Contagious mastitis 101: farm biosecurity and infection control

Most farmers are too trusting about access and movements on their farms, especially given that milk production is their livelihood.

Contagious mastitis can be caused by several different bacteria, some uncommon in NZ, but none should be tolerated here. Vigilance, education and knowledge of contagious mastitis are increasingly important.

The key to identifying contagious mastitis is bacterial culture. Without identification it is impossible to guess which cases are contagious and which are not. Treatments with antibiotics are often not successful, so prevention of new cases should be a focus. 

Elevated somatic cell count (SCC) in individual cows does not always reliably indicate the presence of contagious mastitis. Often suspicion is only raised because milk quality has been negatively impacted, routine treatments are unsuccessful or large numbers of cows are affected. If the problem remains unchecked, economic losses may be substantial and solving the problems can be more difficult.

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

Farmers worried about contagious mastitis, an escalating SCC or grading, must get advice sooner rather than later to rein in costs. Testing the bulk milk can provide a snapshot of current issues and where best to spend time and effort.

Interestingly, some 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 there will be clues to how these bugs turned up in the cows in the first place.

1. Streptococcus agalactiae (Strep. ag.) found in the human gut and genital region in 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 50% of people. (I’ve never seen anyone with gloves on picking their nose.) Treatments can be largely unsuccessful.

3. Corynebacterium bovis is spread from cow to cow but is easy to control with effective post milk teat spraying. Normally resolves in the dry period.

4. Mycoplasma bovis is spread between cows by contact, milking routines, contaminated equipment, movement of affected cows and calf milk. It does not respond to treatment, spreads rapidly and affects multiple quarters of cows. This is an exotic disease 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 by Ruakura in the 1980s* showed that with the pathogen Staphylococcus aureus, an infective level of bacteria could be recovered from liners six 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. 

COW FACTORS

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. Since our farming model means calves are not housed separately, calves suckling each other means even first lactation heifers can be a reservoir for contagious mastitis. Herd screen testing of first-lactation heifers on farms affected by contagious mastitis has confirmed many cases of Staph. aureus carrier cows. (Note: bobby calves should not be fed mastitis milk either due to antibiotic residues).

MILKING ROUTINE AND NON-COW FACTORS

Getting the basics of your milking routine right is worthwhile, since almost all contagious mastitis risks can be reduced by:

Optimised and stable vacuum and pulsation, ensuring equipment is running correctly. ‘Small’ details such as air admission holes matter. Machine checks should be done 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. 

Promote a ‘culture of clean’ in your shed and among your staff. Keeping the shed clean helps everyone and makes the environment safer for your cows and staff. Ensure staff wear milking gloves as cracked hands can be an effective hiding place for pathogens from milk.

Ensure cows are milked out and not overmilked.

Ensure your post-milk teat spraying is effective. Most farmers believe they are already doing this well, but are often overlooking it. Be super fussy; it’s a key aspect of the infection control on your farm. 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.

• Natasha Maguire is a director of Farm Medix Ltd.