbirds - Whose Numbers Are In Freefall - Are Thought To Be Victims of Usutu.

 

Agent

Usutu virus (USUV) is a flavivirus that has emerged in Europe where it poses a potential disease threat to both wild and captive birds. It is transmitted between birds (the primary hosts) by mosquitoes. Different strains of the virus have been found in different locations.

Usutu virus was detected in wild Eurasian blackbirds (Turdus merula) in Greater London in late summer 2020: this is the first time USUV has been found in Great Britain (GB).

USUV infection in avian species (captive or wild) is not a notifiable disease.

Species affected

Hosts: The primary hosts for USUV are birds. Amongst wild birds there are marked differences in susceptibility between species, with Passeriformes (perching birds), Eurasian blackbirds in particular, and Strigiformes (owls) showing the highest levels of mortality.

In addition to birds, the virus has been known to infect mammals, including bats and humans. These are known as incidental hosts (i.e. infection results from incidental exposure through a mosquito bite but mammals are not part of the virus’ normal lifecycle).

Vectors: Whilst USUV has been found in multiple species of mosquito in Africa and mainland Europe, Culex pipiens s.I is considered the main vector in Europe and is present in GB.

Signs of disease

In mainland Europe, the most common sign of USUV infection in birds has been mortality events in the summer months, where sometimes large numbers of wild birds (typically blackbirds) and occasionally captive birds (e.g. Great grey owls (Strix nebulosi)) have been found dead without prior evidence of ill-health.

On rare occasions where sick blackbirds have been observed, they have shown signs such as lethargy, weakness, loss of coordination and seizures.

Disease transmission

USUV is spread to birds and other animals through being bitten by a virus-carrying mosquito.

Distribution and origin

USUV was first discovered in South Africa in 1959 and was subsequently detected in several countries across Africa. It is suspected that migratory birds played an important role in introducing USUV to mainland Europe from Africa. There are multiple distinct lineages (strains) of USUV and several of these have been detected in Europe, suggesting that the virus has been introduced from Africa on multiple occasions.

The first report of the disease in wild birds in a European country was in Austria in 2001, although retrospective analysis subsequently revealed that the virus has been present in Italy since at least 1996. The virus has since been detected in many additional European countries, including Belgium, Croatia, Czech Republic, France, Germany, Greece, Hungary, the Netherlands, Serbia, Spain, Sweden and Switzerland. The precise mechanism for virus spread throughout mainland Europe is unknown but it is likely that mosquitoes and migratory birds have played a role in disseminating USUV between countries.

Since its initial detection, USUV appears to have become established (endemic) in some parts of mainland Europe and has been linked to avian mass mortality events, for example in Austria, Germany and the Netherlands, sometimes associated with blackbird population declines. In some areas it is believed that avian populations may be developing immunity to the virus.

A study conducted in GB from 2005-2011 found that all wild birds tested were negative for USUV. Screening of wild bird tissues for flaviviruses has been conducted each subsequent year in GB with samples testing negative until the first detection of USUV in blackbirds in late summer 2020

. Whilst USUV was only confirmed in wild birds from a single location in 2020, and it is not possible to conclude on the impact of this outbreak with certainty, analyses of disease surveillance and wild bird monitoring scheme data indicate that further blackbird mortality may have occurred as a result of this virus infection on a scale sufficient to cause a regional decline of this species in 2020 in the Greater London area.

Since USUV has been detected in multiple countries in mainland Europe, this incursion of USUV into GB was considered highly likely to occur, most probably as a result of the movement of mosquitoes or migratory birds.

USUV was subsequently detected in a small number of wild birds in the summer of both 2021 and 2022 in Greater London. Molecular work has shown that the detections each year are highly similar, which indicates that USUV is most likely to have overwintered in the UK, rather than being introduced from mainland Europe on multiple occasions. Whether the UK climate will permit USUV to become established remains uncertain. Continued surveillance of wild birds and mosquito vectors is planned.

At present USUV has not been detected in Asia, the Americas or the Australian continent.