A team of researchers has found ivermectin could help save the endangered Australian sea lion after trials on two South Australian islands to treat newborn pups infected with a deadly parasite. 

Key points:

  • Trials have shown ivermectin applied topically onto a sea lion pup’s skin is as effective as being applied by injection
  • Hookworm is transferred to every new Australian sea lion pup, accounting for 40 per cent of pup mortalities
  • Sea lion numbers have dropped to 10,000 and there has been a 64 per cent drop in offspring over three generations

Ivermectin has gained notoriety as an unproven prophylactic and therapy for COVID-19, but it has traditionally been used in human and veterinary medicine for parasitic infections.

Hookworm contributes to up to 40 per cent of sea lion pup deaths.

Pups become infected rapidly after birth by ingesting larvae in their mother’s milk.

The native mammal was classified as endangered in December last year after a 64 per cent reduction in offspring over three generations left the population at fewer than 10,000.

University of Sydney researcher Rachael Gray said an initial study at Dangerous Reef, 12 nautical miles south of Port Lincoln in 2017, treated 85 pups aged between 10 and 21 days with either topical ivermectin, injected ivermectin, or nothing.

The topical ivermectin had an effectiveness rate of 96.4 per cent, which was comparable to the injected formulation rate of 96.8 per cent.

Dr Gray said the topical application was the more accessible option.

Close up of over the shoulder view of people holding furry animal and wiping pen on fur

Dr Rachael Gray and colleagues treating Australian sea lions with ivermectin at Seal Bay Conservation Park, Kangaroo Island.(Supplied: Louise Cooper University of Sydney)

Hookworm slows pups’ growth

The Dangerous Reef results were verified with treatment trials in 2019 and 2020/21 at Seal Bay Conservation Park, on South Australia’s Kangaroo Island.

The Seal Bay trials also investigated whether the treatment of pups improved their overall survival and found optimal outcomes were achieved if pups were treated early, prior to the development of disease.

“Each of these little worms attaches onto the wall of the small intestine and … I think the highest number of worms we found in an individual pup was 8,000. The average for the Australian sea lion usually is about 2,200 worms,” Dr Gray said.

“They cause haemorrhage, they cause inflammation and basically they affect the ability of that wall of the intestine to absorb nutrients and it causes bleeding and diarrhoea.

“Basically the pups are losing blood, they’re losing nutrition and protein and that makes them weaker and makes them lose weight and also slows their growth.”

A blonde woman wearing a blue jacket

Dr Rachael Gray, a senior lecturer in veterinary pathology at the University of Sydney, researches sea lions.(Supplied: University of Sydney)

The balance between the parasite and host appeared to be changing.

“I think hookworm and Australian sea lions have co-evolved, so it’s probably always been an issue and it’s probably always been at a high prevalence,” Dr Gray said.

“But what we’re trying to understand is, it’s not intuitive for a parasite to kill its host at such a high rate and cause such severe disease.

“Obviously if you’re killing your host, then you’re not able to perpetuate your life cycle.

“Part of the things we’re trying to look at is: has there been a change in that interaction between the host and the parasite?”

Reducing eggs on the ground

Dr Gray said pollutants could be playing a role because they affected the immune response of individual animals.

She said the hookworm eggs and larvae were shed in the pups’ faeces and could live for four to eight years in the right conditions before entering adult sea lions through the skin.

“They localise in the abdominal fat and then just before mum is about to give birth there is some physiological trigger, which they think is probably hormonal, then those larvae basically wake up, migrate to the mammary gland and then transfer their larvae onto the next pup,” Dr Gray said.

“If we treat the pup early enough we can reduce the amount of eggs that they shed in their faeces and that will reduce hopefully the parasite load on the ground.”

The trial also found ivermectin was an effective lice killer, reducing infestations by more than 80 per cent.

The findings have been published in the International Journal of Parasitology: Parasites and Wildlife.

Posted , updated 

Ivermectin could save endangered sea lions from deadly parasite
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