Around 400 white storks have died of bird flu in Spain, authorities said on Friday, after their carcasses were found south of Madrid along the Manzanares river.
Officials said the birds likely carried the virus with them during their migration from northern Europe to warmer regions.
Additional cases of birds that died from bird flu have also been detected in several other municipalities around Madrid.
While authorities initially believed the outbreak involved a mild strain of avian influenza that did not require an alert, further tests showed it was a highly pathogenic — and usually fatal — variant.
The findings have since been reported to Spain’s animal disease alert network RASVE and to affected municipalities.
No infections have been detected at poultry farms, where preventive measures have been put in place, authorities said. Spain has required poultry to be kept indoors since November.
Firefighters involved in recovering the carcasses criticized what they described as a delayed warning from authorities.
Madrid’s firefighters’ union called for the dismissal of the region’s director-general for civil protection, Pablo Cristóbal Mayoral, saying the health of the public and of firefighters deployed in the affected area had been seriously endangered.