The Blood and Tissue Bank of the Balearics (BSTIB) has been instrumental in identifying the first indigenous case of Usutu virus (USUV) in a Spanish blood donor. This discovery, which emerged from the West Nile virus screening programme, has been detailed in Transfusion, a prestigious scientific journal on transfusion medicine.
The infection was detected in July 2024 in a donor residing in Mallorca. The initial sample showed reactivity in routine tests for West Nile virus. After being sent to the National Reference Laboratory for Arboviruses at the Carlos III Health Institute, specific and neutralisation tests confirmed infection with Usutu, an emerging arbovirus in Europe transmitted by mosquitoes of the Culex genus, related to West Nile virus.
In addition to the Balearic case, the study records two additional infections in donors from Catalonia, confirmed by molecular biology and genomic sequencing. The three individuals had not travelled outside their community, which indicates local circulation of the virus. The work, published in Transfusion, is the result of a multidisciplinary team of experts. Participants included the BSTIB, the Blood and Tissue Bank of Catalonia, the Institute for Agrifood Research and Technology–CReSA, the National Microbiology Centre of the Carlos III Health Institute, and the Bernhard Nocht Institute for Tropical Medicine in Germany.
The authors emphasise that the usual screening techniques for West Nile virus also allow for the detection of Usutu, thanks to the cross-reactivity between the two viruses. This facilitates the early identification of infected donors and reinforces transfusion safety, especially during periods of increased mosquito activity.
The Usutu virus, identified in Africa in 1959, has undergone a progressive expansion across Europe in recent decades. Although most infections are asymptomatic, there is a risk for immunocompromised individuals, making its monitoring essential for public health. The publication of this study represents recognition of the work carried out in the Balearics and confirms the BSTIB’s ability to detect and analyse emerging agents that could compromise transfusion safety.
USUV has been reported from several African countries including Burkina Faso, Central African Republic, Côte d’Ivoire, Morocco, Nigeria, Senegal, South Africa, Tunisia and Uganda, as well as from Israel. Since 1996, it has also been found in European countries, including Austria, Belgium, Croatia, Czech Republic, Denmark, France, Germany, Greece, Hungary, Italy, Serbia, Spain and Switzerland, and more recently the Netherlands, and was spreading across Europe in the late 2010s. Outbreaks are often simultaneous with those of the related West Nile virus.
USUV’s host range includes primarily Culex mosquitoes and birds. The main bird hosts are blackbirds (Turdus merula), magpies (Pica pica) and owls, including the great grey owl (Strix nebulosa). In addition to humans, the virus has been isolated from Pipistrellus bats, and anti-USUV antibodies have been found in horses, dogs, deer, wild boar, rodents and shrews. Humans and horses are dead-end hosts. The vector is one of several mosquitoes that bite birds, in Europe particularly Culex pipiens.
For example, a 2008–2009 survey of mosquitoes and birds in the Emilia-Romagna region of Italy detected USUV in 89 C. pipiens pools and in 2 Aedes albopictus pools, suggesting the possible involvement of A. albopictus in the virus cycle. The virus was also found in twelve wild birds, primarily Eurasian magpies (P. pica), hooded crows (Corvus cornix), and Eurasian jays (Garrulus glandarius). In Africa multiple Culex species are involved, as well as several species of Aedes and other mosquitoes.
In 2020 the virus was detected in London, and has spread in wild birds since. In 2024 the virus was detected in blackbirds in Denmark for the first time.