Bird flu has claimed its first Australian sea lion and dolphin. How does the virus get into these animals?

A rare Australian sea lion (Neophoca cinerea) on Kangaroo Island has been confirmed to have died from highly pathogenic H5N1 bird flu. The sea lion had recently been attacked by a shark, weakening it. Testing revealed the cause of death was the virus. Another dead sea lion is being investigated.

The first confirmed H5N1 death recorded for this species in Australia marks a significant and worrying moment for Australian wildlife. These sea lions are the rarest pinniped to breed on our coasts. They’re considered endangered, as their population is under 12,000. This species of sea lion breeds slowly, meaning even a limited outbreak of bird flu could be very bad news for breeding colonies.

The strain of bird flu now circulating in Australia and New Zealand has evolved to infect some mammal species as well as birds. Since 2021, it has decimated some seals and sea lion species overseas, especially in South America.

The virus is even capable of infecting and killing dolphins.

But how does the virus pass between these animals? How do seals and dolphins actually get the infection?

There’s much we don’t know. But here’s what we do.

Bird (and mammal) flu?

The highly pathogenic H5N1 strain of bird flu (avian influenza) was first detected in 1996. Highly pathogenic means it is capable of causing severe disease.

In 2020, the virus mutated into a new, more deadly variant capable of infecting more species. It is no longer just a threat to birds.

Since 2021, this strain has caused mass die-offs in pinnipeds (seals, sea lions, walruses and so on). In Argentina, more than 17,000 southern elephant seal pups died. After the virus reached Australia’s Heard Island, drone surveys identified more than 13,000 dead pups from the same species.

Some pinnipeds were able to catch older bird flu strains. But these mainly affected their respiratory tracts. The newer highly pathogenic H5N1 strain can also invade the brain and nervous system of mammals.

Sea lions have been found with encephalitis, brain inflammation and other neurological signs before death.

In Florida, a bottlenose dolphin infected with H5N1 was found to have the highest concentration of virus in its brain.

How do pinnipeds catch the virus?

The way the virus gets into pinnipeds is likely from birds. When sick seabirds infected with the virus die on the islands used by sea lions for breeding, the virus can jump across to the mammal. This can be through direct contact, scavenging carcasses or exposure to contaminated sand, rocks, water, faeces or mucus. Sick birds can and do sneeze and cough, spreading viral particles.

Direct transmission from mammal to mammal is suspected, though not yet conclusively proven in most wild marine mammal die-off events.

But the fact so many elephant seal pups died rapidly suggests the virus is able to spread between them. Transmission between elephant seals is suspected to have occurred in Argentina. It’s highly likely that the virus can spread faster in dense seal and sea lion colonies.

Pinnipeds, too, can sneeze and cough, which would help spread the virus. One researcher testing transmission reportedly had a seal sneeze in their face. They then contracted severe conjunctivitis in the eye. The bird flu virus was detected.

But what about whales and dolphins?

So far, so logical. But how can dolphins and other cetaceans contract the virus?

After a dolphin washed up dead near Goolwa, South Australia, it tested positive for H5N1 – the first case in Australia.

How did it happen? SA chief veterinary officer Dr Skye Fruean told the ABC it was likely the dolphin had “eaten a bird or interacted closely with a wild bird”.

This is possible, but we don’t know for sure. What we do know is that this H5N1 strain has been found in a range of dolphin species around the world.

So far, there’s little evidence of large whales being infected with bird flu. A juvenile dead whale washed ashore at Victor Harbour was found not to have the virus.

How long can the virus survive in the environment?

Viruses can survive outside a host, though typically not for long.

The H5N1 influenza virus generally survives longest in cool, moist conditions. It can remain infectious for days and sometimes weeks in water, depending on temperature, salinity and sunlight.

In hot, dry environments, survival is much shorter.

Carcasses can act as temporary reservoirs of the virus while tissues remain fresh.

This is one reason why beaches and breeding colonies can be transmission hotspots during an outbreak.

What can we do?

There’s no real way to stop the spread of H5N1 now that it’s here. But there are many ways we can reduce its impact.

Removing carcasses safely is crucial, as it removes these viral reservoirs and stops it getting into Australia’s many scavenging animals.

Investing in better monitoring and detection is essential if we are to slow the spread by helping us to target affected populations.

We must ramp up protection for endangered species, reduce human pressures such as disturbance of seal colonies and plan to help depleted populations recover.

Ultimately, the death of this sea lion shows us that the burden of bird flu will reach far beyond birds. This strain of the virus is a wide-ranging threat able to affect entire ecosystems. We must do what we can to protect them.