Workers in protective clothing test a sea lion for bird flu on a beach

The sea lions are believed to have contracted bird flu from pelicans. Image © SERNANP

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Bird flu kills thousands of South American sea lions as outbreak continues

Thousands of sea lions are among the latest victims of bird flu.

An outbreak of highly pathogenic avian influenza (HPAI) has swept the world over the past year, raising fears that humans could become more vulnerable.

There are concerns that bird flu could be getting better at crossing over to mammals after the disease killed 3,500 South American sea lions in Peru.

The Peruvian government has reported that since November 2022 around 3% of the country's population of these animals have died as a result of HPAI, the deadliest form of bird flu.

It is originally believed to have been brought into Peru by pelicans, before killing tens of thousands of birds and making the jump into the marine mammals. The country's agency for protected areas, SERNANP, is warning people to stay away from any animals showing signs of illness.

Roberto Gutierrez, SERNANP's Head of Surveillance, told Reuters, 'We know that bird flu is highly pathogenic in birds, but it can also affect mammals. There was a case in a child in Ecuador in January, and now sea lions are getting infected.'

'It's probable there would be people infected in places with infected animals.'

Dr James Rule, an expert in pinnipeds (seals and sea lions) at the Museum, says that while the species should survive, there is a risk that sea lions and their relatives could spread bird flu further than ever before.

'South American pinnipeds are known to overlap with birds and mammals that travel to Antarctica, and there is a real risk that bird flu could spread to the continent this way,' James says. 'This is a major cause of concern because of Antarctica's large bird populations, and colonial birds such as penguins, would be highly vulnerable.'

A raccoon with its front paws on the side of a tree in woodland

Raccoons are one of the mammal species known to have caught bird flu in the past. Image © L-N/Shutterstock

Bird flu in mammals

While bird flu has been a cause of concern among public health experts for some time, its ability to infect mammals has remained relatively low.

Most HPAI strains can't infect humans, and the ones which can, such as H5N1, can currently only bind to receptors that are uncommon in the upper airways of mammals, making infections less likely.

This has meant that while the virus has spread rapidly among poultry and wild birds, it is not thought to have been spreading between mammals. However, experimental studies in ferrets have suggested that five mutations could allow this to happen more easily.

These mutations may not just be limited to the lab, however. The virus responsible for an outbreak of HPAI at a Spanish fur farm, in which over 50,000 American minks were culled, was found to have acquired a mutation that allowed it to replicate faster in mammals.

While it's not certain if the virus was spreading between the minks, the same study could not rule it out.

In the wild, evidence of HPAI spreading between mammals is even less clear. While cases have been reported in animals including raccoons, bears and foxes, these tend to be sporadic.

Pinnipeds may be particularly at risk due to their lifestyle, as not only do many species live closely together in breeding colonies, but they also have plenty of contact with seabirds.

'While it's not entirely certain how seals and sea lions have contracted bird flu, the leading theory is that they've caught it by eating infected birds,' James says. 'Birds form part of their diet, meaning it's more likely that these mammals will catch the disease than others.'

He adds that although the number of deaths is significant, the large number of different sea lion colonies should help to buffer the species against HPAI.

'Provided that the virus doesn't spread between different sea lion colonies, they should be able to tolerate the disease,' James explains. 'South American sea lions and fur seals have been known to catch tuberculosis, and though on a smaller scale, this does not appear to have severely impacted their populations.'

A worker in protective clothing stands near a sea lion pup on a beach with shelters

Public health agencies continue to monitor wild animals for signs of bird flu and observe any changes in the virus. Image © SERNANP

Is bird flu adapting to humans?

The spread of bird flu into wild mammals has raised concerns that avian influenza viruses could be getting better at adapting to humans as well. This has only been heightened by the recent infection of two people in Cambodia in February, one of whom later died.

While HPAI infections in humans do occur from time to time, it is important to remember that they remain rare.

At the time of writing, there have only been 868 reported human cases of this disease over the past 20 years, which were mostly in people who worked closely with poultry. The recent Cambodian infections, for instance, are thought to have been caused this way.

Genetic analysis of the virus responsible for the deaths in Cambodia showed that they were a local strain of HPAI that isn't closely related to other outbreaks elsewhere in the world. The virus also showed no sign of having any mutations that might make them better at infecting humans.

Public health organisations continue to monitor bird flu for any signs that it could become more threatening, while also taking precautions against it.

This includes the creation of vaccines and antivirals, which should be more rapid than those for COVID-19 simply because influenza is so common. Medicines that work on other influenza viruses should provide at least some protection from bird flu, and provide a starting point for further development.

Even with these viral defences, the best way to avoid getting HPAI is to avoid any infected animals. Symptoms include swollen heads, a lack of coordination and gasping for air in birds, while the virus can cause coughing, shortness of breath and diarrhoea in mammals.

Animals with these symptoms, or which have passed away, should not be touched under any circumstances. Instead, they should be reported as soon as possible – animals found in the UK should be reported to Defra on 03459 33 55 77.