Streptococcus agalactiae primarily infects the cisterns and the ductal system of the mammary gland. An irritant is produced, causing inflammation of the gland which is mostly subclinical with occasional clinical symptoms. Accumulation of bacterial waste products intensifies the inflammatory response, resulting in destruction of milk producing tissue and reduced milk yield or agalactia. Streptococcus agalactiae rarely causes severe illness, but extensive scarring of a quarter may render it unproductive in subsequent lactations. Staphylococcus aureus Staphylococcus aureus is more difficult to eradicate than Strep.
Infected udders are the most important source of infection.
The organism readily colonizes teat skin lesions and the teat canal, and eventually passes into the mammary gland. The organism may also survive at other sites on the cow. Mastitis caused by Staph. Recurring signs of mild clinical mastitis often causes additional losses. High bacteria counts in bulk milk are generally not seen with Staph.