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Antibiotics in food producing animals

By Dr Edgar Ortmann, a veterinary consultant
Antibiotics have been in use in animals for over 50 years. In the last two decades the debates on their safety and possible threats to humans consuming the meat, have risen. Are these concerns valid?Antibiotics inhibit growth of bacteria and related micro-organisms by interfering with essential metabolic functions. In food producing animals these are used for treatment, control and prevention of disease and for enhancing nutritional efficiency (growth promotion by virtue of shifting the population balance in the gastrointestinal tract).
These products take roughly ten years of research and cost hundreds of millions of rands to produce just one new antibiotic. They are brought onto markets only after they have passed stringent science-based regulatory criteria ensuring human, animal and environmental safety, proof of efficacy and manufacturing quality. After approval the monitoring is continued to ensure safety and efficacy and any adverse reactions are reported to the relevant authorities.
Global antibiotic usage is reflected in the proportions of use in Europe and the USA during 1999:

  • 65% used for human health.
  • 35% used for animals.
  • 83% of antibiotics used in animals were for treating disease and 17% to enhance nutritional efficiency (i.e. six percent of total).

– 87% of antibiotics used in food producing animals were used to treat, control and prevent disease.
– 13% were used to enhance nutritional efficiency.
It is clear that by far the greatest slice of antibiotics in the world are used in the human population.
With the intensification of food animal production systems, antibiotics were relied on more and more in disease control and prevention programs. The first concerns raised came from perceptions within consumer organisations that pig and poultry meat contained significant amounts of antibiotic, were based largely on anecdotal evidence. Since then major advances have been made in alternative strategies to reduce and control diseases and antibiotic use has diminished as a result.
The possible human consequences as a result of antibiotic use in animals are:

  • Residues in food animal products
  • Bacterial resistance transfer
  • Food-borne zoonoses transfer.

Antibiotic residues
Controls in the past on antibiotic use was mainly focused on controlling residues in the tissues of treated animals. The food safety issue revolved around allergic reactions and adverse effects on the flora of the human gastro-intestinal tract (selecting for resistance or transfer of resistance).
It is known that penicillin residues may cause allergic reactions in already sensitised humans, but no evidence exists of any individual becoming sensitised by food residues. A rare fatal blood dyscrasia has been reported in humans sensitised to chloramphenicol. This could also be triggered by chloramphenicol in food. Little scientific data is available on effects of antibiotic residues on the bacterial flora of the human gastro-intestinal tract.
Maximum residue levels are modified and often reduced as techniques for detection of residues improve and become more sensitive. Conversely, quite a few have been increased recently as data becomes available that facilitates better risk management.
Antibiotic resistance of bacteria
Exposure of bacteria in animals to antibiotics selects for populations of resistant bacteria, both pathogens and commensals. Molecular studies support the view that resistant genes can spread from animal to human isolates. However, the frequency and extent to which it occurs is still a controversial issue. Resistance of E.coli to tetracycline’s was first reported as early as 1957 and subsequently to many other types of antibiotics. Other documented resistance reports are for Salmonella spp, Campylobacter spp and Enterococcus spp.
A worldwide concern exists regarding the possible spread of resistance from animal isolates to human isolates. This led to the ban on growth promotant antibiotics first in Scandinavian countries and later in the EU. The concerns were first discussed by the World Health Organisation in 1997 and various other bodies since then. Not everyone accepts the links between antibiotic use and resistance in animals and resistance in humans. However, some scientific evidence is mounting for at least some organisms and some antibiotics.
Results on ban of growth promotant antibiotics
The ban on growth promotant antibiotics has led to increased bacterial disease outbreaks and therefore greater therapeutic use of antibiotics.
To date there is no evidence that the ban has resulted in a reduction in the prevalence of antibiotic resistant bacteria isolated from human patients, nor has there been a reduction in the incidence of antibiotic resistant food-borne illness in humans.
The risk and concerns are valid, thus ongoing monitoring and science-based risk assessment studies are important to quantify the extent of this risk as it pertains to ensuring public health.
Prudent use principles

  • Therapeutic antibiotics should only be used in food animals when it is known or suspected that an infectious bacterial pathogen is present.
  • Use the correct dosage only for as long as needed, over as short a dosage period as possible and follow label instructions carefully.
  • Adhere to the stipulated meat withdrawal period before slaughter of the animal.
  • Bacterial culture and sensitivity testing should be utilised.
  • Always use under the supervision of a veterinarian.

It could be argued that most antibiotic resistance problems in people are associated with inadequate controls and misuse of antibiotics in hospitals and in the community in general. There is however, sufficient evidence to support calls for the reduction and improved controls over the use of antibiotics in animals, because antibiotic resistant strains of food-borne pathogens such as Salmonella and Campylobacter do spread from animals to people.
Controls on animal usage will not resolve the current problems in human medicine, but may help extend the useful life of any new classes of antibiotics if and when they are introduced.

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