Contagious Mastitis 101

Farm Biosecurity
and Infection Control.

The publicity around outbreaks of exotic Mycoplasma bovis in New Zealand (2017) could be an important learning exercise for farmers about on-farm biosecurity and contagious mastitis.

Most farmers are simply too trusting with access and movements on their farms, especially given that milk production is their livelihood. Contagious mastitis can be caused by several bacteria, and all of them should receive zero tolerance. Whilst some of them are not common, depending on your location, vigilance, education and knowledge around contagious mastitis is becoming increasingly important.

“The cornerstone of identifying contagious mastitis is bacterial culture.”

Without identification it is impossible to guess which cases are contagious and which are not.
Sadly, treatments with antibiotics are often not successful, so prevention of new cases should be a focus. Elevated Somatic Cell Count (SCC) of individual cows is not always a reliable indicator of the presence of contagious mastitis. Often suspicion is only raised because milk quality has eventually been negatively impacted, routine treatments are unsuccessful or large numbers of cows are affected. If left unchecked, economic losses may be substantial and solving the problems can be more difficult.

Whilst bacterial culture can be easily done on farm, your Veterinarian can also assist. Strategic herd management, getting on top of issues and improving milk quality can only be undertaken with the information gained from such testing.

For those worried about contagious mastitis, an escalating SCC or grading, get advice sooner rather than later as this can make a big difference in costs going forward. Testing the bulk milk can provide a Snapshot of what current issues are and where you are best to spend your time and effort.

It is interesting that some of the contagious mastitis bacteria can be recovered from specific sites in people. Once in cows the bacteria will spread very quickly from clusters cow to cow, but clues are there regarding how these bugs came to be in the cows in the first instance.

  1. Streptococcus agalactiae (Strep. ag.) found in the human gut and genital region in around 30% of people. There’s a reason to include pre-milking hand washing and wearing gloves in the shed. Once in the cows, actions such as stripping cows can promote new infections. Whilst treatment can be successful, this usually involves “blitz treatment” i.e.: all or large numbers of cows.
  2. Staphylococcus aureus (Staph. aureus) found in the nose of around 50% of people. (I’ve never seen anyone with gloves on picking their nose.) Treatments can be largely unsuccessful.
  3. Corynebacterium bovis – spread from cow to cow but easy to control with effective post milk teat spraying. Normally resolves in the dry period.
  4. Mycoplasma bovis Spread between cows by contact, milking routines, contaminated equipment, movement of affected cows and calf milk. Does not respond to treatment, spreads rapidly and affects multiple quarters of cows. (This is an exotic disease for New Zealand with affected farms controlled by MPI.)

Contagious mastitis is often difficult to control since cow-to-cow transfer during milking is an effective means of transmission. A study conducted by Ruakura in the 1980’s* showed that with the pathogen Staphylococcus aureus, an infective level of bacteria could be recovered from liners 6 cows after milking a single infected cow.

*Reference: Phillips DSM. Reduction of pathogen transfer within the milking cluster. Proc Conf Dairy Production from Pasture, New Zealand and Australian Societies of Animal Production, Ruakura, New Zealand, 1982: 81-82.


Cows infected with contagious mastitis carry large quantities of bacteria in their milk, shedding literally millions of bacteria in just a few drops. With an infected cow giving many litres of milk from an affected quarter potential cautions include:

  • New cows on farm for any reason: lease cows; carry over cows; cows from staff run with owner’s herds must be quarantined and ideally tested for contagious pathogens.
  • Sharing clusters of infected cows with non-infected cows. Isolation of contagious mastitis infected cows from other cows including your hospital mob is a must.
  • Feeding milk from contagious mastitis infected cows to heifer calves means calves are exposed to billions of contagious bacteria. If you are trying to avoid these bacteria near cows on your farm, this source is concerning. For some farming models where calves are not housed separately, calves suckling each other means even first lactation heifers can be a reservoir for contagious mastitis. HERDSCREEN testing of first lactation heifers on farms affected by contagious mastitis has confirmed many cases of Staph. aureus carrier cows. (Please note: Bobby calves cannot be fed mastitis milk either due to antibiotic residues).


Be über fussy.

  • Optimised and stable vacuum and pulsation, ensuring equipment is running correctly. “small” details such as air admission holes matter. Machine checks should be conducted frequently by experienced people.
  • Rubberware (liners and jetters) must be in good condition and within the recommended number of hours usage. Ensure liners and shells are compatible.
  • Keep a “culture of clean” with your shed and staff. Keeping the shed clean helps everyone and makes the environment safer for your cows and your staff. Ensure staff wear milking gloves as cracked hands can be a great place for pathogens from milk to hide.
  • Ensure cows are milked out, and not overmilked.
  • Ensure your post milk teat spraying is effective. This is always something everyone believes they are already doing well, but it is often overlooked.

    Be über fussy. As an important part of the infection control on farm it is a key factor in your success. The ratio of teat spray and emollient dilution is important, as is the way the mixture is formulated, stored and applied. We recommend getting professional advice from your chemical rep regularly. Sub-optimal teat condition greatly increases the risk of contagious mastitis spread.

Getting the basics of your milking routine right is worthwhile, since almost all contagious mastitis risks can be reduced by solving the Contagious Mastitis Puzzle.