The relationship between
antibiotic milk concentrations and bacteriological efficacy was investigated in groups of lactating cows with subclinical
mastitis due to either
penicillin G-sensitive or
penicillin G-resistant Staphylococcus aureus. Treatments consisted of the
intramuscular injection of
procaine penicillin G, or its weak base
ester penethamate hydriodide, and
sodium methicillin, or its weak base
ester tamethicillin.
Antibiotics were administered once daily for 2 or 4 days at accepted dosages. After four daily, treatments with
procaine penicillin G and
penethamate hydriodide,
infections were eliminated from 56.5% and 68.8%, respectively, of quarters infected with
penicillin G-sensitive staphylococci, and from 14.3% and 7.7%, respectively, of quarters infected with
penicillin G-resistant staphylococci. After four daily treatments with
sodium methicillin and
tamethicillin,
infections were eliminated from 32.4% and 48.6%, respectively, of quarters infected with
penicillin G-resistant staphylococci. The better efficacy of
penethamate hydriodide and
tamethicillin was considered to be linked to the higher milk
drug concentrations obtained with these drugs as opposed to the lower concentrations measured in the milk
after treatment with the parent drugs. Cure rates were generally higher
after treatment for 4 days than after the 2-day course of
therapy. Treatment efficacy decreased progressively with increasing age of the cows. Intramuscular treatment of subclinical staphylococcal
mastitis in lactating cows can serve as a useful model for screening existing and new
antibacterial agents and
drug products intended for the parenteral treatment of clinical staphylococcal
mastitis.