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This research, with the participation of IDAEA, identifies botulinum toxin as the cause of the paralytic syndrome, which for years has been depleting gull populations
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Removing carcasses and monitoring water quality are key to stopping the disease and preventing damage to biodiversity
A seagull on a beach on the Costa Brava. | CSIC Catalunya
An international scientific team, with the participation of the Institute of Environmental Assessment and Water Research (IDAEA-CSIC), has identified botulism as the most likely cause of the paralytic syndrome, a disease that causes severe paralysis in waterbirds, especially gulls.
The work, published in two articles in the journal Toxins, was carried out in collaboration with the RIAS Wildlife Rehabilitation and Research Centre of Ria Formosa, the Spanish Institute of Oceanography (IEO-CSIC), the Institute for Research on Game Resources (IREC), the Centre for Environment, Fisheries and Aquaculture Science (Cefas), and the Pasteur Institute of the University of Paris.
During the research, 571 samples from 377 birds—both healthy and those showing symptoms of paralysis—were analysed, taken from the southern coast of Portugal. The results show that 100% of the 22 diseased birds analysed tested positive for botulinum neurotoxin, while none of the asymptomatic birds presented this toxin.
The paralytic syndrome manifests with symptoms such as flaccid paralysis of the limbs, breathing difficulties, and diarrhoea, preventing birds from feeding, swimming, or escaping from predators. The RIAS centre attends to hundreds of birds with this clinical condition each year, managing to recover more than half of them, although the cause was previously unknown.
“Aviar botulism is poisoning caused by the toxin produced by the bacterium Clostridium botulinum, which proliferates in aquatic environments with high temperatures and excess organic matter,” explains Rafael Mateo, a researcher at IDAEA-CSIC.
Seagull affected by paretic syndrome. | María Victoria Mena Casero
Birds are usually infected by consuming fly larvae that develop in decomposing carcasses contaminated by the bacterium, although in the specific case of gulls, it is not clear whether it is water or food. “For this reason, the swift removal of carcasses is a good way to stop the cycle. If not, outbreaks occur that kill thousands of birds”, the author points out.
Additionally, the study detected the presence of other environmental toxins in several samples, such as paralytic shellfish toxins and microcystins (produced by microalgae). Although the levels detected would not be enough on their own to cause severe paralysis, the researchers suggest that they could contribute to the general weakening of the birds and exacerbate the effects of botulism.
Two positive cases of avian flu were also identified among apparently healthy birds, highlighting the need to consider multiple possible causes in cases of wild bird mortality.
“The early detection of botulism outbreaks and the rapid adoption of measures, such as the removal of carcasses or the control of water quality, are essential to prevent mass mortality events and protect biodiversity,” says María Victoria Mena, Clinical Director and researcher at the RIAS Wildlife Rehabilitation Centre.
“This study is a clear example of the value of multidisciplinary research and collaboration between scientific institutions and rehabilitation centres, which allows for the identification and addressing of complex environmental problems, using birds as sentinel species,” concludes the scientist.
Mena Casero, M.V., Turner, A.D., Ben-Gigirey, B., Alexander, R.P., Dean, K.J., Hatfield, R.G., Maskrey, B.H., Mazuet, C., Karamendin, K., Mateo, R. 2025. Identifying Causative Agents of a Paretic Syndrome in Waterbirds in Southern Portugal. Toxins, 17 (2). DOI: 10.3390/toxins17020062
Soliño, L., Turner, A.D., Ben-Gigirey, B., Alexander, R.P., Dean, k.J., Hatfield, R.G., Maskrey, B.H., Mena Casero, M.V. 2025. Investigation into Paralytic Shellfish Toxins and Microcystins in Seabirds from Portugal. Toxins, 17 (3), 135. DOI: 10.3390/toxins17030135
