Another piece of the puzzle: analysis of Acanthocheilonema spirocauda larvae from its presumed intermediate host the seal louse (Echinophthirius horridus)

The transmission of the seal heartworm Acanthocheilonema spirocauda (Nematoda: Filarioidea) by the haematophagous seal louse Echinophthirius horridus (Anoplura: Echinophthiriidae) has been proposed for many decades, and several studies based on morphological and molecular data indicate this intermediate host-parasite relationship. However, the larval stages of A. spirocauda have rarely been visualized and studied so far. In the current study, we dissected 151 E. horridus lice obtained from naturally infested seals to investigate the presence of A. spirocauda larvae. Overall, 26.5% of the seal lice proved infected with A. spirocauda. In total, 186 larvae were found, comprising all larval stages. Thereby, 142 larvae were measured, revealing 101 first-stage (L1; 71.1%), 31 second-stage (L2; 21.8%), and 7 third-stage larvae (L3; 4.9%). Larval stages were mainly allocated in the fat bodies (95.1%), but also detected in the caput (2.8%), haemocoel (1.4%), and thoracic muscles (0.7%) of E. horridus. For imaging, we used light microscopy, confocal laser scanning and scanning electron microscopy to provide a more detailed morphological characterization of A. spirocauda larvae. Our findings further support E. horridus as a likely obligate intermediate host in the heteroxenous life cycle of A. spirocauda and call for more research on vector-borne pathogens in pinniped populations.