Somatic cell count (SCC) is a measurement of the number of white blood cells and tissue cells present in a milliliter of milk. It serves as the primary indicator of udder health in dairy animals and directly affects milk quality, shelf life, and processing. A healthy cow typically produces milk with fewer than 100,000 cells per milliliter, while counts above 200,000 signal a likely infection.
What Somatic Cells Actually Are
The word “somatic” simply means “of the body.” In milk, somatic cells are a mix of immune cells and a small number of cells shed from the lining of the udder itself. Those lining cells usually make up less than 15% of the total count. The rest are immune cells: macrophages (which engulf bacteria), lymphocytes (part of the adaptive immune system), and neutrophils (the first responders to infection).
In a healthy cow, macrophages are the dominant cell type. They patrol the udder at low levels as part of normal immune surveillance. When bacteria invade, the body floods the area with neutrophils, and their numbers can spike dramatically within hours. That shift in cell composition is what drives the count upward and signals a problem.
SCC as a Measure of Udder Health
The main reason dairy farmers, veterinarians, and milk processors care about SCC is mastitis, an infection of the udder. Mastitis can be clinical (visible swelling, abnormal milk) or subclinical, meaning the cow looks perfectly fine but bacteria are multiplying inside the gland. Subclinical mastitis is far more common and can only be detected through cell counts or bacterial culture.
The generally accepted threshold for a healthy cow is around 100,000 cells/mL. Once the count crosses 200,000 cells/mL, mastitis is the likely cause, and higher counts correlate with more severe infections. Research published in Veterinary World found that a threshold of 310,000 cells/mL provided the best balance of accuracy for identifying subclinical mastitis at the individual udder quarter level, correctly flagging infected quarters about 92% of the time.
Normal Fluctuations Without Infection
SCC isn’t static, even in perfectly healthy animals. Several non-infectious factors push the count up or down, which is important context for interpreting any single reading.
Stage of lactation is one of the biggest variables. Right after calving, cell counts are naturally elevated. In one study of uninfected cows, counts in the first days after calving averaged 355,000 to 517,000 cells/mL in bucket milk, then dropped rapidly. By day five, no samples exceeded 300,000, and by day ten, counts had fallen below 200,000. Older cows (second lactation and beyond) tend to start with higher counts than first-time mothers, though the difference narrows quickly.
Other factors that temporarily raise SCC in healthy animals include heat stress, recent transportation, changes in milking routine, and the very end of lactation as the udder begins to dry off. A single elevated reading doesn’t necessarily mean infection, which is why regulatory programs look at patterns across multiple samples.
Legal Limits in the U.S. and EU
Governments set maximum SCC thresholds for raw milk entering the processing chain. In the United States, the Pasteurized Milk Ordinance (the federal standard for Grade A milk) caps individual producer milk at 750,000 cells/mL for cows. Goat milk gets a higher allowance of 1,500,000 cells/mL because goats naturally have more neutrophils in their milk even when healthy. Sheep and camel milk share the 750,000 limit.
Enforcement works on a rolling basis. If two of the last four consecutive SCC tests exceed the limit, the producer receives a warning. Three violations in the last five tests trigger an immediate permit suspension, and the herd’s milk supply must test within acceptable limits before the permit is reinstated.
The European Union sets a stricter standard of 400,000 cells/mL for cow’s milk. This tighter threshold means European herds face more pressure to maintain low counts, and studies using Irish herd data have shown that the method used to calculate rolling averages can shift the percentage of herds in compliance by as much as 11 percentage points.
How High SCC Affects Milk Quality
Elevated cell counts don’t just indicate a health problem. They actively degrade the milk itself. The immune cells flooding an infected udder release enzymes that break down fat and protein. This has measurable consequences for both fluid milk and cheese production.
For pasteurized fluid milk, high SCC accelerates spoilage during refrigerated storage. Fat breakdown (lipolysis) occurs about three times faster in high-SCC milk, and protein breakdown runs roughly twice as fast. Low-SCC milk maintains good flavor and quality through a full 21-day shelf life. High-SCC milk develops noticeable rancidity and bitterness between days 14 and 21, cutting its usable life significantly shorter.
Cheese makers take an even bigger hit. The extra enzyme activity causes more protein to escape into the whey rather than staying in the curd. In cottage cheese production, high-SCC milk yielded about 4.3% less curd than low-SCC milk after adjusting for moisture. Protein recovery dropped from roughly 76% to 74%. That may sound small, but at industrial scale, a few percentage points of lost yield translates to significant revenue. The cheese that does get made also degrades faster during storage, with proteolysis accelerating and texture softening more quickly than normal.
How SCC Is Measured
There are two main approaches to measuring somatic cells: lab-based electronic counting and on-farm screening tests.
Electronic counters, the standard for official regulatory testing, use either fluorescent dye methods or flow cytometry to count individual cells in a milk sample. These give a precise number per milliliter and are the basis for the bulk tank counts that determine whether a farm meets legal standards.
The California Mastitis Test (CMT) is a quick, low-cost screening tool used at the cow’s side. A small amount of milk is mixed with a detergent reagent on a paddle. The detergent breaks open cell membranes and reacts with the DNA inside, forming a gel. The thicker the gel, the higher the cell count. Results are scored on a 0 to 3 scale: a score of 0 means no thickening and a low count, while a score of 3 produces a distinct gel and corresponds to very high counts, well into the range associated with active infection. The CMT doesn’t give an exact number, but it’s useful for quickly identifying which quarter of the udder is affected so treatment or further testing can be targeted.
Why SCC Matters Beyond the Farm
For dairy processors, SCC is a purchasing decision. Many cooperatives and milk buyers pay premiums for milk below certain thresholds (often 200,000 or 250,000 cells/mL) and impose penalties as counts climb toward the legal limit. This creates a direct financial incentive for farmers to manage udder health aggressively.
For consumers, SCC is an invisible quality marker. You won’t see it on a milk label, but it influences how long your milk stays fresh, how your cheese tastes, and how efficiently dairy plants operate. Countries and regions with stricter SCC standards generally produce milk with longer shelf life and better processing characteristics. The gap between the U.S. limit of 750,000 and the EU limit of 400,000 reflects different regulatory philosophies, but the biology is the same: lower counts mean fewer destructive enzymes in the milk and a better product on the shelf.