Stranded dolphins show signs of Alzheimer's disease in their brains

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Scientists have discovered markers of Alzheimer’s disease in the brains of three different species of dolphin found dead, stranded on land.

Evidence of mass cetacean strandings exists before our own recorded history, but why dolphins and whales strand in groups is an enduring mystery.

While a direct link between naval sonar and some beaked whales has been found, and some animals washed ashore are clearly unwell, some with their bellies full of plastic waste, most mass strandings provide little or no clue. .

Toothed whales (Odontocetes) share a number of characteristics with humans, including (in at least five species that we know of) menopause. Their ability to live well beyond their reproductive years means they can also be susceptible to late-onset disease.

Alzheimer’s disease is the most common cause of disability in elderly humans, gradually impairing memory, learning and communication. Now, it looks like a similar affliction might affect our aquatic mammal relatives as well.

“I’ve always been interested in answering the question: do only humans suffer from dementia?” says neurobiologist Frank Gunn-Moore of the University of St Andrews in Scotland.

“Our findings answer this question, as they show that the potential pathology associated with dementia is not only seen in human patients.”

Leiden University biologist Marissa Vacher and her colleagues examined the brains of 22 stranded dolphins to look for the biochemical markers present in humans with Alzheimer’s. These include beta-amyloid plaques, which although no longer considered a direct cause of the disease, are still present in high numbers in those who have it; and clusters of tau proteins with hyperphosphorylation – when phosphate groups have been added to all possible binding sites on the protein molecule.

They found accumulations of beta-amyloid plaques and hyperphosphorylated tau in three dolphins, each from a different species: the long-finned pilot whale (globicephala melas), the white-beaked dolphin (Lagenorhynchus albirostris) and the common bottlenose dolphin (Tursiops truncatus🇧🇷 These individuals also showed signs of aging, such as worn or missing teeth and an increased white-to-gray matter ratio in brain tissues.

What’s more, the locations of brain lesions found in dolphins matched equivalent areas seen in humans with Alzheimer’s.

Although it was not possible for the researchers to verify the diagnosis of Alzheimer’s, as they could not test the levels of cognitive impairment in the dead animals, there is no record of accumulations of both proteins in humans without the disease.

“We were fascinated to see brain changes in elderly dolphins similar to those of human aging and Alzheimer’s disease,” says neuroscientist at the University of Edinburgh, Tara Spiers-Jones.

As dolphins are highly social animals, it is possible that they will help other group members who start to struggle with their brains. That means there’s a chance they’ll survive longer, allowing for greater disease progression than in solitary species, the researchers note.

Dolphin stranding is common in one of the studied species, G. melassupporting the ‘sick leader’ theory of this mysterious and fatal behavior.

“In humans, early symptoms of AD-associated cognitive decline include confusion of time and place and a lack of a sense of direction,” Vacher and colleagues explain in their paper.

“If the leader of a group of G. melas suffered from a similar neurodegenerative cognitive decline, which could lead to disorientation, resulting in taking the pod into shallow water and subsequent stranding.”

However, “whether these pathological changes contribute to the stranding of these animals is an interesting and important question for future work”, concludes Spiers-Jones.

This research was published in European Journal of Neuroscience🇧🇷