Mastitis caused by Staphylococcus aureus remains a significant problem in dairy herds. Since the introduction of control programs two decades ago, the disease occurs in less than 5 percent of cows nationally. However, more than 80 percent of dairy farms have Staph aureus-infected animals.

Staph aureus mastitis can be costly to a dairy because of somatic cell count premium losses, clinical treatment, the cost of testing and culling. If Staph aureus affects your herd, meet with your veterinarian and milking crew and develop prevention and resolution protocols, such as the ones listed below. I have highlighted some of these steps below.

Find infected cows
Identifying infected animals requires culturing the lactating herd, plus collecting follow-up cultures from animals that were dry at the initial sampling time. The next-best alternative is to collect multiple bulk tank cultures. If bulk tank cultures consistently test negative, culture only purchased animals and freshening heifers. In one study, 33 percent of new infections came from freshening heifers.

When taking individual cow cultures for Staph aureus, ask your laboratory to increase the amount of milk used in the culture from 0.01 milliliter to 0.1 milliliter. When low bacteria numbers are shed, this more than doubles the chance of identifying Staph aureus. If a 0.01-milliliter sample contains less than 50 bacteria per milliliter, the infection goes undetected 60 percent of the time. Freezing the sample before sending it to the lab also improves the chance of finding the bacteria by releasing bacteria from inside the white blood cells.

Conversely, teat end damage will increase the number of false- positive milk samples. A false positive occurs when the culture identifies a cow as having an infection when she does not. To avoid this, identify the cows with teat-end lesions, disinfect their teats with alcohol and scrub for at least 60 seconds.

Stop disease transfer
Veterinarians classify Staph aureus as a contagious pathogen because the reservoir of new infections is more likely to be a cow than the environment.

Cracked and damaged teat skin can harbor many Staph aureus bacteria. The same bacteria can grow on the skin of milkers’ hands and then be spread to other animals by touch or through contaminated inflations.

Stress and calving also can increase the likelihood of disease transmission. Steroids similar to cortisol, the stress hormone, cause the number of bacteria and somatic cells to increase in Staph aureus-infected animals. Under heat-stress conditions or during herd outbreaks of acidosis, Staph aureus cows typically have elevated somatic cell counts, and are more likely to develop clinical mastitis.

Heifer calves and cows also can become infected by biting flies that attack the teat end. Research shows that pre-calving intramammary antibiotic treatment of infected heifers reduces Staph aureus infections at calving. Cure rates were better than at any time during lactation. However, injecting heifers with an intramammary treatment can be dangerous and may introduce other bacteria. Only use this treatment in groups of heifers with high infection rates.

Get your staff together and discuss how you can remove Staph aureus from your herd.

Marguerita B. Cattell is a consulting veterinarian in Fort Collins, Colo.

Staph aureus mastitis prevention and resolution

1. Identify and segregate infected cows.
2. Milk infected cows last or disinfect units.
3. Use post-milking teat dipping or spraying.
4. Use dry-cow antibiotic treatment in all quarters of all cows.
5. Reduce shedding by controlling stress.
6. Prevent teat lesions.
7. Disinfect teats of dry cows at dry off with a 5 percent tinture (alcohol-based) iodine dip.
8. Milkers should use gloves or disinfect hands between cows.
9. Pasteurize waste cow milk fed to calves.
10. Control biting flies.

1. Use of dry-cow therapy.
2. Treatment of clinical mastitis.
3. Selective culling.