Ingesting plastic is leaving seabird chicks with brain damage “akin to Alzheimer’s disease”, according to a new study – adding to growing evidence of the devastating impact of plastic pollution on marine wildlife.

Analysis of young sable shearwaters, a migratory bird that travels between Australia’s Lord Howe Island and Japan, has found that plastic waste is causing damage to seabird chicks not apparent to the naked eye, including decay of the stomach lining, cell rupture and neurodegeneration.

Dozens of the chicks – which spend 90 days in burrows before making their first journey – were examined by researchers from the University of Tasmania. Many had mistakenly been fed plastic waste by their parents and built up high levels of plastic in their stomachs.

Blood tests indicated that the plastic pollution had left the chicks with severe health issues, disrupting the stomach, liver, kidneys and brain, according to the study published in the journal Science Advances.

“Plastic ingestion in seabirds is nothing new. We’ve known about it since the 1960s, but a lot of plastic research focuses on the birds that are really emaciated: they’re starving, they’re washing up on beaches and not doing too well. We wanted to understand the condition of birds that have consumed plastic but look visibly healthy,” said Alix de Jersey, a PhD student from the University of Tasmania’s School of Medicine, who led the study.

“[In blood tests], we found patterns of the proteins that were very similar to those in people that have Alzheimer’s or Parkinson’s disease. It’s almost equivalent to a small child having Alzheimer’s. These birds are really suffering the impacts from plastic, especially on their neuronal brain health,” she said.

Shearwaters are among the bird species most affected by plastic pollution. Previous studies have found more than 400 pieces of plastic in a single shearwater chick, with plastic sometimes accounting for 5-10% of their total bodyweight.

While chicks are able to vomit up some of the plastic before they migrate, researchers said the sheer quantity meant it was unlikely that all birds could clear it. The young birds that were examined in the study had their stomachs pumped, meaning that they were able to start their migrations to the Sea of Japan without any plastic waste inside.

“It’s almost a death sentence for these chicks, which is unfortunate because they do look really fit and healthy. But knowing the condition which their body is in before they’re starting their migration, it’s quite challenging to imagine that they would make it to the other end,” said de Jersey.

Previous research has found that fewer than 60 multinationals are responsible for more than half of the world’s plastic pollution, with six responsible for a quarter of that.

Find more age of extinction coverage here, and follow the biodiversity reporters Phoebe Weston and Patrick Greenfield in the Guardian app for more nature coverage