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When, Why and How of PCR
Ask the Vets
When, Why and How of PCR in Mastitis Control Programs
The versatility of the Mastitis PCR assay often means there is no cookie-cutter recipe for when, why and how to use it. Even still, many veterinarians incorporate PCR testing into farm-management protocols when they find they need a more convenient, faster, or more sensitive option compared to traditional methods.
United Veterinary Service (UVS), Bear Creek, Wisconsin, has established mastitis management protocols that include a Mastitis PCR assay on several dairy farms. While choosing when, why and how to use PCR instead of culture includes many factors for veterinarians, the UVS team cites PCR’s sensitivity, the ability to detect intermittent shedding of Staph aureus and speed as key reasons for when they choose it. “Anytime a herd is struggling with milk quality issues, clinical mastitis or when contagious pathogens are found in the bulk tank, the Mastitis PCR assay is a good option,” shared Dr. Eric Duch, Dr. Cole Anderson and Dr. Valerie Baumgart at UVS. “Specifically, we like the assays’ ability to detect intermittent shedding of pathogens. Staph aureus can go undetected on culture. We are more likely to find it (Staph aureus) using PCR.” Convenience is one of the main reasons Dr. Jeremy Boge, Bull Creek Veterinary Service, Hastings, Michigan, says PCR fits into farm protocols. “PCR is a “hands-off way” to monitor mastitis pathogens where the producer has little to no effort in the process,” said Dr. Boge. “While culture is easy to train on-farm staff to perform, PCR is convenient. If a farm doesn’t have employees interested and invested in mastitis, I’ll turn to PCR.” Detecting Mycoplasma bovis quickly is another reason veterinarians turn to PCR. “Compared to traditional myco culture, I appreciate the speed of PCR to identify if that is the pathogen in the herd,” said Dr. Boge. Mycoplasma is an extremely contagious pathogen that affects an entire herd – from milking cows to newborn calves. When this pathogen is detected in a herd, the speed of PCR compared to traditional culture, which can take up to two weeks, is imperative for controlling the spread. Due to test sensitivity, the Mastitis PCR assay has the ability to detect even the smallest amount of DNA from an organism, which gives a broader picture of organisms on a dairy. Dr. Boge said, “While my primary focus is watching for contagious pathogens like M. bovis and Staph aureus, I use PCR to track environmental pathogens over time, which may have an impact on things like calf health.” Additionally, bedding and housing management, as well as parlor procedures are areas that can be evaluated based on environmental pathogens detected by PCR.
Results produced by mastitis PCR are sometimes more information than most dairy producers and veterinarians are familiar with, which is where Ct values come into play. “We find the Ct values are very helpful in test and cull management decisions,” said the UVS team. “We have established cut-off points on each farm that state animals below a designated Ct value is an immediate cull, while animals above that threshold should be retested. The exact Ct threshold varies by farm depending on the target species (M Bovis, Staph aureus) and the farm’s goals.“ Understanding the advantages of mastitis PCR is important when determining its role in reducing mastitis on dairy operations. Veterinarians, and producers alike find that the speed, convenience, and sensitivity of mastitis PCR improves their ability to detect and monitor pathogens that cause mastitis. Experience shows that better detection methods can benefit any mastitis control program. Consider the advantages of PCR when trying to improve your mastitis program.
Dr. Jeremy Boge, Bull Creek Veterinary Service
