Valeria Falabella

We present a summary of the initial findings regarding the foraging patterns of southern elephant seals (Mirounga leonina) departing from the small, nascent colony of Jackson Bay, Tierra del Fuego, Chile. Eight juvenile males were satellite tracked while foraging at sea; four of them yielded complete tracks of their trips. The study highlights the importance of both the inland waters in the Chilean fjords and the pelagic waters off the coast deeper than 200 m as preferred foraging areas. Southern elephant seals utilizing the fjords also undertook extensive excursions into the SE Pacific, up to 600 km offshore. In open waters, the study animals engaged in extended loops lasting up to 30 days and covering 2,580 km of horizontal travel, characterized by few focal areas of activity. A comparison with individuals from the Península Valdés colony in Argentina underscores connectivity between colonies and identifies the Strait of Magellan and the Drake Passage as important corridors for southern elephant seal foraging migrations. Overall, high individual variability precludes identifying a dominant strategy for the Jackson Bay southern elephant seals, at least during this early stage of recolonization of the species’ historical range.

H5N1 high pathogenicity avian influenza virus has killed thousands of marine mammals in South America since 2022. Here we report epidemiological data and full genome characterization of clade 2.3.4.4b H5N1 HPAI viruses associated with a massive outbreak in southern elephant seals (Mirounga leonina) at Península Valdés, Argentina, in October 2023. We also report on H5N1 viruses in concurrently dead terns. Our genomic analysis shows that viruses from pinnipeds and terns in Argentina form a distinct clade with marine mammal viruses from Peru, Chile, Brazil and Uruguay. Additionally, these marine mammal clade viruses share an identical set of mammalian adaptation mutations which were also present in tern viruses. Our combined ecological and phylogenetic data support mammal-to-mammal transmission and occasional mammal-to-bird spillover and suggest multinational transmission of H5N1 viruses in mammals. We reflect that H5N1 viruses becoming more evolutionary flexible and adapting to mammals in new ways could have global consequences for wildlife, humans, and/or livestock.

H5N1 high pathogenicity avian influenza (HPAI) viruses of the clade 2.3.4.4b have killed thousands of marine mammals in South America since 2022. In October 2023, following outbreaks in sea lions in Argentina, we recorded unprecedented mass mortality (∼17,000 individuals) in southern elephant seals ( Mirounga leonina ) at Península Valdés. Seal pups were disproportionately affected. Adult seals departed early, disrupting social and breeding structure. Frequent interactions with sea lions and scavenging by seagulls were observed. Deaths of terns concurred with seals but peaked weeks later. HPAI H5N1 was confirmed in seals and terns. Moreover, genomic characterization showed viruses from pinnipeds and terns in Argentina form a distinct clade with marine mammal viruses from Peru, Chile and Brazil. These mammal-clade viruses share an identical set of mammalian adaptation mutations which are notably also found in the terns. Our combined ecological and phylogenetic data support mammal-to-mammal transmission and occasional mammal-to-bird spillover. To our knowledge, this is the first multinational transmission of H5N1 viruses in mammals ever observed globally. The implication that H5N1 viruses are becoming more evolutionary flexible and adapting to mammals in new ways could have global consequences for wildlife, humans, and/or livestock.