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Decoding your mastitis test results

As we see an increase in targets to reduce antibiotic usage, assisting in making appropriate culling decisions, and keep milk in the vat, it’s becoming more and more useful to diagnose clinical mastitis cases.

Having a diagnosis allows us to understand what bacteria we are working against, the prognosis for the cow long term, reduce antibiotic waste, and keep milk in the vat.

There are a number of services available to diagnose clinical mastitis cases; we are very supportive of the increased Mastatest usage on farm and continue to provide a service sending sterile milk samples to the lab to be cultured and diagnosed.

How to collect a sterile milk sample

-          This must be collected before the cow has had ANY antibiotic treatment

-          Clean the affected teat with a teat wipe

-          Discard the first 2-3 squirts of milk onto the ground

-          Using a sterile pottle a couple of inches from the teat end, collect 1-2 squirts of milk

-          Label the pottle with the cow number, affected quarter, and date of collection

-          The sample can be stored in the fridge if coming into the clinic or being tested that day, or frozen if holding


Now that the bacteria has been diagnosed, we can make a decision around treatment.

Mastitis causing bacteria are classed as either gram-positive or gram-negative, and each require a different treatment approach.

Gram-negative mastitis bugs most commonly include E.coli, Klebsiella, Serratia and other Enterobacter. Gram-negative bugs have an outer wall that prevent intra-mammary antibiotics from being effective against them. Therefore, gram-negative cases are best treated with twice daily stripping of the quarter, teat spray, and Ketomax to reduce swelling and pain (nil milk withhold) daily until resolved. This is a good example of how a diagnosis can reduce wasted antibiotics and the associated costs.


Gram-positive bugs commonly include streptococcus (most commonly strep. uberis) and staph aureus. These both require a different approach; Strep. uberis bugs come from the environment so may be seen more commonly early in lactation, and typically respond well to penicillin based intra-mammary treatments like Intracillin.

Staph aureus cases can be a bit more tricky but aren’t necessarily a death sentence. Here’s how to approach them:

  • About 26% of first time, young staph cows can be cured with intra-mammary treatment. Requesting a sensitivity test on your cultured milk sample will give the best indication of what product to use, but as a general rule Orbenin LA or Penclox (both containing cloxacillin) are good options. For any cow diagnosed with S. Aureus, we recommend this treatment plan in an attempt to get a cure. Be sure to give an extended 5 day course to S. Aureus cows. Giving anti-inflammatories like Melovem or Ketomax also help reduce swelling, allowing the antibiotic to better penetrate the affected tissue.

  • For any cows that have had S. Aureus during the season, especially those that have not cured to intra-mammary treatment, we recommend drying off with the long acting dry cow therapy Cepravin. There is evidence to support a 60% cure rate in these cases. Herd test data is very useful in identifying chronically infected cows, or those that have failed to cure. Consistently high SCC even with no clinical mastitis can indicate S. Aureus, and these cows should be treated with Cepravin at dry off. Any Cepravin-treated cows that start the next season with high SCC most likely failed to cure and should be culled to prevent transmission within the herd.

Please don’t hesitate to reach out if you’d like to discuss any individual cases or herd level issues. We are currently booking in our milk quality consults, which provides us with a great opportunity to identify the best steps forward through the dry season and discuss any milk related issues that may have arisen this season.  


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