An outbreak of rabies in seals has been identified by scientists in South Africa, marking what they believe is the first occurrence of the virus spreading among sea mammals.
State veterinarian Dr. Lesley van Helden revealed that at least 24 Cape fur seals, found dead or euthanized across South Africa’s west and south coasts, tested positive for rabies.
Rabies, which primarily affects mammals and can be transmitted to humans, is almost always fatal once symptoms appear. The virus is typically spread through saliva, often by bites, but can also be passed when animals groom or lick each other.
Though rabies has been common in wild animals like raccoons, coyotes, foxes, jackals, and domestic dogs, its transmission among marine mammals had not been previously documented, according to van Helden and other experts.
A case of rabies in a sea mammal was recorded in the early 1980s in Norway’s Svalbard islands, where a ringed seal was likely infected by a rabid arctic fox. However, there was no evidence of the virus spreading among seals at that time.
The discovery of rabies in Cape fur seals was first made in June after a seal bit a dog on a beach in Cape Town, infecting it with rabies. This prompted authorities to conduct rabies tests on brain samples from 135 seal carcasses collected since 2021. Additional tests on 20 new samples revealed more positive cases.
Researchers are now investigating how the virus reached the seals, its spread among large seal colonies, and what measures can be taken to control the outbreak.
There are around 2 million seals migrating along the coasts of South Africa, Namibia, and Angola. Dr. van Helden suggested that the virus may have been transmitted to seals by jackals in Namibia, where the animals hunt seal pups.
Genetic analysis confirmed that the rabies virus found in the seals matched strains found in black-backed jackals in Namibia. It also indicated transmission between the seals themselves, as the virus sequences were closely related.
In South Africa, especially in areas like Cape Town where seals and humans interact closely, authorities have issued warnings. Gregg Oelofse, head of Cape Town’s coastal and environmental management, emphasized the need for caution.
For the past three years, officials have been puzzled by incidents of unusually aggressive behavior in seals, along with a rise in seal attacks on humans, some of which resulted in bites. Fortunately, no human rabies cases have been reported so far.
According to Oelofse, local authorities have begun vaccinating a limited number of seals at two popular harbors in Cape Town, where the animals are a tourist attraction.
A rabies-positive seal carcass from August 2022 suggests the virus has been present in the seal population for at least two years, Oelofse added.
Experts said there were still many unknowns.
It’s hard to predict long-term transmission dynamics, U.S. Centers for Disease Control and Prevention spokesman Dave Daigle said. He noted previous instances of rabies viruses winding up in new hosts and then dying out. In 2021 in the U.S., for example, gray foxes were spreading the raccoon rabies virus variant for two years, and then transmission stopped.
The U.S. public health agency is watching the situation in South Africa, but has yet to see “clear evidence that this is going to be a long-term issue,” Daigle said.
Another uncertainty is whether the vaccine will be effective in seals, as it has never been tested on them. However, experts are optimistic that it could work.
A logistical challenge also exists, according to van Helden: vaccinating a large number of seals, which primarily live in the ocean and migrate across more than 3,500 kilometers (2,170 miles) of coastline. While land animals can be immunized through bait that delivers oral vaccines, seals typically only consume live fish, van Helden pointed out.
South African authorities are working closely with international experts on this issue.