The word antibiotic growth promoter is used to describe any medicine that destroys or inhibits bacteria and is administered at a low, sub therapeutic dose. The use of antibiotics for growth promotion has arisen with the intensification of livestock farming. Infectious agents reduce the yield of farmed food animals and, to control these, the administration of sub-therapeutic antibiotics and antimicrobial agents has been shown to be effective. The use of growth-promoters is largely a problem of intensive farming methods and the problems caused by their use are largely those of developed rather than developing countries.
According to the National Office of Animal Health (NOAH, 2001), antibiotic growth promoters are used to "help growing animals digest their food more efficiently, get maximum benefit from it and allow them to develop into strong and healthy individuals". Although the mechanism underpinning their action is unclear, it is believed that the antibiotics suppress sensitive populations of bacteria in the intestines. It has been estimated that as much as 6 per cent of the net energy in the pig diet could be lost due to microbial fermentation in the intestine (Jensen, 1998). If the microbial population could be better controlled, it is possible that the lost energy could be diverted to growth.
Thomke & Elwinger (1998) hypothesize that cytokines released during the immune response may also stimulate the release of catabolic hormones, which would reduce muscle mass. Therefore a reduction in gastrointestinal infections would result in the subsequent increase in muscle weight. Whatever the mechanism of action, the result of the use of growth promoters is an improvement in daily growth rates between 1 and 10 per cent resulting in meat of a better quality, with less fat and increased protein content. There can be no doubt that growth promoters are effective; Prescott & Baggot (1993), however, showed that the effects of growth promoters were much more noticeable in sick animals and those housed in cramped, unhygienic conditions. . On a world scale, the use of antibiotics as animal growth promoters differs dramatically. Sweden now makes no use of antibiotics for growth promotion purposes; the USA uses a wide range of antibiotics, including some considered to be "medically important". The following information is taken from the Report of the Joint Expert Advisory Committee on Antibiotic Resistance (JETACAR, 1999) on the use of antibiotics in food producing animals.
Poultry is one of the world's fastest growing sources of meat, representing nearly one-fourth of all the meat produced. The modern production unit can produce market ready broiler chickens in less than six weeks. This development comes from genetic improvement, feeding and health management practices by using of of antibiotics as therapeutic agents to treat bacterial diseases in rural poultry farming. The recommended levels of antibiotics in feed were 5-10g kg-1 in the 1950's and have increased by ten to twenty folds since then. In many modern countries, antibiotics used in poultry is for treatment of infections. The characteristics of resistant becteria transfer from poultry products to human population may occur through consumption or handling meat contaminated with the pathogens (Van den Bogaard and Stobberingh, 2000). According to WHO the resistance to antibiotics is an ability of bacterial population to survive the effect of inhibitory concentration of antimicrobial agents (Catry et al., 2003). The resistant bacteria can damage the human intestine and the genes coding resistance to antibiotics can be transferred to other bacteria belonging to the endogenous flora of humans, thereby jeopardizing effective treatment of bacterial infections (De Leener, 2005). These resistant bacteria then increasing their numbers a million fold a day, becoming the micro-organism in the population. Such bacteria transmit their genetically defined resistance characteristics to their offspring's of the strains and to other bacterial species via mutation(Gould, 2008).
Antibiotics may reduce the maintenance cost associated with turn our of the intestinal epithelium.(visek 1978) sugests that up to 20% of nutrient requirement for maintenance are directed to epithelial resupply.because antibiotic cause the thinning of epithelium,this is up to 40% in this maintenance components.(Visek 1978) concluded that a for a 1000 gram Broiler gaining at 50% per day reduce maintenance needs for epithethial regeneration caused by feeding , Anitibiotics could account for the 4-5% improvements in growth,offen seen with in these products.
