Seal louse and heartworm: Time trends and impact on harbour seals in the North and Baltic Sea

Abstract text

Pinnipeds provide a livelihood for numerous parasites, showing unique morphological and physical adaptions to the semiaquatic lifestyle of their host. Echinophthirius (E.) horridus, the seal louse and the heartworm Acanthocheilonema (A.) spirocauda outline a possible unique vector-parasite congregate transported to the marine environment. Commonly mild health effects are reported, yet they can cause anaemia, alopecia or arteritis and obstruction of vessels in severe cases. E.horridus is discussed to play an obligatory role as vector for heartworm filariae (A. spirocauda) of harbour seals (Phoca vitulina). In this study we examined the prevalence of A.spirocauda and E.horridus in harbour seals (n = 672) between 2014 and 2021 to analyse time trends and life history traits using a unique sample set collected within a stranding network along the North and Baltic Sea coast comprising necropsy results, histological data and tissue archive. Additional Confocal Laser Scanning Microscopy (CLSM), nanoCT and attachment force measurements were performed to characterise physical solutions for life on marine hosts.Infection of harbour seals with A.spirocauda was found in 11% (n = 76). The prevalence of A.spirocauda varied over the seven-year-study period, and showed a notable increase compared to previous decade. Seal lice were found in 4% (n = 26) of the examined harbour seals. Seal lice prevalence remained relatively low. For both parasites, there were no sex related differences in infection patterns but a higher prevalence in yearling and young–of-the-year seals. Histological screening of seal lice revealed larval nematode stages in the hemocoel supporting the hypothesis of seal lice functioning as intermediate host and vector of A.spirocauda. Additional histology showed lesions affecting the health status of their host. CLSM of E.horridus indicates sclerotized structures and nanoCT aided in 3D reconstruction of musculoskeletal system, providing first understandings of attachment mechanisms. Increased prevalence of A.spirocauda over the study period may reflect growing seal populations, higher densities on haul-outs, thereby facilitating transmission by lice vectors. Higher prevalence in younger age groups might reflect the social behaviour of harbour seals within the different age groups; it could also indicate a vertical transmission pattern. The results underline, parasites are useful indicators for host ecology and ecosystem changes in the North and Baltic Sea.