Rabies is estimated to kill around 60,000 people worldwide each year mainly in Asia and Africa. While it can be stopped by vaccination or early treatment, once the full-blown disease develops, it is always fatal.
The virus is endemic among some wildlife in South Africa, though human cases in the country are not common.
Almost all worldwide cases in humans come from dog bites or scratches.
Tamsin Lovelock, an infectious disease expert at Stellenbosch University and Tygerberg Hospital, said while only one other outbreak in seals had ever been recorded before, the animals are not routinely tested, so it could remain undetected in the wild.
The only other recorded case in seals was in the Svalbard islands of Norway in 1980.
She said a seal was likely to have been infected by a bite from either a dog or jackal, which are both reservoirs for the virus.
She said: “Interactions between wild jackals and seal colonies are well described on the West Coast of South Africa and many people take their dogs to the beach so such an interaction is highly plausible.”
Potential for spread
Prof Sarah Cleaveland, from the School of Biodiversity, One Health and Veterinary Medicine, at the University of Glasgow, said all mammals are susceptible to rabies.
Viruses have previously been seen spreading from terrestrial carnivores to seals. Canine distemper has killed large numbers of seals in Lake Baikal and the Caspian Sea after spreading from dogs.
She said: “It is likely that Cape Fur seals caught rabies from the bite of either a domestic dog or jackal, with rabies cases known to have been occurring in both these species in the Western Cape.
“Both dogs and jackals have the opportunity to come into close contact with Cape fur seals along much of the coastal range in southern Africa and seals would be vulnerable to an attack from a rabid dog or jackal while hauled out.”
Cape Fur seals can gather on land in large colonies and also travel large distances when feeding, raising the prospect the virus could spread widely within the population.
Dr Lovelock said: “Seals live in large family groups so the potential for spread is certainly there. Any animal with rabies may become very aggressive, attacking and biting indiscriminately, and if a seal with rabies bites and injures other seals in its social group they may also become infected.”
Prof Cleaveland said it is unclear how efficiently the virus spreads from seal to seal however and in most outbreaks where rabies jumps from a reservoir species to a different species, transmission chains are short.
In a video shared widely in late May, several surfers at Cape Town’s Muizenberg beach reported being attacked and bitten by a seal.