Ivermectin could throw a lifeline to endangered Australian sea lions

Ivermectin has been burdened with a seriously bad reputation in recent times. Infamously – and incorrectly – touted as a wonder cure for COVID-19 by, the drug has emerged from the turmoil of the pandemic as something of a snake-oil equivalent in the collective psyche.

But this image is undeserved – despite being thoroughly debunked as a COVID treatment, ivermectin is one of the most effective parasitic infection treatments currently available in both human and veterinary medicine. And now, researchers at the University of Sydney are directing ivermectin treatment at a severely parasite-burdened species – Australia’s iconic sea lion.

The research team, led by Dr Rachael Gray, has been investigating the impact of parasitic loads on Australian sea lion populations since 2006. Publishing in the International Journal of Parasitology – parasites and wildlife, the team has documented an extraordinarily encouraging success rate of ivermectin treatment for hookworm in sea-lion pups, with a reported effectiveness rate of over 96%.

This treatment could prove to be a desperately needed lifeline for Australian sea-lion populations, which were reclassified from ‘vulnerable’ to ‘endangered’ on the IUCN Red List last year, and are the only marine mammal to be included on the government’s 100 priority species for conservation.

While threats to the population are many – including competition with fisheries, marine debris and pollution and climate change – it’s perhaps the loss of significant numbers of pre-weaning age pups each breeding season, often due to hookworm infestation, that’s most strongly holding back population recoveries.

Dr. Rachael Gray and colleagues treating Australian sea lions with ivermectin at Seal Bay Conservation Park, Kangaroo Island. Credit: Louise Cooper/USYD.

Every Australian sea-lion pup born will inevitably become infected with hookworm, which is passed directly from mothers to pups through milk. This infection can have devastating consequences – hookworm burrow into the intestinal wall, leading to blood loss, diarrhoea and inflammation. When infestations are severe, this stress can overwhelm the bodies of tiny pups and lead to death.

Gruesomely, even those pups who can survive infestation are not out of woods. In their weakened state, they are much more vulnerable to the leading cause of death in Australian sea lions – trauma inflicted by fellow sea lions.

Luckily, ivermectin is an effective treatment for hookworm. The team has been experimenting with various methods of administering the drug and have found that topical applications show comparable effectiveness (96.4%) to the more traditional, but more invasive, injection method (96.8%).

“Given there are multiple threats to the species’ recovery, the goal was to see if we could test a safe, effective and minimally invasive hookworm treatment,” says Gray.

Treatment is theoretically simple – ivermectin is daubed onto the back of the pup’s neck, in the same manner that you might apply a flea treatment to your dog or cat. Compared to domestic pets though, treating sea-lion pups comes with significant logistical challenges. The sea-lion research team must come prepared for self-sufficient camping in remote locations – such as Dangerous Reef off the coast of South Australia – as well as performing the necessary, but decided unglamorous, task of collecting faeces for hookworm assessment.

Sea lion pups
Australian sea lion pups at Dangerous Reef, SA. Credit: USYD

Initial results of this new treatment method have been very promising, with significantly improved survival in pups up to the age of five months. Helping to boost the numbers of pups that make it past this milestone could vastly improve the long-term trajectory of sea lion population densities.

“If pups can survive to weaning, they should have a greater chance of getting to reproductive age and directly contributing to population recovery,” says Gray.

Even with a simplified application approach, the aim will not be to treat the entire population. Logistical issues aside, there are good reasons to leave some pups to the mercy of hookworm.

“One thing we have to consider is reducing the risk of worms developing resistance to ivermectin,” notes Gray. The team is attempting to address this by leaving a portion of each colony untreated. Pups there will hopefully gain some benefit from the treatment program, given a reduced overall environmental load of hookworm eggs and larvae.

Over the longer term, the research group will be looking into the factors that affect the balance between this parasite and its sea lion host. Theoretically, it isn’t in the best interest of a parasite to overwhelm its host to the degree observed in sea-lion pups. Determining which additional environmental factors are skewing the balance in favour of hookworm – such as toxicants that can suppress the immune response of pups – could be key in focusing environmental remediation efforts.

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