Clinical vs. Subclinical Mastitis: What Dairy Farmers Need to Know

Clinical mastitis gets attention because you can see it. Subclinical mastitis often costs more because you cannot. Knowing the difference helps you act on both.

Key points

Clinical cases show; subclinical cases are found by testing.
Subclinical infections often cost more because they go unnoticed.
Sort cows by cell count history before deciding what to do.
Milking routine, bedding and equipment affect both forms.
Follow withdrawal times exactly for any treated cow.

Two forms of the same problem

Both forms are infection and inflammation of the udder. Clinical mastitis produces visible signs: abnormal milk, a swollen quarter or a sick cow. Subclinical mastitis looks like a normal cow giving normal-looking milk, but tests show an immune response in the udder.

Because most herds have far more quiet cases than loud ones, the subclinical form is where much of the lost milk and quality premium trouble comes from. Check with your milk buyer on how quality is measured and paid.

What clinical mastitis looks like

Watch for flakes, clots, watery or discolored milk. The quarter may be hot, firm and sore. In more serious cases the cow runs a fever, stops eating and may lie down and refuse to get up.

Clinical cases are handled with your vet's guidance. Ask your vet to help you set a plan for mild, moderate and severe cases so milkers know what to do at three in the morning.

Abnormal milk in one or more quarters
Hot, swollen or hard quarter
Drop in milk and appetite
Fever or a dull, depressed cow
Cow that is down and will not rise

How subclinical mastitis hides

There are no obvious signs at the udder. The clues come from the numbers: somatic cell count on the test day report, bulk tank cell count, and screening tests your vet may recommend. A cow with repeated high counts is likely carrying infection even if she looks fine.

Subclinical infections can last a long time and the infected cow acts as a source for others, usually at milking. That is why the milking routine matters so much.

Using testing to sort your cows

Individual cow cell counts help you sort the herd into groups: cows that have always been low, cows that are newly high and cows that stay high. Each group needs a different plan.

Ask your vet which cows are worth culturing. Culture results can show whether the problem is a contagious type, spread from cow to cow, or an environmental type, coming from bedding and wet areas.

Low all lactation: keep doing what you do
Newly high: recheck and consider culture
Chronically high: discuss with your vet, since some cows are best managed by separate milking or culling
Heifers high at first test: look at pre-calving housing and flies

Controlling both forms

The basics are the same for clinical and subclinical cases. Use clean, dry bedding. Prepare teats consistently before attaching the unit. Dip teats after milking with a product your vet or milking equipment dealer supports. Keep milking equipment tested and in good working order. Milk known infected cows last or with a separate unit.

Dry cow management and fresh cow care also set up the next lactation. Cows that go dry with an infection often start the next lactation with trouble.

What to ask your vet and milk buyer

Bring your last several test day reports and your bulk tank results to a herd visit. Ask your vet to help you sort cows by history and to explain which ones are worth culturing. Ask your milk buyer how quality is measured and paid, since the rules differ and change over time.

The cost of subclinical mastitis is rarely a single event. It shows up as slightly less milk from many cows over many weeks, plus extra culling and a bulk tank that drifts upward. Write down your bulk tank trend and review it with your vet each visit, so you notice drift before it becomes a quality problem.

Which cows are probably carrying quiet infections?
Is our problem contagious, environmental or both?
What cut points should we use for cell count in our herd?
Which cows should be milked last?
How do heifers compare with older cows?

Common mistakes

Treating only the cows you can see while ignoring the cell count report is the usual error. Others include moving infected cows through the same milking order as clean cows and using dirty towels from one cow to the next.

Keep treated cows' milk out of the tank for the full withdrawal time on the label or set by your vet. Never shorten it to save milk.

When to get help: Ask your veterinarian to review your cell count reports and set a written case plan. Contact your milk buyer or extension dairy specialist if bulk tank quality is slipping and you cannot find the cause.

Sources and further reading

Organization homepages are linked so you can search them for current guidance on this topic.

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