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FEATURE

FUNCTIONAL FEED ADDITIVES

Functional feed additives can reduce the impact of an Enteromyxum leei infection on performance and disease severity: evidence from an experimental challenge with gilthead sea bream

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by Palenzuela O1, Del Pozo R.1, Piazzon M.C1, Isern-Subich M.M2, Ceulemans S2, Coutteau P2, SitjàBobadilla A.1 1 Institute of Aquaculture Torre de la Sal (IATS-CSIC). Castellón, Spain (oswaldo.palenzuela@csic.es) 2 Nutriad International, Dendermonde, Belgium (mm.isern@nutriad.com)

oncern on the impact of parasite infections in aquaculture has increased in recent years. In addition to visible mortality episodes and increased running costs, some estimates of the world annual grow-out loss in finfish farming due to parasites ranges from one to 10 percent of harvest size, with an annual cost that can reach up to US$9.58 billion (Shinn et al., 2015). In Mediterranean fish farming, one of the major parasitic diseases is enteromyxosis, caused by Enteromyxum leei (Figure 1). This microscopic parasite infects the intestinal tract of fish and sometimes-associated organs like gall bladder and liver. Enteromyxum species belong to the Myxozoa, a group of parasites related to Cnidarians that produce economically important fish diseases like whirling disease, proliferative kidney disease (PKD), milky fish liquefaction, proliferative gill disease (PGD), gill sphaerosporosis, swim bladder inflammation (SBI) or ceratomyxosis. In contrast to the complex, two host life cycle described for about 50 myxozoan species, spontaneous direct fish-to-fish transmission has been demonstrated for the genus Enteromyxum. This unique mode of horizontal transmission favours the spread of enteromyxoses in cultured fish stocks. E. leei has a wide host and geographical range, including economically important aquacultured species in the Mediterranean and worldwide, like tiger puffer, Japanese flounder, parrot fish, malabar grouper, various sea breams, or Peruvian fine flounder. The virulence and mortality caused on each host is quite variable, and largely affected by the species susceptibility and the rearing system and environmental conditions. In gilthead sea bream (GSB) (Sparus aurata), enteromyxosis has a chronic course leading to a cachectic syndrome with anorexia, anaemia, weight loss, severe epaxial muscle atrophy and, eventually, death (Sitjà-Bobadilla and Palenzuela, 2012) (Figure 1). Direct mortality due to enteromyxosis in GSB raised in sea cages is most often moderate, whereas the serious economic impact of enteromyxosis in these facilities is largely due to arrested growth and inability to reach commercial size. This effect is most patent in advanced stages of the grow-out period. There are neither vaccines nor effective prescription medicines for enteromyxosis and its control measures are limited to avoidance of risk factors, early diagnosis, and good farm management practices. Therefore, the farming industry needs other solutions to minimise the impact of the infection. Healthpromoting feed additives are a crucial component of effective disease prevention strategies. A wide range of additives with

different modes of action is currently offered including yeast extracts, phytobiotics, probiotics, prebiotics, organic acids and their derivatives. Functional feeds containing gut health promoters deliver with every meal an adequate concentration of natural compounds, which can work through multiple mechanisms to reduce the success of parasitic infestations. Natural compounds with antiparasitic activity can work directly on gut parasites and/or reach the blood and/or mucus to affect ectoparasites, whereas immune modulators can change the composition and thickness of the mucus (Coutteau et al., 2011, 2014, 2016). Many of these strategies target the gut as a primary focus for health and offer maximum benefits in chronic or subchronic infection processes, and thus GSB enteromyxosis constitutes an excellent model to study and to evaluate their potential. The present study evaluated the capacity of a functional feed additive to prevent or mitigate the effect of enteromyxosis in gilthead sea bream experimentally infected with E. leei. All the experiments were run at the indoor experimental facilities of the Institute of Aquaculture Torre de la Sal (IATS) using fibreglass tanks in an open flow-through seawater system.

Feeds and feeding protocol

Naïve GSB fingerlings, free of intestinal parasites were obtained from a local hatchery at four grammes and grown up to 12.9g before starting the feeding trial. They were allocated into 90-L tanks (25 fish/tank) and acclimated to the basal diet for 11 days until the experimental feeding started. Three diets were tested in two replicate tanks/treatment: a basal control feed (Diet A) and the same diet with two different inclusion levels of SANACORE® GM, a natural health promoting feed additive (Nutriad International, Belgium); Diet B “low dosage” and diet C “high dosage” of SANACORE® GM. The basal diet was representative of a commercial feed formulation (45/20% crude protein/fat; 15% LT fishmeal, 12% poultry by-product meal, soybean meal 25%, soybean protein concentrate 10%, corn gluten 8%, wheat gluten 3.4%, wheat flour 10.5%, soybean oil 9.7%, fish oil 6%, amino acids, vitamin and minerals premix). Fish were fed manually ad libitum twice a day for weekdays and with automatic feeders on weekends, during the whole experiment. Daily food intake was recorded and SGR (specific growth rate) and FCR (feed conversion ratio) calculated. Water temperature ranged from 18 to 26.5 °C over the feeding trial (post-challenge period between 22°C and 26.5°C; see Figure 2A). The salinity of the seawater was 37.5 g/l.

Experimental infection and samplings

After five weeks on the experimental diets, fish from groups A, B & C were inoculated with 0.2ml of a homogenate from

14 | July 2017 - International Aquafeed

Jul 2017 - International Aquafeed magazine