Mutual adaptations between hosts and parasites determine stress levels in eels

Abstract

Invasive parasites may severely affect their new hosts. Two invasive parasites occurring in the European eel ( Anguilla anguilla ) are the Asian swim bladder nematode Anguillicola crassus and the Ponto-caspian acanthocephalan Pomphorhynchus sp., which were introduced to the river Rhine in the early 1980/90s. The Japanese eel ( Anguilla japonica ), as the native host of A. crassus , developed mutual adaptations to the swim bladder parasite, which are lacking in the European eel. Therefore, after its spread to Europe, infestations of European eels with A. crassus were found to be more severe and caused massive swim bladder wall damages mainly due to the feeding activity of the adult nematodes. A suppression of the immune system also appears to be likely, which allows secondary infections e.g. by bacteria or other parasites in European eels. Acanthocephalans of the genus Pomphorhynchus have not been described so far in Japanese eels, in contrast to European eels, which regularly show infestations with Pomphorhynchus sp. By using these differentially adapted host-parasite associations for experimental studies, host stress responses were examined in the present study in relation to the degree of mutual adaptations between eel hosts and parasites. Under laboratory conditions, Japanese and European eels were each inoculated with A. crassus and Pomphorhynchus sp., respectively, to investigate their stress responses against differently adapted parasites. The stress response was determined by analyzing plasma levels of cortisol, which is the main corticosteroid hormone during stress response of fish. The results show a strong cortisol release in European eels after infestation with A. crassus whereas Japanese eels only react against Pomphorhynchus sp. infestations. These results are consistent with the initial hypothesis that a low degree of host-parasite adaptations lead to stronger host stress responses against the parasite.

Monitoring for Anguillicoloides crassus, Anguillid herpesvirus 1, aquabirnavirus EVE and rhabdovirus EVEX in the European eel population of southern Spain

“…An illustrative example is the introduction of the nematode A. crassus through Japanese eels ( Anguilla japonica ) imported into Europe for commercial purposes (eel farming and consumption) depleted stocks of the native European eel ( A. anguilla ). The parasite expanded throughout Europe, because of the spread from infected farms or by the movement of live eels from infected areas (Honka & Sures, 2021).…”

“…Globalization has dramatically increased worldwide trade of goods and live organisms, often entailing the introduction of allochthonous species into new habitats, including pathogens (Dangel et al, 2015), resulting in new host–parasite combinations between autochthonous and non‐native hosts (Galli et al, 2005). The arrival of invasive species often affects the fitness of the native populations (Honka & Sures, 2021) and biodiversity, causing important changes in the structure of populations (Hatcher & Dunn, 2011; McGeoch et al, 2010). An illustrative example is the introduction of the nematode A. crassus through Japanese eels ( Anguilla japonica ) imported into Europe for commercial purposes (eel farming and consumption) depleted stocks of the native European eel ( A. anguilla ).…”

“…It has been proven that stressors such as high water temperatures, low oxygen concentrations, contamination with toxic substances such as polychlorinated biphenyls (PCB's) or inorganic elements, silvering, exhaustion from migration to spawning grounds in the Sargasso Sea (Haenen et al, 2010;Kullmann et al, 2017;Lortholarie et al, 2021;Muñoz et al, 2019;van Beurden et al, 2012;Van et al, 2005) and especially, infection with A. crassus, might reactivate AngHV1 infection in persistent, non-clinical carriers, causing high mortalities (Haenen et al, 2010;van Beurden et al, 2012;Van Nieuwstadt et al, 2001). In this sense, several authors demonstrated high cortisol levels in the blood derived from the presence of A. crassus (Honka & Sures, 2021;Sures & Streit, 2001).…”

“…Several studies have shown that A. crassus causes oedema and hyperplastic and fibrotic changes in the swimbladder mainly due to the hematophagous activity of the adult nematodes and the migration of the L3 larvae through the swimbladder wall (Haenen et al, 1989; Kirk, 2003). This leads to an increase in stress parameters leading to immunosuppression and a reduction in the body condition of eels (Abdelmonem et al, 2010; Barry et al, 2017; Costa‐Dias et al, 2010; Dezfuli et al, 2021; Honka & Sures, 2021). The consequence of this health loss is that the eels' success in completing their migration to the Sargasso Sea for spawning may be compromised (Kirk, 2003).…”