Impacts on Human Health Potentially Caused by Exposure to an Unprecedented Ostreopsis spp Bloom in the Bay of Biscay, French Basque Coast.

Introduction: In recent years, climate change and human activity have modified marine biotopes, including the widening distribution of harmful algal blooms (HABs). Bloom events predominated by microalgae of the genus Ostreopsis have been described on the French Mediterranean coast, but in 2021 an unprecedented bloom occurred on the French Basque coast. The objective of this study is to describe the health impact of the Ostreopsis spp bloom that occurred on the French Basque coast in 2021.

Methods: A historical cohort was conducted, including cases of possible exposure to Ostreopsis spp registered at the Centre Antipoison de Nouvelle-Aquitaine between July 1 and September 30, 2021.

Results: Of 674 patients with possible toxicity due to Ostreopsis spp, 96.9% had bathed in contaminated waters. Most of them developed respiratory tract symptoms (64.4% of patients). The time to the onset of symptoms was <6 h for 73.6% of 174 short-term (<24 h) exposed patients. The median duration of symptoms was 7.5 days for occupational (e.g., lifeguards and surfing instructors) and 3 days for recreational exposures. There were no severe cases. In total, 3% of the cases were of moderate severity, and 97% were of minor severity, according to the Poisoning Severity Score.

Conclusion: Toxic reactions caused by Ostreopsis spp are mostly benign. The clinical picture is similar to that described following exposures to Ostreopsis cf. ovata blooms in the Mediterranean area since the end of the 20th century. Ostreopsis spp are present on the Basque coast. The ecological factors promoting its blooms remain to be clarified.