|MARK WUSTENBERG, DVM, works with Tillamook County Creamery Association in Tillamook, Ore.
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One of the basic tools needed for the successful management of milk quality is accurate identification of the bacterial types involved.
This is especially true for controlling mastitis infections and for focusing treatment strategies. But it can also be very helpful when establishing likely causes of herd-based problems as well as defining nonanimal-related milk quality issues. Basic plate and media culturing methods have long been the "gold standard" for bacterial identification. However, these techniques are time-consuming, take varying lengths of time for results and require considerable experience to do well.
Several new technologies provide significant improvement over traditional culture strategies. These technologies are based on isolating and identifying the DNA from organisms, then growing the organisms and screening them with a series of biochemical tests.
One of these techniques, called real-time polymerase chain reaction (rt-PCR), is rapidly becoming the new "gold standard" in the human medical field. It is currently being adapted for application in identifying bovine mastitis-causing organisms.
The advantage of rt-PCR is rapid turnaround—three to four hours for results versus days to weeks, depending on the organism. And samples do not need to be refrigerated since the process can be run on preserved samples.
There appears to be a significantly higher recovery of organisms from samples (in other words, fewer no-growth samples). The currently available panels cover most but not all of the major mastitis-causing pathogens.
The disadvantages include cost and the limited availability of the rt-PCR technology. Both of these situations should improve as more laboratories adopt the technology. Also, the identification process is specific for certain organisms, so it will not identify organisms that are not currently included in the panels. This may be more important when screening bulk tank samples than when doing individual cow samples.
Yet another disadvantage of the new technology is that because it detects an organism’s DNA, it shows positives for dead and nonviable organisms as well as for those that are alive and still capable of growing. This may complicate the interpretation of results.
For example, you may get a positive result on a sample from a cow that has actually managed to cure the infection herself. This would be helpful in showing what caused the infection, but it could also lead to treating cases that might not need to be treated.
Testing strategies are also being developed that would potentially allow either individual or bulk milk samples to be screened for persistently infected cows.
Similar strategies are being developed that will help troubleshoot microbial problems with manufactured products such as cheese and fluid milk. These tests will allow us to more rapidly identify if the source of a problem is related to the incoming milk supply or to manufacturing.
- May 2011