Authorities have expressed serious concern after multiple dogs fell critically ill and died following swimming in the IJmeer near Almere and Muiderberg. The Environmental Service Flevoland & Gooi and Vechtstreek (OFGV) has issued an immediate swimming ban for the designated dog area at Almeerderstrand, NOS reports.

The incidents are affecting both sides of the IJmeer. At the dedicated dog beach on the Almeerderstrand, at least one dog has died, and several others have fallen ill. Veterinarians in the area report at least 11 cases of dogs showing similar symptoms after visiting the beach.

Earlier last week, incidents were also reported on the opposite side of the IJmeer at Het Voorland in Muiderberg. Two Dalmatians became severely ill after swimming in an inlet and died later that evening, NH Nieuws stated. Het Voorland is not an official swimming site, so the water quality there is not regularly monitored or tested beforehand.

Affected owners report that symptoms develop rapidly after swimming, including extreme lethargy, paralysis-like symptoms, sudden illness, and, in some cases, death within a matter of hours.

The exact cause has not yet been officially established, though multiple investigations are ongoing. Specialists and former veterinarians suspect acute poisoning linked to blue-green algae. When dogs disturb the water by running or splashing, toxins from the sediment may be released and subsequently ingested.

Tests are also being conducted for E. coli and enterococci bacteria. While earlier routine measurements at Almeerderstrand indicated that levels were well below critical limits, the environmental service has not yet excluded them as possible factors.

A necropsy is currently being carried out on a deceased dog to establish the exact cause of death. Results from both water samples and medical tests are expected later this week.

The closure of the dog section at Almeerderstrand applies to both dogs and people. The environmental agency warns that the water may present significant health risks.

A warning is in effect for the remaining parts of Almeerderstrand and the official beach at Muiderberg, where swimming is currently at your own risk.

Dog owners are strongly advised not to allow their pets to swim in the IJmeer near these areas or drink the water. If a dog has been exposed, it should be rinsed thoroughly with clean tap water as soon as possible to prevent it from ingesting any contaminants from its coat, and a veterinarian should be contacted immediately if any symptoms develop.