Trematode-Associated Renal Lesions in Stranded Humboldt Penguins (Spheniscus humboldti) Along the Chilean Coast

Humboldt penguins (<i>Spheniscus humboldti</i>) are classified as Vulnerable and listed in Appendix I of CITES, making the investigation of stranding and mortality causes essential for their conservation. This study describes the post-mortem findings of five Humboldt penguins stranded along the Chilean coast during 2025, focusing on renal and ureteral lesions associated with trematode infection. Gross examination revealed multifocal to coalescing renal lesions, including intrapelvic white purulent material and marked thickening of the ureteral walls. Histopathological analysis demonstrated moderate to severe renal and ureteral inflammation, characterized by intratubular and intraureteral trematodes associated with tubular degeneration, interstitial inflammation and fibrosis, vascular alterations, occasional glomerular changes, and severe ureteritis. Morphological and morphometric analyses of adult parasites obtained from histological renal sections confirmed the presence of trematodes. On clinical evaluation, stranded animals presented with lethargy, no evidence of external trauma, and body condition ranging from normal to emaciated. These findings confirm the presence of trematode-associated nephropathy and ureteropathy in Humboldt penguins. To our knowledge, this is the first report worldwide linking renal and ureteral parasitosis to disease in this species. Further molecular analyses are required to achieve definitive etiological identification and to support the recognition of this condition as an emerging disease relevant to conservation strategies for Humboldt penguins.