Spirochiidiasis in Stranded Olive Ridley Turtles (Lepidochelys olivacea) Long the Northern and Central-Northern Chilean Coast

1. Introduction

The olive ridley turtle (Lepidochelys olivacea) is a circumtropical sea turtle species currently listed as "Vulnerable" by the International Union for Conservation of Nature (IUCN) [1]. Throughout its distribution, this species faces significant risks that lead to frequent stranding events, including habitat degradation and diverse anthropogenic pressures as highlighted in global and regional recovery assessments [2,3]. Specific threats impacting survival include predation of eggs and hatchlings by birds [4], domestic dogs [5], and large felids [6], as well as infectious diseases such as fibropapillomatosis [7]. Furthermore, environmental stressors like heavy metal accumulation [8], oil pollution [9], vessel collisions [10], and other direct human impacts [11] play a critical role in population declines. Beyond these traumatic and environmental causes, infectious and parasitic diseases are fundamental to the health of these chelonians, often acting as determining or contributory factors in the mortality of stranded individuals [12]. Among the parasitic diseases affecting olive ridley turtles, infections caused by blood flukes of the family Spirorchiidae are recognized as a significant pathological condition and have been associated with mortality in some cases [13,14]. These endoparasites inhabit the cardiovascular system and are responsible for spirorchiidiasis, a multisystemic pathology characterized by the presence of adults in large vessels and the dissemination of eggs toward various vital organs [15]. The Spirorchiidae family comprises approximately 21 genera, 10 of which exclusively parasitize marine turtles, including the olive ridley turtle [16]. Although spirorchiidiasis is considered near-cosmopolitan, it has been documented in only five of the seven marine turtle species, with the pathology and host specificity being topics of active molecular and clinical research [15,17]. The host's response to these embolized eggs could trigger chronic granulomatous inflammation that can severely compromise tissue function in multiple systems [18]. In severe cases, this infection leads to arteritis, thrombosis, and systemic debility, which may cause the direct death of the animal [12]. Despite the importance of this parasitosis, reports in the South-Eastern Pacific are extremely limited. While lesions associated with Spirorchiidae have been described in olive ridley turtle and green turtles (Chelonia mydas) in the South Atlantic (Brazil) [19] and Central America (Costa Rica) [20], there are no previous records documenting the impact of these trematodes on sea turtles in Chile. The inherent nature of these blood flukes inhabiting blood vessels often complicates their detection, particularly in regions where systematic monitoring is limited. The National Fisheries and Aquaculture Service of Chile (SERNAPESCA) conducts constant monitoring of strandings, but the histopathological characterization of internal parasitic diseases remains an area with critical knowledge gaps regarding the prevalence and diversity of these etiological agents [21]. Therefore, the objective of this communication is to report, for the first time, the presence of histopathological lesions associated with spirorchiidiasis in six adult female olive ridley turtle specimens stranded along the Northern and Central-Northen Chilean coast between 2024 and 2026. This study describes the multisystemic distribution of lesions, evaluates the impact of infection on the health and mortality of these individuals, and contributes to the epidemiological knowledge of these parasites in the South Pacific.

2. Materials and Methods

2.1. Stranding Records and Sample Collection

This study included six female adult olive ridley turtles that stranded either alive (n = 4) or dead (n = 2) along the Northern (Atacama) and Central-Northern (Coquimbo, Valparaíso) Chilean coast between 2024 and 2026. These stranding events occurred across three administrative regions: Atacama, Coquimbo, and Valparaíso. All specimens were donated for scientific research under official authorizations issued by the National Fisheries and Aquaculture Service (SERNAPESCA) for the Coquimbo and Atacama regions. Data recorded for each individual included identification number, stranding condition (live or dead), body condition, stranding date, date of death, and geographic location. Body condition score (BCS) was determined following the criteria established by Stacy et al. [12].

2.2. Necropsy and Histopathology

Complete necropsies were performed at the Laboratory of Veterinary Science Research (LiCiVet), Universidad del Alba, La Serena, Chile. A systematic gross examination of all organs was conducted to identify possible causes of death or associated contributory factors. Representative tissue samples from the gastrointestinal tract, liver, reproductive tract, heart, adrenal glands, spleen, kidney, and major vessels were collected and fixed in 10% neutral buffered formalin (DiaPath, Martinengo, Italy). These fixed tissues were submitted to the Veterinary Histopathology Center (VeHiCe) in Puerto Montt, Chile, for routine histological processing. Tissue sections (4 µm) were stained with hematoxylin and eosin (H&E) and subsequently digitized using a Motic Easy Scan system (Motic, Xiamen, China) at magnifications up to 80× for detailed pathological evaluation.

