Staphylococcus aureus intramammary infections in the face of routine contagious mastitis pathogen control procedures: The role of Staphylococcus aureus strain and possible intervention strategies

Objectives of this research were (1) determine if management strategies beyond milking time hygiene could reduce mastitis prevalence following an outbreak with a single strain of Staphylococcus aureus, (2) determine whether herds that imported cattle for expansion or replacement were more likely to have a higher prevalence of Staphylococcus aureus mastitis and have a greater number of strains of Staphylococcus aureus than closed herds that reared their own replacements, and (3) determine whether strains of Staphylococcus aureus causing bovine mastitis differed with regard to pathogenicity. Segregation of cattle with Staphylococcus aureus mastitis, therapeutic cessation of lactation of infected mammary quarters in mid-to-late lactation, and culling multiple-quarter infected cattle in combination with milking time hygiene reduced prevalence and incidence of Staphylococcus aureus mastitis in a herd with a single strain outbreak from 20% to 8% and 3.4 cases/100 cows months to 0.35 cases/100 cow months, respectively, after one year. Permanent cessation of lactation of Staphylococcus aureus infected mammary quarters was best achieved using intramammary 5% povidone-iodine. Herds that purchased replacement heifers had a higher prevalence of Staphylococcus aureus mastitis than herds that purchased lactating cattle for expansion. Herds that purchased replacement heifers had more Staphylococcus aureus strains and acquired more new strains of Staphylococcus aureus than closed herds. A predominant strain of Staphylococcus aureus in one herd caused a significant decrease in milk production in the infected mammary quarter relative to an uninfected control quarter (P = 0.01), whereas quarters infected with sporadic strains of Staphylococcus aureus produced similar quantities of milk to control quarters ( P = 0.83). Finally, no significant differences were found for milk somatic cell count and N-acetyl-β-D-glucosaminidase activity from mammary quarters infected with different strains of Staphylococcus aureus (P > 0.05). However, the coefficient of variation approximated 1.0 suggesting the response to infection with a given strain of Staphylococcus aureus is highly variable.