Mastitis in Dogs: Signs, Causes and What It Looks Like

Mastitis in dogs is inflammation of one or more of the mammary glands, almost always because bacteria have entered the gland and infected the milk sitting inside it. If you can see or feel a hot, hard or discoloured gland, or there is pus or blood coming from a teat, call your veterinarian today, and if your dog is also off her food, lethargic, shivering or feverish, treat that as an emergency and go in now. Any new lump or swelling in a dog's mammary chain should be examined by a veterinarian, because only a hands on exam and a sample taken from the lump can tell you what it actually is.

This page is about recognition: what the condition is, why it happens, what the early signs are, and what an affected gland looks and feels like. Diagnosis and the plan that follows belong to your vet, and the sooner they see the dog, the smaller the problem usually is.

What the word means and which dogs are affected

Mastitis simply means inflammation of mammary tissue. It can involve a single gland or several at once, and it can appear anywhere along the two rows of mammary glands that run from a dog's chest down to her groin. In most situations there is infection as well as inflammation, because bacteria have travelled up through the teat opening into a gland that is full of milk.

The dogs owners worry about first are nursing mothers. The glands are producing, the teat canals are open, and puppies are feeding around the clock, so there are plenty of opportunities for bacteria to get in. It can show up within days of whelping, at any point during nursing, or around weaning, when milk stops being drawn off and backs up inside the gland.

It is not only a nursing problem. A dog going through a false pregnancy can produce milk and develop exactly the same picture. A dog who has never been bred, and even a spayed dog, can develop an infected mammary gland if the skin is broken by a bite, a scratch or a graze, or if bacteria reach the tissue through the bloodstream. That is one reason a swollen gland in a dog who is clearly not nursing still deserves an appointment rather than a wait and see approach.

Why a mammary gland becomes infected

Two things usually line up. The first is milk that is not moving. Milk sitting in a gland is warm and rich, which makes it a comfortable place for bacteria to multiply. Milk pools when a litter favours certain teats and ignores others, when a puppy is lost, when weaning happens abruptly, or when the dam is sore and stops letting the litter feed properly on one side.

The second is a way in. Puppy claws scratch the delicate skin around the teats. A teat can be nicked, bruised or chapped. Bedding that stays damp or soiled holds bacteria right against open teat canals, and a dog lying on rough ground can graze the underside of her belly without anyone noticing. The bacteria involved are often ordinary organisms that live harmlessly on skin or in the environment, and they only need a small opening.

Anything that lowers a dog's general resilience, such as poor body condition, a difficult whelping, another illness or a heavy parasite burden, can make an infection easier to establish and quicker to spread. None of this means an owner has done something wrong. It happens in clean, carefully managed whelping boxes.

Early signs and the full symptom picture

Early signs are quiet, which is why they get missed. A dog in the first hours of a problem usually looks normal from across the room. What owners notice first tends to be one of these:

• One gland that feels slightly firmer or warmer than the gland beside it or opposite it
• Mild tenderness, so she shifts position or stands up when a particular teat is used
• Puppies becoming restless, crying more between feeds, or one teat being avoided
• A litter that stops putting on weight the way it had been

As it progresses, the picture becomes much clearer:

• Visible swelling, noticeably larger than the matching gland on the other side
• Heat you can feel with the back of your hand
• Redness, or a darker, bruised look to the skin
• Obvious pain, shown as flinching, licking at the area, restlessness, growling or snapping when the belly is touched
• Milk or discharge that is thick, yellow, brown, grey, stringy or blood tinged instead of white
• Refusing to let the litter nurse at all
• Whole body signs: fever, shivering, dullness, loss of appetite, vomiting, dehydration

In a female dog who is not nursing, the symptom list is the same minus the milk changes: one swollen, hot, painful gland, sometimes with discharge from the teat. Once any of the whole body signs appear, this has moved past something to monitor at home and become something to have assessed the same day.

What an affected gland looks like and feels like

Visually, an affected gland is usually larger than its opposite number, and the difference is often the first thing that catches your eye. The skin may be pink, red, or in more advanced cases purple, blue or close to black. It can look tight, stretched and shiny, the teat itself may be enlarged, and there may be crusting, scabbing, a weeping patch or a frank open wound.

To the touch, the gland is warm or hot. Firmness ranges from a slightly doughy, swollen feel through to a tight, hard lump that sits like a golf ball under the skin. Sometimes the whole gland is involved; sometimes there is one discrete hard knot deep in the tissue while the rest of the chain feels completely normal. Almost always the dog reacts to pressure, and that reaction is a meaningful sign in itself.

