Animal diseases have a large impact on the economy of the livestock industry having total estimated global losses up to $2773 million annually. The disease may have a direct effect on performance through decreased production of milk, wool, or eggs, reduced fertility, delays in reaching maturation of reproductive system, decreased draught power or decreased weight of cull animals. These epidemics have the direct effect on farmers, consequently affecting agricultural sector and the national economy.
There is no doubt in the fact that certain dietary antibiotics have been playing a pivotal role in promoting good growth and health in animal production and feed conversion efficiency. However, the emergence of drug resistance among pathogens and public concerns of antibiotic residues in meat has led the European Commission (EC) to draw a critical decision to ultimately ban (January 1st, 2006), the marketing and application of antibiotics growth promoters (AGP) as feed additive (EC Regulation No. 1831/20031). This verdict was taken on the basis of the precautionary principle: ‘Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation’ (Principle 15 of the Rio Declaration, 19922).
Will the ban on antibiotic growth promoters in animal feed have the desired effect of reducing the overall level of antimicrobial use in animals? At the moment, this looks to be highly unlikely, as statistics continue to show a sudden rise in the use of AGP used for remedial purposes, even though there is a significant reduction in the usage of antibiotic growth promoters. In countries such as the USA the use of AGP in the poultry industry has been formally declined. This has led to increase in FCR and rise in the incidence of necrotic enteritis. Necrotic enteritis (NE) is an acute infection caused by Clostridium species. This is characterized by excessive dehydration, severe intestinal tissue necrosis with an increase in death rate. Such sick birds that do not have access to appropriate treatment are likely to carry much heavier loads of bacteria at the time of processing. The escalating usage of treatment antibiotics is essential for the effective control of prevalent diseases such as coccidiosis and necrotic enteritis in poultry, diarrhoea and inflammation of ileum in pigs, which have serious consequences and may badly affect production performance. From a veterinary point of view, withdrawing AGPs and in-feed anticoccidials may open the door to more enteric pathogen invasion.
In light of recent publicity surrounding the use of antibiotics by livestock and poultry industries, there are reports claiming to be alternatives to antibiotics which have the direct or indirect effect on microflora. Ideally, the alternatives to AGP should function in a same way as growth promoters without hindering its beneficial effects i.e.
(1) By reducing the amount of toxins and other metabolites produced by gram-positive bacteria;
(2) By increasing the intestinal wall permeability thereby improving absorption of nutrients;
(3) By blocking enzymes secreted by the pathogens thereby reducing the incidence and severity of sub-clinical infections.
Few of the most widely used alternatives for AGP along with their mode of action is briefly discussed:
Organic acids are widely distributed in animal tissues formed through catabolism of carbohydrates by microbial fermentation mainly in the caecum of the poultry intestine. The antibacterial nature of organic acid is mainly due to the low pH. The environmental pH change affects the dissociation of the acid which enhances the antimicrobial effect. The dissociated form of acid molecules freely diffuses through the semi-permeable membrane of microorganisms into the cell cytoplasm. Once in the cell, the acid molecules will disrupt the cell enzymes and nutrient transport mechanism.
To read more: Adams, C., 1999. Nutricines: Food Components in Health and Nutrition. Nottingham University Press, UK.
Probiotics and Prebiotics
Probiotics are mixed cultures of live bacteria which improve the growth and characteristics of the indigenous microflora thereby benefiting the host’. The most well-known group of probiotics is lactic acid bacteria. Most of the probiotic strains produce specific metabolites such as hydrogen peroxide, secondary metabolites, and organic fatty acids. It also stimulates the immune system while interacting with the various receptor sites. On the other hand, Prebiotic are non-digestible feed ingredients with selective effects on the intestinal microflora. The mechanism of action of prebiotics as an alternative is dependent on the nature of the compound. They are feed ingredients that improve the activity of the gut microflora because of selective stimulation of the growth or metabolic activity of a limited number of intestinal micro-biota species, such as Bifidobacteria and Lactobacillus sp. Thus, they may have a similar mechanism of action as probiotics.
Herbs and essential oils
It is not an unknown fact that the herbs are used for beneficial therapeutic properties derived from their specific bio-active components. Most of the bioactive compounds of the plant are mostly secondary metabolites such as terpenoids (mono- and sesquiterpenes, steroids, etc.), phenolics (tannins), glycosides and alkaloids (present as alcohols, aldehydes, ketones, esters, ethers, lactones, etc).Essential oils are concentrated products with very strong aromatic fragrance produced by these secondary metabolites. The mechanism of action of bio-active compounds on chemical constituents, biological factors and so on. Bio-active constituents derive from plants play important role in inhibiting the bacteria by inactivation of the protein, loss of function, neutralizing free radicals. The study of microbial activity of essential oils and other bio-active compounds on micro-organisms still remains area of future research.
Alternatives for AGPs are only of practical significance which improves animal performance as compared to AGPs. Microflora stimulating and immunomodulatory compounds have potential and are used as feedstuff of feed additives. Acids, probiotics, prebiotic and herbs or essential oils are some examples of different naturally available products which are used as alternatives for AGPs. Within each product class, numerous other products are available in the market, of which some products are potentially good, for others the efficacy is not clear. Hence, this brings us to a point to describe the mechanism of actions of these useful vital compounds in a whole scientific manner that meets high grades for AGP alternatives for poultry.