The Canary Islands are considered a hot spot for marine species biodiversity. Each stranded cetacean has provided important scientific, biological and pathological information. The morphological identification of parasites in these stranded cetaceans is the main aim of the present article. An investigation to identify parasites was carried out in 233 stranded animals (2018–2022), which were submitted for necropsy. Adult and sub-adult/juvenile animals were the most parasitized age categories, and eighteen parasite species were morphologically identified, including nematodes (Crassicauda grampicola, Crassicauda anthonyi, Halocercus delphini, Anisakis simplex, Stenurus globicephalae, Stenurus ovatus), trematodes (Nasitrema delphini, Oschmarinella rochebruni, Brachycladium atlanticum, Pholeter gastrophilus), cestodes (Clistobothrium delphini, Clistobothrium grimaldii), acanthocephalans (Bolbosoma vasculosum, Bolbosoma capitatum), protists (Toxoplasma gondii) and crustaceans (Conchoderma auritum, Xenobalanus globicipitis, Pennella balaenoptera), as well as nine genera: Crassicauda sp., Halocercus sp., Anisakis sp., Stenurus sp., Nasitrema sp., Oschmarinella sp., Dyphyllobothrium sp., Cyamus sp., Conchoderma sp., and Sarcocystis sp. These could not be identified at the species level. Differences in the host specificity of some parasites were observed, varying in prevalence. The accurate morphological identification of the parasites affecting these marine mammals is essential to understanding disease epidemiology, assessing cetacean populations’ health status, and developing effective conservation and management strategies.

Crassicauda nematodes in cetaceans infect various organs, including the genitourinary system, subcutaneous tissues, circulatory system, and cranial structures such as the pterygoid sacs. In the cranial region, they may induce osteolytic lesions, potentially leading to neurological impairment, disorientation, and contributing to stranding and death. This study investigated the epidemiology and pathology of cranial crassicaudiasis in cetaceans stranded in the Canary Islands between 1999 and 2024, with emphasis on associated central nervous system (CNS) lesions. The pterygoid sacs of 438 cetaceans were systematically examined, and Crassicauda sp.-induced sacculitis was detected in 49 individuals (11.2 %) of the family Delphinidae. Prevalence was highest in Risso's dolphins (Grampus griseus; 11/18, 61.1 %), and juvenile and subadult animals were more frequently affected (24/49, 49 %), with no sex-related differences. Molecular characterization targeting COX1 and ITS2 gene regions was performed on nematodes from an Atlantic spotted dolphin (Stenella frontalis) and a Risso's dolphin, confirming C. grampicola in both species. Histopathology in selected cases (26; 5 %) revealed epithelial hyperkeratosis, desquamation, and mild lymphoplasmacytic inflammation in parasitized pterygoid sacs. CNS inflammation was observed in 16/49 cases (32.6 %), including co-infections with Brucella spp., Cetacean Morbillivirus (CeMV), Herpesvirus and Nocardia otitidiscaviarum. No statistically significant association was found between cranial crassicaudiasis and CNS lesions. These findings highlight cranial crassicaudiasis as a relevant pathological condition in stranded cetaceans, suggesting that parasite-associated lesions may contribute to morbidity and mortality, and provide the first molecular confirmation of C. grampicola in S. frontalis, contributing novel sequences to GenBank.