If you have been searching for pictures, it is worth knowing what photographs can and cannot tell you. Images online skew heavily towards dramatic, advanced cases, because those are the ones that get photographed. Early inflammation photographs as a gland that looks almost normal. Pigmented skin and a thick coat hide colour change completely, so a black or heavily coated dog can have significant inflammation with nothing visible, while the same degree of change is obvious on a pale bellied dog. The most useful comparison is not an image on a screen, it is the gland on the other side of your own dog's body.

Photograph the area yourself, in the same light, once a day, and bring those pictures to the appointment. A record of how fast the swelling and colour are changing is far more useful to your vet than any stock photo.

Severe cases, open wounds and when to go in immediately

When inflammation cuts off blood supply, tissue starts to die. The gland turns dark red, purple, blue or black, areas of skin may feel cool rather than hot, and the area can develop a foul smell. An abscess may form and rupture, leaving an open wound that drains pus, brown fluid or blood tinged discharge. Skin over the gland can slough away and expose raw tissue underneath.

At the same time, infection can spill into the bloodstream. Watch for fever or a temperature that drops below normal, weakness, wobbliness or collapse, gums that look pale or brick red, fast breathing, vomiting, and refusal to drink. A dog in that state needs emergency care straight away, not a morning appointment.

Puppies are exposed as well. A dam who feels unwell may stop feeding them, and milk from an infected gland is not safe for them to drink. A litter that goes quiet, cold or floppy is its own emergency.

An open or draining wound over a mammary gland should not be probed, squeezed or managed at home. Keep the dog quiet and comfortable, stop the puppies accessing that gland if you can do so safely, and get her seen.

Hard lumps: how mastitis is told apart from other causes

A hard lump in the mammary chain is not automatically an infection, and this is exactly where owners benefit from a professional opinion rather than an internet guess.

What you notice	How it tends to present	Why an exam settles it
Infected gland (mastitis)	Appears over hours or days, hot, firm, painful, often with discoloured milk or discharge, dog may feel unwell	Infection can spread quickly and the dog needs assessment the same day
Milk build up without infection	Firm, uncomfortable glands around weaning or after a litter is lost, often on both sides, no fever	Difficult to separate from early infection by feel alone
Abscess	A pocket that feels softer or squashy in the centre, may be hot, may burst and drain	Needs veterinary handling, never home drainage
Mammary mass or tumour	Usually slower growing, often neither hot nor painful, may be a single firm nodule present for weeks	A vet rules this in or out with sampling; a painless lump is not a reassuring lump
Skin lump, cyst or insect reaction	Sits within the skin rather than deep in gland tissue, tends to move with the skin	Only an exam can place it accurately

Only a needle sample or biopsy, read in a laboratory, can say what a lump is actually made of. Age, spay status, whether she has nursed and how quickly the swelling appeared all shift what is likely, and none of them are conclusive on their own. In an older dog who is not nursing, a mammary tumour is one of the things a vet will want to rule out before anything else. If you are reading around other canine problems, the conditions library covers them in the same plain terms.

Key Takeaways

• Mastitis means inflammation, nearly always with infection, in one or more mammary glands.
• Nursing dogs are affected most often, but false pregnancy, injury and bloodborne infection mean non pregnant and spayed dogs can develop it too.
• Early signs are subtle: one gland slightly warmer or firmer, mild tenderness, a teat the puppies avoid, a litter that stops gaining.
• Typical appearance is a swollen, tight, discoloured gland that is larger than the one opposite, sometimes with crusting or an open draining area.
• To the touch it feels hot and firm, from doughy swelling to a hard lump, and the dog reacts because it hurts.
• Dark purple or black skin, a foul smell, an open wound, fever, collapse or pale gums mean emergency care now.
• Photographs online mostly show advanced cases and colour change is hidden on dark or heavily coated dogs, so compare left to right on your own dog.
• A hard lump is not automatically infection, and only sampling can identify it.

Your veterinarian owns the diagnosis and the plan here. They can examine the gland, work out whether milk is simply backed up or genuinely infected, decide what needs sampling, judge how the puppies should be managed, and handle pain properly. Recovery timelines and how a dog responds vary from dog to dog, so talk to your vet about what you are seeing, bring your daily photos, and go back sooner than scheduled if the swelling, colour or her general demeanour changes.

Related reading: Dog Emergency Signs.