News & Advice

What’s the big deal with subclinical mastitis?

Sep 1, 2022 | Dairy, Milk Quality

Subclinical mastitis occurs on every farm and is much more common than clinical mastitis. But it’s tricky because it is much harder to detect.

Subclinical mastitis affects your bottom line by reducing production and decreasing milk quality i.e., increasing bulk milk somatic cell count (BMSCC). It can often go on for a long period of time, ultimately having long term effects on milk yield and somatic cell count (SCC) in individual cows or at the herd level. Individual cows can get subclinical mastitis at any time during the season, but herd level problems tend to occur later in the season as more cows become infected and production levels drop.

If you’re aiming for high milk quality for the season to qualify for your suppliers’ reward programmes e.g. Fonterra’s Co-operative difference, it is really important to keep on top of subclinical cows and other factors that contribute to BMSCC to achieve “Milk Quality Excellence” and be rewarded! We know that cell count will always rise later in the season, so it is important to be starting low now so the increase in cell count doesn’t hurt later. 


How is it different to clinical mastitis?

A high cell count occurs when white blood cells move to the udder to defend it against infection. It is this movement of white blood cells which causes the rise in cell count. It is completely normal for there to be cells in the udder of a normal cow but an increase above normal occurs due to mastitis, stress, disease or trauma. Lower milk volumes also mean a rise in somatic cell count as there is less dilution. So an increased somatic cell count in a cow, doesn’t always mean she is infected with mastitis.

Okay, so you’ve got a higher BMSCC than you would like but don’t have clinical cows that have been contributing to this– time to find some high somatic cell count subclinical cows!

Subclinical mastitis often goes undetected for a long time because it is not as easy to identify affected cows. To find cows with subclinical mastitis you need to use tools to help as there are no clinical signs in the cow to help you.

1. Rapid Mastitis Test (RMT) paddling

2. Herd test individual somatic cell count results


Identification of infected cows and quarters is the first step

The big question – what do I do with subclinical cows?

So, you’ve found some RMT positive cows in the herd and/or used your herd test records to identify high SCC cows. Now what?

Mark and record these cows. Ideally keep them separate to prevent bugs spreading to healthy cows. If you are grading, or close to grading, these cows should be removed from supply, as they are a big factor in increasing the bulk tank SCC.

It’s important to know what bugs (if any) are causing subclinical mastitis as this will help guide treatment decisions. Cows with subclinical mastitis may get chronic infections and can maintain a reservoir of infection for the rest of the herd and increase exposure of healthy “clean” cows to contagious bugs. Identifying the cause can go a long way to improving milk quality in your herd.

Take sterile milk samples. Take samples from RMT positive quarters. If all quarters are positive, you can squirt milk from all four quarters into the same pottle (composite) or take samples from all four quarters individually. If you are taking samples from cows identified as high on herd test, you can either RMT the cows to find the offending quarter(s) or take a composite sample.  

Once you’ve taken your samples, refrigerate and submit them ASAP to your clinic for testing or if you can’t get to the clinic that day, freeze and submit them at your next convenience. Remember that it becomes more economical to test samples in bulk (≥ 9 samples), so you may wish to freeze samples until you have a bigger batch to test at the same time. Culture results from milk samples will help your vet to identify potential reasons for subclinical mastitis and help limit the spread of infection.

In general, it is not recommended to treat subclinical cows so it is important to wait until the lab results come back before starting treatment. Your vet might make recommendations to treat certain cows based on the causative bug, cow age, SCC history as well as previous Dairy Antibiogram results for your farm. 


Options based on test results decided by you and your vet will be:

  1. Do nothing – let the cow’s immune system deal with it
  2. Treat with an anti-inflammatory only 
  3. Treat with antibiotics (± an anti-inflammatory)
  4. Dry off the quarter
  5. Cull the cow

Knowing which bugs are causing issues on your farm can help identify areas to work on e.g. teat spraying practices may need to be improved or perhaps there is a contagious Staph aureus problem in your herd.


What bugs might you find?

Staph aureus – contagious mastitis bug that can hide away in the udder tissue and cause nasty clinical mastitis.

Strep uberis – the classic “mud bug” – usually clinical but can be subclinical.

Coagulase Negative Staph (CNS) – teat end bug that mostly causes subclinical mastitis that can become chronic.  Proper teat spraying usually takes care of this.

Corynebacterium – a bug that likes to live on teat skin – rarely causes clinical mastitis and usually self-cures. Proper teat spraying usually takes care of this.

There are many other bugs that can cause subclinical mastitis. Without sampling, we have no way of knowing what the best course of action for the individual cow or herd.


What if it’s a no growth?

A ‘no growth’ result means no bug was able to be cultured. This can be because the bug has been killed already by the cow’s immune system (self-cure) or the bug may be difficult to culture even if it’s still there (sometimes the case with Staph aureus). It can also mean there was no bug to begin with and the reason for a cow to have a high SCC is due to stress, disease, teat or udder injury, or even low milk volumes later in the season.

Don’t wait until it’s too late. Early identification and intervention are key. Make sure you are on top your bulk milk somatic cell count – don’t let it get on top of you!


Anexa resources you might find helpful:

Anexa Milk Quality Resources and Services

How to collect a milk sample.pdf 

How to Rapid Mastitis Test.pdf

Hand Hygiene.pdf


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