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Subclinical mastitis
Subclinical mastitis is an infection of the udder that cannot be seen. The milk appears normal, the area is not hot, hard or painful, the cow is eating and behaving as usual. However, inflammation is well established inside the udder. Only...
What is it?
Subclinical mastitis is an infection of the udder that cannot be seen. The milk appears normal, the area is not hot, hard or painful, the cow is eating and behaving as usual. However, inflammation is well established inside the udder.
Only two clues betray its presence: milk production which drops without explanation, and an increase in the number of cells in the milk. This is why it is not detected by eye but with tools.
It is by far the most common form of mastitis in dairy herds: for one cow with visible mastitis, several others are infected without anything showing it. It constitutes the silent reservoir which fuels clinical mastitis in the herd.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
No visible signs on the udder: no swelling, no redness, no heat, no pain on palpation.
Normal-looking milk, without lumps or color change, including the first jets.
Gradual, unexplained decline in quarter or cow production, often the only thing the farmer notices.
Increased cell count, on individual milk recording analysis or on tank milk.
Positive reaction to the field test carried out at milking, with formation of a gel in the cup corresponding to the affected quarter.
At the herd level: tank milk whose cell count rises, visible mastitis which increases, deterioration in the quality of the milk delivered.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Bacteria enter the udder through the teat canal and settle there without causing a reaction strong enough to be visible.
Certain bacteria live from one udder to another and are transmitted especially during milking: they are the ones that most often give rise to long-lasting and invisible infections, and which cause the cells of the entire herd to rise.
Others come from the environment — litter, dung, stagnant water, mud from exercise areas — and enter between two milkings.
A teat whose sphincter remains open after milking, or whose skin is damaged, leaves the door open to bacteria.
A poorly adjusted or poorly maintained milking machine damages the teats and makes it easier for bacteria to pass from one cow to another.
Lack of regular cell monitoring: without screening, the infection remains unknown.
Uncontrolled milking machine, worn sleeves, unsuitable settings.
Insufficient milking hygiene: poorly prepared teats, collective wiping, bare and unwashed hands between cows.
Lack of disinfection of teats after milking.
Poorly managed dry-off, without protection of the udder during the dry period.
Damp housing, soiled litter, muddy waiting areas.
Cows with injured teats, teat tip lesions, hyperkeratosis.
Older cows and cows that have already had mastitis.
Heat and humidity, which encourage the multiplication of bacteria in the litter.
Transmission
Transmission from one cow to another occurs mainly during milking: the milker's hands, rags or cloths used for several cows, milking liners passing from one udder to the next.
The milk droplets that remain in the sleeves carry the bacteria to the next quarter.
Infected cows without visible signs are the main sources of contamination of the herd, precisely because there is no indication that they should be milked last.
Environmental bacteria are not transmitted from one cow to another but from the litter and the ground to the udder, between milkings.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Screening is based on two simple and complementary tools.
Cell counting: on tank milk, it gives an image of the udder health of the entire herd; on individual milk, as part of milk control, it designates the cows concerned.
The field test carried out at milking, with a reagent and a cup with four compartments, allows us to go further: it indicates which quarter is affected, immediately, without a laboratory. Milk that gels indicates an infection.
Once the cow and the neighborhood have been identified, the veterinarian can have a milk sample analyzed in the laboratory to determine the bacteria in question. This sample must be taken properly, before any treatment, on a disinfected teat.
This identification is decisive: it indicates whether the herd has a contamination problem during milking or an environmental problem, and therefore what measures to take.
Treatment
There is no automatic treatment: treating all high-cell cows in full lactation is neither effective nor reasonable.
The veterinarian decides, cow by cow, whether to treat during lactation, treat when drying off, or cull a long-term infected cow which is maintaining the problem.
Drying off treatment is often the most effective time to clean an udder, because the cow does not produce and the product stays longer in the area.
The antibiotics used are chosen according to the bacteria identified, which requires prior analysis and a prescription.
Treating without identifying the cause leads to failures, relapses and promotes resistance.
After any treatment, the waiting period for milk and meat set by the veterinarian must be respected without exception: the milk must not be delivered or consumed before the end of this period.
At the same time, infected cows must be milked last, with equipment that is then disinfected.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Milking hygiene and properly managed drying off are the heart of mastitis prevention. Everything else comes next.
Prepare each teat: clean, eliminate the first jets in a bowl with a black bottom, then wipe with a paper or a clean cloth per cow - never the same cloth for the whole herd.
Disinfect the teats after each milking with a suitable product.
Wash your hands and wear gloves while milking.
Milk cows known to be infected last.
Have the milking machine checked regularly and replace the sleeves as recommended.
Carry out the dry-off methodically: complete stop of milking, protection of the udder decided upon with the veterinarian, clean housing and monitoring for the first days.
Keep cubicles and bedding dry and clean, avoid mud in circulation areas.
Follow the cell count of the tank and the cows: it's your dashboard.
Cull chronically infected cows that remain a source for the herd.
Possible complications
Transition to visible mastitis, sometimes serious, from a silently infected quarter.
Progressive destruction of udder tissue and permanent loss of quarter production.
Decrease in the quality of milk delivered, collection penalties, less suitability for processing.
Continuous spread of infection to the rest of the herd with each milking.
Premature culling of otherwise healthy cows.
When to see a veterinarian
Your tank's cell count is increasing without you seeing any mastitis.
A cow produces less for no apparent reason.
The field test reacts on one or more neighborhoods.
Visible mastitis is increasing in the herd.
You don't know how to interpret your cell results: ask the veterinarian to analyze the herd's situation.
Before treating a high-cell cow: have the bacteria identified rather than treating blindly.
Prognosis
The outcome depends a lot on the bacteria involved: some infections clear up well, others are resistant and persist from one lactation to the next despite treatment.
The chances of recovery are better in young cows, in recent infections, and during dry-off treatment.
A neighborhood that has been infected for a long time can permanently lose part of its production capacity, even after the bacteria has been eliminated.
At the herd level, a plan combining milking hygiene, machine control, dry-off management and cell monitoring makes it possible to sustainably reduce the infection rate.
Frequently asked questions
How do you know that a cow has subclinical mastitis if the milk is normal?
Solely by screening: milk cell counting, individual or tank, and field test carried out at milking with a cup. To the eye and the hand there is nothing to see or feel.
Can this milk be delivered?
Milk from an untreated cow is not removed from the collection, but it increases the cell count in the tank and degrades the overall quality. As soon as a treatment is administered, milk must be kept aside for the duration of the waiting period set by the veterinarian.
Should all high cell cows be treated?
No. Treatment is decided on a case-by-case basis with the veterinarian, after identification of the bacteria. Many situations are better resolved by drying off or by hygiene measures than by treatment during full lactation.
How does this sheet differ from usual mastitis?
Visible mastitis can be seen in the udder and milk; the subclinical form is only visible during analyses. These are the same infections, but this one goes unnoticed and silently maintains the problem in the herd.
References
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 09 August 2026
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