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Scientists in South Africa have identified what they believe to be the first outbreak of rabies among sea mammals, specifically Cape fur seals. At least 24 seals found dead or euthanized along South Africa’s west and south coasts tested positive for rabies, according to state veterinarian Dr. Lesley van Helden.
Rabies, a deadly virus that affects mammals and can be transmitted to humans, typically spreads through saliva via bites or close contact like licking. While rabies is commonly found in animals like raccoons, jackals, and domestic dogs, this marks the first recorded instance of the virus spreading among marine mammals. The only other known case of rabies in a sea mammal occurred in the 1980s in Norway, where a ringed seal was likely infected by an arctic fox.
The rabies outbreak in South African seals was first discovered in June after a dog was bitten by a seal on a Cape Town beach and subsequently contracted rabies. This prompted authorities to test brain samples from 135 seal carcasses collected since 2021, along with 20 new samples, revealing further rabies-positive cases.
Researchers are now investigating how the virus was transmitted to the seals, whether it is spreading widely among the colonies, and what measures can be taken to control the outbreak. With an estimated 2 million seals migrating between South Africa, Namibia, and Angola, the most likely transmission route involves jackals in Namibia, which are known to hunt seal pups along the coast.
Genetic testing confirmed that the rabies virus found in seals closely matches that found in black-backed jackals in Namibia, suggesting that transmission is occurring between seals as well. This is supported by the fact that many virus sequences from the infected seals are closely related.
The seals’ proximity to populated areas, particularly beaches around Cape Town, has raised concerns. Gregg Oelofse, Cape Town’s head of coastal and environmental management, stated that city authorities have issued warnings to residents and begun vaccinating small numbers of seals at two popular harbors.
Reports of unusually aggressive seals and increased seal attacks on people over the past three years had puzzled authorities, but no human cases of rabies have been recorded so far. One positive rabies test from a seal carcass collected in August 2022 suggests that the virus has been circulating in the population for at least two years.
There are still many uncertainties surrounding this outbreak. Experts, including the U.S. Centers for Disease Control and Prevention (CDC), are monitoring the situation but have yet to determine if the virus will persist long-term in seals. In some cases, rabies variants in new hosts die out over time, as was seen in the U.S. in 2021 when raccoon rabies spread among gray foxes but eventually stopped.
A major unknown is whether rabies vaccines will be effective in seals, as they have never been tested on marine mammals. Even if the vaccine works, vaccinating large numbers of seals, which live mostly in the ocean and migrate along a 3,500-kilometer (2,170-mile) coastline, presents logistical challenges. Unlike land animals, which can be vaccinated through bait, seals typically only eat live fish.
South African authorities are collaborating with international experts to address these challenges and better understand the dynamics of this unprecedented outbreak.
