Heavy infection with Contracaecum rudolphii A larvae in the intestine of Anguilla anguilla: Pathobiology and inflammatory response

Abstract text

Introduction

The European eel, Anguilla anguilla, is a major warm water fish species and Contracaecum rudolphii uses eel and several other fish species as paratenic host. Fish response to extra-intestinal parasites induce the formation of granuloma and granulomas are chronic inflammatory lesions that appear as nodules in one or more organs. Larvae (L3) of C. rudolphii A were encapsulated within the thickness of the eel intestinal wall and within a granuloma on the external visceral peritoneum (serosa). Information on the status of the eel immune system is of importance to maintain good health throughout the grow out period in eel fisheries.

Objectives

There is limited knowledge in terms of both cellular and humoral immune responses of A. anguilla to helminths. Identification of the host immune cells only with histology appeared to be insufficient. The current investigation was undertaken to gain information on the presence and distribution of 5 antibodies in different types of immune cells of A. anguilla intestine against C. rudolphii A (L3) larvae.

Materials & Methods

From January to October 2022, a subsample of 40 eels from the Comacchio lagoons were examined, several 15 x 15 mm pieces (from infected-uninfected intestine) were excised and fixed in 10% neutral buffered formalin for 24 h and were assessed by histology techniques. For immunohistochemistry following molecules were tested: antibodies against immunoglobulin E (IgE)-like receptor (FCεRIγ), histamine, serotonin, tryptase and inducible-nitric oxide synthase (i-NOS).

Results

Twenty-four eels (60%) out of 40 harboured larvae (L3) of the nematode C. rudolphii A, the intensity of infection ranged from 5 to 76 parasites per eel. Conspicuous granulomas were visible on outer surface of the intestine. The outer part of the granuloma contained macrophages, macrophage aggregates, and mast cells (MCs) scattered among fibroblasts. The middle layer was rich in MCs and fibroblasts and the inner layer consisted mainly of dark epithelioid cells. Within the granulomas cells of different types were positive to antibodies anti-FCεRIγ, -histamine, -serotonin, -tryptase and -i-NOS.

Conclusion

Immune evasive mechanisms render the parasites able to survive even years in tissue despite the high abundance of host immune cells in vicinity. This survey might suggest an evolutionary trade-off between eel tolerance and relatively inactive nematode larva within granuloma.