Alzheimer's Brain Damage May Help Explain Dolphin Strandings Linked to Algal Blooms

Could brain damage associated with Alzheimer's be one reason dolphins become disorientated and strand themselves? That possibility is examined in a new study of 20 common bottlenose dolphins (Tursiops truncatus) that stranded in Florida's Indian River Lagoon between 2010 and 2019.

The researchers also connect evidence of neurodegeneration in dolphins with climate change. Their proposed link involves toxic algae and bacterial blooms, which are becoming more prevalent and extensive as waters warm.

Examination of the stranded dolphins' brains identified alterations in gene expression linked with Alzheimer's in humans, alongside disease-typical damage such as aggregated proteins.

Alzheimer's-like brain changes in stranded dolphins

The dolphins that stranded during algal bloom seasons showed a striking difference. Their brains contained concentrations of the neurotoxin 2,4-diaminobutyric acid (2,4-DAB) that were 2,900 times higher than those found in dolphins that beached outside periods of algal blooms.

This points to the damaging impact of blooms containing cyanobacteria, and may account for losses of memory and navigational ability that could cause dolphins to strand.

"Since dolphins are considered environmental sentinels for toxic exposures in marine environments, there are concerns about human health issues associated with cyanobacterial blooms," says toxicologist David Davis, from the University of Miami.

It is important to recognise that, as dolphins age, they naturally develop brain abnormalities that closely resemble Alzheimer's. Toxins released by cyanobacteria are also known to damage neurons in animals and humans, although their links with neurodegenerative diseases in people remain under investigation.

Climate change, algal blooms and dolphin neurodegeneration

The team's current suggestion is that harmful algal blooms may speed up and intensify these problems in dolphins. The research provides further detail on the neurotoxins responsible for the damage, their main effects in dolphin brains, and seasonal differences.

"The co-occurrence of Alzheimer's disease neuropathological changes and the natural accumulation of algal toxins observed in dolphins allows a unique opportunity to study the impact of these two converging events on the brain," write the researchers in their published paper.

The danger is not confined to dolphins. These blooms are harming many other forms of marine life, with effects that travel up the food chain and ultimately reach humans.

Earlier research has already associated algal blooms with toxins capable of causing memory loss, a central feature of Alzheimer's. Should these chemicals enter food supplies in sufficiently large quantities, they could pose a serious concern.

Potential implications for human health

Although this research concerns dolphins rather than people, the researchers say some of the underlying Alzheimer's-like brain changes are shared. No direct connection has yet been demonstrated, but the evidence warrants additional investigation.

Several members of the same research team have previously studied cyanobacteria and the neurotoxins produced in cycad trees. They found that the toxins can remain in the environment and build up through the food chain. This offers a possible route by which toxin exposure could contribute to several forms of neurodegeneration in humans, including dementia.

"Although there are likely many paths to Alzheimer's disease, cyanobacterial exposures increasingly appear to be a risk factor," says Davis.

The research has been published in Communication Biology.