Usutu virus: how to report and spot the disease

Usutu virus (USUV) is a mosquito borne flavivirus first detected in South Africa in 1959. USUV belongs to the Japanese encephalitis virus serocomplex and is similar to other human and animal pathogens such as West Nile virus. 

USUV is maintained through a cycle involving birds (mostly passerines as blackbirds) and mosquito species as vectors (mainly Culex species). 

Mammals, including humans, can become infected but can not transmit the infection to others. This is because viraemia in an infected mammal does not reach high enough levels to ensure transmission via mosquito bites and continuation of the transmission cycle.  

Clinical signs

In humans, USUV has an estimated incubation period of between 3 to 12 days following the bite of an infected mosquito. Most human USUV infections are mild or asymptomatic. Although the full spectrum of clinical presentation of USUV infections in humans has yet to be fully described, it ranges from mostly asymptomatic or mild symptoms (characterised by fever, myalgia, headache, asthenia and rash) to a rare severe neuroinvasive presentation most often observed in immunocompromised individuals or patients. 

Usutu virus is usually seen as a precursor of West Nile Virus, which is a more serious disease which also affects wild birds, but also, humans and horses. The UK has surveillance and research projects for both, USUV and WNV. 

USUV can have a severe impact on populations of wild birds, in particular blackbirds where cases are often be fatal. Infected blackbirds may be found dead however there are key behavioural signs that can be observed:

• lack of flight response: The bird may appear tame, failing to flee when approached
• poor coordination: with signs such as difficulty walking, unsteady movements, or loss of balance
• neurological issues: with signs such as head tremors, twisted neck (torticollis), drooping wings and seizures

Reporting dead or sick birds

If you suspect birds are affected by USUV, or have discovered a dead blackbird with no clear explanation of death (such as accident, predation or impact), please report through the gov.uk Report Dead Birds service or by calling the Defra helpline at 03459 33 55 77 

• do not touch or pick up any dead or visibly sick wild birds
• keep dogs and other pets away from carcases as they can also catch and spread the disease
• if found in a public area, contact your local authority so they can arrange safe removal

Human health implications

USUV is an emerging infection in humans, but the number of globally confirmed clinical infections to date is very low.

A risk assessment on Usutu virus has been published by the Human Animal Infection and Risk Surveillance Group (HAIRS)

The assessment of the risk of human infection in the UK is considered low. As of June 2026, despite the virus being present since 2020 in the UK, there have not been any human cases reported.