Scientists have found markers for Alzheimer’s disease in the brains of three different species of dolphins found dead, stranded ashore.
Evidence of mass cetacean strandings exists before our own history, but why dolphins and whales strand in groups is a lingering mystery.
While a direct link has been found between naval sonar and some beaked whales, and some animals stranded on shore were clearly sick, some with bellies full of plastic waste, most mass strandings provide little or no clues.
Toothed whales (Odontocetes) share a number of traits with humans, including (in at least five species we know of) menopause. Their ability to live well beyond their childbearing years means they also have the potential to be susceptible to late onset diseases.
Alzheimer’s disease is the most common cause of disability in aging humans, progressively impairing memory, learning and communication. Now, it looks like a similar condition can also affect our water-dwelling mammal relatives.
“I’ve always wanted to answer the question: do only humans have dementia? says neurobiologist Frank Gunn-Moore of the University of St Andrews in Scotland.
“Our results answer this question because they show that the potential pathology associated with dementia is indeed not only seen in human patients.”
Leiden University biologist Marissa Vacher and her colleagues examined the brains of 22 stranded dolphins to look for biochemical markers present in humans with Alzheimer’s disease. These include amyloid-beta plaques, which although no longer thought to be a direct cause of the disease, are still present in high numbers in those who have it; and tau protein clusters with hyperphosphorylation – when phosphate groups have been added to all possible binding sites on the protein molecule.
They found accumulations of amyloid-beta plaques and hyperphosphoryl tau in three dolphins, each of a different species: the long-finned pilot whale (Globicephala melas), the white-beaked dolphin (Lagenorhynchus albirostris) and the common bottlenose dolphin (Tursiops truncatus). These people also had signs of old age such as worn or lost teeth and an increased ratio of white matter to gray matter in brain tissue.
Additionally, the locations of the brain lesions found in the dolphins matched equivalent areas seen in humans with Alzheimer’s disease.
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 deceased animals, there is no evidence of accumulations of the two proteins in humans without the disease.
“We were fascinated to see brain changes in older dolphins similar to those of human aging and Alzheimer’s disease,” says Tara Spires-Jones, a neuroscientist at the University of Edinburgh.
Since dolphins are very social animals, they may help other group members who begin to struggle with their brains. That means there’s a chance they’ll survive longer, allowing for more disease progression than solitary species, the researchers note.
Dolphin strandings are common in one of the species studied, G. melassupporting the “sick leader” theory of this mysterious and fatal behavior.
“In humans, early symptoms of cognitive decline associated with Alzheimer’s disease include confusion of time and place and a poor sense of direction,” Vacher and colleagues explain in their paper.
“If the leader of a group of G. melas suffered from a similar cognitive decline related to neurodegeneration, which could lead to disorientation resulting in the pod being driven 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,” Spires-Jones concludes.
This research was published in the European Journal of Neuroscience.
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