Infection of mammary glands which, if allowed to continue, can render the infected teat useless. Mastitis milk is not good for puppies to ingest, either.
Physical Symptoms
Red or discolored, swollen, sensitive mammary glands
Cause/Transmission
Bacterial infection. Post-partook females may be more susceptible to infection due to stress.
Treatment
Consult your veterinarian. Antibiotics are usually prescribed.
