The paper “Heartworms in Halichoerus grypus: first records of Acanthocheilonema spirocauda (Onchocercidae; Filarioidea) in 2 grey seals from the North Sea,” published in Parasitology is freely available to read online. 

The two resident species harbour and grey seals are sharing the same habitat in the North- and Baltic Sea. They can therefore be observed regularly resting on sandbanks in the Wadden Sea on low tide, on the island Heligoland in the North Sea or along the Baltic Sea coast at Greifswald Bay. They are charismatic ambassadors of their marine environment and still rely on land for breeding, parturition, pup care and moulting.

In the beginning of the 19th century, grey seals had become almost extinct due to human over-exploitation by hunting and pollutant exposure. Since the 1990’s, after hunting and pollutants like polychlorinated biphenyls (PCBs) were banned, seal populations are recovering and have increased, with grey seals recolonising and reproducing in the North Sea and recently Baltic Sea waters of Germany.

Like their terrestrial carnivore relatives, harbour seals are infected by heartworms. The filarial nematode Acanthocheilonema spirocauda parasitizes the cardiac ventricles and blood vessel lumina of harbour seals, although infections occur rarely and the probability is low that A. spirocauda causes severe health impacts. In dogs, heartworms are known to be transmitted by mosquitoes, which resembles a typical parasite assembly of an insect as vector or carrier and filarial worms in terrestrial ecosystems.

In the marine environment, however, insects are rare. Therefore, parasitologists were puzzled about transmission pathways. Fur is probably what enabled a much specialised ectoparasite to cling on to seals even when they returned to the ocean in the Eocene: the seal louse Echinophthirius horridus is common on grey and harbour seals around the Northern hemisphere. It is a blood-sucking insect, which is transmitted directly through social interactions between seal hosts and is assumed to transmit stages of heartworms to its final seal host.

While seal louse infections are common findings in both grey and harbour seals, heartworms were so far only found in harbour seals. Scientists wondered about the reason – as both harbour and grey seals have very similar life styles, share the same geographic area, eat the same prey and have many parasites in common. In the frame of monitoring health in marine mammals in German waters, dead stranded animals are submitted for post mortem investigations since 1996. When taking stock of archived samples from decades of health monitoring, it took researchers by surprise when 2 cases of grey seals infected with heartworms were detected. Because the samples from 2018 and 2020 contained damaged nematode specimens, nucleotide sequences of a part of the mitochondrial DNA were used for unambiguous parasite identification.

This first record of heartworm infection with A. spirocauda in grey seals intrigued scientists and produced new questions about the species-specificity of filarial nematodes as well as immune system traits of grey seals. If transmission occurs directly via seal louse vectors on mixed resting places of grey and harbour seals, increasing seal numbers in the North- and Baltic Sea could have density dependent effects on the frequency of occurrence of heartworm and seal louse infections in both host species when grey and harbour seals interact during their time on land.