Gastrointestinal parasites of harbour seal (Phoca vitulina L.) in Danish marine waters: Prevalence, abundance, intensity and reproductive potential

Populations of harbour seal (Phoca vitulina) in Danish marine areas have increased markedly during recent decades, but their associated parasite populations have remained unknown. In February 2024 we collected 13 harbour seals from the western part of Limfjorden (Denmark), a marine area connecting the North Sea with the Kattegat Sea. The animals were subjected to a parasitological examination (including morphological and molecular methods), which confirmed that this seal species acts as a definitive hosts for the gastrointestinal nematodes Contracaecum osculatum s.s., Anisakis simplex, Phocanema decipiens s.s. and P. krabbei; all nematodes were at prevalences of 100 %. The seals also harboured the intestinal acanthocephalan Corynosoma strumosum (100 % prevalence) and the cestode Dibothriocephalus schistochilus (7.7 % prevalence). The nematode intensities ranged from 4 to 1790 individuals per host, whereas acanthocephalans occurred in lower numbers (range 4–222 per host) and a single seal was infected with three cestodes. The reproductive potential of the nematodes was evaluated by counting mature eggs in female worms, which indicated that each of the female worms could release between 7202 and 72,810 eggs per day. Infection intensities revealed that a single harbour seal on average releases more than 3 × 10⁵ eggs per day. In more heavily infected seals, the potential release of anisakid eggs to the environment exceeds 2.4 × 10⁶ eggs per day, each of which, after hatching and invasion of the first paratenic host, represents a potential infection risk for fish and humans. Some of the anisakid parasites are potentially infective to human consumers, and we discuss the potential spread of infection to fish and humans at different infection levels and seal population sizes.