Final answer:
Streptococcus agalactiae is the mastitis pathogen with the highest rate of successful treatment using intramammary antibiotic therapy during lactation due to its typical sensitivity to common antibiotics.
Step-by-step explanation:
The major mastitis pathogen that has the highest rate of successful treatment with intramammary antibiotic therapy during lactation is Streptococcus agalactiae. This bacterium is typically sensitive to many of the antibiotics used to treat mastitis. Other pathogens such as Staphylococcus aureus are known for their more resistant properties and lower treatment success rates, especially because some strains have acquired resistance to antibiotics like methicillin. Additionally, organisms like Escherichia coli may cause mastitis but typically have a varied response to antibiotics, and Mycoplasma bovis is known for being particularly difficult to treat with antibiotics during lactation due to its lack of a cell wall and resistance to many antibacterial drugs.