Spain has confirmed its first locally-acquired case of Usutu virus in humans, detected in a blood donation in Mallorca in July 2024. This discovery, made by the Banc de Sang i Teixits de Baleares (BSTIB), has drawn attention from the health sector as it marks the first indigenous case of this African-origin virus in the country.
The case involves an asymptomatic individual who had not travelled outside the Balearics. The sample initially tested positive in routine screenings designed to detect West Nile Virus, which are regularly conducted between April and December to prevent transmission through blood transfusions. However, after more specialised testing at the Carlos III Health Institute, researchers precisely identified it as the Usutu virus, related to but distinct from the initially suspected pathogen.
This cross-detection occurred because Usutu virus shares an estimated genetic similarity of 76-78% with West Nile virus, explaining the initial misidentification. Teresa Jiménez, medical director of the Balearics Blood Bank, explained to Efe news agency that this genetic similarity was key to identifying Spain's first human infection with this virus, marking a significant moment in the country's epidemiological surveillance history.
What is Usutu virus and its relationship with West Nile virus
Usutu virus is an arbovirus belonging to the flavivirus family, closely related to West Nile virus. Its natural cycle involves birds as reservoirs and mosquitoes of the Culex genus as transmission vectors. The infection primarily affects avian fauna and, although uncommon in humans, can cause significant complications.
While most human infections are asymptomatic or mild, Usutu virus can manifest with symptoms including fever, headaches, skin rashes, muscle discomfort or joint pain. In more severe cases, especially in people with weakened immune systems, the virus can lead to neurological complications such as encephalitis, hepatitis or severe neurological dysfunction.
Transmission and vectors
The primary transmission of Usutu virus occurs through the bites of infected mosquitoes, particularly those of the Culex genus. These mosquitoes acquire the virus when feeding on infected birds and then transmit it to other birds, completing a natural cycle that maintains the virus within local wildlife.
Rarely, this mechanism can lead to transmission to humans, who are accidental hosts. There is no evidence of direct transmission between people or through other means such as physical contact or airborne routes. Therefore, controlling mosquito populations is essential to prevent outbreaks and potential infections.
Signs and symptoms in humans
The vast majority of Usutu virus infections in humans are silent or present very mild symptoms. Many people may be infected without knowing it and without developing noticeable symptoms, as occurred in the case detected in Mallorca.
In other patients, common symptoms may include:
- Moderate fever
- Intense headache
- Skin rashes or eruptions
- Muscle and joint pain
- Persistent general malaise
In rare situations, and mainly in individuals with compromised immune systems, serious complications may manifest, such as:
- Encephalitis, an inflammation of the brain that can lead to neurological sequelae
- Hepatitis, with inflammation and liver damage
- Severe neurological alterations requiring urgent medical attention