The Spread of High Pathogenicity Avian Influenza H5N1 to Antarctica

The ongoing global HPAI panzootic is unprecedented in scale, with mass mortality events causing population level effects across numerous bird and mammal species (1-3). The vast majority of avian influenza viruses are low pathogenicity avian influenza, causing no clinical disease in birds, and have likely co-evolved with wild birds through their evolution. In contrast HPAI viruses (restricted to H5 and H7 subtypes) are the highly virulent forms that emerge in poultry production systems, and can spill over into wild species causing disease outbreaks and, in many cases, substantial mortality (4).    

While HPAI H5N1 has been circulating in poultry populations since 1996 (4, 5), the current panzootic started in 2021, and with it a dramatic increase in outbreaks in wild birds and mammals, broad geographic spread, and a dramatic increase in host range (1-3, 6). HPAI has caused deaths in at least >400 species of wild birds (2, 7) and lead to the culling of hundreds of millions of poultry. HPAI has also been detected in 70 species of wild mammals (3, 8) as well as domestic ruminants (9), and infected >70 people since 2021 (10). As of October 2025, only Oceania remains free from HPAI H5N1 (11). 

A. Worldwide transfer of HPAI 

Although the initial emergence of HPAI was in Asia, Europe has been identified as the epicenter of the ongoing global panzootic (12) . The virus spread from Europe across the North Atlantic and arrived in northeastern North America in October 2021, with Iceland as a likely bridge (13). There have been several incursions into North America since 2021, across both the Atlantic and Pacific Oceans (14, 15). The virus rapidly spread across the continent, including into the Arctic (14).    

Approximately one year after arriving in North America, HPAI spread to South America (16). HPAI spread along the Pacific coast, following outbreaks of seabirds as well as marine mammals, particularly, South American sea lions (Otaria byronia) in Peru and Chile (16-18). Spread was rapid, with HPAI H5N1 reaching Tierra del Fuego approximately 6 months after first occurring on the continent in October 2022; this was followed by both an inland spread into Argentina, as well as coastal northbound spread along the Atlantic coast of Argentina and Uruguay and up to Brazil (16, 19-21). In addition to seabird mortality, South America also experienced unprecedented mortality in marine mammals including South American sea lions, South American fur seals (Arctocephalus australis) and southern elephant seals (Mirounga leonina) (16, 22). To date, more than 660,000 wild birds and ~57,000 marine mammals have died as a result of HPAI in South America (16). Although HPAI has been recorded in marine mammals in both Europe and North America, the scale of mortality of marine mammals in South America was unprecedented.  

Coinciding with outbreaks on the Atlantic coast of South America in October 2023 was the first detection of HPAI on islands in the southwest Atlantic and sub-Antarctic. Specifically, the first detection of HPAI on South Georgia Island (Islas Georgias del Sur) occurred on 16 September 2023 in brown skuas (Stercorarius antarcticus), and on the Falkland (Malvinas) Islands on 30 October 2023 in a southern fulmar (Fulmarus glacialoides) (16, 23, 24). It is hypothesized that the arrival of HPAI was coincident with the arrival of brown skuas to these areas from South America. Introduction events into South Georgia Island (Islas Georgias del Sur) and the Falkland (Malvinas) Islands occurred independently (24), followed by local transmission on both islands, impacting brown skuas, kelp gulls (Larus dominicanus), southern fulmars, gentoo penguins (Pygoscelis papua), king penguins (Aptenodytes patagonicus), southern giant petrels (Macronectes giganteus), Antarctic terns (Sterna vittata), wandering (snowy) albatrosses (Diomedea exulans), southern rockhopper penguins (Eudyptes chrysocome), variable hawk (Geranoaetus polyosoma), South Georgia shag (Leucocarbo georgianus) and black-browed albatrosses (Thalassarche melanophris), as well as southern elephant seals and Antarctic fur seals (23, 24). In South Georgia, brown skuas, southern elephant seal weaners, Antarctic fur seals and wandering (snowy) albatrosses were the most impacted with high mortality rates observed in pinnipeds at multiple locations (23, 25). Indeed, it has been estimated that 48% of all breeding female southern elephant seals on South Georgia died in 2023/24 due to HPAI (26) . In the Falkland (Malvinas) Islands, most reported cases were in single individual animals or small outbreaks, except for Jason Steeple Island which experienced high mortality in black-browed albatrosses (25, 27).  

