Why are Cape fur seals in South Africa infected with rabies? | Wildlife News

In May of this year, a seal bit five surfers in a matter of minutes at a popular beach in Cape Town, South Africa. The surfers laughed it off with characteristic nonchalance, but seal experts were concerned because the extremely unusual behavior appeared to be part of what was becoming a pattern.

Six days earlier, a seal had washed up on the other side of town with horrific facial injuries that could only have been caused by a very aggressive animal.

From late 2021, authorities noted with concern an increase in seal aggression. While most seals continued to ignore humans, a few seemingly “deranged” animals began biting humans or other animals without any provocation.

“Although the behaviour looked ‘mad’, our best scientific knowledge is that seals do not get rabies,” said Dr Tess Gridley, founder of Sea Search, an NGO specialising in marine mammal research.

After the May attacks, as public speculation mounted, it was decided to test four seals (the two previously mentioned and two others) for rabies. “We were desperately hoping it wasn’t rabies,” Gridley said.

The answer was shocking: three of those four seals tested positive for rabies, a number that has now risen to 17.

How far along is the outbreak now?

At the time of writing, 17 seals have tested positive for rabies along a 650km (404mi) stretch of coastline between Cape Town and Plettenberg Bay. Some of these positive tests have come from animals euthanised for aggressive behaviour since the first case was confirmed, while others have come from retrospective testing of 130 samples biobanked (preserved) by Sea Search over the past three years as part of an unrelated study. The number of positive cases – both from retrospective and future testing – is certain to rise.

Although the investigation is still ongoing, the latest analyses show that the seals have a strain of the virus that also occurs in wild animals.

“Our best guess is that the seals got it from black-backed jackals,” said Dr. Brett Gardner, a veterinarian with a special interest in disease epidemiology based at the University of Melbourne in Australia. Rabies is endemic among southern African jackals, which prey on seal cubs in land-based colonies on the west coast of South Africa and Namibia.

At least one pet dog in Cape Town appears to have contracted rabies from a seal bite. So far, none of the people bitten by rabid seals have developed rabies.

Why are experts so concerned?

It is the first outbreak of rabies in a marine mammal in the world. The only other known case of a seal contracting rabies occurred on the Norwegian island of Spitsbergen in 1980 and was considered an isolated incident.

“We just don’t know how the disease is going to evolve,” Gardner said. “And we have a lot of questions about things like the transmission rate. Is this going to look like what we’re used to seeing in conventional land mammals, or is it going to look more like the unexpected mass mortality that’s historically been seen in rabies-infected kudus?”

Two million Cape fur seals live along a 3,000 km (1,864 mi) coastline that stretches from southern Angola to Algoa Bay on the east coast of South Africa. The seals spend days or weeks at sea, but on land they live in crowded colonies where their need to defend their personal space results in frequent fights and altercations – which is not ideal, since rabies is mainly transmitted through saliva.

A potential positive is that seals have less saliva than land mammals, so they can swallow slimy fish underwater without much lubricant.

“We’re encouraged that no humans have developed rabies yet,” says Gardner, who is curious about why. “Does the saltwater reduce the viral load or partially inactivate the virus? Do people’s neoprene wetsuits clean the seals’ teeth before they take blood?”

“We don’t know any of these answers yet.”

Is it safe to go to the beach in South Africa?

Surfers, swimmers, fishermen and other water users are urged to continue to enjoy the ocean, but do so with caution and keep your dog on a leash at all times.

“There is no need to panic if you see a relaxed seal,” says Gregg Oelofse, who leads the City of Cape Town’s coastal management team. “But if an animal looks ferocious or aggressive, move away and alert fellow beachgoers and the authorities.”

According to Gardner, there are other, nonspecific signs that an animal may have rabies: incoordination and other neurological signs.

Rescue workers and shark spotters in the affected areas have been ordered to close beaches at the slightest sign of doubt, while companies offering snorkeling tours with seals have no choice but to close their operations.

Rabies is a slowly progressive disease of the nervous system – it can last for months or even years. However, if it becomes symptomatic, it is almost always fatal, in all species.

If someone is bitten, the wound should be washed with soap and water for 15 minutes. The next step is to see a doctor for an injection of rabies immunoglobulin (which binds to the virus) and a series of rabies vaccines. With this course of action, it is extremely unlikely that you will contract rabies.

What is the most likely outcome of the outbreak?

While Gardner and Gridley emphasize that there is no scientific precedent for rabies outbreaks in marine mammals, experience with the disease in land animals suggests three possible scenarios.

• The disease could be eradicated through vaccination programs. However, with two million seals spread across three countries (Angola, Namibia, South Africa), this is not feasible, especially since the rabies vaccine requires multiple doses to be fully effective and oral administration of vaccines, as is done for raccoons and coyotes, is out of the question.
• The disease is becoming endemic in Cape fur seals, with occasional flare-ups such as those currently occurring. “The impact on the seal population is unknown,” a City of Cape Town press release said. “However, in other animals, rabies tends to progress ‘slowly’, with flare-ups and declines, rather than leading to mass mortality.”
• The disease is becoming more virulent, causing more deaths and spreading more widely, as happened with the kudu in Namibia in the 1970s.

“Based on what we know so far, the second scenario is by far the most likely,” Gardner said, adding that the public can rest assured that scientists around the world are giving this their full attention.

Is the entire ecosystem at risk?

While the outbreak is certainly not ideal, it doesn’t seem apocalyptic either—at least for now. Rabies only affects mammals, so there’s no reason to worry about seagulls or penguins getting it. And while dolphins or whales could technically get it from seals, their behavioral patterns make that extremely unlikely.

Scientists are particularly concerned about stray seals from subantarctic waters – particularly elephant seals, which come quite close to Cape fur seals – contracting the disease and bringing it back to their habitats. While this is considered highly unlikely (there are only a handful of strays each year), there is a plan to vaccinate all strays that visit, Oelofse says. There are also plans to vaccinate “harbor seals,” Cape fur seals that live in harbors and are often fed by people.

One local species that may be at risk is the Cape clawless otter. This species frequently comes into contact with Cape fur seals. The total population is much smaller and more vulnerable, between 21,000 and 30,000.

What now?

“Coastal authorities will continue to work closely with state veterinarians and scientists to implement ongoing proactive measures to responsibly manage the rabies outbreak,” Oelofse said.

They are particularly concerned about seals congregating in large groups when the breeding season begins in October.

“I’m really reassured by the proactive attitude I’ve seen from everyone in South Africa,” Gardner says. “And I hope they can euthanize problem animals before they get into the breeding colonies, but I’m a little nervous if they don’t.”