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Algal bloom triggered by sewage discharges could be causing in dolphins the same form of brain degeneration seen in humans with Alzheimer’s disease, scientists warned in a new study. Scientists found a type of disorientation consistent with Alzheimer’s to be affecting dolphins due to the chronic exposure to toxic molecules produced by cyanobacteria, also known as blue-green algae. And just like some humans with dementia, who are occasionally found wandering far from their homes, dolphins with this condition may also become disoriented, leading to their strandings, researchers suspect. Earlier research established that cyanobacterial blooms are increasing with global warming and nutrient inputs into waterbodies in the form of agricultural runoff and sewage discharges – all products of human activities. Previous studies have found the toxin β-N-methylamino-L-alanine (BMAA) to be extremely toxic to nerve cells. The latest study, published in the journal Communications Biology, assessed the brains of twenty common bottlenose dolphins stranded in the Indian River Lagoon in eastern Florida and found that they contained BMAA. Dolphins have increased death rates during summer months when harmful algal blooms (HABs) are most common, the study found. “Since dolphins are considered environmental sentinels for toxic exposures in marine environments, there are concerns about human health issues associated with cyanobacterial blooms,” said David Davis at the Miller School of Medicine. Researchers also found that the concentration of another algal toxin was 900 times higher in dolphin brains during bloom seasons compared to non-bloom seasons. This toxin, known as neurolarthyrogen, or 2,4-DAB, has been shown previously to provoke hyperirritability, tremors, and convulsions within hours of exposure. Many of the stranded dolphins assessed in the study also had brain tissue anomalies similar to Alzheimer’s patients, such as β-amyloid plaques and tau protein clusters. Some brain protein changes characteristic of a severe form of Alzheimer’s were also found in the dolphin brains, researchers say. The study also found that these same dolphins showed 536 differentially activated genes associated with Alzheimer’s disease. Earlier studies have shown that at least one half of stranded bottlenose dolphins have severe to profound hearing loss. Researchers also suggest that hearing loss is one of the main risk factors for Alzheimer’s disease and can accelerate dementia in humans. “Although there are likely many paths to Alzheimer’s disease, cyanobacterial exposures increasingly appear to be a risk factor,” Dr Davis said. “As our climate warms, harmful algal blooms will continue to intensify. Understanding the impact of HAB exposures will help to identify populations at risk for neurological illnesses,” researchers wrote. They call for future studies to understand the role played by algal toxins in dolphin brain diseases.
