Abstract
The Amazon basin is the largest on the planet, and its freshwater ecosystem diversity promotes a wealth of life forms, including four cetacean species.1 Three of them belong to the Inia genus, comprising the Amazon river dolphin (I. geoffrensis), Bolivian river dolphin (I. boliviensis), and Araguaia river dolphin (I. araguaiaensis), and one is a delphinid, represented by tucuxi (Sotalia fluviatilis).2,3 The Amazon freshwater ecosystems are facing multiple anthropogenic threats (e.g., mining, over-harvesting, agropastoral pressure, climate change), and, therefore, those species are classified as threatened with extinction.1,4
From September through December 2023, an unprecedented, unusual mortality event (UME) affecting Amazon river dolphins and tucuxis was registered in Lake Tefé and Lake Coari (Amazonas State, Brazil), involving over 270 river dolphins. A national and international partnership was created to respond to this UME, and an emergency was initiated by the Chico Mendes Institute for Biodiversity Conservation of the Brazilian Ministry of the Environment. Herein, we describe the preliminary histopathology, immunohistochemistry, and pathogen screening of the stranded river dolphins of Lake Tefé (n=154).
Standardized necropsies were performed (n=104), with tissue samples obtained for histologic, immunohistochemical, and molecular analyses.5 Immunohistochemistry for myoglobin detection was performed in the kidneys of fresh individuals. Total RNA and DNA were extracted from frozen tissue samples and tested by qPCR or RT-PCR for Toxoplasma gondii; toxin C, D, and C/D of Clostridium spp.; morbillivirus; SARS-Cov2; influenza virus A/B; and respiratory syncytial virus.6,7,8
We analyzed samples from 23 river dolphins, including 14 I. geoffrensis and nine Sotalia fluviatilis. Primary gross findings were generalized passive congestion, serosanguinous fluid in body cavities, pulmonary congestion, diffusely pale epaxial muscle, and the presence of petechiae on epicardial muscle. On histopathology, generalized vasodilation and congestion were observed, with acute necrotizing pneumonia (n=8) and cardiomyopathies with necrosis and degeneration (n=7). Immune-histochemistry essays were positive in 9/11 cases, indicating myoglobinuria. All individuals were negative for the tested infectious agents and toxins by PCR.
The pathological findings do not indicate an infectious agent as a primary cause of the UME. Thus, we hypothesize the hemodynamic disturbances, myoglobinuria, and bacteria translocation are likely related to environmental stress that may be triggered by the extreme drought and low water level, high-water temperatures ranging daily between 27–40°C, unhealthy to hazardous air quality values due to forest fires in the region, and/or potential presence of biotoxins. Of note is that the pneumonia observed, which is characterized by epithelial necrosis, may be associated with the inhalation of irritative substances. Ongoing studies are performing complementary exams for toxicology and correlating environmental factors, and samples from Lake Coari are under analysis to compare with the findings observed in Lake Tefé.
Acknowledgments
We would like to thank all institutions and its volunteers that collaborated in the emergency response of the UME: ADAF Coari—Agência de Defesa Agropecuária e Florestal do Estado do Amazonas, Aiuká Consultoria em Soluções Ambientais, Aqua Viridi, Aquasis, Corpo de Bombeiros de Tefé, Conselho Regional de Medicina Veterinária do Amazonas, European Association for Aquatic Mammals, Exército Brasileiro, Friends of Nuremberg Zoo Association, Fundação Mamíferos Aquáticos, Fundación Mundo Marino, GRAD—Grupo de Resgate de Animais em Desastres, Greenpeace, IBAMA—Instituto Brasileiro do Meio Ambiente e dos Recursos Naturais Renováveis, INPA—Instituto Nacional de Pesquisas da Amazônia, Instituto Aqualie, Instituto Baleia Jubarte, Instituto Oswaldo Cruz (IOC/Fiocruz), International Fund for Animal Welfare, IPAAM—Instituto de Proteção Ambiental do Amazonas, LAPCOM-USP, Loro Parque Fundación, Marinha do Brasil, National Marine Mammal Foundation, Nuremberg Zoo, Oceanogràfic València, Planète Sauvage, Polícia Militar do Amazonas, Prefeitura de Coari, Prefeitura de Tefé, R3 Animal, Rancho Texas, Sea Shepherd Brasil, Sea Shepherd France, SeaWorld and Busch Gardens Conservation Fund, SEMMAC—Secretaria Municipal de Meio Ambiente e Conservação de Tefé, Universidad de Las Palmas de Gran Canaria, WWF-Alemanha, WWF-Brasil, YAQU PACHA e Zoomarine Portugal. Particularly, we would like to thank Claudia de Lima Souza, Mariana Lobato de Carvalho Martins, Karen Carolina da Silva, Maria Clara Cauassa, Isadora Safira Carvalho Dias, and Kellen Karoline Lopes for their work in sample collection and necropsies.
*Presenting author
+Student presenter
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