Paul McMullin MVB DPMP MRCVS 
31^st October 2002 

The natural bacterial population of the intestines of farm animals is extremely complex and variable among species, ages and types of animal. The use of low doses of a wide range of anti-bacterial compounds has been shown to significantly enhance digestion of nutrients in a range of species. The size of this effect varies with species and many other factors. That these compounds are exerting their effect by modulating the bacterial population is confirmed by the fact that they have no such effect when given by non-oral routes or in animals which are germ-free.

Many studies have been conducted on the normal flora of chickens using culture techniques. It is recognised that the predominant bacteria present in the chicken cecae are obligate anaerobes (1011/g) (Barnes, 1972) and that the flora is composed of a broad range of species and strains (Barnes et al., 1972, Mead, 1989). However, only a proportion, estimated at 10-60%, of the bacteria present actually grow with currently available culture techniques (Barnes et al. 1972; Barnes, 1972; Salanitro 1974). New techniques for the assessment of gut flora are being developed and include PCR's and DNA profiling and G:C ratio profiling (Netherwood et al. ,1999, Apajalahti et al.,1998, Zhu et al. 2002). Some components of the intestinal flora can contribute to the pathogenesis of severe clinical disease. The involvement of Clostridium perfringens in clinical necrotic enteritis, is probably the best example of this. The same organism has been implicated in subclinical enteritis, impaired feed conversion and retarded growth (Kaldhusdal and Hofshagen, 1992) and severe liver lesions, often identified at slaughter (Onderka et al., 1990, Løvland and Kaldhusdal, 1999). Necrotic enteritis has been reproduced experimentally but usually with atypical diets (Al-Sheikhly and Truscott, 1977ab, Prescott et al., 1978, Brennan et al., 2001). Protozoal parasites of the gut are ubiquitous in poultry production systems of all sorts and commonly interact with the bacterial flora (the most important being Eimeria sp, the cause of coccidiosis). A range of other gut problems associated with disturbances of the flora have been designated "Dysbacteriosis". These are commonly associated with dramatic increases in faecal water content, with consequent "wet litter" and resultant contact dermatitis of feet, hocks and sometimes breast skin. These conditions are considered to be a threat to the welfare of the stock.

Antimicrobial products have been used for more than 50 years to improve productivity in pigs and poultry. They are commonly called "growth promoters" but as their mechanism of action is primarily by improvements in feed conversion efficiency it may be more appropriate to call them Antimicrobial Digestive Enhancers (ADE's).

In the late 1990's publicity and campaigning with respect to the use of avoparcin as a growth promoter caused considerable pressure to eliminate all growth-promoting uses of antimicrobials even though they had been:
- carefully assessed for safety, quality and efficacy
- that their use provides benefit to producers and consumers,
- they also improve animal welfare, and generally reduce variability in the production system.

The consequences of removal were fairly easy to predict:
"Elimination of the use of these products will tend to increase the use of therapeutic antimicrobials. Animal production is likely to decline in countries which implement a unilateral ban on these products, to the benefit of those which do not." (McMullin 1997)

In the UK a number of organisations representing farmers, feed compounders, veterinarians, retailers came together in 1998 to address the issues being raised with respect to antimicrobial use in agriculture. It was subsequently named the Responsible use of Medicines in Agriculture Alliance (RUMA). It commissioned and published prudent use guidelines. The first two sets of guidelines published dealt with poultry and with pig production. The RUMA guidelines went to some lengths to emphasise the need for good management, hygiene, and vaccination as measures that help reduce the need for antibiotics. These guidelines also document a range of beneficial effects of antimicrobial feed additives but have as one of their objectives:

"Enabling poultry producers to discontinue routine antimicrobial use without adversely affecting either the welfare of their animals of the viability of their business."

However they also accept that the above can only be achieved by "encouraging a commercial environment in which the very considerable burdens of some of the measures required are shared equitably and that, primarily, UK producers are not placed at an unfair disadvantage with respect to competitors in other EU and non-EU countries."