2.3. Lesion Grading and Spirorchiid Impact Rating

Vascular damage, specifically arteritis, was evaluated through integrated gross and microscopic findings and categorized into four distinct grades. Grade 0 was defined by the absence of gross lesions and normal parameters upon microscopic examination. Grade 1 included focal lesions less than 1.5 cm in diameter with histological evidence of minimal inflammation and/or fibrosis. Grade 2 comprised focally extensive lesions between 1.5 and 4 cm in diameter, or a maximum of three lesions, characterized microscopically by moderate inflammation and intimal proliferation. Grade 3 was assigned to focally extensive or multiple lesions greater than 4 cm in dimension or those involving one or more major blood vessels, with histological criteria including thrombi, aneurysm, or medial necrosis. Enteric spirorchiidiasis was similarly graded based on the microscopic identification of distinct egg masses: Grade 0 (no lesions), Grade 1 (1–10 egg masses), Grade 2 (20–50 egg masses), and Grade 3 (more than 50 egg masses). Based on these integrated pathological findings and concurrent clinical data, the final impact rating was classified as none detected, incidental infection (low parasite load and mild inflammation), contributory (significant parasite burden associated with complications like thrombosis or secondary infections), fatal (severe lesions directly linked to mortality), or unknown, where significant damage was present but the definitive cause of death could not be determined due to concurrent factors [12,22].

3. Results

3.1. Stranding Records and Biological Data

A total of six olive ridley turtles were analyzed, all of which were identified as adult females. The strandings occurred between November 2024 and January 2026 across the regions of Atacama (n = 2), Coquimbo (n = 3), and Valparaíso (n = 1). Body condition was classified as intermediate (2/6) or poor (4/6). Two specimens were found dead on arrival and were delivered directly by SERNAPESCA from the Atacama and Coquimbo regions. The four turtles that stranded alive were transferred to the Humboldt Conservation Rescue Center for general clinical examination and quarantine; however, all died within 24 hours of arrival due to their severely deteriorated physiological state (Table 1).

3.2. Postmortem findings

Post-mortem examination of the six specimens revealed significant lesions across multiple organs, predominantly associated with infection by these blood parasites.

3.2.1. Gross findings

All turtles (6/6) exhibited significant gastrointestinal alterations characterized by varying degrees of mucosal thickening and intestinal edema. Multi-focal to coalescing transmural granulomatous lesions were observed in the subserosa, with the largest masses measuring up to 10 × 12 × 7 cm (Figure 1a). In the liver, multifocal nodular granulomas were identified in three specimens (Animal IDs 3, 4, and 6). Specifically, Animal ID 3 presented a large granulomatous lesion (20 × 10 × 6 cm) adjacent to a mature oviduct; upon sectioning, the mass exhibited a friable, whitish-to-yellowish appearance (Figure 2a). Cardiac examination revealed serous atrophy of fat and prominent lymphatic vessels (Figure 3a). Vascular evaluation confirmed the presence of adult trematodes within the lumen of major vessels (Figure 3b). Morphologically, these parasites were consistent with the flukes of family Spirorchiidae, appearing either attached to the vessel wall or forming entangled clusters. Concurrent findings included a severe cranial fracture and predator bite wounds in Animal ID 2, and an embedded fishhook in the esophageal mucosa of Animal ID 3, surrounded by chronic fibrous tissue.

3.2.2. Microscopic findings

Microscopic analysis confirmed multisystemic spirorchiidiasis in all specimens. The most consistent finding was chronic granulomatous inflammation associated with trematode eggs. In the intestine, duodenal villi were markedly thickened due to dense lymphoplasmacytic infiltration in the lamina propria. Numerous elongated eggs compatible with Spirorchiidae family were observed, often surrounded by histiocytes and multinucleated giant cells (MGCs). Multifocal to coalescing granulomas of variable diameter were distributed throughout the submucosa, muscularis, and serosa; these lesions featured necrotic centers containing parasite eggs and, in some cases, the developed miracidium inside them (Figure 1b). Vascular lesions included the formation of parasitic thrombi and emboli. Affected vessels showed significant intimal proliferation, medial necrosis, and adventitial thickening. In Cases 4 and 6, high egg concentrations within the vascular walls triggered severe endarteritis. Extra-vascular involvement included granulomas and MGCs surrounding spirorchiid eggs in the uterus (Figure 2b), spleen (Cases 4 and 6), and adrenal glands (Case 2). Additionally, granulomatous inflammation without visible eggs was identified in the lungs (Case 3) and heart (Case 4). Secondary bacterial aggregates were noted in one individual (Case 3).

