Heartworm and seal louse: Trends in prevalence, characterisation of impact and transmission pathways in a unique parasite assembly on seals in the North and Baltic Sea.

ABSTRACT

The ectoparasitic seal louse, Echinophthirius horridus infects harbour (Phoca vitulina) and grey seals (Halichoerus grypus) in the North and Baltic Sea. The endoparasitic heartworm Acanthocheilonema spirocauda parasitizes the right heart and blood vessels of harbour seals. The complete lifecycle of the heartworm is not entirely understood although the seal louse is assumed to serve as vector for its transmission. Knowledge about the impact of both parasite species on host health are scarce. In this study, necropsy data and archived parasites of harbour and grey seals in German waters were analysed to determine long-term seal louse (SLP) and heartworm prevalence (HWP) from 2014 to 2021. Histology, microbiology and scanning electron microscopy (SEM) were applied on seal louse infected and uninfected skin to investigate associated lesions and the health impact. During the study period, HWP in harbour seals was 13%, the SLP in harbour seals was 4% and in grey seals 10%. HWP of harbour seals was significantly higher during the winter months compared to the summer. SLP in adults was significantly higher in comparison to juvenile harbour seals. SLP varied significantly between grey seals from the North and Baltic Sea. Filarial nematodes were detected in the haemocoel, pharynx, and intestine of E. horridus highlighting the seal louse as vector for heartworms. Alopecia and folliculitis were associated with the attachment posture of E. horridus and microbiological investigations isolated bacteria commonly associated with folliculitis.