Effect of antimicrobials in food-producing animals on pathogen load Systematic review of published literature

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Effect of antimicrobials in food-producing
animals on pathogen loadSystematic review of
published literature

• Michael Goodman MD, MPH
• Exponent, Alexandria, VA
• January 23, 2002

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Issue

• Antibiotics in food-producing animals could
  decrease microbial shedding. (NRC 1999)
• However,
• Most antibiotics only effective against certain
  bacteria (e.g., grampositive cocci).
• Therefore,
• Antibiotics may disturb normal microbial
  ecosystem resulting in an increase in certain
  bacteria (e.g., Salmonella). (CVM 1999)

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Concern

• Animals carrying increased amounts of pathogens
  (pathogen load) at the time of slaughter may
  present an increased risk for contamination of
  food and resulting human illness (CVM 1999).

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Objective

• To review and analyze published literature
  evaluating the effect of antimicrobials in
  food-producing animals on pathogen load.

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Methods Literature search

• 33 literature databases searched
• Medical (e.g., Medline, Toxline)
• Agricultural (Agricola, Agris, etc.),
• Food literature (Foodline, Food Adlibra),
• Veterinarian and zoological (e.g., Zoological
  Record Online)
• General scientific
• Search terms
• antibiotics AND pathogen AND food
• animals AND food AND pathogen load
• food-producing animals AND antibiotics
• other combinations

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Methods Data extraction

• Animal species under study
• Antibiotic in question
• Dose of antibiotic
• Study design
• Bacterial species evaluated
• Results

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Classification of studies
Two studies conducted separate experiments on
three species chickens, swine and calves.
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Challenge studies swine calves

• Antibiotics included
• chlor- and oxytetracyclin
• Apramycin
• Neomycin
• Penicillin
• Ceftiofur
• Carbadox.
• All studies used S. typhimurium as inoculum.
• No evidence of antibiotics increasing Salmonella
  shedding. (Table 1, handout)

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Challenge studies poultry

• All 6 studies from the Houghton Laboratory, UK
  showed significant increase in Salmonella
  shedding in chickens that received antibiotics.
• Shedding in these birds was also longer than in
  controls.
• Results were particularly strong for avoparcin.
  (Table 2, handout)
• These results were not replicated by others.

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Challenge studies poultry (continued)

• Gustafson (1981)
• Avoparcin increased Salmonella shedding only
  after single inoculation early in life.
• Serial inoculations and inoculations later in
  life showed no impact of avoparcin.
• Holmberg et al. (1984)
• No increase in Salmonella shedding after
  avoparcin alone.
• Combined use of avoparcin and coccidiostat
  monensin caused increased Salmonella shedding.
  (Table 2)

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Challenge studies poultry (continued)

• Other studies examined effect of
• virginiamycin (Abou Youssef et al. 1983) on
  Salmonella
• nosiheptide (Benazet et al. 1980) on Salmonella
  and E. coli
• flavophospholipol and salinomycin (Bolder, 1999)
  on Salmonella, Campylobacter and Clostridium
• No increase in pathogen load found. (Table 2)

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Observational studies

• Little evidence that antibiotics added to animal
  feed substantially affect bacterial shedding.
• Only exceptions are studies that used penicillin.
  (Table 3)

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Limitations of studies

1. Only Salmonella studied extensively.
2. Challenge studies may not represent real life
   conditions.
3. Only swine and chickens underwent sufficient
   study.
4. The diet or genetic line of animals that may
   affect the pathogen load and may explain data
   variability are not considered.

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Conclusions

• Most data indicate that the use of antibiotics in
  food-processing animals is generally not
  associated with increased pathogen load.
• Studies of Salmonella inoculation in chickens
  show disagreement.
• Overall the data have to be considered too
  limited to draw firm conclusions.