4. Discussion

The findings presented in this study constitute the first formal report of lesions associated with spirorchiid trematodes in olive ridley turtles within the South-Eastern Pacific region. While spirorchiidiasis is considered a near-cosmopolitan disease, it has been documented in only five of the seven marine turtle species, with no confirmed reports to date in the leatherback (Dermochelys coriacea) or the flatback (Natator depressus) turtles [15]. Within the American continent, previous reports in olive ridley turtle were restricted to the Atlantic coast of Brazil [19] and the Pacific coast of Costa Rica [20]. Our findings in Chile significantly expand the known geographical distribution of these parasites in the Pacific, suggesting a wider presence in temperate latitudes than previously estimated. The most characteristic presentation of spirorchiidiasis is chronic granulomatous inflammation around embolized eggs, a finding observed in all the cases analyzed in this study. According to Chapman et al. [15], the organs most frequently affected by egg-associated granulomas include the lungs, spleen, gastrointestinal tract, and thyroid gland. Our results align with these observations, particularly regarding the high prevalence of pulmonary and splenic involvement. While the thyroid gland is often reported as a common site of infection in loggerheads (Caretta caretta) and green turtles, the documentation of granulomas in the adrenal glands (Animal ID 2) and uterus (Animal ID 3) in the present study is a notable finding. The presence of eggs in the adrenal glands could interfere with the host's endocrine response to stress, while uterine involvement might impact reproductive health, highlighting the high invasive potential of these parasites in olive ridley turtle [12,18]. Genus Hapalotrema, previously recorded from the South Atlantic, has been associated with severe arterial damage, including aneurysms and endarteritis [23]. Evaluating the impact of spirorchiidiasis on stranding events requires distinguishing between incidental, contributory, and fatal infections. In this study, infection was classified as fatal in Animal ID 6 due to the severity of vascular occlusion and subsequent multiorgan failure. In other individuals (Animal IDs 3 and 4), spirorchiidiasis was considered a contributory factor alongside anthropogenic stressors, such as fishhook ingestion. This cumulative stress model suggests that chronic infections by these cryptic parasites cause systemic debility, increasing host vulnerability to traumatic events or secondary infections [15].

Histologically, the granulomatous lesions observed must be differentiated from other causes of chronic systemic inflammation, such as systemic fungal or bacterial diseases [15]. The presence of secondary bacterial aggregates in Animal ID 3 underscores the importance of considering concurrent opportunistic infections in cases of severe tissue damage, which may exacerbate the primary parasitic pathology. For definitive confirmation and species-level identification of these cryptic parasites, molecular techniques remain essential due to the high level of diversity within the family Spirorchiidae [17]. To contextualize these findings within the regional and global conservation of sea turtles, it is relevant to consider the ecology of these species in Chile [24]. Furthermore, these results complement pathological records from other regions such as Indonesia [25], Australia [26], Taiwan [27], and Brazil [28]. The pathological patterns observed here are consistent with lesions described in other species, such as green turtles in the Caribbean [29] and the Mediterranean [30], where specialized qualitative and quantitative methods are increasingly utilized to estimate parasite burden and health impact [22]. Future research in the South-Eastern Pacific should prioritize the integration of these advanced diagnostic tools with systematic necropsies to clarify the species composition, transmission dynamics, and true clinical significance of spirorchiids in this region.

5. Conclusions

This study provides the first evidence of spirorchiidiasis in olive ridley turtles in the South-Eastern Pacific, significantly extending the known distribution of these parasites in this host species from temperate latitudes. The documentation of multisystemic lesions, particularly the involvement of the adrenal glands and uterus, underscores the high invasive potential of these trematodes and suggests that their impact on sea turtle health may be more complex than previously recognized, potentially affecting endocrine stability and reproductive success. Our findings demonstrate that spirorchiidiasis is a significant factor in sea turtle strandings along the Northern and Central-Northern Chilean coast, acting either as a primary cause of death or as a severe contributory stressor in individuals already compromised by anthropogenic factors. Given the lesions observed in the analyzed specimens, it is imperative to integrate systematic histopathological and molecular screening into national stranding response protocols to better monitor these and other parasites. Future research should prioritize the use of advanced diagnostic tools, such as a molecular approach, to clarify the species diversity within the family Spirorchiidae. Such knowledge is essential for developing effective, evidence-based conservation strategies for these vulnerable marine populations in the South Pacific.