Clinical mastitis caused by different types of pathogens:differences in milk production loss, recovery, age at onset, and milk NAGase

Milk samples were collected at the onset of clinical signs from 338 cases of mastitis on a 1700-cow Michigan dairy farm. Infections were categorized according to etiology as contagious (Staphylococcus aureus), coagulase-negative staphylococci (Staph. Spp.), environmental, or dual pathogen infections. Daily milk production and disease events were recorded for all cows in the herd. Time until clinical recovery and daily milk production loss were greatest with dual infections. Milk NAGase was highest for environmental agents and lowest for dual infections. Duration of antibiotic therapy, baseline NAGase, and Wisconsin Mastitis Test (WMT) were not significantly different among etiologic agents. First lactation cows contracted clinical mastitis later in lactation than older cows and had a greater proportion of clinical cases caused by environmental pathogens; older cows had a greater proportion caused by contagious bacteria and Staph spp. Correlation between NAGase and WMT in mastitic quarters was 0.40. Greater production loss was associated with increased NAGase and increased WMT. Increased NAGase was associated with longer time until clinical recovery and longer duration of antibiotic therapy