Phocid seals, seal lice and heartworms: a terrestrial host–parasite system conveyed to the marine environment

Abstract

Adaptation of pinnipeds to the marine habitat imposed parallel evolutions in their parasites. Ancestral pinnipeds must have harboured sucking lice, which were ancestors of the seal louse Echinophthirius horridus. The seal louse is one of the few insects that successfully adjusted to the marine environment. Adaptations such as keeping an air reservoir and the ability to hold on to and move on the host were necessary, as well as an adjustment of their life cycle to fit the diving habits of their host. E. horridus are confined to the Northern Hemisphere and have been reported from 9 species of northern phocids belonging to 4 genera, including land-locked seal species. The transmission from seal to seal is only possible when animals are hauled-out on land or ice. Lice are rarely found on healthy adult seals, but frequently on weak and young animals. The seal louse is suggested to play an important role as an intermediate host transmitting the heartworm Acanthocheilonema spirocauda among seals. However, the evidence is restricted to a single study where the first 3 larval stages of the heartworm were shown to develop in the louse. The fourth-stage larvae develop in the blood system of seals and eventually transform into the adult stage that matures in the heart. Since all other studies failed to confirm the presence of heartworm larvae in seal lice, other unknown intermediate hosts could be involved in the transmission of the heartworm. Transplacental transmission of microfilariae in seals has been suggested as an additional possibility, but is not likely to be important since the occurrence of heartworms in adult seals is very rare compared with juveniles. Furthermore, there are no findings of the first 3 larval stages in seals. This review shows that the heartworm infects nearly the same species of seals as the seal louse, except for the grey seal Halichoerus grypus, where the heartworm is absent. Prevalence and intensity of infection differ among regions in the Northern Hemisphere. As for seal lice, heartworms mainly infect immature seals, and after infection the prevalence seems to decrease with increasing age of the host.

Heartworm (Acanthocheilonema spirocauda) and seal louse (Echinophthirius horridus) infections in harbour seals (Phoca vitulina) from the North and Baltic Seas

“…Although they share the same habitat and prey species, grey seals apparently do not get infected by heartworms (Leidenberger et al, 2007;Measures et al, 1997) while both seal species are infected by seal lice (Durden and Musser, 1994). In contrast to cetaceans seals have retained an amphibian life-style (Scherf, 1963a(Scherf, ,1963b and rely on terrestrial habitat for reproduction and moulting (Boyd et al, 1999;Riedmann, 1990;Siebert et al, 2012).…”

“…It is assumed that the hematophagous seal louse is essential for the development of microfilaria and transmission of the heartworm larvae to their final host, the harbour seal (Geraci et al, 1981). During their adaptation to the marine environment some parasite species of the harbour seal's terrestrial ancestors were lost, but presumably the heartworm A. spirocauda and seal louse E. horridus (Anoplura; Insecta) underwent a coevolution with their host and remained throughout its transmission to the aquatic milieu (Leidenberger et al, 2007). The life-cycle of the heartworm is still not fully understood and little is known about the role of the seal louse as a vector (Conlogue et al, 1980;Dunn and Wolke, 1976;Mehlhorn et al, 2002;Taylor et al, 1961).…”

“…Subsequent studies failed to confirm the presence of heartworm larvae in seal lice. Therefore, mosquitos or simuliids as potential vectors (Dunn and Wolke, 1976) or transplacental transmission of microfilariae in seals has been discussed (see Leidenberger et al, 2007 for a review). The parasite host system could be assumed to be highly host-specific, but heartworms are found in several pinniped species, like harp, hooded and ringed seals from the arctic (Measures et al, 1997), as well as seal lice (Durden and Musser, 1994).…”

Characterization of the heartworm Acanthocheilonema spirocauda (Leidy, 1858) Anderson, 1992 (Nematoda: Onchocercidae) in Scandinavia

“…Additional hosts P. v. stejnegeri, ribbon seal P. fasciata, harp seal P. groenlandica, ringed seal P. hispida, Baltic ringed seal P. h. botnica, Ladoga seal P. h. ladogensis, spotted seal P. largha, hooded seal Cystophora cristata, and bearded seal Erignathus barbatus (Westerling et al 2005;Leidenberger et al 2007). …”

“…a Distance from anterior extremity b Distance from anus to posterior extremity c Following the curvature of the structure Present localities Danish North Sea (Agger Tange), Limfjord (Thy), Skagerrak (Lysekil) and Kattegat (Rungsted, Lille Vrøj) and the Baltic Sea (Lunds høje, Falster). Northern Hemisphere, Holarctic distribution(Westerling et al 2005;Leidenberger et al 2007).Site Right and left ventricle of the heart, pulmonary vein, pulmonary artery, vena cava.Intensity of infection Up to 44 specimens per infected heart.Present material Twenty-two voucher specimens (nos. 94821-94842) deposited in the Invertebrate Collections of the Swedish Museum of Natural History, Stockholm.…”