Scientists say rabies spreads among South African seals

For the past three years, scientists in South Africa have been trying to solve a grim marine mystery: what was happening to the Cape fur seals?

The boisterous marine mammals that are common along the country’s coastline, started washing away the dead in huge numbers. Pregnant females gave birth to stillborn pups prematurely. And some seals began to exhibit unusually aggressive behavior, attacking people, dogs, and each other.

Some scientists suspected that a neurotoxin produced by algae might be to blame. But in recent weeks, another specter has emerged: rabies.

So far, 17 seals have tested positive. because of the virussaid Tess Gridley, founding director of Sea Search Research and Conservation, which investigated the seal deaths. The cases, which date back to at least August 2022 and cover hundreds of miles of coastline, could be the first documented sustained outbreak of rabies in marine mammals.

“What I do is sit here and collect all the reports of aggressive interactions between seals and dogs and seals and humans over the last few years,” Dr. Gridley said. “And it tells a pretty horrifying story..”

Rabies, which is almost always fatal once symptoms appear, is spread through the saliva of infected animals.. There have been no recorded human cases so far, but Dr Gridley estimates that at least 72 people in South Africa have been bitten or scratched by Cape fur seals since 2021; eight people have been bitten by seals since then and have been confirmed to have rabies.

Scientists and officials gathered in Cape Town this week to discuss what they know about the outbreak and how to proceed. They agreed to euthanize seals suspected of having rabies and vaccinate those that have regular contact with humans, Dr. Gridley said.

However, many issues related to the epidemic and how best to deal with it remain uncertain.

“It’s a really tough situation,” said Gregg Oelofse, the city of Cape Town’s coastal manager. “There’s no global benchmark for dealing with rabies in a marine mammal species.”

Rabies is sporadic in marine mammals, and when immense numbers of dead seals began to appear in 2021, the disease wasn’t high on the list of possible causes. Instead, Dr. Gridley suspected domoic acid, a toxic compound produced by a type of algae. The toxin has been linked to the deaths of sea lions in California and is known to cause neurological symptoms. (The researchers saw no signs of bird flu, which has killed immense numbers of seals and sea lions in North and South America.)

But a series of aggressive seal attacks this spring brought rabies back into the spotlight, and Dr. Gridley and her colleagues decided to test the four seals for the virus, even if it was unlikely. “Everyone told us that rabies didn’t occur in Cape seals and that we were wasting our time,” said Dr. Luca Mendes, a veterinarian at False Bay Veterinary Clinic.

But three of the seals tested positive. “That just opened up a whole modern Pandora’s box,” Dr. Mendes said.

The research team has collected brain samples from a number of seals that have died in the past few years; they are currently testing the samples for rabies. The virus likely does not account for all the deaths, some of which may still be related to domoic acid, the researchers said.

Virus samples taken from infected seals suggest the marine mammals were infected with the canine version of the virus, said Dr Lesley van Helden, state veterinarian at the Western Cape Department of Agriculture. In South Africa, this form of rabies occurs not only in domestic dogs but also in wild red-eared foxes and black-backed jackals.

“The viruses obtained from seals appear to be more closely related to our wildlife viruses than those currently circulating in dogs,” Dr. van Helden said in an email. However, she cautioned that “the information we have so far is very preliminary and we need more samples.”

Genomic analysis also suggests that once the virus got into the seals, it started circulating between them. “It definitely looks like the seals are transmitting it between each other,” said Dr. Johan Steyl, a veterinary pathologist at the University of Pretoria.

Seals are very social and gather on beaches in huge numbers during the breeding season. They have been known to bite each other when fighting and mating.

They also often cross paths with humans, hang out at docks, share waters with surfers and attract crowds of tourists. Dr Gridley said that of all the marine mammals that get rabies, “the Cape fur seal is the real deal.”

Authorities warn to keep a secure distance from seals and urge anyone bitten to seek immediate medical attention.

Experts also fear the virus could spread to some species of Antarctic seals that sometimes visit South Africa. “We desperately don’t want rabies to come back,” Mr Oelofse said.