Morphological and molecular characterisation of pathogenic helminths in common dolphins (Delphinus delphis) stranded along the Irish coast

Parasitic infections are a major cause of stranding and death of cetaceans. An increase in their prevalence and pathology has been linked to rising sea temperatures, making understanding and monitoring of marine mammal host-parasite interactions ever more important. In this case series, postmortem examinations were conducted on five short-beaked common dolphins (Delphinus delphis) stranded on the Irish coast between 2023 and 2026. Nematode parasites, considered to be the main cause of stranding in all cases, were characterised morphologically and by sequence analysis of a fragment of the internal transcribed spacer 2 (ITS-2). The lungworm Halocercus delphini was detected in three of the five dolphins, causing more severe pathology, characterised by marked granulomatous pneumonia, than previously reported for the North-East Atlantic. Anisakis simplex sensu lato was recovered from four of the five dolphins and was associated with severe ulcerative to pyogranulomatous gastritis; all ITS-2 sequences of A. simplex (s.l.) were identical to each other and to specimens reported worldwide. Crassicauda sp. was found in the subcutis, mammary gland and prescapular lymph node of two dolphins. Sequences generated from these specimens were identical to each other and represent the first Crassicauda sp. sequences from common dolphins. Using Bayesian inference analysis, these formed a genetically distinct group from other Crassicauda spp. Subcutaneous co-infection of Clistobothrium delphini and C. grimaldii was observed in one case. ITS-2 sequences of these cestode species are published for the first time from a common dolphin. C. delphini sequences from this study were at least 99.7% identical to each other and to C. delphini from long-finned pilot whales in Spain. The same pattern was observed for C. grimaldii. This study highlights the pathological significance of parasite infections in common dolphins and provides critical new information on their phylogeny, which will inform future marine mammal monitoring.