Staphylococcus lugdunensis and Staphylococcus simulans super shedders in dairy goats.

The bulk milk total bacterial count (BMTBC) is a key milk quality indicator because bacterial byproducts reduce the shelf life of milk and dairy products and increase the risk of bacterial overgrowth before consumption. Exceeding BMTBC limits results in milk price penalties, motivating farmers to keep it under control. Factors influencing BMTBC include milking hygiene and milking technique, but also goats with IMI can shed bacteria into the milk. How many bacteria are shed during IMI, and to what extent individual goats can contribute to BMTBC is not known yet. From January to October 2023, a Dutch dairy goat farm experienced persistently high geometric mean BMTBC and bulk milk somatic cell count (BMSCC). In a cross-sectional study of 98 goats on this farm, we evaluated whether goats with mastitis could significantly affect the BMTBC and BMSCC. Also, the relation between mastitis pathogens in individual goat milk and udder health parameters such as clinical mastitis and asymmetry were investigated. Milk was cultured on blood agar plates and bacteria were identified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Additionally, the numbers of bacteria in the milk samples were determined using both flow cytometry and conventional culture methods (total blood agar plate count with track dilution). We found that goats with IMI caused by Staphylococcus lugdunensis or S. simulans shed extremely high numbers of bacteria into the milk with a maximum of approximately 700 million cfu/mL. Moreover, presence of S. lugdunensis or S. simulans was associated with having a clinical mastitis and an asymmetric udder, a clinical sign often observed in goats with a history of mastitis. Research into host-specific reactions and which virulence factors of S. lugdunensis and S. simulans contribute to this high bacterial shedding could help in developing preventive measures.