Influence of adult Anguillicoloides crassus load in European eels swimbladder on macrophage response

Towards an in-situ non-lethal rapid test to accurately detect the presence of the nematode parasite, Anguillicoloides crassus, in European eel, Anguilla anguilla

Anguillicoloides crassus is an invasive nematode parasite of the critically endangered European eel, Anguilla anguilla, and possibly one of the primary drivers of eel population collapse, impacting many features of eel physiology and life history. Early detection of the parasite is vital to limit the spread of A. crassus, to assess its potential impact on spawning biomass. However accurate diagnosis of infection could only be achieved via necropsy. To support eel fisheries management we developed a rapid, non-lethal, minimally invasive and in situ DNA-based method to infer the presence of the parasite in the swim bladder. Screening of 131 wild eels was undertaken between 2017 and 2019 in Ireland and UK to validate the procedure. DNA extractions and PCR were conducted using both a Qiagen Stool kit and in situ using Whatman qualitative filter paper No1 and a miniPCR DNA Discovery-System™. Primers were specifically designed to target the cytochrome oxidase mtDNA gene region and in situ extraction and amplification takes approximately 3 h for up to 16 individuals. Our in-situ diagnostic procedure demonstrated positive predictive values at 96% and negative predictive values at 87% by comparison to necropsy data. Our method could be a valuable tool in the hands of fisheries managers to enable infection control and help protect this iconic but critically endangered species.

Survival of metazoan parasites in fish: Putting into context the protective immune responses of teleost fish

Defence mechanisms of fish can be divided into specific and non-specific that act in concert and are often interdependent. Most fish in both wild and cultured populations are vulnerable to metazoan parasites. Endoparasitic helminths include several species of digeneans, cestodes, nematodes, and acanthocephalans. Although they may occur in large numbers, helminth infections rarely result in fish mortality. Conversely, some ectoparasites cause mass mortality in farmed fish. Given the importance of fish innate immunity, this review addresses non-specific defence mechanisms of fish against metazoan parasites, with emphasis on granulocyte responses involving mast cells, neutrophils, macrophages, rodlet cells, and mucous cells. Metazoan parasites are important disease agents that affect wild and farmed fish and can induce high economic loss and, as pathogen organisms, deserve considerable attention. The paper will provide our light and transmission electron microscopy data on metazoan parasites-fish innate immune and neuroendocrine systems. Insights about the structure and functions of the cell types listed above and a brief account of the effects and harms of each metazoan taxon to specific fish apparati/organs will be presented.

The impact of Anguillicoloides crassus (Nematoda) on European eel swimbladder: histopathology and relationship between neuroendocrine and immune cells

The swimbladder functions as a hydrostatic organ in most bony fishes, including the European eel, Anguilla anguilla. Infection by the nematode Anguillicoloides crassus impairs swimbladder function, significantly compromising the success of the eel spawning migration. Swimbladders from 32 yellow eels taken from Lake Trasimeno (Central Italy) were analysed by histopathology- and electron microscopy-based techniques. Sixteen eels (50%) harboured A. crassus in their swimbladders and intensity of infection ranged from 2 to 17 adult nematodes per organ (6.9 ± 1.6, mean ± s.e.). Gross observations of heavily infected swimbladders showed opacity and histological analysis found a papillose aspect to the mucosa and hyperplasia of the lamina propria, muscularis mucosae and submucosa. Inflammation, haemorrhages, dilation of blood vessels and epithelial erosion were common in infected swimbladders. In the epithelium of parasitized swimbladders, many empty spaces and lack of apical junctional complexes were frequent among the gas gland cells. In heavily infected swimbladders, we observed hyperplasia, cellular swelling and abundant vacuolization in the apical portion of the gas gland cells. Numerous mast cells and several macrophage aggregates were noticed in the mucosal layer of infected swimbladders. We found more nervous and endocrine elements immunoreactive to a panel of six rabbit polyclonal antibodies in infected swimbladders compared to uninfected.

Fish innate immunity against intestinal helminths

Most individual fish in farmed and wild populations are infected with parasites. Upon dissection of fish, helminths from gut are often easily visible. Enteric helminths include several species of digeneans, cestodes, acanthocephalans and nematodes. Some insights into biology, morphology and histopathological effects of the main fish enteric helminths taxa will be described here. The immune system of fish, as that of other vertebrates, can be subdivided into specific and aspecific types, which in vivo act in concert with each other and indeed are interdependent in many ways. Beyond the small number of well-described models that exist, research focusing on innate immunity in fish against parasitic infections is lacking. Enteric helminths frequently cause inflammation of the digestive tract, resulting in a series of chemical and morphological changes in the affected tissues and inducing leukocyte migration to the site of infection. This review provides an overview on the aspecific defence mechanisms of fish intestine against helminths. Emphasis will be placed on the immune cellular response involving mast cells, neutrophils, macrophages, rodlet cells and mucous cells against enteric helminths. Given the relative importance of innate immunity in fish, and the magnitude of economic loss in aquaculture as a consequence of disease, this area deserves considerable attention and support.

Influence of nematode Anguillicoloides crassus infestation on the cellular and humoral innate immunity in European eel (Anguilla anguilla L.)

Parasitic invasions are recognized as one of the primary factors responsible for decreasing populations of European eel. The aim of the present study was to determine the influence of infestation with the nematode Anguillicoloides crassus on the innate immunity in European eel (Anguilla anguilla). Anguillicoloides crassus parasitizes the swim bladder of this fish. Levels of the following immunological parameters were measured: spleen phagocyte respiratory burst activity, spleen phagocyte potential killing activity, pronephros lymphocyte proliferation stimulated by concanavaline A or lipopolisaccharide, plasma lysozyme and ceruloplasmin activity, total protein and immunoglobulin (Ig) serum levels. The analyses of the results of humoral and cellular immunity indicate that all studied parameters were statistically significant higher (p < 0.05) in non-infested fish compared to the ones with anguillicolosis except for ceruloplasmin level. These data suggest that the A. crassus infestation in European eel is responsible for a decreased immune response what could result in higher susceptibility to other pathogenic conditions.