Tuesday, June 10, 2014

Aquaculture view: Does Better Nutrition Equals Better Disease Resistance? A Paradigm Shift May Be Occurring

Aquaculture view

Aquaculture view is a column in each edition of International Aquafeed magazine (IAF), written by Dominique P Bureau.

Part of the IAF editorial panel, Dom has a PhD in Nutritional Sciences from the University of Guelph, Canada.

Today he teaches various undergraduate and graduate courses on animal nutrition and agriculture at the University of Guelph. Between 2007 and 2009, he coordinated the “Paris Semester”, a study abroad program for undergraduate students at the University of Guelph.

He serves on a number of international committees, including the US National Research Council Committee on Nutrient Requirements of Fish and Shrimp.

Does Better Nutrition Equals Better Disease Resistance? A Paradigm Shift May Be Occurring

The emerging treat of antibiotic resistance among pathogenic bacteria and the creation of "super bugs" (bacteria resistant to almost all the currently available antibiotics) is leading some medical experts to predict a very dim future for our health care system.  Through careless use of these wonderful tools that are antibiotics, we may have been shooting ourselves in the foot.  To some experts, including Dr. Gerry Wright, Scientific Director of the Michael G. DeGroote Institute for Infectious Disease Research at McMaster University (Hamilton, Canada), we will very likely face catastrophic consequences in the not too distant future.

This grim perspective need to be a wake-up call for all of us, including us working in the aquaculture industry.  When I hear of very powerful "human" antibiotics illegally used in fish and shrimp culture in Asia, I cringe!  We truly need to invest in novel solutions to deal with bacterial disease in aquaculture species.  These solutions may need to be based on completely different approaches.  Nutritionists probably have an important role to play in addressing this challenge. However, we may need to experience a paradigm shift.

As a student, I have always been taught the adage that "better nutrition results in better disease resistance". In aquaculture, in response to a disease outbreak or simply to prevent one, we often assume that one should resort to supplementation of the diet with a variety of nutrients and other dietary compounds (vitamin C, vitamin E, omega-3 fatty acids, selenium, zinc, glucans, nucleotides, alginates, etc.) that may positively influence the immune system and thus the ability of the animal to resist pathogens.  I have been following pretty distantly the whole area of research on the effects of the large array of nutritional supplements on disease resistance.  I may be a little too skeptical at times but so far, I have been largely unimpressed.

What if the solution was not in better nutrition (more, more, more) but rather in less good nutrition (less is more)?

Would that be a change in paradigm for our field?!
Think about it. What is good "motherly" advice when we catch a cold or suffer from the flu? Isn't it "get plenty of rest, drink plenty of fluids and eat very little solid food"?   The equivalent approach was common practice in animal agriculture before the advent of antibiotics.
I was told by a friend that in the old days, when pigs were sick, they would simply be taken out of the herd and allowed to heal on their own with very simple diets (grass, vegetable residues, etc.). I am not sure it worked perfectly but it probably did not too much harm and perhaps even prevented the spread of disease to the entire herd.

As a graduate student, I was fascinated by the work of the research group of Dr. W.D. (Bill) Woodward at the University of Guelph who showed that the very significant depression of the immune of highly malnourished mammals could be reversed through simple interventions (notably thyroid hormone injections) and this prior to addressing the malnutrition problems, something that common wisdom took as an essential corollary of improving the immune response of the animals.
Almost 20 years ago, Dr. Hugh Ferguson, the former head of the Fish Pathology Laboratory at the Ontario Veterinary College (OVC) told me his team had found the nutritional "silver bullet" to cure bacterial gill disease  in salmonid fish species. What was this silver bullet? Fasting the fish for several days!

Dr. Ferguson's team found that fasting fish that were challenged with the bacterial disease agent (Flavobacterium branchiophyllum) provided absolute protection to these animals. The fish that were fed during and after the challenge with the pathogens died rapidly.  Refeeding the fasting animals prior to clearing of the bacterial load from the gills resulted in resurgence of mortality (for detailed results, readers can consult: McPhee et al. 1995. Diseases of Aquatic Organisms, 21: 163-170).

My research group recently collaborated with the team of Dr. John Lumsden who now heads the Fish Pathology Laboratory at OVC on the effect of the mycotoxin deoxynivalenol (DON) on resistance of rainbow trout to cold water disease (Flavobacterium psychrophyllum).  Previous research from my group had shown that rainbow trout was extremely sensitive to DON.  Surely, this extreme sensitivity would be translated into a decrease resistance to disease when DON was included in the diet of this species. What we saw was the exact opposite!  The incorporation of DON in the diet of rainbow trout appeared to significantly increase resistance of the animal to the disease agent, in part through reduction in the appetite of the animal and in part through another mechanism.

What are the mechanisms at play? I have not carefully looked into this myself. However, so far through some consultation with a number of experts, my feeling is that nobody really knows.  Pathogens are living organisms. When we are feeding the animal we may also be feeding the pathogens?  However, attempts to restrict the dietary supply of nutrients (iron, glucose, etc.) to infected fish had very limited effect so far.

It is strange thing to write but poorer nutrition may, in some cases, improve disease resistance of animal. We definitely need to look more carefully at this new paradigm!
Agree or disagree? Please don't hesitate to contact me. dbureau@uoguelph.ca

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