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

## TL;DR

This study reports the first case of kidney disease caused by trematode parasites in Humboldt penguins, suggesting it could be a new threat to their survival.

## Contribution

The first global report of trematode-induced renal and ureteral disease in Humboldt penguins.

## Key findings

- Post-mortem examinations revealed severe kidney and ureter inflammation linked to trematode parasites in stranded penguins.
- Histopathological analysis confirmed trematode presence and associated tissue damage, including fibrosis and ureteritis.
- Clinical signs were subtle, but lesions were severe, indicating a potential emerging health threat for the species.

## Abstract

Humboldt penguins are a vulnerable species whose population is continuously declining along the Chilean coast. Although infectious diseases may play a role in penguin mortality and stranding events, renal diseases caused by parasites have not been previously reported in this species. In this study, we describe renal and ureteral inflammation associated with trematode parasites in five stranded Humboldt penguins. While clinical examinations of penguins that were stranded alive did not show obvious signs of renal disease, post-mortem examinations revealed severe lesions affecting the kidneys and ureters. These findings suggest that parasitic renal infections may represent an emerging health threat for Humboldt penguins and could contribute to stranding and mortality. Improved recognition of this condition may support better diagnosis, rehabilitation, and conservation strategies for this vulnerable species.

Humboldt penguins (Spheniscus humboldti) 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.