Edwardsiellosis in Pangasius – Its Cause, Its Symptoms, and Its Treatment
Enteric septicaemia of catfish (ESC), is a devastating disease responsible for significant economic losses throughout the catfish industry worldwide. Limited success in improving fish resistance through vaccination and breeding programs has led the industry to look for alternative solutions. Recent trials with prebiotic feed supplements revealed an incremental improvement in this situation.
The production of commercial quantities of fish by the aquaculture industry has a persistent difficulty: bacterial infections that easily spread among intensively cultured fish in aquaculture tanks, ponds or cage systems. One of these threatening diseases is enteric septicaemia of catfish (popularly known as ESC) caused by Edwardsiella ictaluri, a gram-negative, pleomorphic and rod-like-shaped bacterium with flagella. Surveys conducted by the USDA in 2010 revealed rather than being an occasional appearing intruder, E. ictaluri is actually prevalent in catfish stocks year-round, with almost 20% of fingerlings carrying the pathogen and more than 35% of the adult fish having it.
Outbreaks of ESC can occur at any water temperature between 20 and 30°C. Major outbreaks are generally common twice a year, during spring and again during autumn. When a fish’s immune system is compromised by the bacteria, red spots may appear under their heads and under bellies, red and white ulcers develop, and red pimples appear at the cranial foramen. These symptoms can combine to create a “hole-in-the-head” condition, in which parts of the brain are exposed, followed by granulomatous inflammation of this sensitive tissue.
Researchers have sought ways to mitigate or eliminate this disease and the huge associated economic losses (more than 60 mio. dollar in the US) for at least thirty years. This effort has had two main branches. Scientists have attempted to develop vaccines to protect eggs and fingerlings from infection by E. ictaluri, with some success. Fish breeders have worked on creating hybrid catfish strains with a stronger natural resistance to E. ictaluri.
The vaccination effort commenced in earnest in the late 1980s and early 1990s. Research studies show that exposing catfish eggs and catfish fingerlings to vaccine formulations significantly increases the catfish survival rate and increased output. Work on more advanced vaccines continues at the time of this writing.
It must be noted that while the vaccines have high levels of effectiveness in the laboratory setting, under practical conditions of a pond farm, the outcome metrics are significantly murkier. A research survey carried out in 2009 asked catfish producers with vaccinated fish whether they observed a change in survival rates. 41.9% of respondents did say that they believed survival rates had improved with the vaccines, but 37.5% said that they were unable to see any difference. Vaccines are a well-known mitigating treatment, but most vaccination methods mean manual handling of the fish, which causes stress (resulting reduced feed intake and growth) and which requires considerable labor by the farmer.
Considerable work has also been done in the field of genetics and selective breeding. There are more than 4000 catfish species worldwide, of which about 30 are commercially exploited. Among these, researchers and breeders have found highly-ESC-resistant strains, which they successfully hybridized to tougher fish . Today between 50 and 75% of catfish being grown in the United States are one variety of ESC-resistant fish or another. However, the researchers have largely reached the limits of what can be achieved by improved genetic resistance. While breeding efforts will continue, major breakthroughs in genetic disease resistance of fish are likely to be achieved far into the future. As the overall species resistance stands, ESC remains a major problem – particularly in light of recent research suggesting that ESC may spread to economically critical fish species such as tilapia or rainbow trout.
A third alternative has presented itself beneficial to increase the bacterial resistance in aquaculture species: the supplementation of yeast cell wall-based prebiotics in aquafeeds. TechnoMos® is a cell wall-extract of the primary grown yeast, Saccharomyces cerevisiae, and thus consists of highly concentrated mannan-oligosaccharides (MOS) and β-1,3-1,6-glucans. The ß-glucans act as non-pathogenic polysaccharides which are able to stimulate protective cascades within the animal resulting in an overall higher immune status. Moreover, ß-glucans have been reported to increase the expression of mucin and defensin coding genes for improved mucus production within the intestine, but also at the outer skin level – a strong physical barrier against pathogens. The MOS content in TechnoMos® is able to block pathogenic adhesins and receptors like type-IV-fimbriae (e.g. Vibrio spp.) and thereby prevents them to bind to and to proliferate at the intestinal wall. Additionally, MOS acts as substrate for probiotic bacteria and hence increases feed efficiency and ensures a well-balanced microbiota. In combination, this leads to an improved intestinal morphology, which also has a supporting effect on feed utilization.
It sounds good in theory – but does it work? Over a period of six weeks, two different diets were fed to two groups of young striped catfish (Pangasianodon hypophthalmus). The first group was fed on a standard diet, while the second had 1 g/kg TechnoMos® added to its feed. All of the fish were then given injections of a 0.15 ml E. ictaluri solution at a previously tested 50%-lethal dose (LD50). A third group was given the injection of NaCl solution as a negative control. 14 days post infection, 71% of the TechnoMos®-treated fish survived in comparison to only 47% of the untreated control group. This trial was not designed to emulate a natural outbreak, in which the path of infection would follow water transmission, using the channels through the intestine and gills. The protective features of TechnoMos® such as increased mucus production, improved intestinal morphology and the ability of MOS to bind and remove pathogens in the intestine would have significantly reduced the risk of infection.
The method of direct injection guarantees by 100% that the pathogen invades the organism. In spite of these difficult conditions and due to the sustained stimulation of the immune system, the survival rate could be increased by almost 25%! An experiment from a practical point of view, where the pathogens have to pass all external and internal transmission barriers, which TechnoMos® augments, should show even better results. These results confirmed the protective effect of TechnoMos® against and in dealing with bacterial diseases like ESC in striped catfish.