Bernard Dixon

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Specific policies aimed at limiting use of antibiotics in animals in some European countries are positive examples for the rest of the world

Bernard Dixon

From the beginning of the new millenium, Denmark's pig producers have relinquished entirely the use of antibiotics as growth promoters. They took this unprecedented step as a voluntary response to increasing professional and public concern about the proliferation of drug-insensitive bacteria such as salmonellae and their spread from farm animals to humans.

The Danish move is the most recent in a series of initiatives that have placed the northern European countries in the forefront of the worldwide battle against antibiotic resistance. In 1997, for example, Finnish microbiologists announced their success in halving the rate of recovery of erythromycin-insensitive group A streptococci from throat swabs and pus samples. Helena Seppala and her colleagues (N. Engl. J. Med. 337:441, 1997) attributed this change to stringent new national guidelines on the prescribing of macrolides, instituted four years previously.

The contributions made by medical applications and misapplications of antibiotics to the burgeoning global pool of drug resistance are probably far greater than those attributable to animal husbandry. Nevertheless, the practical consequences in terms of untreatable infections are so serious that any significant diminution is welcome.

This year's initiative in Denmark is the latest of several steps taken voluntarily by that country's pig and poultry farmers to curb the mass dissemination of antibiotics to promote growth. The government too has imposed increasingly strong controls over veterinary products.

In 1995 it ruled that these would in future be available only from pharmacists. Veterinarians could no longer sell them for profit (except in emergencies). The removal of their financial interest in what they prescribe, and in what quantities, has been a key policy limiting the use of antibiotics in Denmark. Since the restriction of these and other drugs to pharmacies, the quantity prescribed to combat animal disease has fallen significantly.

Five years ago likewise saw the creation of a nationwide program to tackle a serious problem of Salmonella spp. in pig meat. Particular attention focused on the S. enterica serotype Typhimurium known as definitive phage type 104 (DT104). This is usually insensitive to ampicillin, chloramphenicol, streptomycin, sulfonamides, and tetracyclines, and an increasing proportion of isolates are also relatively resistant to fluoroquinolones.

Why Give Drugs to ``Healthy Animals?''

Now, if meat inspectors find one pig that is infected with this organism, the entire herd has to be slaughtered. Inspectors test the animals in a random but intensive screening program at abattoirs. Over the past four years, they have tested 16,000 herds every month. The current incidence of S. enterica in pork is now only 1%, with a target of 0.5% by the end of next year.

It was also in 1995 when researchers at the Danish Veterinary Laboratory in Copenhagen published evidence that avoparcin, then included in pig feed as a growth promoter, could cause resistance and cross-resistance to vancomycin. Against a feverish background of parliamentary and public opposition to the very idea of giving antibiotics to ``healthy animals,'' the government immediately banned avoparcin from pig fodder.

Contrary to some farmers' fears, the removal of all growth-promoting antimicrobials from pig food (which was actually in progress before the 1 January 2000 deadline) has been accomplished without any serious ill effects. The only significant repercussion has been that the animals tend to produce slightly looser dung, and in some cases they develop diarrhea. But these changes (attributable to shifts in the intestinal flora following the removal of antibiotics) usually abate within a month and otherwise can be prevented by dietary modification.

More importantly, these alterations in feedstuffs plus those necessitated by the termination of artificial growth promotion have not loaded farmers with greatly increased costs. The additional annual outlay is less than $0.65 U.S. per pig.

It is now 35 years since the British bacteriologist E. S. Anderson, a pioneer of microbial drug resistance, called in Nature (206:579, 1965) for ``a re-examination of the whole question of the use of antibiotics and other drugs in the rearing of livestock.'' His concern was based on meticulous research in which he traced the emergence of ampicillin-resistant S. typhimurium strains to the use of this agent to treat and/or prevent infections in calves.

Working for the Public Health Laboratory Service in London, Anderson presented evidence that a particular phage type of the organism had been distributed around Britain by livestock movements and had caused human infections. He argued that if farmers improved their standards of animal husbandry, they could stop using antimicrobial drugs for the twin purposes of disease prophylaxis and growth promotion.

Good Husbandry Is the Real Answer

Anderson's work led to a government enquiry four years later, and to restrictions on the incorporation of tetracyclines and penicillins into feedstuffs. Yet opposition to controls of this sort has continued, both in the United Kingdom and elsewhere, to the present day. Critics, questioning the contribution of intensive agriculture to bacterial resistance, have argued that it would be financially prohibitive for farmers to stop dosing entire herds and flocks with antibiotics.

The Danish experience is showing that Anderson was right. ``The standard of animal husbandry management in Denmark has been the critical factor in removing antibiotic growth promoters,'' write Debra Robertson-Welsh and Verner Wheelock in their report Food Safety and Pig Production in Denmark (Verner Wheelock Associates, Shipton, United Kingdom, 2000). ``Moreover, there has been no corresponding increase in the therapeutic or prophylactic use of antibiotics during or since the removal of growth promoters from feed.''

Of course, the new situation in Denmark requires continued monitoring. One purpose of the surveillance now in place is to ensure that farmers do not simply obtain antimicrobials from another source (as was widely rumored after the curbs on penicillins and tetracyclines in the United Kindom three decades ago). The second aim is to gather evidence on the consequences of the voluntary ban for the pattern of resistance among enterobacteria in farm animals and in the community at large.

Given Denmark's preeminence as a bacon exporter, particularly within Europe, the new arrangements bring benefits for both public health and public relations. It's strange that many food campaigners are less interested in concrete measures of this sort--adopted to combat the considerable, proven menace of drug resistance--than in fostering hysteria over remote conjectural hazards of genetically modified food.