Mastitis in cows shows up as visible changes in both the udder and the milk, though the signs range dramatically depending on severity. In mild cases, the milk itself may be the only thing that looks off. In severe cases, the udder becomes visibly swollen, red, and hot to the touch, and the cow may stop eating entirely.
What the Milk Looks Like
The earliest visible sign of mastitis is usually abnormal milk. Instead of the smooth, white appearance you’d expect, milk from an infected quarter can look watery, yellowish, or discolored. Flakes, clots, or stringy clumps may appear, especially in the first few squirts (the foremilk). In more advanced infections, the milk may contain visible blood or take on a pinkish tint.
Checking the foremilk before every milking is one of the simplest ways to catch mastitis early. Squirting the first streams onto a dark surface or strip cup makes it much easier to spot flakes or clots that you’d miss against a white background. Normal milk flows freely and looks uniform. Anything chunky, watery, or off-color is a red flag.
What the Udder Looks Like
As the infection progresses beyond the milk, the affected quarter of the udder starts showing physical changes. The classic signs are swelling, heat, redness, and pain when touched. The infected quarter often looks noticeably larger than the others, and the skin may appear stretched or shiny from the swelling. When you touch it, the tissue feels firm or hard compared to the soft, pliable feel of a healthy quarter.
In chronic or long-standing infections, the udder tissue changes permanently. Scar tissue (fibrosis) builds up inside the quarter, and you can sometimes feel hard lumps or nodules through the skin. Over time, a chronically infected quarter may actually shrink and atrophy as the milk-producing tissue is replaced by scar tissue. At that point, the quarter produces little or no milk and feels distinctly different from the healthy ones.
Mild, Moderate, and Severe Cases
The International Dairy Federation classifies clinical mastitis into three severity levels, and knowing the differences helps you gauge how urgently a cow needs attention.
- Mild (Grade 1): The only visible change is in the milk itself. You’ll see differences in color, consistency, or texture, such as flakes or a watery appearance. The udder looks and feels normal, and the cow behaves normally.
- Moderate (Grade 2): The milk is abnormal, and now the udder shows local inflammation. The affected quarter is swollen, warm, hard, red, or painful to the touch. The cow may flinch or kick during milking but otherwise seems fine.
- Severe (Grade 3): The cow is visibly sick beyond just the udder. Signs include fever (or sometimes abnormally low body temperature), loss of appetite, lethargy, and difficulty standing. The udder is often dramatically swollen and painful. This is a systemic infection that can become life-threatening.
How Different Infections Look
Not all mastitis looks the same, partly because different bacteria cause different patterns of disease.
E. coli infections tend to hit fast and hard. The udder becomes red and swollen quickly, and the cow can go from looking fine to running a high fever within hours. These infections are often dramatic but usually short-lived if the cow’s immune system responds well.
Staph. aureus works differently. It doesn’t trigger as strong an immune response, so the signs are subtler. The infection tends to smolder for weeks or months as a chronic, low-grade problem. You might notice a gradual drop in milk production and occasional flare-ups of mild clinical signs, but the cow rarely looks severely ill. This makes it easy to miss, and the bacteria can quietly damage udder tissue over time.
Strep. agalactiae often causes subclinical mastitis, meaning there are no visible abnormalities in the milk or udder at all. The only clue is a rising somatic cell count and declining milk yield. Mycoplasma infections, while less common, are among the most destructive. They damage the milk-producing tissue, cause fibrosis throughout the gland, and can lead to abscesses inside the udder.
Subclinical Mastitis: When There’s Nothing to See
One of the trickiest things about mastitis is that the most common form, subclinical mastitis, has no visible signs at all. The milk looks normal. The udder looks normal. The cow acts normal. But white blood cells are flooding into the milk in response to infection, and milk production is quietly dropping.
The only way to detect subclinical mastitis is by measuring somatic cell count (SCC), which reflects how many white blood cells are present in the milk. Healthy milk typically contains fewer than 200,000 cells per milliliter. Research has identified 310,000 cells per milliliter as an effective threshold for distinguishing subclinically infected cows from healthy ones, with about 92% accuracy in catching true infections.
Using the California Mastitis Test
The California Mastitis Test (CMT) is a quick, low-cost way to check individual quarters without sending samples to a lab. You squirt a small amount of foremilk into each cup of a paddle, add the test reagent, and gently rotate the paddle for 10 to 15 seconds.
What you’re watching for is gel formation. Milk from a healthy quarter stays liquid and flows freely around the paddle. A slight, temporary thickening that disappears with movement suggests a borderline result, roughly 150,000 to 500,000 cells per milliliter. A distinct thickening that doesn’t gel corresponds to a moderate infection (400,000 to 1.5 million cells). When the mixture forms a visible gel with obvious clumps, you’re looking at 800,000 to 5 million cells. The most severe reaction produces a thick, sticky gel that clings to the paddle and forms a peak in the center, indicating over 5 million cells per milliliter.
The greater the infection, the more white blood cells are present, and the thicker the gel becomes. Running CMT on fresh cows, newly purchased animals, or any cow with a sudden production drop helps catch infections before clinical signs appear.
Behavioral Signs in the Cow
Beyond what you can see on the udder and in the milk, an infected cow’s behavior often changes. With moderate mastitis, she may kick at the milking unit, hold her leg up to protect the sore quarter, or shift her weight away from the affected side. She might be reluctant to lie down if the swollen quarter is painful under pressure.
In severe cases, the behavioral shift is more obvious. The cow may stand apart from the herd, stop eating, appear dull or depressed, and move stiffly. Rapid weight loss can occur surprisingly fast with certain aggressive infections. A cow that was eating well yesterday and is now off feed with a hot, swollen udder needs immediate attention.