Mastitis in dogs shows up as swollen, red or purple mammary glands that feel firm and hot to the touch. It most commonly affects nursing mothers, and the visual signs change dramatically depending on how far the infection has progressed. In early cases, the changes can be subtle enough that the first clue is puppies not gaining weight. In advanced cases, the skin over the gland can turn dark purple or black.
Early Signs You Might Miss
Mild mastitis doesn’t always look alarming. The affected mammary gland may be only slightly swollen compared to the others, with minor inflammation that’s easy to overlook if you’re not checking regularly. The gland might feel a bit firmer or warmer than the surrounding tissue, but the dog often seems perfectly healthy otherwise.
The biggest early red flag is often the puppies, not the mother. If one or more puppies aren’t gaining weight at the expected rate, or if they seem restless and unsatisfied after nursing, that’s a reason to examine the mother’s mammary glands more closely. Subclinical mastitis, where bacteria are present but haven’t yet caused visible inflammation, can only be confirmed through lab testing. But slow puppy growth is the practical signal most owners notice first.
What Moderate Mastitis Looks Like
As the infection takes hold, the visual changes become unmistakable. The affected gland swells noticeably and turns red or purple. It feels hard, hot, and painful. Your dog will likely flinch or pull away when you touch it. One gland or several can be involved, and without treatment the infection tends to spread from gland to gland.
The milk itself may also change. In some cases it still looks normal, but it can become blood-tinged or thick with pus. If you gently express a small amount of milk from a suspect gland and see discoloration, streaks of blood, or a chunky texture, that’s a clear sign of infection. Open wounds, scabs, or ulcerated skin on the surface of the gland can also develop at this stage.
Gangrenous Mastitis: The Emergency
The most severe form of mastitis causes the tissue in the mammary gland to die. The skin over the gland turns dark purple or black as blood supply is cut off by the overwhelming infection. The gland is still hot, swollen, and extremely painful, but the color shift to dark or blackened tissue is the hallmark of gangrenous mastitis.
Dogs at this stage are visibly, seriously ill. Expect to see fever, dehydration, complete refusal to eat, depression, and in some cases a rapid heart rate or dangerously low blood pressure. Sepsis, a life-threatening bloodstream infection, can develop quickly. Gangrenous mastitis is a veterinary emergency. The affected gland tissue may need surgical removal, and the dog will need aggressive supportive care.
Behavioral Changes to Watch For
Beyond the physical appearance of the glands, mastitis changes how a mother dog acts. Early on, she may seem restless or uncomfortable when puppies latch on. As the pain worsens, she may refuse to nurse entirely, pushing puppies away or standing up and walking off when they try to feed. Loss of appetite, lethargy, and a general lack of interest in the puppies are all behavioral signs that accompany the more visible physical changes.
A mother dog who was attentive and calm with her litter and suddenly becomes avoidant or agitated during nursing is telling you something is wrong, even if the glands don’t look dramatically swollen yet.
How to Check Your Dog at Home
If your dog is nursing a litter, get in the habit of gently feeling her mammary glands every day. You’re comparing each gland to the others. Run your fingers along the full chain of glands on both sides of her belly. What you’re feeling for is any gland that’s noticeably firmer, hotter, or more swollen than its neighbors. A healthy nursing gland is soft and pliable. An infected one feels hard, sometimes almost like a rock.
Watch your dog’s reaction as you touch each gland. Pain on one gland but not the others points to a problem. Also check the skin surface for any redness, bruising, open sores, or scabbing. If the skin looks discolored or broken, or if milk from that gland looks abnormal in any way, those are signs that need veterinary attention quickly. Mastitis can escalate from mild swelling to abscess or tissue death within days if left untreated.
Can Puppies Still Nurse Safely?
In mild cases, puppies can often continue nursing from unaffected glands while the mother receives treatment. Continued nursing actually helps keep milk flowing and can prevent further engorgement. But there are situations where puppies need to be weaned early or switched to a milk replacer: if the mother refuses to let them nurse, if multiple glands are infected, or if the milk is visibly abnormal. Puppies nursing from a severely infected gland risk ingesting bacteria and pus, which can cause them to become ill or fail to thrive.
If you notice that puppies who were previously healthy start losing weight, becoming lethargic, or crying more than usual, consider that contaminated milk may be the cause, even if the mother’s glands don’t look severely affected yet.
What Causes It
Bacteria enter the mammary gland, typically through the teat opening. Tiny scratches from puppy nails or teeth create entry points, and the warm, moist environment of a nursing gland is ideal for bacterial growth. Poor hygiene in the whelping area increases the risk. Milk that sits stagnant in a gland, whether because a puppy dies, a litter is too small to drain all the glands, or weaning happens abruptly, also creates conditions ripe for infection.
Mastitis occasionally occurs in dogs that aren’t nursing. A false pregnancy can trigger milk production, and that milk with no puppies to drain it can become a breeding ground for bacteria. Less commonly, trauma to the mammary area or a compromised immune system can set the stage for infection.