Dr Rachael Gray has the seal of approval


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Dr Rachael Gray
Photo: Nick Cubbin

Every Australian sea lion pup is infested with debilitating hookworms. Dr Rachael Gray wants to know if effective treatment is beneficial for the species’ recovery. By Kerryn Ramsey

In 2006, Dr Rachael Gray visited Seal Bay on Kangaroo Island for the first time to evaluate the health status of Australian sea lion pups. 

“I expected to collect some samples and find that everything was normal,” says Dr Gray. “I thought I would do this for a couple of years then walk away having confirmed a healthy sea lion population.”

Things didn’t go according to plan. One of the first necropsies Dr Gray performed revealed the intestines of a sea lion pup full of worms. She looked back over the literature and found the first report of hookworms in Australian sea lions by parasitologist Professor Ian Beveridge in 1980.

“I realised we were dealing with hookworm and it was very widespread,” says Dr Gray, who graduated with a Bachelor of Veterinary Science from the University of Sydney in 1996. After working in small animal practices in Australia and the UK, she settled in Sydney to complete a PhD on the health status of leopard and Weddell seals in Antarctica. This included spending two summer research seasons at Davis Station. “The pups only weigh between seven and 10 kilos but have thousands of worms attached to the wall of their intestine, causing bleeding and protein loss. The highest worm count for an individual was greater than 8000 worms. All these pups are in very poor condition and essentially starving.”

In 2013, the Recovery Plan for the Australian Sea Lion was developed by the Australian Government. One of its objectives was to investigate threats to these native pinnipeds, including disease and pollution. With her research directly aligning with the goals of the species’ recovery plan, Dr Gray has been working with sea lions ever since that first trip to Kangaroo Island.  

Seal Bay colony 

Over the next few years, a packed cell volume baseline was established, revealing the red blood count of Australian sea lions was lower than all other species. In 2011, a treatment trial at Dangerous Reef in Spencer Gulf, SA, found that the injection of ivermectin subcutaneously would successfully treat the animals for hookworms. In 2019, the team concentrated their research on the more accessible sea lions of Seal Bay and started assessing the effectiveness of a topical application of ivermectin. The colony is unique in one important detail—they have been part of a microchipping program since 2002.

Working alongside Dr Gray is Melanie Stonnill, the research and operations coordinator at Seal Bay. She monitors the species while assisting with the operational requirements of the educational tours run by the Department for Environment and Water SA, the managers of the Seal Bay colony. Every season, Stonnill and other research groups microchip as many pups as possible. 

“We estimate that 80 per cent of the colony is now microchipped,” says Stonnill. “The oldest known-aged female to pup at Seal Bay was born in 2003 and she pupped for the first time at age nine. She had her sixth pup this year, though not all have survived.”

By utilising the microchip information, Dr Gray can track the impact of the disease while minimising the number of times an animal needs to be caught.


Hookworms infest every Australian sea lion pup with a mortality rate as high as 40 per cent in some breeding seasons. To further complicate things, the elimination of the hookworms may not lead to the recovery of the population. 

“It’s almost certain hookworms co-evolved with Australian sea lions and, generally, that’s a positive thing,” says Dr Gray. “The problem is that there are all types of other impacts on the existing population. Australian sea lion numbers are declining due to fisheries interactions and habitat change. Climate change will also have an impact because they live on low-lying areas for a majority of the time. Their low reproductive rate, with pupping occurring once every 18 months, further limits their capacity to recover from historical commercial harvesting.

The pups only weigh between seven and 10 kilos but have thousands of worms attached to the wall of their intestine, causing bleeding and protein loss. The highest worm count for an individual was greater than 8000 worms.

Dr Rachael Gray, Sydney School of Veterinary Science

“We’re also looking at heavy metal intoxication and persistent organic pollutants. We’ve found very high mercury concentrations in pups and mercury causes immunosuppression. So, not only is there a terrible endemic parasite but human impact is dampening down the pup’s ability to respond to that infection.”

Life cycle

The life cycle of the hookworm is a masterful example of integration between two species. Eggs are shed into the faeces of an infected pup and those eggs turn into larvae that can survive in soil, sand or substrate for up to four years. The larvae invade through the skin and blubber of a sea lion and sit dormant. When a female is about to give birth, a physiological trigger, likely hormonal, sees those larvae migrate to the mammary glands. As the newborn pups drink antibody rich and nutritious colostrum, the larvae are also transferred. The hookworms attach to the intestines and start feeding between 11 and 14 days later.

Dr Gray and her team are hoping to show that the treatment of pups for hookworm will positively impact on the length, weight and growth of the pup. They also need to prove that treating pups improves overall survival and that more animals go into a reproductive cohort. While the main cause of mortality for pups is trauma by other animals, that may also be related to their hookworm infestations. When an animal is anaemic and lethargic, they’re less able to escape from the trauma. 


Dr Gray and her team are being very cautious with their intervention. At present, the trial consists of 50 per cent treated pups and 50 per cent untreated. This is to reduce the development of resistance to ivermectin while ensuring this refugia of parasites will still be susceptible and part of the co-evolution process.

“We have a treatment that successfully removes the hookworm but we have to also be certain it doesn’t produce a negative impact on the intestinal bacterial microbiome,” says Dr Gray. “The species is considered endangered and the population is declining 2.4 per cent every breeding season.”

Working together

Kangaroo Island and the Seal Bay colony are popular tourist destinations and Melanie Stonnill ensures the research information is used to personalise the educational component of the guided tours. At the same time, she helps ensure the Australian sea lion colony is only minimally impacted by outside forces.

“This season we had six pups born within our touring range,” says Stonnill. “One of the greatest challenges we face, besides the wild weather, is that we can only observe, learn and appreciate the colony. We can’t interfere when we see shark injuries, pups wandering in the heat with no protection, conspecific trauma, orphaned pups, distressed females looking for their offspring, weaned juveniles struggling to make it on their own, and all other raw patterns of their lives.”

The long game

Female sea lions don’t birth their first pup until six years of age while males aren’t reproductively active until age 10. Dr Gray is in it for the long haul.

“The female pups we treated in 2019 will need to be followed for years,” she says. “I’ll be returning to the colony every 18 months to scan juveniles and mums, and follow their development. My involvement will run for at least another 10 years.”

Funding for Dr Gray’s research has come from the Hermon Slade Foundation and Sydney School of Veterinary Science bequests. Further funding supporting the current treatment trial is supplied by the Department for Environment and Water SA but that ceases at the end of the 2021 season. After that, Dr Gray will be back on the treadmill, applying for grants and support. Fortunately, her enthusiasm for the project knows no bounds—she spent last year’s Christmas eve on Kangaroo Island with her husband and kids, analysing blood and faecal samples collected from Seal Bay pups. 

Dr Gray is hopeful for the future of Australian sea lions. “I’m positive about the long-term viability of Australian sea lions and hope our work will ultimately benefit the species.”

Melanie Stonnill agrees. “There’s a large number of people and organisations who are dedicated to protecting the species, and that gives me hope.”


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