Mastitis therapy and pharmacology

Effective and economical mastitis control programs rely on prevention rather than treatment. Herds practicing mastitis prevention produce higher quality milk at less cost than herds that do not. Nonetheless, therapeutic intervention is an important part of a control program for bovine mastitis. This article describes the strategies of therapy for bovine mastitis, with an emphasis on antibacterial and anti-inflammatory therapy. The concept of administering intramammary infusions of antiseptic solutions as a treatment for mastitis caused by infectious agents has been present for at least a century. Widespread availability and use of antibacterials in animal agriculture in the 1950s cultivated the development of a wide variety of commercial products for intramammary infusion in the 1960s and 1970s. Initial successes suggested 75% efficacy (cures) in both lactating and dry-cow formulations; however, there has been growing skepticism that in many cases, therapeutic reality falls short of expectations. Chronic intramammary infections (IMI) with extensive fibrotic change caused by pathogens such as Staphylococcus aureus pose difficult therapeutic problems. It is unlikely that typical labeled-dose regimens, despite providing antibacterial concentrations above minimal inhibitory concentrations (MIC) in milk for 24 to 48 hours, will eliminate the pathogen from infected mammary glands. In addition, the major developmental thrust for antibacterials as a treatment for mastitis has been directed against gram-positive organisms, particularly staphylococci and streptococci. Many herds, however, have seen the emergence of pathogens with greater resistance to antibacterials such as gram-negative rods and Mycoplasma bovis as a substantial cause of mastitis losses. Nonetheless, mastitis remains the most frequent cause of antibacterial use on dairy farms and contributes to a substantial portion of total drug and veterinary costs incurred by the dairy industry. In addition, application of management practices that decrease the prevalence of contagious pathogens such as Streptococcus agalactiae and Staphylococcus aureus has shifted the focus of mastitis control and economic losses to environmental pathogens that are associated with more frequent episodes of clinical mastitis. The cost of clinical mastitis is greater than $100/case and averages $40 to $50 per cow in herd per year [1,2]. Discarded milk following treatment may account for as much as 70% of lost marketable milk, and in herds that do not have a judicious treatment program, losses from discarded milk alone can exceed $100 per cow in herd per year. Thus, there is increased awareness among producers of treatment-related costs and the economic costs of extensive antibacterial therapy for mastitis. Michigan Department of Agriculture records, collected from Michigan dairies that had an occurrence of inhibitory residues in marketed milk from 1995 to 2000, revealed that antibacterial therapy of mastitis accounts for 90% of the occurrences. Furthermore, both the consuming public and the regulatory sector have increased their awareness of the possible health hazards posed by antibacterials and other drugs in milk used for human consumption. Whether these concerns are real or perceived, frequency of milk testing and regulatory control of drug use on dairy farms is increasing and likely to increase in complexity as time continues. Thus, because of accountability to dairy producers and the consuming public, two key issues must be addressed in addition to efficacy: therapy must be economically viable and it must not increase the risk of drug residues in marketed milk.