Disease remains a major bottleneck for the expansion of aquaculture. This is particularly true as aquaculture intensifies and production stressors negatively impact the animal, increasing disease susceptibility and denting the profits of producers . This has created the need for prophylactic disease management, through the use of functional feed. There is often a misconception that additives can solve all problems, and these unrealistic expectations have resulted in skepticism about feed additives.
This is particularly true when combatting bacterial disease. There are two types of bacteria, which differ in membrane structure. Gram-negative bacteria have an outermost layer of lipopolysaccharides (LPS) whilst the external layer in Gram-positives is comprised of peptidoglycans . Both types contain pathogens and not surprisingly, different management approaches are needed to combat them . In tilapia this is particularly relevant, as the fish are exposed to many bacterial pathogens, including Aeromonas hydrophila , Edwardsiella tarda , Francisella noatunensis etc. (G-ve) and Streptococcus spp. (G+ ve).
Organic acids and their salts have been used extensively in aquaculture, and are a ‘go-to’ additive to combat fish disease. It is often thought organic acids act to reduce the pH in the gut, but this is not always true unless a very high dosage is used. More important is to explore antimicrobial properties where lower doses might work, but are highly formulation specific. For example, a n enhanced acidifier (Biotronic® PX Top3) was found to be more effective than similar products, despite having a 4x lower inclusion rate (0.5 kg/t vs 2.0 kg/t). After an eight-week feeding period, tilapia (initial weight = 10.96 ± 0.02g) were exposed to an Aeromonas hydrophila challenge via IP injection (7.5 x 105 CFU/ fish). After monitoring for 20 days, highest survival was seen in the Biotronic® treatment (85.7%), followed by the competitor products (50-64.58%) and lowest in the control (43.75%). These data demonstrate the value that organic acids can bring in the fight against G-ve pathogens, but also highlights the importance of product formulation.
On the other hand, phytogenics have not been used so widely, but their benefits are particularly interesting against Gram-positive pathogens. To demonstrate this, tilapia fingerlings (initial weight = 10.63 ± 0.01g) were randomly split into two treatments and stocked into 12 tanks (n = 6). Fish were fed either a control diet, or one supplemented with a phytogenic feed additive (PFA, Digestarom® P.E.P. MGE) at 0.2 kg/ t. After a 57-day growth trial, where the PFA significantly improved biomass, weight gain and FCR (p < 0.05) fish were exposed to an S. agalactiae challenge via immersion (5 x 106 CFU ml-1 for 1 hr). Following a 15-day monitoring period, a survival of 52.5% was seen in the control treatment. This compared to 70.0% survival in the PFA treatment, a real term increase of 33.3% . This demonstrates that phytogenics can be a useful tool in the continual fight against Streptococcus spp.
These studies demonstrate that in order to see the value of feed additives, the additive type and active components should be carefully selected for the specific field challenge. In addition, they are not ‘silver bullets’, and their use should be complemented with other management considerations, including biosecurity, vaccinations, water quality etc.