It has been the experience in the United Kingdom, just as it was in Sweden some years ago, that blanket bans on the use of legally approved products can be counter-productive. Specifically, the suspension of the use of approved feed additives is increasing the need for the use of therapeutic antimicrobials to help control both classical diseases, such as necrotic enteritis, and some ill-defined intestinal dysfunctions. The clinical signs and pathology associated with these conditions has recently been described (Pattison, 2002). Other practitioners report broadly similar findings though it must be accepted that there is considerable variability in their manifestation, company-to-company, farm-to-farm and even house-to-house. For necrotic enteritis, and, to a lesser extent, wet litter, it is common to observe periods of deterioration interspersed with periods with fewer problems. This is entirely to be expected given the known effects of feed raw materials on intestinal physiology and disease (Apajalahti and Bedford 2000, Kaldhusdal, 1999). These problems are usually associated with poor and uneven growth as compared to normal commercial performance (which is well below that achievable by birds of similar genotype under ideal conditions).

The economics of poultry production in the UK and other European countries are such that the great majority of home-produced meat is sold as fresh whole birds and portions. These have a short shelf life and producers must provide retailers with products within tight weight specifications, often at short notice. Loss of predictability of growth results in increased within- and between-flock variability which reduces the ability of producers to satisfy market demands. Flocks which "miss their slot" will be held on to the next required target weight. Given that the next flock to be placed on the farm will already be in the incubators, this results in shortened "turn-around" period between flocks which further degrades performance as well as limiting the opportunity for cleaning and disinfection and hence increasing the difficulty in controlling important human pathogens such as Salmonella sp.

None of these problems are insoluble. However, in a world where politicians are intent on encouraging free trade of all commodities, it is foolish to ignore the consequences varying policies with respect to the use of medicines and zootechnical additives on the viability of agricultural industries. European reliance on wheat-based rations puts our industries at a disadvantage as compared to countries with low-cost maize-based diets (Apajalahti and Bedford 2000). This difference is exacerbated by non-use of "growth-promoter" feed additives. Time is required to either develop and approve products with similar health-promoting effects to the existing zootechnical additives, or develop methods of processing the available feedstuffs to eliminate the need for such products, or, perhaps a combination of both approaches.

The majority of UK specialist poultry veterinarians joined with retailers and poultry companies in making submissions in the Spring of 2002 to the technical committee of "Assured Chicken Production", emphasising the increased use of therapeutic antimicrobials in an effort to control the welfare problems of suspension of ADE's. This resulted in a change in the scheme rules to allow, provisionally for a 1 year period, the use of approved ADE's in accordance with their licensed dose, in circumstances where the attending veterinarian feels that such use can be expected to mitigate welfare problems.

The major lesson to learn from the UK experience of the voluntary withdrawal of growth promoters from the poultry industry was 'first reduce the need for antibiotics in the production system before attempting to remove growth promoters'. The position of the broiler industry in each of the Member States in the EU is very different due to varying feed raw materials, management systems and disease challenges. A blanket ban of growth promoters will only lead to an increase in the use of therapeutic antibiotics related to human medicines. If the problems have surfaced in Sweden and Denmark where the need for antibiotics in their birds & animals is perhaps the lowest in the EU how much greater will be the problem in other countries.

References:

Al-Sheikhly, F. and Truscott, R.B. (1977a). The pathology of necrotic enteritis of chickens following infusion of broth cultures of Clostridium perfringens into the duodenum. Avian Diseases, 21, 230-240.

Al-Sheikhly, F. and Truscott, R.B. (1977b). The interaction of Clostridium perfringens and its toxins in the production of necrotic enteritis in chickens. Avian Diseases, 21, 256-263.

Apajalahti, J. H. A., L. K. Sarkilahti, B.R.E. Maki, J. P. Heikkinen, P.H. Nurminen, W.E. Holben. 1998. Effective recovery of bacterial DNA and percent-guanine-plus cytosine-based analysis of community structure in the gastrointestinal tract of broiler chickens. Appl. Environ. Microbiol. 64:4084-4088.

Apajalahti, J.H.A. and Bedford, M. . 2000 Impact of Dietary and Environmental Factors on Microbial Communities of the Avian GI Tract Worlds Poultry Science Congress, Montreal, in Proceedings.

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