Paretic Syndrome in Gulls in Spain and Portugal: Causes, Consequences, and Current Understanding
What is paretic syndrome in gulls?
Paretic syndrome (PS) is a condition affecting aquatic birds — particularly gulls — and is characterized by ascending flaccid paralysis, difficulty breathing (dyspnea), and diarrhea.
Birds affected by the syndrome lose the ability to feed, swim, or escape from predators, which greatly increases their risk of mortality.
Although the syndrome has been observed on the Iberian Peninsula for decades, its precise cause remained unclear until recently.
Origins and Causes — New Scientific Evidence
A recent study (2025), led by an international research team involving the Spanish Institute of Oceanography (IEO-CSIC), IDAEA-CSIC, IREC, and the RIAS Wildlife Rehabilitation and Research Center, has identified avian botulism — caused by botulinum neurotoxin — as the most likely origin of the syndrome.
Botulinum toxin is produced by the bacterium Clostridium botulinum, which thrives in warm aquatic environments rich in organic matter — conditions that may occur in coastal waters, stagnant ponds, or contaminated sites.
It is believed that gulls may become infected by ingesting fly larvae that develop on decomposing carcasses contaminated with the toxin, although the exact exposure route in the Iberian context has not yet been fully confirmed.
The study also investigated other potential causes — including marine biotoxins (from shellfish), microcystins, and viral agents — but found no significant evidence linking them to the syndrome.
Symptoms and Clinical Presentation
Gulls affected by paretic syndrome typically show:
- Progressive flaccid paralysis of the limbs (usually legs and wings)
- Respiratory distress (dyspnea)
- Diarrhea
- Inability to feed, swim, or escape predators
These symptoms severely weaken the birds and explain the high mortality rates observed in affected regions.
The RIAS wildlife rehabilitation center treats dozens or even hundreds of these cases annually. In many instances, more than half of the affected birds can be successfully rehabilitated.
Ecological Consequences and Conservation Concerns
- Population decline: Deaths linked to paretic syndrome can reduce gull numbers in coastal colonies, altering the structure of local seabird communities.
- Ecosystem imbalance: Gulls play key ecological roles — scavenging, controlling marine pests, and contributing to food webs. Their decline may disrupt these processes.
- Environmental warning indicator: The syndrome may signal underlying issues such as pollution, eutrophication, or poor management of organic waste in coastal environments.
- Urgency for conservation: Effective management measures, water-quality control, and wildlife health monitoring are essential to protect vulnerable gull populations.
Recommendations: Monitoring and Prevention
Based on the study’s findings, several key actions can help reduce or prevent the occurrence of the syndrome:
- Remove bird or marine-animal carcasses from beaches and coastal areas as quickly as possible to prevent the growth of fly larvae that may carry botulinum toxin.
- Monitor water quality and prevent the accumulation of organic matter that favors bacterial growth.
- Implement wildlife health surveillance programs with regular sampling of coastal bird populations.
- Promote collaboration among rehabilitation centers, universities, and conservation institutions to collect data and identify high-risk areas.
- Raise awareness among the public and local authorities about the importance of keeping beaches and intertidal zones free of organic waste, litter, and carcasses.
