Mastitis in dogs is an infection or inflammation of one or more mammary glands, most common in females that are nursing puppies. It ranges from mild swelling that’s easy to miss to a severe, potentially life-threatening emergency. The condition is almost always caused by bacteria entering the mammary tissue, and it can develop quickly, sometimes within hours.
What Causes Mastitis
Bacteria are the primary cause. They typically enter through the teat opening, often introduced by scratches from nursing puppies’ nails or through cracks in the skin. The warm, moist environment of a lactating mammary gland is ideal for bacterial growth. The most frequently isolated pathogens are Staphylococcus aureus (which accounts for roughly 56% of positive cultures in studies), E. coli, and Streptococcus species.
Several situations raise the risk. Milk that isn’t fully drained from a gland, whether because of a small litter, early weaning, or puppies favoring other teats, creates a buildup that encourages infection. Unsanitary whelping areas, trauma to the mammary tissue, and a weakened immune system all contribute. In rare cases, mastitis can occur in dogs that aren’t nursing, including during a false pregnancy when the body produces milk without an actual litter.
Recognizing the Symptoms
Early mastitis can be subtle. You might notice only slight swelling or firmness in one mammary gland, and your dog may flinch when the area is touched. At this stage, the gland may still look relatively normal, which is why mild cases sometimes go undetected until they worsen.
As the infection progresses, the signs become more obvious. The affected gland becomes increasingly swollen, hot to the touch, and visibly red or purple. Milk expressed from the gland may look abnormal: cloudy, thickened, or containing visible blood or pus. Your dog may also show systemic signs of illness, including fever, lethargy, loss of appetite, and reluctance to let puppies nurse. She may lick at the affected gland excessively or seem restless and uncomfortable.
In some cases, the skin over the gland becomes ulcerated, with open wounds or scabs forming on the surface.
When Mastitis Becomes an Emergency
Most cases of mastitis respond well to treatment when caught early. But a severe form called gangrenous mastitis is a genuine emergency. It develops when the blood supply to the mammary tissue is compromised, causing the tissue to die. The telltale sign is a gland that turns dark red, purple, or black.
Gangrenous mastitis can progress to septic shock, a life-threatening condition where the infection overwhelms the bloodstream. If your dog’s mammary gland has any dark discoloration, or if she suddenly becomes very weak, vomits, or seems disoriented, that warrants immediate veterinary care, not a wait-and-see approach.
How It’s Diagnosed
A veterinarian will typically start with a physical exam, feeling the mammary glands for heat, swelling, and pain. Even if the milk looks normal to the naked eye, it can still be infected. Microscopic examination of a milk sample will reveal elevated levels of white blood cells (specifically a type called neutrophils), which confirms inflammation. A milk culture identifies the exact bacteria involved and which antibiotics will be effective against them. Your vet may also recommend blood work to check whether the infection has spread beyond the gland.
Treatment and Recovery
Antibiotics are the cornerstone of treatment. Your vet will prescribe a broad-spectrum antibiotic that covers the range of bacteria most likely responsible. If a milk culture was submitted, the antibiotic choice may be adjusted once results come back, usually within a few days.
Beyond antibiotics, treatment involves getting milk flowing out of the infected gland to prevent further bacterial buildup. Warm compresses applied to the gland before gently expressing milk can help. One approach recommended by the American Kennel Club is applying cabbage leaf compresses to reduce pain and swelling. The leaves are secured against the affected gland with a bandage or a fitted T-shirt and left on for two to four hours, then removed for three to four hours before reapplying. Puppies can nurse from the gland during the periods when the compress is off.
In severe or gangrenous cases, surgical removal of the affected mammary tissue may be necessary. Dogs with septic shock require intensive care, including IV fluids and aggressive infection management.
Can Puppies Still Nurse?
This is one of the first questions most owners have, and the answer depends on severity. In mild to moderate cases, continued nursing is actually encouraged because it helps drain the infected gland. The antibiotics prescribed are generally chosen to be safe for nursing puppies.
However, if the milk is visibly abnormal (containing pus, blood, or an unusual color), or if the mother is too painful to allow nursing, puppies should be redirected to unaffected glands or supplemented with a milk replacer. In cases of gangrenous mastitis, puppies should not nurse from the affected gland at all. Subclinical mastitis, the kind with no obvious symptoms, is a hidden danger for puppies. Neonatal deaths in litters have been linked to undetected infections, particularly those caused by Staphylococcus aureus and certain streptococcal species.
Prevention
Keeping the whelping area clean is the single most important preventive step. Bedding should be changed frequently, and the mother’s mammary area should be gently wiped down regularly, especially if the environment is damp or soiled. Trimming puppies’ nails starting in the first week or two reduces the small scratches on the teats that give bacteria an entry point.
Monitor the mammary glands daily during the entire nursing period. Gently feel each gland for firmness, heat, or asymmetric swelling. Ensuring all glands are being nursed from helps prevent milk stasis, so if you notice puppies ignoring one or two teats, rotate them or manually express a small amount of milk to keep things flowing. If your dog is going through a false pregnancy and producing milk, your vet can advise on how to manage the milk production and reduce the infection risk.