Mastitis in dogs is a bacterial infection of the mammary glands that requires prompt treatment, typically a combination of veterinary-prescribed antibiotics and supportive care at home. Most cases respond well when caught early, but delayed treatment can lead to serious complications including tissue death in the affected gland. If your dog’s mammary glands are swollen, hot, or painful, getting a veterinary diagnosis is the essential first step before starting any treatment.
What Causes Mastitis in Dogs
The infection is caused by bacteria that normally live on your dog’s skin migrating into the mammary tissue. The three most common culprits are Staphylococcus species, E. coli, and Streptococcus species. Among these, Staphylococcus aureus is the most frequently isolated, accounting for roughly 56% of positive cultures in affected dogs.
Mastitis most commonly develops in nursing mothers. Bacteria enter through small cracks or scratches on the nipple caused by puppy teeth, or through milk ducts that stay open during lactation. It can also occur in dogs experiencing a false pregnancy, where hormonal changes cause milk production even without puppies. Less commonly, trauma to the mammary area or poor hygiene in the whelping environment sets the stage for infection.
Recognizing the Signs
The affected mammary gland will typically look swollen, feel firm or hard, and be warm to the touch. Your dog may flinch or snap when the area is touched. The milk itself often changes: it may appear discolored (yellowish, greenish, or blood-tinged) instead of its normal white or slightly off-white color. In mild cases, only one gland is involved, but multiple glands can be affected at once.
Beyond the local signs, watch for changes in your dog’s overall behavior. A dog with mastitis often becomes lethargic, loses interest in food, and may run a fever. She might refuse to let her puppies nurse or seem restless and uncomfortable when lying down. In some cases, the puppies themselves are the first clue: if a litter suddenly fails to gain weight, stops thriving, or develops diarrhea, subclinical mastitis (infection without obvious symptoms in the mother) could be the cause. Subclinical mastitis associated with Staphylococcus aureus and certain streptococci is a common cause of neonatal puppy death.
Veterinary Treatment
Your vet will likely start with a physical exam and may take a sample of milk from the affected gland. Examining the milk under a microscope and culturing it helps identify which bacteria are involved and which antibiotics will be most effective. Broad-spectrum antibiotics are usually started right away while waiting for culture results, then adjusted if needed once the specific bacteria are identified.
The antibiotic course typically lasts 2 to 3 weeks. It’s important to finish the full course even if your dog looks better within a few days, because stopping early allows resistant bacteria to survive and the infection to return.
Pain Relief
Your vet may prescribe anti-inflammatory pain medication to keep your dog comfortable. This is a judgment call when the dog is still nursing. Research on one commonly used veterinary anti-inflammatory found that it passes into milk at low concentrations in healthy mammary tissue, making short-term use likely safe for nursing puppies. However, inflamed mammary glands allow higher concentrations of the drug into the milk, potentially up to several times more than a healthy gland would. No anti-inflammatory pain medications are officially approved for use in lactating dogs, so your vet will weigh the risks and benefits for your specific situation.
Home Care That Helps
Supportive care at home plays a real role in recovery alongside antibiotics. Warm compresses applied to the swollen gland for 10 to 15 minutes several times a day help improve blood flow to the area, reduce swelling, and encourage the gland to drain.
Cabbage leaf compresses are another option that may reduce pain and inflammation. To use them, place a clean cabbage leaf over the affected gland and hold it in place with a snug bandage or a fitted t-shirt. Leave the leaves on for two to four hours, then remove them for three to four hours before reapplying. If your dog is still nursing, the puppies can feed from the affected gland during the uncovered intervals.
Gentle hand-milking of the infected gland can also help. Expressing milk keeps the gland from becoming overly engorged, which reduces pain and helps flush bacteria out. Your vet can show you the proper technique so you avoid causing further discomfort or damage.
Can Puppies Still Nurse?
In mild to moderate cases, puppies can usually continue nursing from the affected gland. Continued nursing actually helps by keeping the gland draining, which speeds recovery. The antibiotics your dog is taking will pass into the milk in small amounts, but this is generally considered safe for the puppies.
There are situations where nursing should stop. If the milk is visibly abnormal (thick, discolored, or foul-smelling), if the gland has abscessed, or if the mother is severely ill, the puppies need to be separated and hand-fed with a commercial milk replacer. Puppies that have been nursing from an infected gland should be monitored closely for diarrhea, poor weight gain, or lethargy. If they start showing these signs, switch to bottle feeding and have your vet check them.
When Mastitis Becomes Dangerous
Most mastitis cases resolve with antibiotics and supportive care. But when treatment is delayed or the infection is especially aggressive, the tissue in the affected gland can begin to die. This is called gangrenous mastitis, and it’s a veterinary emergency. The gland turns dark purple or black, feels cold instead of warm, and may develop open wounds that ooze foul-smelling discharge. Your dog will likely be visibly sick: feverish, weak, and refusing food.
Gangrenous mastitis requires aggressive treatment. Your dog will need intravenous fluids, strong antibiotics, and in many cases surgical removal of the dead mammary tissue. Without surgery, the infection can spread to the bloodstream and become life-threatening. If you notice any darkening of the skin over a swollen gland, or if your dog’s condition worsens despite being on antibiotics, get veterinary attention immediately.
Preventing Mastitis
Keeping the whelping area clean is the single most effective preventive measure. Wash bedding frequently, keep the nesting box dry, and clean up soiled materials promptly. Bacteria thrive in warm, moist environments, and a dirty whelping box is the perfect breeding ground.
Trim puppies’ nails regularly starting at about one week of age. Their sharp little claws scratch the skin around the nipples, creating entry points for bacteria. Check the mother’s mammary glands daily during nursing. Catching early swelling or tenderness before it progresses to full infection makes treatment far simpler.
During weaning, reduce the mother’s food intake gradually to slow milk production. An abrupt stop in nursing while the glands are still producing heavily leads to engorgement, which creates a perfect setup for infection. A gradual weaning process over one to two weeks, removing puppies from nursing sessions progressively, lets milk production taper naturally.
Mastitis in Non-Nursing Dogs
Though less common, mastitis can develop in dogs that aren’t nursing. False pregnancy (pseudopregnancy) triggers hormonal changes that cause milk production, and that milk can become a growth medium for bacteria if it pools in the glands. Spayed dogs occasionally develop mastitis if mammary tissue remains active due to residual hormonal activity.
Treatment follows the same principles: antibiotics for the infection and supportive care to manage symptoms. In dogs with false pregnancies, your vet may also address the underlying hormonal issue to stop milk production and prevent recurrence. For dogs that experience repeated episodes of false pregnancy with mastitis, spaying is often recommended as a long-term solution.