The effect of a combined intramammary and intramuscular treatment of mastitis on excretion of antibiotics in milk

Most farmers use intramammary antibiotics to treat clinical mastitis in dairy cows.During the acute phase of mastitis, the swelling of the intramammary tissue causes occluded milk ducts.This may prevent distribution of the infused antibiotic and consequently result in failure of the intramammary therapy.A combination of intramuscular and intramammary therapy may overcome these problems.Therefore this combination therapy is a recognised and accepted practice among many veterinarians.Combination therapy has resulted in better bacteriological cure rates of mastitis (Ziv, 1980;Owens et al, 1988;Owens et al., 1990).But combination therapy is considered off label use and problems with residues in milk may occur, as the prescribed withdrawal period is not reliable anymore.The prescribed withdrawal period of an antibiotic preparation is based on the excretion time of antibiotics in milk after treatment of healthy cows according to the label.Oliver et al. (1990) and Pedersoli et al. (1995) showed that the combination of intramammary and intramuscular therapy was associated with longer excretion periods of antibiotics in milk compared to single intramammary therapy.However, these studies used healthy cows instead of cows with mastitis.To gain better insight in the possible risks of exceeding the prescribed withdrawal period when using combination therapy under practical circumstances, it is necessary to use cows with mastitis instead of healthy cows.The objective of our study was to investigate the effect of combining intramammary and intramuscular mastitis therapy on the excretion time of antibiotics in milk.