Staphylococcus aureus — A real bugger

Our veterinary practice runs an in-house laboratory where we perform several types of cultures, including milk cultures from the bulk tank and individual cows.  Bulk tank cultures are an economical tool for dairy producers to monitor for contagious mastitis pathogens in their herd. The most common contagious pathogens we see in our lab include Staphylococcus aureus and Prototheca.

Over the past 12 months, we have had an uptick in the number of S. aureus positives. S. aureus may be difficult to eradicate in a herd, though I believe it can be well-controlled. These are just a few thoughts from my review of literature and experience in practice when it comes to S. aureus.

I’m not sure that total eradication is a good goal. In fact, S. aureus can appear in closed herds. Rather, I would encourage milk quality management strategies to keep contagious pathogens as low as possible. Even though I said eradication might not be a good goal, I would not ignore the detection of S. aureus on individual mastitis cultures or bulk tanks. Bulk tank bacteria measurements are usually reported as colony-forming units per milliliter, or cfu/ml. Unfortunately, it can be hard to correlate the cfu to the number of cows infected. In our lab, I usually start to have alarm bells when the number reaches 15-20 cfu/ml though I don’t really know how many infected cows that would be, except more than one. Occasionally, I come across bulk tank reports from creameries that do not report zero, but call S. aureus less than 10, which isn’t very specific. I might consider running a bulk tank culture outside of the creamery if the somatic cell count is at or over 200,000 to get a more accurate bacteria measurement.

Mastitis caused by S. aureus produces more damage to milk-producing tissues than other types of bacteria and can decrease milk production by up to 45% per quarter and 15% per infected cow. So not only does it have an impact on SCC, but it can also impact milk production.

Infected cows appear to be an important and permanent source of infection for other cows. However, S. aureus can also be found in areas outside of the udder in the environment, including the hock and udder skin. Infected cows also do not shed S. aureus bacteria every day.

Once a staph cow, always a staph cow? While I’d like to think that we stand a good chance of beating S. aureus, cure rates are low. This is because the bacteria form micro abscesses that are eventually walled off by scar tissue and difficult to reach with antibiotics. One literature review suggests cure rates of only 20%-30% overall across all lactations. Better cure rates are associated with young cows early in lactation who do not have clinical mastitis at the start of treatment. Because S. aureus is shed intermittently, it takes multiple cultures after treatment to determine if a cow is cured.  It is highly unlikely that S. aureus-infected cows self-cure. If you have previously identified a cow infected with S. aureus and she has a negative culture without any treatment, it is more likely that she just isn’t shedding bacteria the day you sampled her.

Parlor procedures are very important in controlling spread, such as creating a staph pen and milking that pen last, changing gloves after milking an infected cow, and using one towel per cow in the prep procedure to avoid the transfer of bacteria.

Fresh heifers can be a significant source of infection in dairy herds. Why? I suspect culprits such as cross sucking and fly pressure cause teat end damage that allows S. aureus found on the skin to enter the teat canal and settle in the udder.

While S. aureus might be a bugger to manage, I do believe it can be dealt with economically in most herds. Monitoring through some sort of bulk tank culture is a good first step and I encourage all dairy operations to do so periodically and review the results with their farm team.

Megan Weisenbeck is one of six veterinarians at Northern Valley Livestock Services in Plainview, MN. She practices primarily dairy production medicine in Minnesota and Wisconsin. Megan can be reached at [email protected].