Contracaecum osculatumand other anisakid nematodes in grey seals and cod in the Baltic Sea: molecular and ecological links

Abstract

Populations of grey seals (Halichoerus grypus), sprats (Sprattus sprattus) and cod (Gadus morhua) in the Baltic Sea are relatively stationary. The present work, applying classical and molecular helminthological techniques, documents that seals and cod also share a common parasite, the anisakid nematode Contracaecum osculatum, which uses seals as the final host and fish as transport hosts. Sequencing mitochondrial genes (COX1 and COX2) in adult worms from seals and third-stage larvae from livers of Baltic fish (sprats and cod), showed that all gene variants occur in both seals and fish. Other anisakid nematodes Pseudoterranova decipiens and Anisakis simplex are also found in both seals and cod in the Baltic Sea, but at much lower rates. The Baltic grey seal population was left at a critically low level (comprising a few hundred individuals) during the latter part of the 20th century, but since the year 2000 a marked increase in the population has been observed, reaching more than 40,000 individuals at present. Ecological consequences of the increased seal abundance may result from increased predation on fish stocks, but recent evidence also points to the influence of elevated parasitism on fish performance. Contracaecum osculatum larvae preferentially infect the liver of Baltic cod, considered a vital organ of the host. Whereas low prevalences and intensities in cod were reported during the 1980s and 1990s, the present study documents 100% prevalence and a mean intensity of above 80 worms per fish. Recent studies have also indicated the zoonotic potential of C. osculatum larvae in fish, following the consumption of raw or under-cooked fish. Therefore the present work discusses the impact of parasitism on the cod stock and the increasing risk for consumer health, and lists possible solutions for control.

Endoparasitic helminths in Baltic salmon Salmo salar: ecological implications

“…Larvae are obtained by the final host fish by ingestion of invertebrate or piscine transport hosts (Køie 1993), but the broad distribution and low infection means it is a less interesting biological tag (Williams et al 1992, MacKenzie 2002. The relatively low prevalence and intensity of third-stage nematode larvae of Contracaecum osculatum is noteworthy because the parasite is abundant in sprat and cod in the investigated area (Mehrdana et al 2014;Zuo et al 2018). The size of the larva in cod liver may reach 21 mm (Fagerholm 1982), but larvae in Baltic salmon liver all had stunted growth (body lengths: 6−10 mm) and were located in small granulomas, which suggests that the immune reactions in salmon towards C. osculatum larvae are more effective compared to cod.…”

“…A part of each individual parasite (10 parasite specimens per species) was taken for molecular analysis using lysis, DNA purification, PCR with specific primers (Table 1), product purification, sequencing and BLAST analysis. The lysate was prepared with a QIAGEN ® DNeasy Blood & Tissue Kit, and PCR performed in 60 µl PCR mix which contained 2 µl sample, 6 µl forward primer (10 mM), 6 µl reverse primer (10 mM), 6 µl 10× PCR buffer, 6 µl dNTP (4 × 10 mM), 1.8 µl MgCl 2 (50 mM), 0.25 U DNA polymerase (Bioline) and H 2 O (Zuo et al 2018). The PCR outline was pre-denaturation at 95°C for 5 min; amplification by a number of cycles starting with denaturation at 95°C for 30 s, followed by annealing at an assay-specific temperature for 30 s, and ending with elongation at 72°C; and finally a post-elongation step at 72°C for 7 min.…”

Metazoan Endoparasites as Biological Indicators of Baltic Cod Biology

“…The distribution in the Baltic of these major animal groups is dependent on biological and hydrographical conditions in various salinity zones, and their distribution will to some extent determine the parasite fauna found in the Baltic cod whereby these can be applied as biological indicators. The use of parasites in fish as indicators has been suggested and documented by numerous authors [4][5][6][7][8][9][10][11][12][13]. Although many parasite species display a high degree of host specificity, some parasite species infect a broad variety of fish.…”

Immunomodulatory effects of excretory/secretory compounds from Contracaecum osculatum larvae in a zebrafish inflammation model

“…osculatum (Rudolphi, 1802) comprises adult worms in marine mammals, e.g. seals [6, 7], and infective third-stage larvae in invertebrates and teleosts serving as intermediate/transport hosts. Humans may accidentally obtain third-stage larvae of the parasite through consumption of raw or under-processed seafood which causes anisakidosis associated with gastrointestinal symptoms [8–10] and experimental C .…”

“…osculatum is increasing in certain localities, e.g. the Baltic Sea [7, 17] and may have an increasing influence on health in fish and mammals including humans. It is, therefore, worthwhile to investigate the immunogenic properties of C .…”