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How to treat mastitis in dogs 
Start your dog on antibiotics. 
Since mastitis is a bacterial infection of the mammary glands, the vet will prescribe antibiotics for your dog to take. This will treat the cause of the mastitis and the signs of mastitis should soon disappear.
If she's still nursing the puppies, the vet will choose an antibiotic that is safe for the pups to ingest through her milk. Be sure to give the full course of antibiotics, even if your dog's mastitis improves.