A year after the H5N1 outbreak devastated elephant seals in Argentina, scientists estimate that only around one-third of the usual seal population has returned.
The distinctive barks of elephant seals have returned to the breeding grounds of Península Valdés, Argentina, but the atmosphere is subdued. Approximately one year after a deadly H5N1 avian influenza outbreak led to the deaths of over 17,000 elephant seals, including nearly 97% of their pups, estimates suggest that only a third of the typical seal population has come back.
“It’s heartening to walk the beaches and hear the seals again,” said Marcela Uhart, who leads the Latin America Program at UC Davis’ Karen C. Drayer Wildlife Health Center within the UC Davis School of Veterinary Medicine. “However, it’s unsettling to walk past heaps of carcasses and bones, noticing very few harems of seals.”
A recent study published in the journal Nature Communications, co-led by UC Davis and Argentina’s National Institute of Agricultural Technology (INTA), shows mammal-to-mammal transmission during the 2023 outbreak. This research indicates that H5N1 spread rapidly among marine mammals, marking the first cross-border transmission of the virus among these animals across five countries in southern South America.
The genomic analysis from the study revealed that when the virus entered South America, it evolved into distinct strains for birds and marine mammals, a first for this type of situation.
“We’re illustrating the evolution of H5N1 viruses related to genotype B3.2 since they arrived in South America in late 2022,” remarked Agustina Rimondi, a virologist and co-author from INTA, who is also affiliated with the Robert Koch Institute. “This virus can adapt to marine mammals, as evidenced by the mutations consistently identified in this type of virus. Importantly, our findings suggest that H5 marine mammal viruses can revert to infecting birds, highlighting the urgent need for enhanced monitoring and collaborative research in the region.”
Elephant Seal Population Count
The Wildlife Conservation Society in Argentina (WCS Argentina) has projected the number of returning elephant seals as part of a long-term monitoring initiative.
“We were shocked by the severe impact the avian influenza outbreak had on this population,” stated Valeria Falabella, WCS Argentina director of coastal and marine conservation. “It seems that more than half of the breeding population succumbed to the virus. It will take many years for the population to recover to its 2022 levels.”
Falabella pointed out that the outbreak has reversed years of conservation work for the species, noting that most adult males and many experienced, fertile females have not returned.
Efforts to track the virus in elephant seals are collaborative. While WCS Argentina undertakes its census, Uhart and her team at UC Davis document and sample deceased seals for H5N1 signs, while Rimondi and her colleagues analyze and sequence samples at the Institute of Virology, INTA.
Thus far, no elephant seals have tested positive for the virus this breeding season. However, there are still lingering uncertainties from last year’s outbreak. For instance, scientists are unsure whether the virus was transmitted through aerosol, saliva, feces, or other means, nor do they know if the surviving seals have developed immunity through protective antibodies. These questions remain crucial.
Throughout the past year, the virus has appeared extensively in wild birds and caused outbreaks among poultry and dairy cows in the U.S., with recent cases among workers in those industries. On October 30, the U.S. Department of Agriculture reported the first occurrence of H5N1 in pigs in the country. There have been no known cases of person-to-person transmission of H5N1.
The Journey of a Virus
The current H5N1 variant, clade 2.3.4.4b, began causing significant global issues in 2020. While the world focused on the COVID-19 pandemic, H5N1 began decimating seabird populations in Europe before reaching South Africa. In 2021, the virus spread to the United States and Canada, threatening both poultry and wild birds, and reached South America by late 2022.
By February 2023, highly pathogenic avian influenza was identified in Argentina for the first time, primarily affecting poultry in the central inland regions for five months. After two months without poultry outbreaks, the virus appeared in sea lions off the southern coast of Tierra del Fuego island in August 2023. It then spread rapidly northward, initially wreaking havoc on marine mammals followed by seabirds.
In October 2023, following sea lion outbreaks, the study’s authors conducted a survey of the elephant seal breeding colony at Punta Delgada, documenting unprecedented levels of mortality. Testing confirmed the presence of HPAI H5N1 in the seals, as well as in several terns found deceased concurrently.
The authors emphasized the need for ongoing monitoring and research to deepen understanding of the virus’s evolution. Its growing adaptability to new hosts could have serious implications for human health, wildlife conservation, and ecosystems.
Co-authors of the study include Ralph Vanstreels from the UC Davis School of Veterinary Medicine; Martha Nelson from the National Institutes of Health; Valeria Olivera from INTA-CONICET; Julieta Campagna, Victoria Zavattieri, and Claudio Campagna from WCS Argentina; and Philippe Lemey from the Rega Institute in Belgium.
Funding for the study was provided by WCS Argentina, UC Davis, and INTA.
