Tracing a Mysterious Rabies Outbreak to a Surprising Source

Robert Sivewright doesn’t remember the blood “but they say I was completely drenched in it,” he says.  

In February, Sivewright was leading a kayaking tour of Hout Bay Harbor on South Africa’s Cape Peninsula when a Cape fur seal with one bloodshot eye drifted up to his group. After bumping into one of the kayaks a couple of times, “it just lost it and started attacking the client's kayak,” biting it repeatedly, says Sivewright. He helped his guests scramble out of the water, but the seal turned its attention to him. It “managed to sink its teeth into me a good couple of times,” he says. 

Sivewright was taken to the hospital for reconstructive surgery and multiple post-exposure prophylaxis shots, and the seal was later euthanized from shore. Authorities couldn’t recover the body from the ocean for testing, but Sivewright is convinced it was rabid. “From the behavior, there's not much doubt that it was rabies,” he says. “There was definitely something not right with that seal.”   

Rabies is a viral disease that causes fatal inflammation of the spinal cord and brain. It is transmitted between mammals through saliva, usually from a bite or scratch from an infected animal. Before this outbreak—which began in 2024—no one knew that marine mammals could spread the illness. 

The epidemic left experts scrambling for answers: How did the virus get into seals and what might happen if they can’t stop it from spreading?

In 2021, an uptick in negative interactions between seals and humans signaled something was wrong. “Suddenly we saw this unexplained, very aggressive seal behavior,” says Gregg Oelofse, coastal manager for the city of Cape Town. In the past 20 years, he’d recorded only a couple of instances of seals biting humans.

At that time, there was only one recorded case of rabies in seals—a ringed seal in Svalbard, Norway, in 1980—and the virus hadn’t spread to other seals. 

Some experts wondered if an algal bloom might be behind the biting uptick. Domoic acid—a neurotoxin produced by certain algae—“causes neurological clinical signs in the infected animal” like aggression or erratic movements, says Ernest Ngoepe, a researcher at the Agricultural Research Council–Onderstepoort Veterinary Research in South Africa and the author of a study on the emergence of rabies in seals. “Nobody thought it could be rabies.”

Other illnesses, including a mysterious 2021 mass mortality event, also “masked this slow emergence of this rabies in the Cape fur seals,” says Tess Gridley, founding director of Sea Search in Cape Town, and author of a pre-print study on the outbreak.

The puzzle pieces didn’t fall into place until May 2024 when a pet dog caught rabies after a seal attack. Rabid dogs are rare in the Western Cape, prompting experts to consider whether the seal was the vector. Testing four seals confirmed that three animals that had displayed abnormally aggressive behavior were positive for rabies. 

This triggered retrospective testing, which confirmed more than 100 positive cases of rabies in seals on the Cape since 2024. Experts don’t know how many of the region’s 2 million seals may be infected. The only way to confirm rabies is by analyzing brain samples of deceased animals. “You're basically looking for carcasses and dragging them around the beach and chopping their heads off and getting the brains,” says Gridley. “It's not fun.” 

But how did seals contract the virus? “Initially, we thought it could be [from domestic dogs],” says Ngoepe, “but we don't have dog rabies in Cape Town.”

To solve the mystery, experts had to work backwards. Using genetic sequencing and phylogenetic analysis—mapping out how animals are related through evolution—they traced the source to black-backed jackals, a species known to carry rabies in South Africa. 

Jackals live close to seal colonies where they hunt pups and scavenge carcasses, so it’s plausible that rabies was transmitted during a fight. Sequencing reveals the virus first entered seal populations around 2015, long before anyone knew such a thing was possible.

Now, experts are desperately trying to contain the outbreak by vaccinating habituated seals and euthanizing animals suspected of having the disease. While individual seals can be vaccinated, injecting every seal from southern Angola through Namibia to South Africa's eastern cape is logistically impossible. “How do you vaccinate two million Cape fur seals?” says Gridley. “We're working in a complete unknown.”

For the vaccine to be effective, every seal needs a booster. “[All the seals] look the same, so you wouldn't know who's had a vaccine,” Oelofse says. “It's just not viable.” And seals eat their prey alive, so oral vaccinations—delivered via bait to inoculate scavengers—can't be hidden in food. 

A seal’s lifestyle also contributes to the spread of disease. “Cape fur seals are hugely gregarious,” says Gridley. “They just love hanging out with each other, and they're really wide-ranging.” Seals can swim 100 kilometers, or about 60 miles, in one day, so an affected individual could carry the disease between colonies. 

Around 59,000 people globally die from rabies every year. Quick medical intervention can protect humans from the virus, which can cause numbness, hallucinations, breathlessness, and paralysis up to one year after infection. Once symptoms set in, it's almost always fatal. 

The earliest confirmed attack on a human in Cape Town was in July 2022, and people are still getting bitten. On a single day in September 2026, a seal with presumed rabies bit nine people, including six children, at Muizenberg Beach. “There are about 176 bite cases that have been recorded,” says Ngoepe.

Still, experts don’t want people to be scared to enjoy the ocean. At the time of reporting, there has been no transmission from seals to humans. The disease is now endemic in the population, so it can’t be wiped out, but marine managers are doing what they can to keep it in check through vaccinations and euthanasia. “It's this slow burn in the population—little flares now and then,” says Oelofse.

Although the chances of getting bitten are low, experts advise the public to always give wildlife space. Infected seals often display “aggression, not wanting to go back in the water, and these weird head shakes,” says Oelofse. “If it's behaving strangely in any way, leave the water or leave the beach,” he says. “Call the authorities.”