A. Veterinary antibiotics in soil
The concentration of antibiotics in various soil layers is termed “terracumulation” 34. Terracumulation will occur if an antibiotic is deposited in the soil at a rate that exceeds its rate of degradation.
How do veterinary antibiotics reach the soil?
Antibiotics administered to animals are not completely absorbed by the animals to which they are administered. Depending on the antibiotic, 30-90% of the antibiotic can be excreted via urine or feces as intact bioactive substances or as antibiotic metabolites that may still have some antimicrobial activity. The excretion rate varies greatly, depending on the pharmacokinetics of the administered antimicrobial, the route of application and the animal species involved. Animal antibiotics can also reach the soil through medical wastes, improper drug disposal or via dust from pens or barns.
Has it been proven that active forms of veterinary antibiotics are indeed present in the soil?
A growing number of studies worldwide provide evidence of the presence of many of veterinary antibiotics in the soil at concentrations reaching as high as 9,990ug kg -1. Examples include: oxytetracycline and sulfachlorpyridazine35, sulfamethazine and chlortetracycline36.
What happens to antibiotics in the soil?
Excreted compounds can be adsorbed, leached, degraded (through biotic or abiotic processes) and in some cases may revert back to the parent compound37. Degradation in soil is mainly from microbial action on the antibiotic. Although antimicrobials may remain in the upper layer of the soil, sorptive affinity and other properties of the antibiotic and soil may cause the antibiotic to reach the groundwater layer.
Do antibiotics in the soil remain active?
Once in the environment, any continued antibiotic efficacy depends on its physical-chemical properties (molecular structure, size, shape, solubility and hydrophobicity), prevailing climatic conditions, soil types and other environmental factors33. Antibiotic potency is mostly decreased by dilution, sorption and fixation, but antimicrobial activity may persist for long periods of time38 . No one answer is correct for all classes of antibiotics.