QMPS: Staphylococcus aureus, A new twist on an old storyPrint
By Jessica C. Scillieri Smith, DVM & George Cudoc
After decades of dealing with Staphylococcus aureus, it may feel like the dairy industry has not made any progress. Many herds across the country still struggle to keep the number of infected cows to a minimum, and it always seems like the organism is targeting one’s best and highest-producing cows.
Over the years, our industry has adopted changes and standards that have helped control the spread of this organism. Research has been done to determine the most effective treatment options, and yet some still struggle. Humans are smarter than bacteria, but we haven’t quite outsmarted Staph. aureus ... yet.
Farmer “Ted” has a small dairy in upstate New York. He milks about 50 cows, and is struggling to keep his somatic cell count (SCC) down. He’d like to eventually get premiums, but just trying to keep it under 400,000 cells/ml is his current goal. Six months ago, he cultured all the cows in the herd and seven were identified with Staph. aureus. By aggressively culling and purchasing replacements, Ted hoped he had eliminated the problem. His SCC immediately dropped, but then rose again.
Veterinarians and dairy scientists have been studying Staph. aureus for years and the wealth of information on how the organism behaves is still growing. Staph. aureus lives in the mammary gland of the cow and can wall itself off by creating abscesses. It will intermittently release itself back into the gland, resulting in flare-ups of clinical disease.
This “skill” is one of the main reasons we struggle to identify infected cows, and why treatments are not more successful. When Staph. aureus is intermittently shed from the microabscesses in the mammary gland, it is difficult to call a cow free from infection with just one milk culture, or to evaluate attempted treatment after just one negative culture. The organism also frequently causes subclinical infections, potentially escaping detection until the increased SCC is noted.
Historically, the textbooks told us Staph. aureus is considered a contagious pathogen, and certainly standard practices in the modern dairy industry have helped curb the spread. Effective teat dips, gloved hands, individual towels and fly control reduce the risk of spreading the organism from an infected cow to a non-infected cow. Farms that identify cows with Staph. aureus and milk them at the end of milking (or with a separate unit) reduce the risk further. But even with all these control measures, new cases develop – sometimes even in fresh heifers, supposedly unexposed to infections due to milking or handling.
Here’s where the story takes a turn. We know that Staph. aureus can be found outside the mammary gland – on the skin, vagina and nose – and that it has also been cultured from barn bedding and even the air. Previously, we have considered these environmental sources to pose minimal risk for intramammary infections.
But bugs don’t read books, and the environment can be a source of infection as well. The concept of Staph. aureus not simply being spread from cow to cow, but also infecting cows from the environment, makes control measures that much more difficult. Why can’t Staph. aureus just choose – contagious or environmental? Maybe this is why we have such difficulty eliminating this problem.
Three possible sources of infection
Now, with strain typing, we can get a better picture of which pattern the Staph. aureus in a herd is mimicking. Strain typing allows us to determine if one strain of Staph. aureus is spreading from cow to cow, or if more than one strain is being picked up from the cow’s environment.
Back to Farmer Ted and his uphill battle: There are three possible sources of infection:
1) cows in the herd that were not identified with Staph. aureus,
2) new cows purchased that were already infected, and
3) environmental sources.
One of the first things to look at are his HiFresh rate, or the percentage of cows with a linear score over 4 at the first test after freshening (Figure 1). By interpreting his elevated HiFresh rate on Dairy Comp 305, most of the cows freshened in with a linear score over 4 during many months. When breaking out the mature cows and heifers, both groups mirror the herd trend. This indicates that cows and heifers are potentially being infected from the environment while they are not lactating.
To know if these infections are a contagious or environmental form of Staph. aureus, we need to determine if the strains of the organism are all the same (contagious) or different (environmental) through strain typing. Once we determine this, we can focus our efforts on Ted’s farm.
Figure 2 shows RAPD PCR patterns from the Staph. aureus isolates that were identified in Farmer Ted’s cows. The patterns are created by fragments of DNA, and the distance and intensity of each band is determined by the volume and length of the fragments. As you can see, there is variation between the patterns when comparing the different samples of Staph. aureus cultured from Farmer Ted’s cows. The different patterns in all but the first two columns (F1 and F2) indicate that there are different strains for each of the Staph. aureus infected cows. This is suggestive of an environmental source of infection, as opposed to contagious spread where the same strain (clone) would travel from cow to cow and result in identical patterns.
To deal with the current infections, we can look at individual cows and determine their suitability for treatment. With the low success rate for treatment of Staph. aureus, it makes sense to only spend the time and financial input on cows that have the best chance to cure. With an extended course of antibiotic therapy, first-lactation cows with only one infected quarter have the best chance of being cured (40%-50%). Older cows or cows with more than one quarter affected are less likely to be cleared of the infection, and should be milked last and potentially culled from the herd at the next opportunity. If treatment is attempted, three negative milk cultures are used to definitively call a cow cured.
For Ted, the focus was on his dry cow and heifer management. Adding an internal teat sealant to the dry cow protocol was recommended to help prevent infection during this period. The bedding in the dry cow and heifer facilities needed to be changed more frequently and overcrowding addressed. New cows should always be sampled for milk culture to ensure that Staph. aureus is not being brought into the herd unknowingly.
With close monitoring and strategic changes in management, it will be possible for Ted to outsmart those bugs, start getting those premiums and begin feeling proud of his quality milk again!