Words by: Lisa Whitfield

A vast array of products are available for the treatment of mastitis in dairy cattle. From the red box to the pink box, the green box, the blue box to the black boxes, everyone has their favourite product.

There is the box you reach for to cure those pesky spring clinical cases, the box you reach for to treat that millionaire from the herd test, and the extra strong box to treat those hard to cure cases.

In terms of the agents commonly causing mastitis in New Zealand, we are relatively unusual in the world. Our pasture-based farming system and island-isolation means that we have a relatively small number of bacterial types causing the majority of our mastitis problems, with few non-bacterial causes. Streptococci, Staphylococci and E.coli bacteria cause more than 90% of clinical cases of mastitis here.

The following table presents antibiotic products which are available for use in the treatment of mastitis in dairy cattle in New Zealand. The table classification is organised based on guidelines presented in “Antibiotic Judicious Use Guidelines – Dairy” produced by the New Zealand Veterinary Association, published in September 2018.

The NZVA traffic light classification system is based on but differs from World Health Organisation guidelines. As discussed in Jane Lacy-Hulberts’ article this month, the NZVA classification system provides advice to help us make prudent decisions on the use of antimicrobials in veterinary patients.

So bearing in mind the most likely bacteria that we are treating – are the first line therapies any good? Are we able to prudently use a ‘green light’ antibiotic for the average case of mastitis?

Yes! Thankfully, Streptococci show very good susceptibility to treatment with ‘green’ antibiotics. Staphylococci are slightly more difficult. This group contains both Coagulase-negative Staphylococci (CNS) as well as Staph aureus.

Clinical cases of CNS are generally well covered by treatment with ‘green light’ antibiotics however some farms have Staph aureus strains (~30%) which are resistant to ‘green light’ antibiotics. Staph aureus mastitis, while less commonly causing clinical cases, requires a more broad management plan to be developed. You can easily find out whether you have a susceptible strain of Staph aureus on your farm by getting a few milk samples cultured, and asking for the antibiotic sensitivity if a Staph aureus bacteria is grown.

E.coli mastitis is interesting in that it can present as a very mild clinical case, through to cases where the cow is extremely sick. The use of antibiotics to treat E.coli mastitis is controversial. Mild cases will recover despite antibiotic therapy. In a very sick cow, fluid therapy and non-steroidal anti-inflammatories are critical therapies to save the cow – these cows should usually be assessed by a veterinarian.

So onfarm, while practically speaking you are not going to know the bacterial agent causing mastitis in the cow in front of you when you initially want to treat her, reaching for a ‘green light’ antibiotic is a good choice for the majority of clinical cases, regardless of the time of year, on most farms. If you have penicillin-resistant Staph aureus on your farm, you will need to develop a broader management plan to control this bacteria. Work with your veterinarian to develop a mastitis control plan, do some milk cultures so you get to know the mastitis-causing bacteria on your farm, and have a serious discussion about which product you are reaching for as your first-line therapy.