All antibiotics control growth and proliferation ofmicroorganisms; however, all antibiotics do not accomplish this control by the same mechanism (Ferket, 2004). Consequently, antibiotics differ with regard to their ability to influence certain disease states or improve growth and feed efficiency. Much of the work with antibiotic growth promoters continues to be from the standpoint of studying the effects on easily cultured bacterial populations such as lactobacili and Clostridium perfringes and poultry health rather than resulting physical changes to the GIT (George et al., 1982; Engberg et al., 2000; Sims et al., 2004). Several researchers have studied intestinal morphology in poultry during the last decade but predominantly from the standpoint of normal development and not with regard to effects of antibiotics (Uni et al., 1995, 1998, 1999; Geyra et al., 2001).
Discussion:
Use of antibiotics in animals:
Antibiotics are used largely for three purposes in animals: therapeutic use to treat sick animals;
prophylactic use to prevent infection in animals; as growth promoters to improve feed utilisation
and production. In general, therapeutic treatment involves treatment of individual animals
over a short period with doses of antibiotic exceeding the minimal inhibitory
concentration of the known or suspected pathogen. Sometimes, with intensively-farmed animals,
therapeutic treatment is delivered by feed or drinking water; however, this treatment can
be of doubtful ef®cacy in some situations, as sick animals often do not drink or eat. Prophylactic
treatment again involves moderate to high doses of antibiotic, often given in feed or
water for a de®ned period to a group of animals. Antibiotics used as growth promoters tend to
be given in feed at subtherapeutic levels over extended periods to entire herds and ¯ocks, and
are available for purchase over the counter by feed manufacturers and farmers. It is important to
note that subtherapeutic levels generally still exceed the minimal inhibitory concentration of
enteric organisms such as Clostridium perfringens and Enterococcus spp. (van den Bogaard &
Stobberingh, 1999).
Concern about use of antibiotics in animals and the possible impact on human health
covers two major issues: the antibiotic agents that are used; the way in which they are used.
There is a view that antibiotics that are important in human medicine should not be used
therapeutically in food-producing animals, particularly for mass medication. Prophylactic use
presents a problem on two grounds: the antibiotic agents used; the lack of de®nition of what is
the appropriate duration of prophylactic use. Growth-promotant use is probably the area of
highest concern, as some of the antibiotics used are now regarded as compromising the ef®cacy
of some key human antibiotics and the duration of treatment may be for the whole life of the
treated animals.
Regulatory controls
Controls vary from country to country. For example, in Australia there are three points of
control of antibiotic use in animals. First, all importations are controlled by a permit system (no
280 M. D. Barton
antibiotics are produced in Australia). Second, at the registration level, there are strict regulatory
guidelines over which antibiotics can be used in food-producing animals. Since 1970,antibiotics intended for animal use have been assessed for their potential to compromise human health. As a result, ¯uoroquinolones, amphenicols, colistin and gentamicin have not been registered for use in food-producing animals because of concerns about antibiotic resistance, and the registration of carbadox was withdrawn in the late 1980s and of nitrofurans in 1992 because of concerns about carcinogenicity (Joint Expert Technical Advisory Committee on Antibiotic Resistance, 1999). Finally, there is control-of-use legislation that restricts antibiotics registered for therapeutic or prophylactic use to registered veterinary surgeons, but allows overthe- counter sales to farmers or stock-feed companies of products registered for use as growth promoters. Agricultural use of antibiotics in the USA and Canada is also regulated. There are three categories of use: as feed antibiotics; as over-the-counter drugs; as veterinary prescription drugs. Feed antibiotics include antibiotics used as growth promotants and those used for subtherapeutic (including prophylactic and some growth-promotant use) and therapeutic purposes (Prescott, 1997). Feed antibiotics are licensed for speci®c uses such as for meat chickens or young pigs or calves or feedlot cattle. In the USA preregistration assessment speci®cally addresses human health issues relating to antibiotic resistance in enteric coliforms, salmonella excretion and increased resistance in salmonella, increased virulence and pathogenicity of bacteria, animal disease that is dif®cult to treat, and residues and risk of hypersensitivity in consumers (Sundlof et al. 1997). In Canada, the risk of development of antibiotic resistance is not assessed at this stage (Joint Expert Technical Advisory Committee on Antibiotic Resistance,1999).
