Rabies Jumps to the Sea!

The Cape fur seal (Arctocephalus pusillus), an iconic marine mammal along South Africa’s coastline, has recently faced an alarming health challenge: outbreaks of rabies. Rabies, typically associated with terrestrial mammals like dogs and bats, has crossed into the marine realm, raising questions about its origins, transmission, and broader ecological implications.

The outbreak of rabies detected in Cape fur seals in South Africa in June 2024 is believed to be one of the first documented outbreaks of rabies in marine mammals. The particular strain of rabies found in the Cape fur seals is most similar to the one occurring in rabid blackbacked jackals.

The terrestrial habitat use of Cape fur seals on beaches in Namibia and South Africa overlaps with that of the black-backed jackal. In fact, these jackals often scavenge on Cape fur seal carcasses and predate on pups in these colonies.

It is very likely that the virus originated in the seal colony via a bite from a black-backed jackal. Once introduced into a seal colony, rabies spreads primarily through bites, as the virus is present in the saliva of infected individuals.

Seal populations in South Africa are particularly vulnerable to the spread of rabies due to their social nature. Thousands of seals haul out together on beaches and islands, making close contact inevitable. This high density facilitates the rapid spread of the virus, leading to localized outbreaks that can have devastating effects on seal colonies.

Historically, great white sharks were abundant along South Africa’s coastline, preying on seals and keeping their populations in check. A robust population of great white sharks could act as a natural regulatory force, limiting the size and density of seal colonies.

Smaller, less crowded seal populations might reduce the risk of rabies transmission by decreasing the frequency of close interactions. Furthermore, sharks often target weak or sick individuals, which could help remove rabid seals from the population before they spread the virus further.

However, in recent years, South Africa’s great white shark population has declined dramatically, due to overfishing of their prey species, improper or illegal fishing practices that target sharks and/or tuna, intentional capture in shark nets and drumlines in kwa-Zulu Natal, and predation by orcas.

A fascinating question is if the white shark population in South Africa was strong and healthy, would they have managed to control the spread of rabies within the Cape fur seal population? It is important to note that this is a speculative hypothesis.

The relationship between predator and prey dynamics and disease control in marine ecosystems is complex and influenced by numerous factors, including environmental conditions, human activities, and the behaviour of both seals and sharks. But it is an interesting hypothesis to think about nonetheless!

Addressing the issue of rabies in Cape fur seals requires a multi-faceted approach. Enhanced monitoring and research are essential to understand the virus’s spread and its impact on seal populations.

Public education campaigns can also play a crucial role in reducing the risk of human exposure to rabid seals, particularly for coastal communities and tourists. Additionally, efforts to conserve and restore great white shark populations could have broader ecological benefits, potentially influencing the dynamics of rabies outbreaks in seals.

Protecting these apex predators is not only crucial for maintaining marine biodiversity but may also contribute to healthier, more balanced ecosystems. As we strive to protect South Africa’s marine wildlife, understanding the intricate connections between species, diseases, and ecosystems will be key to ensuring a sustainable future for both seals and sharks.