In 2023, an unprecedented outbreak of highly pathogenic avian influenza (HPAI) H5N1 resulted in the death of thousands of pinnipeds along the Argentinean coast, raising concerns about its ecological and epidemiological impact. Here, we present clinical, pathological, and molecular findings associated with HPAI H5N1 infection in pinnipeds from Chubut, Argentina. Necropsies were conducted on three South American Sea Lions (SASLs) ( Otaria flavescens ) and one Southern Elephant Seal (SES) ( Mirounga leonina ), followed by histopathological, immunohistochemical and RT-sqPCR analyses. Neurological clinical signs were observed in two SASLs, with one also exhibiting respiratory distress. Neuropathological findings included lymphoneutrophilic meningoencephalomyelitis and choroiditis, neuronal necrosis, gliosis, hemorrhages, and perivascular cuffing. Viral antigen was localized in neurons, glial cells, choroid plexus epithelial cells, ependymal cells, and the neuropil. Systemic manifestations included HPAI-related necrotizing myocarditis in the elephant seal and placental necrosis in a sea lion, with fetal tissues testing positive for HPAIV. Pulmonary lesions were minimal, limited to bronchial glands in one individual. RT-sqPCR confirmed HPAI H5 in all tested animals. Our findings highlight the neurotropism of HPAI H5N1 in pinnipeds, and expand the known systemic effects of the virus, revealing new tissue tropism and vertical transmission.

The pituitary gland regulates essential physiological processes in mammals. Despite its importance, research on its anatomy and ultrastructure in dolphins remains scarce. Using non-invasive imaging technology (MRI) and a novel skull-opening and dissection protocol, this study characterizes the dolphin pituitary through immunohistochemistry (IHC) and transmission electron microscopy (TEM). A total of 47 pituitaries were collected from stranded common bottlenose dolphins (Tursiops truncatus). common dolphins (Delphinus delphis), and Atlantic spotted dolphins (Stenella frontalis). as well as from captive common bottlenose dolphins. MRI allowed visualization of the gland’s anatomy and its spatial relationship with the hypothalamus and surrounding structures. A modified skull-opening and pituitary extraction protocol ensured the preservation of the adenohypophysis and neurohypophysis for detailed analysis. Histological, immunohistochemical, and ultrastructural studies confirmed the gland’s structural organization, identifying eight distinct adenohypophyseal cell types: corticotrophs (ACTH), somatotrophs (GH), gonadotrophs (FSH and LH), lactotrophs (LTH), melanotrophs (MSH), thyrotrophs (TSH), follicular cells, and capsular cells. This study presents the first immunolabelling of thyrotrophs in cetacean adenohypophysis and the first detailed ultrastructural characterization of adenohypophyseal cells in cetaceans, providing baseline data for future research. By integrating multidisciplinary techniques, it advances the understanding of dolphin neuroendocrinology and highlights broader implications for cetacean health, welfare, and conservation.

The South American sea lion (SASL, Otaria flavescens) is an endemic pinniped native to South America. Its breeding period is notably seasonal and highly synchronized, occurring during the austral summer. Its breeding behavior has been extensively documented; therefore, any unusual observations warrant careful attention and monitoring. In late 2022, highly pathogenic avian influenza (HPAI) H5N1 emerged in South America, rapidly spreading among marine birds and mammals and causing mass mortality events in SASL. Between August and November 2023, mass mortalities and clinical signs consistent with HPAI were reported in several SASL colonies in Argentina. This study documents and quantifies an atypically high number of abortions and abnormal reproductive behaviors observed in SASL during the 2023 HPAI outbreak in Northern Patagonia. A total of 73 abortions events were recorded, some involving females exhibiting clinical signs of avian influenza. Additionally, unusual sexual behaviors were observed, including males retaining and mounting females outside the breeding season and instances of necrophilia documented for the first time in this species. We discuss potential factors associated with reproductive failure and abnormal behaviors in pinnipeds and suggest a high likelihood that these patterns could be related to the HPAI/H5N1 outbreak in the Atlantic. These findings underscore the need for continued research to understand the impacts of emerging diseases on marine mammal health and behavior, emphasizing the importance of sustained population monitoring.

In this work we report Plagiorhynchus (Plagiorhynchus) crassicollis from Patagonia, Argentina. Specimens were collected from the small intestine of a charadriid shorebird with Neotropical distribution, the Two-banded Plover (Charadrius falklandicus). Both morphological and molecular characterization, plus pathological aspect of this species is provided. Plagiorhynchus (Plagiorhynchus) crassicollis is characterized by having a proboscis with 18–20 longitudinal rows of hooks and 11–14 hooks per row. Sequences of the small subunit (SSU) and large subunit (LSU) of the nuclear ribosomal DNA were obtained and compared with other sequences available in GenBank. Phylogenetic analyses inferred with each molecular marker consistently showed that P. (P.) crassicollis is sister taxa to Plagiorhynchus (Plagiorhynchus)aznari, a parasite of the long-billed curlew (Numenius americanus) from northern Mexico. Pathologic findings associated with the parasites include ulcerative enteritis, granulomatous inflammation, diffuse lymphocytic infiltration, serositis, and peritonitis. This record expands the host and geographical record of P. (P.) crassicollis, provide baseline information on its pathological aspects, and represents the first molecular characterization of P. crassicollis in the Neotropics.

In 2023, high pathogenicity avian influenza (HPAI) H5N1 outbreaks caused mass mortality events affecting pinnipeds throughout South America. Here, we present the clinical, pathological, immunohistochemical, and molecular findings associated with HPAI H5N1 infection in 3 South American sea lions ( Otaria flavescens ) and 1 southern elephant seal ( Mirounga leonina ) stranded during the outbreak in Chubut, Argentina. Neurological signs were observed in 2 sea lions, with 1 also exhibiting respiratory distress. Necropsies were conducted on all 4 animals. Neuropathology revealed mild-to-severe lymphohistiocytic (2/4) to mixed neutrophilic (2/4) meningoencephalitis with neuronal necrosis, neuronophagia, glial cell proliferation, multifocal hemorrhage, and perivascular cuffing in all animals. Novel findings for HPAI H5N1-infected pinnipeds included choroid plexitis (2/4) and myelitis (2/4). Immunohistochemistry for viral nucleoprotein was positive in 3 of 4 animals and involved neurons (3/3), glial cells (3/3), and ependymal cells (2/3). Viral-related systemic findings included multifocal necrotizing myocarditis in the elephant seal and multifocal necrotizing placentitis in a sea lion, with HPAI virus detected within fetal tissues. Pulmonary lesions were minimal and were limited to multifocal necrosis of bronchial glands in 1 sea lion. HPAI H5 clade 2.3.4.4b virus was confirmed by polymerase chain reaction and sequencing in all animals. These findings indicate a predominance of central nervous system involvement in HPAI H5N1-infected pinnipeds and expand the recognized spectrum of virus-associated lesions, identifying novel tissue tropisms and transmission routes.