B. Incursion of HPAI to the Antarctic Treaty Area 

Shortly after detections in the sub-Antarctic, reports (December 2023) of brown skua with clinical signs consistent with HPAI were reported at Laguna Skua Sur Isla Laurie, Archipelago Orcadas  (28). The first confirmation of HPAI on the Antarctic Peninsula occurred in February 2024 in brown skuas at Cierva Cove near the Argentine Primavera Base (29) and on James Ross Island near the Czech Johann Gregor Mendel Base (30). Through the austral summer of 2023/24 HPAI was both suspected and confirmed at multiple locations in the South Shetland Islands, and along the Antarctic peninsula, from the Northern Weddell Sea region as far south as Marguerite Bay (25, 28) (Figure 1).  Most cases were reported in brown and south polar skuas (28, 30, 31), but also confirmed in a snowy sheathbill (31), and an Antarctic fur seal (31, 32). In the South Shetlands, HPAI was confirmed in a southern elephant seal and kelp gull (33, 34). Adélie penguins have also been implicated (35), but the role of penguins remains unclear and further research is needed. The final detection in multiple species including Antarctic fur seal, snowy sheathbill, Adelie penguin, and skuas occurred in March 2024, coinciding with the end of the tourist and research summer season (Figure 1) (28).  

Analysis of virus genomes indicates that South America acted as the primary source of initial viral introductions in the 2023/24 austral summer, with independent incursions into (1) the Falkland (Malvinas) Islands, (2) South Georgia, and (3) the South Shetland Islands (24, 34) (Figure 2). South Georgia appears to have been a key steppingstone for subsequent dissemination to Antarctica and other sub-Antarctic islands evidenced by genomes from the Northern Weddell Sea being most closely related to those from South Georgia (36). Together this demonstrates virus introduction and movement in the region to be complex and not unidirectional. 

C. Resurgence of HPAI in the Antarctic Treaty Area

Unfortunately, HPAI was again detected in the austral summer of 2024/25 in both the Falkland (Malvinas) Islands/South Georgia Island, as well as in the Antarctic Treaty Area.  In general, there was a resurgence of HPAI in many of the sites that reported positives in the 2023/24 austral summer, including those in the South Shetlands, northern Weddell Sea, Gerlache Strait, and Margeurite Bay (28, 39, 41, 43-45) (Figure 3).  The number of sites with confirmed HPAI increased from 11 in the 2023/24 field season to 32 in the 2024-2025 season (28). Throughout the 2024/25 season, the HPAI was confirmed in 11 species of birds and 5 species of mammals. HPAI is also suspected to be the cause of three unusual mortality events that occurred in the Northern Weddell Sea in crabeater seals, where reports suggest 50-100  individuals had died on fast ice (28).  

Viral genome data helped illuminate our understanding of HPAI in the 2024/25 austral summer. First, the viral genomes from the Antarctic Peninsula in the summer 2024/25 are not closely related to those from 2023/24 which suggest that HPAI was likely re-introduced to the Peninsula. It is mostly likely that South Georgia was the source, however, it is noteworthy that there are few genomes available from the 2023/24 summer other than from South Georgia, which may bias the result (40, 42). All the genomes generated in 2024/25, which include those from the South Shetlands and the Peninsula, fall into a single clade, suggesting a single introduction to the region, followed by spread (Figure 2).  

We expect that the 2025/26 austral summer will bring similar trends of detection in the South Shetlands and Antarctic Peninsula. An up-to-date list of the sites and species where HPAI has been detected in the Antarctic region is available through the SCAR “Sub-Antarctic and Antarctic Highly Pathogenic Avian Influenza H5N1 Monitoring Project” website (https://scar.org/library-data/avian-flu).  

D. Long distance spread in the sub-Antarctic

In the austral summer of 2024/25 HPAI spread considerably, with confirmations at Possession Island (Crozet), Kerguelen Island, Marion Island, and Gough Island (Figure 2,3) (40, 42, 46). HPAI was confirmed on Heard Island in November 2025 (47), but it more than likely arrived in the 2024/25 season. As a result of this spread, HPAI now has an almost circumpolar distribution of the virus in the sub-Antarctic, being present in the south Atlantic and southern Indian Ocean islands. This poses a considerable risk for further spread in the Antarctic, with these islands potentially acting as a steppingstone to spread in the Antarctic, potentially mirroring the role that South Georgia Island has played for virus introduction to the Antarctic Peninsula. 

Overall, HPAI has had a considerable impact on Antarctic wildlife, however, those impacts are not homogeneous. In general, brown and south polar skuas are being heavily impacted: they often comprise the first species in which HPAI is detected in a new location, and there have been recorded declines (23, 28, 31, 42, 44, 45). Other scavenging species such as kelp gulls and southern giant petrels have had widespread notifications. The impact of HPAI in penguins is very heterogeneous – king penguins (40) and gentoo penguins (23, 25, 28) have been heavily impacted in the sub-Antarctic and South Atlantic, but there have been no indications of mass mortalities in the Antarctic Treaty Area. The possibility that penguins may be infected without clinical disease has been proposed (35), with HPAI being reported in live Adelie penguins, however it is uncertain of the fate of these animals post sampling. Finally, HPAI has been detected in five species of marine mammals in the Antarctic, southern elephant seals and Antarctic fur seals are being heavily impacted in the sub-Antarctic (23, 26, 28, 40) (Figure 4).   

