Subclinical Mastitis in Cows: How to Catch It Early

Subclinical mastitis is udder infection without any visible sign — the milk looks normal, the udder feels normal, but somatic cell count is raised and milk yield is quietly falling. On most Indian dairy farms it is several times more common than clinical mastitis, and the only way to find it is to test for it: a cowside California Mastitis Test (CMT) on every quarter, or a laboratory somatic cell count. Catching it early protects yield, shortens treatment, and stops the infection spreading through the milking line.

What subclinical mastitis actually is

Mastitis is inflammation of the udder, almost always caused by bacteria entering through the teat canal. In clinical mastitis you can see it: clots or flakes in the milk, a hot swollen quarter, watery secretion, sometimes fever. In subclinical mastitis the same infection is present, but the cow's immune response has contained it. Milk looks and strips normally. What has changed is inside the milk — the number of somatic cells (mostly white blood cells sent to fight the infection) has risen sharply.

Because there is nothing to see, the quarter keeps being milked as usual, the farmer keeps expecting full yield, and the bacteria keep multiplying and passing to other cows on milkers' hands, cloths and cluster liners.

Why it costs more than clinical mastitis

• Silent yield loss. Damage to milk-secreting tissue reduces output from the affected quarter, and the loss is permanent for that lactation.
• Poorer milk quality. Higher SCC is associated with lower casein, reduced shelf life and poorer curd yield — relevant if you sell to a co-operative that pays on quality, or make paneer, curd or ghee. (See our guide on how milk is priced on fat and SNF.)
• It becomes clinical later. A subclinical quarter is a reservoir. Under stress — calving, heat, monsoon mud, a missed milking — it flares into clinical mastitis.
• It spreads. Contagious organisms such as Staphylococcus aureus and Streptococcus agalactiae move cow-to-cow during milking.

How to detect subclinical mastitis on your farm

1. California Mastitis Test (CMT) — the practical cowside test

The CMT is the single most useful tool a farmer can keep in the milking shed. It is cheap, takes under a minute per cow, and needs no laboratory.

• Discard the first two or three streams from each teat (foremilk).
• Draw about 2 ml of milk from each quarter into the four cups of the CMT paddle.
• Add an equal volume of CMT reagent.
• Swirl gently in a horizontal circular motion for about 10 seconds and read immediately — the reaction fades after roughly 20 seconds.

2. Somatic cell count (SCC)

If your co-operative or a nearby veterinary college laboratory offers SCC testing, use it. As a working guide, a composite cow SCC below about 200,000 cells/ml suggests an uninfected udder; consistently above that level suggests infection in at least one quarter. Bulk-tank SCC tells you how the whole herd is doing and is the cheapest herd-level monitor available.

3. Strip cup and electrical conductivity

A black-plate strip cup at every milking catches early clinical cases. Handheld electrical conductivity meters detect the ionic changes of inflammation and can flag a quarter before CMT does, though they are less specific.

Reading CMT results

• Negative: mixture stays liquid — quarter is very likely uninfected.
• Trace: slight slime that disappears on continued swirling — borderline; re-test in a week.
• 1 (weak positive): distinct thickening, no gel — subclinical infection likely.
• 2 (distinct positive): mixture thickens and moves towards the centre of the cup — clear subclinical infection.
• 3 (strong positive): a gel forms and a central peak stands up — heavy infection; treat as a priority case.

What to do when a quarter tests positive

1. Mark the cow and milk her last. Milking order is free and it works — positive cows at the end of the line stop passing bacteria to clean cows.
2. Do not self-medicate with intramammary antibiotics. Ask your veterinarian to sample the quarter for culture and sensitivity where possible. Blind antibiotic use during lactation drives resistance, risks residue in milk, and often fails against the organism you actually have.
3. Fix the cause, not just the cow. Check liners, vacuum level and pulsation if you use a machine; check hand-milking hygiene if you do not.
4. Consider dry-cow therapy at drying off, under veterinary guidance. The dry period is the best window to clear an established subclinical infection. See our guide to dry period management.
5. Cull the chronic. A cow positive at high CMT score across several months, unresponsive to treatment, is usually costing more than she earns.

Preventing subclinical mastitis: the routine that works

Milking hygiene

• Wash hands with soap before milking and between cows; use a separate clean cloth or paper towel per cow for udder drying.
• Pre-dip or wash teats, then dry them completely — a wet teat is a highway for bacteria.
• Post-milking teat dip with an approved iodine or chlorhexidine dip after every milking. This is the highest-return single change most Indian farms can make.
• Milk out fully. Residual milk in the quarter feeds bacteria.
• Keep cows standing for 20–30 minutes after milking (offer fresh feed) so the teat sphincter can close before she lies down.

Housing and bedding

• Keep the standing and lying area dry; environmental organisms such as E. coli and Klebsiella multiply in wet dung and mud. This matters most in the rains — see preventing monsoon mastitis.
• Clean and lime the floor daily; slope it so urine and wash water drain away.
• Control flies, which carry organisms to teat ends and udder sores.

Nutrition and routine udder support

Udder defence depends on the cow's overall condition. Adequate energy, protein, and trace minerals — particularly zinc, copper, selenium and vitamin E — support teat-end keratin and neutrophil function. A balanced chelated mineral supplement such as Cattle Remedies Verymin helps close routine trace-mineral gaps in Indian rations; see our guide to choosing a mineral mixture.

For herds with a recurring udder-health problem, Cattle Remedies Uddicin-H is a herbal udder-health supplement used alongside — never instead of — good milking hygiene and veterinary treatment. Our buyer's guide to udder health supplements for cattle in India explains what to look for.

A simple monitoring calendar

• Every milking: strip cup on the first squirts.
• Every month: CMT on all quarters of all lactating animals. Record scores by cow and quarter.
• At drying off and after calving: CMT every quarter, without exception.
• Quarterly: bulk-tank SCC if available.

When to call the veterinarian

Call immediately for a hot, hard or swollen quarter, watery or blood-tinged milk, a cow that is off feed or febrile, or any animal that goes down. Also call if CMT scores across the herd are rising month on month — that points to a milking-routine or machine problem that needs professional assessment rather than more medicine.

Frequently asked questions

Can I detect subclinical mastitis without any test kit?

Not reliably. That is exactly what makes it subclinical — the milk and the udder look normal. Rising SCC or a positive CMT is the only dependable signal. A gradual, unexplained drop in a cow's yield, or a quarter that gives noticeably less than its neighbours, is a reason to test, not a diagnosis on its own.

How often should I run a CMT?

Once a month on every lactating animal is a realistic and effective routine for a small or medium Indian dairy, plus a compulsory test at drying off and again within a week of calving. Weekly testing is worthwhile in a herd with a known mastitis problem.

Is milk from a subclinically infected cow safe to sell?

If the cow is not under antibiotic treatment, the milk is not unsafe, but it is lower quality — higher SCC, altered composition and shorter shelf life — and may be penalised where payment is quality-linked. If the animal is on any antibiotic or intramammary treatment, observe the withdrawal period your veterinarian specifies and discard that milk.

References and further reading

• Peer-reviewed research on bovine subclinical mastitis prevalence and control: PubMed Central
• Indian Council of Agricultural Research — animal science research and extension material: icar.org.in

This article is general educational information for livestock keepers. It is not a substitute for veterinary diagnosis or treatment. Always consult a registered veterinarian for a sick animal, and observe milk withdrawal periods for any medicine used.