In the UK and other EU countries, antibiotics are authorised as either veterinary medicinal
products or zootechnical feed additives. Veterinary medicinal products and growth promoters
are subject to assessment for safety, including residues (veterinary medicines) and the risk of
emergence of antibiotic resistance, cross-resistance to therapeutic antibiotics and selection for
transferable resistance (both veterinary medicines and growth promoters; Rutter, 1997). Other
European countries outside the EU have their own regulations.
China has regulated the use of antibiotics in animal feeds since 1989 and only non-medical
antibiotics are permitted as feed additives. Antibiotics used include monensin, salinomycin,
destomycin, bacitracin, colistin, kitasamycin, enramycin and virginiamycin. However, in
practice, other antibiotics such as tetracyclines are used and the mycelial by-products from the
production of antibiotics are incorporated into animal feeds (Jin, 1997). Russia also restricts
feed antibiotics to non-medical drugs; bacitracin, grizin, ¯avomycin and virginiamycin are
registered for use in this way (Panin et al. 1997).
Growth promoters:
Antibiotics used for growth-promotant purposes constitute a large proportion of the total
antibiotic usage, but the scale of the problem is dif®cult to estimate since there is little published
information on the overall quantities of antibiotics used in animals or human subjects. In
Australia import statistics for the years 1992±3 to 1996±7 show that 55_8% of antibiotics
imported were for use in stock feed, 36_4% for human use and 7_8% for veterinary use (Joint
Expert Technical Advisory Committee on Antibiotic Resistance, 1999). Prescott (1997)
reported that 40% of antibiotic production in the USA was for use in stock feeds, including 55±
60% of penicillin G and tetracycline production. European Federation of Animal Health (1998).
The availability of antibiotics in poultry production depends on greater understanding of their risks and benefits. The regulatory status and use of antibiotics in poultry production were addressed in research presented The use of antibiotics as preventative growth promotants is probably the most misunderstood by the general public. The use of antibiotics as preventative growth promotants is probably the most misunderstood by the general public. Many people interpret the use of antibiotics to be the same, whether it is a therapeutic treatment of disease or preventative control of subclinical disease such as necrotic enteritis.When reading the summaries that have been published by both domestic and foreign organizations on antibiotic use in food animals, the reader should be aware of whether the reports include one or a combination of the three antibiotic categories. For example, some reports may only include therapeutic and preventative growth promotants, whereas others will include therapeutics, preventative growth promotants and ionophore coccidiostats.( Fairchild and C.L. Hofacre, 2011)
Antibiotics benefits has in feed include increasing growth and efficiency, preventing or reducing the incidence of infectious disease treating clinically sick animals.The major use of antibiotics among these. Medicine increased feed efficiency and feed conservation ratio, also improved growth rate of animals. In poultry ration e.g. tetracycline and penicillin show best results in improvement in egg production, feed conservation ratio and hatchability of eggs, but have no effect on mortality. Oxytetracyclin and penicillin also show result on growth and have little effect on mortality. Antibiotics in animal feed used regularly for increased efficiency and growth rate than to combat specific diseases.( The Rise of Antibiotic-Resistant Infections" (1995) FDA Consumer, 29).
The recognition of the dangers of antibiotic resistance prompted the ban on sub-therapeutic antibiotic usagein Europe and the potential for a ban in the United States and many developed countries, there is increasing interest in using probiotics that have potential to reduce enteric disease in poultry and subsequent contamination of poultry products (Patterson and Burkholder, 2003).
It is very difficult to grow broilers without the use of growth promoters, since clostridial organisms often proliferate and clinical necrotic enteritis develops. While some countries have a ban on sub-therapeutic growth promoters in the feed, their use is escalating as water additives. Without the use of such 'antibiotics', there will undoubtedly be greater risk of bacterial overgrowth in the bird's digestive tract and especially when 'poorly' digested ingredients are used since these provide substrates for microbial fermentation in the lower gut.(Commercial poultry Nutrition) .There is a lack of definitive information on the overall of antibiotics used in feed as growth promotor, and there are obstacles to this information since feed formulations are considered confidential business information under U.S. law. our data are consistent with studies highlighting the prevalence of resistant enterococci and staphylococci in the poultry environment (Hayes et al., 2004; Lu et al., 2003).
