Bovine mastitis is a very common disease in dairy herds worldwide. In Switzerland, the incidence density is 32.7 cases per 100 cow years (Stärk et al., 1997), resulting in total costs of about CHF 130 Mio. per year (Heiniger et al., 2014). Udder diseases are the most common causes for slaughtering primiparous cows in Germany (Brade and Brade, 2007). Many of these losses are directly related to intramammary infections caused by Staphylococcus aureus (Staph. aureus). This pathogen normally causes subclinical chronic mastitis in several to many cows within one herd (Sears and McCarthy, 2003). Although the clinical changes are normally mild, the cure rates (30%) for antimicrobial treatment are low (Gruet et al., 2001). Diagnosis of Staph. aureus by bacteriological testing of quarter milk samples is not satisfactory. The diagnostic sensitivity under the routine conditions reaches an overall diagnostic sensitivity of only 75% for single sampling (Sears et al., 1990; Studer et al., 2008). In some cases, this sensitivity may be as low as 21.4% (Studer et al., 2008). Based on these results, a satisfying diagnostic sensitivity is only achieved, if at least 3 consecutive milk samples are analysed. As triple sampling is normally too expensive and too laborious, routine testing is accomplished with single sampling. This fact, however, is a major reason why sanitation of Staph. aureus-infected herds frequently fails.
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