With only a short list of mastitis-inducing bacteria to deal with, New Zealand dairy farmers can reach for antimicrobials falling under the ‘green’ classification. Veterinarian Lisa Whitfield reports.

A significant knowledge shift is happening on farms around New Zealand, in part due to new systems and technology allowing farmers to rapidly identify which bacteria are causing mastitis cases and which are resulting in high cell count cows on farms. The rapid turnaround of results able to be achieved by these systems means a new standard of care for the individual cow has become achievable. As soon as you know which bacteria you are dealing with, you can have a high level of confidence in how best to manage the affected cow.

This is an achievement farmers should be proud to be part of. It is very clear that there is a large farming community out there wanting to do the right thing – not only by improving the outcome and performance of their cows but also by being responsible guardians of antimicrobials.

For those who are using these culture systems routinely – how do you apply the new knowledge that you have available? Have you been able to adjust your onfarm approach to mastitis management to reflect best practices in antibiotic use? After having collected a milk sample for culturing, what do you do next when managing a new mastitis case in the herd?

We are very lucky here in NZ that we have a short list of mastitis-causing bacteria to deal with. The environmental bacteria – Strep uberis – dominates as the most common cause of clinical mastitis. Staph aureus and E.coli are two other commonly encountered pathogens causing clinical cases. Aside from these three, the other bacteria are relatively infrequently encountered.

In contrast, when it comes to what is available to treat mastitis cases, there is an inexplicably large number of products available, most of which are hard to justify having on the shelf.

When we apply the traffic light system for responsible antimicrobial use to mastitis treatments, antimicrobials falling under the ‘green’ classification should be what you are reaching for first when you choose to treat a mastitis case. So how do the ‘green’ classified antimicrobials stack up in their effectiveness against our most common mastitis causing pathogens?

Strep uberis: ‘Green’ classified antimicrobials are highly effective against Strep uberis, in most cases showing over 95% efficacy. As a first-line therapy, you can be confident that these antimicrobials will do the job.

  • Clots in the milk will often take up to six days to clear up regardless of the duration of treatment – this doesn’t mean the treatment isn’t working.
  • If you see a clear improvement in clinical signs over the first three days of treatment, then extended therapy is probably unnecessary.


Staph aureus: 70% of Staph aureus isolates show sensitivity to ‘green’ classified antimicrobials. In most cases, these antimicrobials are a good first-line therapy, but there are a few other things to think about once you know you are dealing with Staph aureus.

  • Understanding the antimicrobial sensitivity of Staph aureus isolates on your farm is key to whether you need a second-choice antimicrobial on hand.
  • Consider how long the cow has likely had the infection for – chronic infections are a poor choice for antimicrobial therapy. For cows with very new infections, it is possible to achieve good cure rates.
  • Other management strategies to consider include making the cow into a three-titter, or in some cases consider culling the cow.

Coli: 99% of strains of E.coli are resistant to ‘green’ classified antimicrobials, however E.coli is also resistant to most other mastitis antimicrobial treatments as well. When you are dealing with a case of E.coli mastitis it is important to understand:

  1. The cow’s own immune system is very effective at responding to E.coli infection – so in most cases, antimicrobial treatment is actually unnecessary.
  2. Anti-inflammatories play an important role in E.coli mastitis – they help the cow by reducing high fevers and dampening down the strong inflammatory response generated by the dying bacteria. This means she is more likely to want to eat.
  3. Oral Fluid-therapy – which can be readily done on-farm with the right training and equipment – is critical to support the sick E.coli cow while she is unable to hydrate herself.

Having rapid access to knowing which bacteria you are dealing with when treating mastitis is a game-changer. A number of systems now allow you to get results back in 24 hours or less, rather than having to wait for days and getting results back after the cow has already finished her treatment course. When it comes to choosing a treatment for a new mastitis case, you can have confidence that a ‘green’ classified antimicrobial is a really good place to start.

  • Lisa Whitfield is a Manawatu-based production animal veterinarian.