III. The environmental impact of imprudent antimicrobial use in animals
Another area of human health concern is the long-term effect of
antibiotic residues in the environment. Although human
antimicrobial usage may be the primary source for aquatic and
terrestrial antibiotic contamination, antibiotic applications in
livestock, poultry and aquaculture also contribute significantly to
this growing problem.
A varying proportion of administered antibiotics may remain active in excreted biological matter (generally feces or urine) after passing through the animal. Along with antimicrobials used for humans, the livestock, poultry and aquaculture sectors are important contributors to aquatic and terrestrial contamination with antibiotics. Antibiotics and their metabolites (degradation products) reach the environment via the application of antibiotic-laden manure or slurry on agricultural lands, or direct deposition of manure by grazing animals. This can be followed by surface run-off, driftage or leaching into deeper layers of the earth33. A proportion of the antibiotics that reach the environment will remain biologically active. Low subtherapeutic concentrations of antibiotics that accumulate over time may have profound effects on some ecosystems. Environmental antibiotic concentrations may exert selective pressure on environmental bacteria and may also foster the transfer of resistance genes, helping create the “resistome” mixing pot of genetic AMR traits.