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The Pays de la Loire region is the fourth largest producer of pond fish with a surface area of between 1700 and 6000 hectares. Different species of fish are produced, the main one being carp, with a production mainly intended for restocking and fishing routes. In addition, steps are being taken to diversify production towards human food, mainly carp.

Fish farming, like other animal productions, is faced with the presence of infectious agents and must control as best it can the associated diseases that affect production. In this context, it is important to provide knowledge on bacteria belonging to the genus Aeromonas. Indeed, the Aeromonas genus is composed of several bacterial species that have colonized aquatic ecosystems, some of which have been shown to be opportunistic or pathogenic. For example, furunculosis is a bacterial disease of worldwide distribution due to Aeromonas salmonicida sub salmonicida. It is responsible for heavy economic losses in fish/salmon farms through haemorrhagic septicaemia (acute form) or through depreciation of fish due to the development of boils in the muscles (chronic form). Erythrodermatitis in carp, also caused by bacteria of the genus Aeromonas, leads to similar clinical signs and consequences.

The first objective of AEROFISH was to gain a better understanding of the presence of the bacteria of the genus Aeromonas and potentially of the circumstances in which they appear in the farm. To do this, interviews were conducted with fish farmers with a stocking centre to collect their perception on the presence or not of this bacterial genus. Thus, a questionnaire was developed and submitted to the pond operators during farm visits. It covered the areas of site structuring, surface areas and volumes involved, zootechnical and biosecurity management practices, subsequent sanitary episodes, the perceived impact of Aeromonas and factors that appeared to favour clinical episodes. The analysis of the questionnaire shows that the pond fish farming sector in the Pays de la Loire region is characterised by heterogeneity in rearing structures and practices. There are common points: management of batches in ponds (stocking at the end of winter, fishing at the end of autumn), low level of intensification, species present (90% cyprinids, 10% carnivores) and a high level of predation. However, on other points, variability is high, complicating comparisons between farms: different complementary activities (trout trading, ornamental fish, on-site fishing routes), biosecurity management at the grazing areas (mixing of different species, cleaning and disinfection). The factors favouring Aeromonas disease mentioned by the farmers are, in decreasing order of occurrence: prolonged storage, high temperature, contact with ornamental fish or trout trade and handling stress.

During these visits, samples were taken from batches of animals, specifically in the case of apparent lesions. Autopsies were performed on these animals as well as microbiological samples of mucus, gills, spleen and digestive tract. A protocol for the identification of bacteria of the genus Aeromonas was developed on the basis of a literature search on the phenotypic and molecular characteristics of the genus. This protocol was tested on 29 reference strains (genus Aeromonas and other genera used as negative controls) for validation and then used on field isolates obtained after autopsies. Of the 141 field isolates tested, eight were identified as non-Aeromonas and one isolate remains doubtful. Some isolates were further investigated (VITEK®2 and/or MALDI-TOF) in order to estimate the contribution of these two techniques.

It was not possible to go further in bacterial identification at either the species or subspecies level due to the complexity of the genus and the lack of an efficient methodology. Therefore, the analysis has been considered for the bacterial genus Aeromonas. The identification of the genus Aeromonas thus resulted in an average carriage rate of about 65% of Aeromonas spp. on the sampled fish. This carriage rate varies from 45.2% to 86.7% depending on the rearing sites and from 47.8% to 90% depending on the fish species.

Antimicrobial resistance was studied on certain isolates by determining the minimum inhibitory concentrations of six antibiotics. The results show a great diversity in sensitivity/resistance depending on the isolates studied.

The second objective of our study was to evaluate a so-called alternative method of controlling this infection via food. For this purpose, the efficacy of a feed additive based on essential oils on the development of fish was evaluated by studying the zootechnical performance, immunocompetence and resistance of fish fed with this additive. The first work carried out under experimental conditions did not reveal any significant difference in the clinical expression of furunculosis (mortality and morbidity) between batches of rainbow trout fed or not with the additive. Analyses are in progress to evaluate the impact of feeding with or without additives on the immune response in trout, at the level of zootechnical performance in trout and carp.

This project has made it possible to assess the risk in relation to Aeromonas in the Pays de la Loire region in the pond sector (perception of the presence of the agent, associated mortality, confirmation of carriage).

Contact:Ségolène Calvez