H5N1 kills an endangered Australian sea lion on Kangaroo Island for the first time

Drone surveys on Australia's Heard Island counted more than 13,000 dead southern elephant seal pups after H5N1 reached the island ^[8]. That is more pups than there are Australian sea lions alive anywhere ^[22]. These are different hosts in a different place, and nothing in the record predicts a die-off of that size on Kangaroo Island. The strain now circulating in Australia and New Zealand has evolved to infect some mammal species as well as birds ^[5].

Where the virus goes inside a mammal changed with the 2020 variant. Older bird flu strains in pinnipeds mainly affected the respiratory tract; this one can also invade the brain and nervous system ^[9]. Sea lions have been found with encephalitis and other neurological signs before death ^[10]. In a Florida bottlenose dolphin infected with H5N1, the highest concentration of virus was in the brain ^[11].

How the virus gets into pinnipeds is an inference built on plausible contact. Sick seabirds die on the islands sea lions use for breeding, and the virus can cross by direct contact, by scavenging the carcasses, or through contaminated sand, rocks, water, feces or mucus ^[12]. Mammal-to-mammal spread is suspected and, in most wild marine mammal die-off events, has not been conclusively proven ^[13]. In Argentina the case for it is epidemiological: the pups died rapidly and in numbers that suggest the virus moved between them, and spread is thought to be faster in dense colonies ^[14]. Pinnipeds sneeze too, and a researcher testing transmission reportedly took a seal sneeze in the face, then developed severe conjunctivitis in that eye with bird flu virus detected ^[15].

The Kangaroo Island sea lion had recently been attacked by a shark and was weakened; testing found the virus caused its death ^[1][2]. The dolphin near Goolwa is the country's first confirmed cetacean case ^[16]. South Australia's chief veterinary officer, Dr Skye Fruean, told the ABC it was likely the dolphin had "eaten a bird or interacted closely with a wild bird" ^[17]. The strain has been found in a range of dolphin species around the world ^[21]. A dead juvenile whale that washed ashore at Victor Harbor tested negative, and evidence of infection in large whales remains slight ^[18].

Outside a host the virus persists for days and sometimes weeks in water, depending on temperature, salinity and sunlight, and much shorter where conditions are hot and dry ^[19]. Carcasses act as temporary reservoirs while the tissues remain fresh ^[20].

Australia's confirmed marine mammal tally is two animals, one sea lion and one dolphin ^[23]. Another dead sea lion is being tested ^[3]. South Australia has not published how many carcasses have been sampled to produce those two positives ^[24]. Two positives out of a handful of tested strandings means something different from two out of several hundred, and that difference decides whether the virus has just arrived or has been circulating undetected.