A Bit of Background...
Macrolides have been used in both human and animal medicine since their discovery
in the 1950s. Their common mechanism of action is the inhibition of bacterial
protein synthesis with the activity of the macrolides directed primarily at
Gram-positive bacteria and mycoplasma.
Macrolides in human medicine are used primarily in outpatient medicine, and
are not considered “drugs of last resort”. In veterinary medicine,
macrolides are excellent tools used in the treatment and prevention of disease.
Macrolides are important in maintaining healthy livestock and poultry.
Macrolides were used in the European Union for growth promotion up until July
1, 1999. Based on a political decision, the legislation prohibiting the use
of the macrolide tylosin (one of the antibiotics used in veterinary medicine
that is not used in human medicine) for growth promotion was contrary to scientific
advice given by the European Union’s Scientific Committee on Animal Nutrition
(SCAN). In February 1998, SCAN concluded that “...the laboratory data
and the literature cited do not provide sufficient evidence that the use of
macrolides as feed additives presents a significant risk to human or animal
health. It is presumptuous to claim that any risk from allowing macrolides as
feed additives have been proven. In the absence of sufficient research data
on the epidemiology and spread of macrolide resistance, both among farm animals
and from them to man, there is no reason for a general ban on the use of macrolides
as feed additives”. Unfortunately, the ban was implemented for antibiotic
growth promoters, although they are still used for therapeutic purposes.
Commonly Used Animal Macrolides…
Tylosin
Tylosin is a bacteriostatic antibiotic essentially active against Gram-positive
bacteria and some Gram-negatives. It may also be active against some mycoplasmal
species. It is one of the antibiotics used in veterinary medicine that is not
used in human medicine. Tylosin is available as an injectable solution, water-soluble
powder, and feed additive premix. Tylosin is approved for use in the treatment
of pneumonia and metritis in beef cattle, erysipelas, pneumonia, porcine proliferative
enteropathy (ileitis) and dysentery in swine and respiratory diseases in poultry.
Tilmicosin
Tilmicosin is indicated for the treatment of bovine respiratory disease complex
associated with Pasteurella haemolytica and Pasteurella multocida. Tilmicosin
is administered as a subcutaneous injectable antibiotic for use in cattle (not
lactating dairy) and sheep.
Erythromycin
Indicated for the treatment of respiratory syndromes and as an aid in the
management of mastitis, metritis and leptospirosis in swine, for the treatment
of pnuemonia and shipping fever in cattle, and as an aid in the prevention of
lamb dysentery, erythromycin is a valuable macrolide for veterinary use. An
antibiotic of low toxicity, it is an effective antibiotic against Gram-positive
organisms with a wide range of activity, particularly against staphylococci,
streptococci and pneumococci.
How Antibiotics Are Used In Animals...
Antibiotics are used in animals for three basic purposes:
Disease Treatment: Any specific treatment regime used for
the cure or amelioration of a disease.
Disease Prevention: This is accomplished by hindering the
occurrence of a disease in a susceptible population. A prime example is when
illness is induced by stress such as a major weather change, which leaves
animals more susceptible to disease. Antibiotics are used strategically to
provide a barrier against infection.
Growth Promotion: Through a number of different mechanisms,
including disease suppression in sub-clinical carriers, the use of non-therapeutic
antimicrobials results in increased feed efficiency and average daily gain.
The Danish Experience...
We have learned valuable lessons from other countries that have banned the
use of animal antibiotics for non-therapeutic purposes. In Denmark in 1999,
certain antibiotics were legislatively banned for enhancing growth promotion
in food animals in an attempt to protect public health. This action has not
achieved expected results:
- No reduction has been noted in the incidence of antibiotic resistant hospital
isolates in humans.
- Disease has increased among animals, producing adverse animal welfare effects.
- In response to the increase in diseased states, it has been necessary to
increase the use of antibiotics for therapeutic treatment in animals.
North American Reality...
For over 40 years, antibiotics have played a critical role in keeping animals
healthy. Eighty-seven percent of antibiotics used in animal agriculture are
for disease treatment, with only about 13% used to maintain the health of animals
as measured by feed efficiency and average daily gain.
Healthy animals may play a role in providing environmental benefits as well.
Antibiotics help farmers maintain healthy, productive animals with less strain
on the environment. Without antibiotics, more natural resources (including land
base) would be required because more animals would be needed to produce the
same amount of food.
It is the nature of bacteria to mutate and develop resistance whether antibiotics
are used in food animal production or not. Although proper processing and handling
minimizes the levels of bacteria on food products, it is ultimately the responsibility
of the consumer to ensure that any remaining bacteria are killed by cooking
to proper endpoint temperatures.
Did You Know...
- Although banned for growth promotion purposes in the European Union, tylosin
is still used as a therapeutic treatment.
- To date, there is no evidence that the ban of antibiotic growth promotion
uses in European food animals has resulted in a reduction in the prevalence
of antibiotic resistant bacteria isolated from patients.
- The European experience shows that once certain antibiotics were phased
out or banned, an increase in bacterial disease outbreaks occurred so that
the elevated use of therapeutic antibiotics was needed for treatment.
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