CONDITION
Mastitis
Mastitis is inflammation of one or more mammary glands, most commonly seen in female dogs and cats during lactation or in the period shortly after weaning. The affected gland may become swollen, warm, firm, or discoloured, and milk expressed from it can appear abnormal—thickened, bloodstained, or otherwise changed in appearance. Owners often notice these changes when a nursing mother seems uncomfortable, when puppies or kittens appear restless or are not gaining weight as expected, or when routine handling reveals a gland that feels different from the others. Mastitis can range from mild localised changes that resolve with supportive measures to more significant inflammation that affects the mother's comfort and the offspring's ability to feed. In some cases, signs may extend beyond the gland itself—the nursing animal may seem quieter, less interested in food, or reluctant to allow feeding. This page explores the signals that can accompany mastitis, the biological processes that contribute to inflammation in mammary tissue, the ways in which the condition is investigated, and the range of approaches used to support affected animals and their offspring during the nursing period.
Why this matters now
Mastitis most commonly appears during lactation, typically in the first few weeks after birth when milk production is at its peak and the mammary glands are under greatest demand. The condition can also occur during the weaning period, when changes in nursing frequency alter the balance between milk production and removal. Whilst any nursing dog or cat can develop mastitis, it tends to be seen more often in animals with large litters, those experiencing their first lactation, or in cases where milk removal is incomplete—whether because offspring are not feeding evenly from all glands, because one or more puppies or kittens have been lost, or because weaning is happening quickly.
The course of mastitis varies considerably between individuals and depends on factors including the extent of initial inflammation, the presence or absence of infection, and how quickly the imbalance between milk production and removal is addressed. In some cases, mild localised swelling and discomfort may settle over a few days as nursing patterns adjust or supportive measures are introduced. In others, inflammation may progress, with the affected gland becoming more painful, the milk increasingly abnormal, and the nursing animal showing broader signs of being unwell. Occasionally, more significant changes develop within the gland tissue itself, and these may take longer to resolve even after the acute phase has passed.
Signals & patterns
Early signals
One gland firmer than others
During routine handling or when puppies or kittens are feeding, one mammary gland may feel noticeably harder or more swollen than its neighbours. This can reflect early inflammation and fluid accumulation within the tissue.
Localised warmth or redness
The skin over the affected gland may feel warmer to the touch than surrounding areas, and in animals with pale or lightly pigmented skin, a degree of redness or colour change may be visible. These changes indicate increased blood flow as part of the inflammatory response.
Discomfort when gland is touched
The nursing animal may flinch, pull away, or vocalise when the affected gland is handled, or may position herself in ways that discourage offspring from latching onto that particular teat. This suggests the gland is tender or painful.
Milk appears thickened or discoloured
If a small amount of milk is expressed from the affected gland, it may look different from milk from other glands—cloudier, more viscous, tinged with blood, or containing flecks or strands. This reflects changes in the composition of the secretion as inflammation develops.
Puppies or kittens avoid one gland
Offspring may instinctively avoid nursing from a gland that is uncomfortable for the mother or producing abnormal milk, leading to uneven feeding patterns. This can sometimes be the first sign an owner notices.
Later signals
Mother appears quieter or less engaged
As inflammation progresses, the nursing animal may become less active, show reduced interest in food, or seem reluctant to interact with her offspring in the usual way. These changes can reflect discomfort or a broader systemic response to inflammation.
Gland becomes visibly swollen or darkened
The affected gland may enlarge further and take on a dusky or bruised appearance, particularly if blood flow to the tissue is compromised or if inflammation is more extensive. The overlying skin may feel tight or appear shiny.
Offspring not gaining weight as expected
If one or more glands are producing little or abnormal milk, and if the litter is large or nursing is otherwise uneven, puppies or kittens may fail to gain weight at the expected rate, or may seem restless and vocal between feeds.
Click to read about the biological mechanisms
How this is usually investigated
Options & trade-offs
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