Clinical mastitis treatment decisions

Abstract

Mastitis is the most common bacterial disease of lactating cows1,2 and throughout the world accounts for the majority of antibiotic doses given on dairy farms.3-8 Many of these antibi­otic doses are unnecessary because of high spontaneous cure rates for many intramammary infections (IMI) affecting other­wise healthy cows. Mastitis is recognized based on the extent of the inflammatory response after IMI is established. It is consid­ered “subclinical” when the immune response causes increased somatic cell count (SCC), but the cow remains healthy with no visible changes in the milk or udder. Milk from cows with sub­clinical mastitis can be comingled with milk from healthy cows and sold, thus treatment during lactation is not usually cost ef­fective and most of these cases are treated with antibiotics at dry off. Clinical mastitis occurs when the immune response to IMI results in visually abnormal milk with or without second­ary signs. Fortunately, few cases of clinical mastitis cause sys­temic signs that require immediate therapy, but abnormal milk may not be sold for human consumption, so farmers emphasize interventions (such as antibiotic therapy) that are perceived to hasten the return to normal milk. Non-severe cases of mastitis that present with abnormal milk, or abnormal milk accompa­nied by localized swelling of the udder account for about 85% of all CM cases.9-11 Mastitis caused by many opportunistic bacteri­al pathogens often have high rates of spontaneous bacteriologi­cal clearance, so on many farms, most cases of clinical mastitis occurring in otherwise healthy cows do not benefit from antibi­otic therapy. Treatment protocols for non-severe clinical masti­tis should include an assessment of the immune capabilities of the cow, the likely etiology of the case, a review of the medical history of the cow and an assessment of the probability that she will remain productive long enough to recoup the costs (includ­ing discarded milk) associated with treatment. The purpose of this paper is to review treatment decisions for non-severe clini­cal mastitis, emphasizing evidence-based criteria that indicate when antibiotic therapy is beneficial to the cow.