Researchers in Argentina have confirmed that the highly pathogenic avian influenza (HPAI) H5N1 virus causes severe neurological damage in marine mammals and is capable of crossing the placenta to infect fetos. According to a report published by the National Council for Scientific and Technical Research (CONICET), the findings explain the mass mortality events and high rate of miscarriages observed among sea lions and elephant seals along the Patagonian coast during the 2023 outbreak.
Neurological Impact and Placental Transmission in Patagonian Wildlife
During the 2023 outbreak of HPAI H5N1 along the coast of Chubut province, thousands of South American sea lions (Otaria flavescens) and southern elephant seals (Mirounga leonina) washed ashore dead, according to CONICET. Their findings were published in the peer-reviewed journal Veterinary Pathology, as reported by CONICET.
The research revealed that the circulating H5N1 strain is neurotropic, meaning it possesses a high affinity for the nervous system. Investigators detected the virus in virtually all analyzed brain regions, including the choroid plexus, which produces cerebrospinal fluid. This distribution suggests the virus spreads through cerebral fluid, accounting for lesions found in the spinal cord and explaining the severe neurological symptoms observed in the field.
Furthermore, scientists documented vertical transmission of the pathogen from mother to offspring. During the necropsy of a pregnant female sea lion, researchers observed severe placental lesions and detected the virus within fetal organs using molecular techniques. According to CONICET, this marks the first confirmed case of vertical transmission of the avian influenza virus in any wild mammal species, providing a direct biological explanation for the high number of spontaneous abortions recorded during the outbreak.
Cardiac Damage and Implications for Public Health Surveillance
In addition to respiratory and neurological pathology, the study identified cardiac lesions in a southern elephant seal pup. While heart tissue damage from H5N1 has previously been documented in birds and other mammals, CONICET noted this is an unprecedented finding in pinnipeds.
According to CONICET, identifying these specific target organs allows field researchers to optimize diagnostic protocols. Earlier in the outbreak, diagnostic efforts focused primarily on respiratory tracts, which occasionally resulted in false negatives. Incorporating brain tissue analysis corrects this diagnostic blind spot.
Fiorito emphasized the broader public health implications under the “One Health” framework, noting that approximately 70 percent of emerging human diseases originate in wildlife. Continuous monitoring of zoonotic pathogens like H5N1 remains vital for both wildlife conservation and human health protection.