CAPE TOWN, South Africa — Scientists in South Africa say they have identified an outbreak anger in seals which is believed to be the first time the virus has spread in marine mammals.
At least 24 Cape seals that were found dead or euthanized in various places on the South African coast had rabies, said state veterinarian Dr Lesley van Helden.
Rabies, which affects mammals and it can be passed on to peopleit is almost always fatal once symptoms appear. Rabies is spread through saliva, usually through biting, but also sometimes when animals lick and groom themselves.
The virus has long been seen in wild animals such as raccoons, coyotes, foxes, jackals and in domestic dogs. But it had never been recorded spreading among marine mammals, van Helden and other experts said this week.
The only other known case of rabies in a marine mammal was in a ringed seal in Norway’s Svalbard Islands in the early 1980s. That seal was likely infected by a rabid arctic fox, the researchers said, and there was no evidence of rabies spreading between the fires.
Authorities in South Africa first discovered rabies in the Cape fires in June after a dog was bitten by a seal on a Cape Town beach. The dog became infected with rabies, which prompted rabies tests on brain samples from 135 seal carcasses that researchers had already collected by 2021. Around 20 new samples were also collected and more positives came back in subsequent tests.
Scientists are trying to find out how rabies was transmitted to the seals, whether it is widely spread among their large colonies and what can be done to contain it.
“It’s all very, very new,” said Greg Hofmeyr, a marine biologist who studies seals in South Africa. “A lot of research is needed … there are a lot of unknowns here.”
There are approximately 2 million seals that migrate back and forth between South Africa, Namibia and Angola along the south and west coast of Africa. The most likely possibility, van Helden said, is that rabies was first transmitted to seals by jackals in Namibia, where the wolf-like animals hunt seal pups on the coast.
Rabies virus genes found in seals match rabies in black jackals in Namibia. It also showed that rabies was transmitted between seals, because most of the virus sequences were closely related, he said.
“So it’s basically established in the seal population and it’s maintained by them biting each other,” van Helden said.
Seals live close to people in places in South Africa, especially on the beaches around the city of Cape Town. The city has issued warnings to locals, said Gregg Oelofse, Cape Town’s head of coastal and environmental management.
Authorities have been mystified for the past three years by reports of overly aggressive seals and an increase in seal attacks on people, some of whom had been bitten. No human cases of rabies have been recorded as a result.
Oelofse said city authorities had begun vaccinating the small number of seals in two popular ports in Cape Town, where they are considered an attraction.
One of the positive rabies tests was on a seal carcass collected in August 2022, meaning rabies had been in the seal population for at least two years, Oelofse said.
“He’s been here for a while longer than we’ve known,” he said.
Experts said there were still many unknowns.
It is difficult to predict the dynamics of long-term transmission, the spokesman for the US Centers for Disease Control and Prevention Dave Daigle said. He has noted previous cases of rabies viruses breaking out in new hosts and then dying. In 2021 in the United States, for example, gray foxes spread the variant of the raccoon rabies virus for two years, and then the transmission stops.
The US public health agency is looking at the situation in South Africa, but has yet to see “clear evidence that this is going to be a long-term problem,” Daigle said.
Another unknown is whether the vaccine will be effective in seals. It’s never been tested, but experts think it should work.
There’s also a logistical question, van Helden said: How to vaccinate a significant number of seals that live largely in the ocean and migrate back and forth along a coastline more than 3,500 kilometers (2,170 miles) long. Land animals can be vaccinated by dropping bait that releases oral vaccines when eaten, but seals generally only eat live fish, he said.
South African officials collaborated with international experts.
Seal researcher Hofmeyr said some other seal species came into contact with the Cape seals and then traveled to other parts of the world and that was a concern for further spread.
“The odds of that happening are very low, but the implications of it happening are very important,” he said.
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