HPAI causes systemic infections with a wide variety of disease signs. In many cases, outbreaks in wildlife have manifested as sudden and large-scale mortality, often with little evidence of diseased birds observed in the preceding weeks (e.g. (23). Clinical signs are generally observed hours or days before death, and involve neurological or respiratory manifestations, or a combination of both, meaning some animals maybe infectious without showing any visible clinical signs of disease. In the sub-Antarctic, ocular changes have also been observed in marine mammals, with elephant seals appearing to have opaque milky eyes (40). Importantly, clinical signs of HPAI are not specific and can overlap with other infectious and non-infectious conditions; it is impossible to determine whether an animal has HPAI without laboratory testing. All of this means that detecting outbreaks and monitoring the sources and dispersion of HPAI in populations of Antarctic birds and mammals is especially challenging. 

Despite widespread outbreaks and large numbers of marine mammals killed by HPAI there have been very few human infections. To date, almost all detected cases have occurred in people who have come into direct contact with infected animals (48). Overall, the risk of human infection with HPAI for the general population is still considered to be low, however for those who are occupationally exposed (i.e., people in direct contact with infected animals) the risk is moderate-low (48). Unlike some lineages of HPAI, the estimated case fatality rate of human infections caused by clade 2.3.4.4b (the particular lineage/variant of HPAI H5Nx causing the panzootic) is low; there are only 3 reported deaths in the >70 human cases since 2020 (10). Regardless, as HPAI is known to survive for extended periods of time in the environment (soil, feces, water, etc.), especially at cold temperatures, humans conducting science or visiting as tourists in Antarctica may act as passive hosts transporting the virus on feces/soil laden boots, clothing, or instruments, between colonies (49). In response to this risk, SCAR, COMNAP and IAATO have provided a suite of recommendations and guidelines for enhanced cleaning and biosecurity measures to reduce the risk of transmission from clothing and equipment (49-52). This includes thorough cleaning of soiled material and disinfection using an appropriate biocide and limiting access to sites to essential activities where HPAI is suspected or confirmed. Although humans can facilitate the movement of HPAI from colony to colony, to date all outbreaks in Antarctica are believed to have been via natural migration of wildlife.  

Our understanding of virus ecology and epidemiology in Antarctica is hampered by a lack of available genome data, clarity on virus spread, longevity of the virus in the environment and host species which play a role in long distance spread. Recent observations in the sub-Antarctic and Antarctic Peninsula have shown there is no observable differentiation between ‘natural’ mortality (from starvation, end of season mortality, head trauma, or other non-virus-related causes) and HPAI in both seabirds and seals without laboratory testing. Some individuals which appeared to be ‘normal end of season’ mortality have tested positive for HPAI.  Therefore, only investigating ‘above baseline mortality’ may result in positive cases being missed, especially in the absence of human presence to collect data. Nonetheless, based on the impacts of HPAI on wildlife from other continents and recent reports from the south Atlantic, sub-Antarctic, and Antarctic Peninsula, there are some overarching patterns that suggest what may continue to unfold in Antarctica: 

• Skuas (Stercorariidae), gulls and terns (Laridae) are highly susceptible to HPAI, and large-scale mortalities have been reported for these species in several continents; Skuas are the most impacted species within the Antarctic Treaty Area.  

• Avian predators and scavengers can be exposed to HPAI viruses through the consumption of infected prey, and therefore skuas, gulls, giant petrels and sheathbills may be at increased risk.  

• Penguins (Spheniscidae) and cormorants (Phalacrocoracidae) have shown high susceptibility in some regions but not in others, so it is difficult to predict whether (and which of) these species may also be impacted in Antarctica.  To date there have been no mass mortality events for penguins in the Antarctic Treaty Area due to HPAI. However, there have been reports of high mortality in gentoo penguins in the Falkland (Malvinas) Islands and king penguins in the sub-Antarctic.  

• Pinnipeds (Otariidae and Phocidae) in South America experienced severe outbreaks with high mortality, as well as increased abortion rates and disruption of social structure during the breeding season. In the sub-Antarctic regions both southern elephant seals and Antarctic fur seals have been heavily impacted by HPAI. In the Antarctic region three mass mortality events in crabeater seals were reported on fast ice in the northern Weddell Sea but were never confirmed. However, HPAI was confirmed in crabeater seal carcasses located nearby at Tay Head.  So, the risk of high mortality in pinnipeds in the region is high.