Editor’s note: This is the first of a three-part series. Traditional bacterial culture has been the “gold standard” method for the identification of mastitis-causing bacteria for many years. However, traditional culture is a slow and time-consuming process that allows dairy cattle with subclinical or clinical mastitis to commingle with their herdmates, potentially allowing for the spread of contagious mastitis bacteria.
The simplest traditional culturing tests require 24 to 48 hours for the growth of gram-positive or gram-negative mastitis bacteria, whereas the detection of fastidious bacteria such as Mycoplasma species may take between 10 to 28 days with traditional culture methods and requires the use of specialized culture media.
Additionally, bacteria do not always grow as part of traditional culture in a substantial proportion of mastitic milk samples – leading to the reporting of culture-negative results. A culture-negative result for a milk sample is not only frustrating for the dairyman and the veterinarian, but also for the laboratory responsible for providing the culture results.
There are many possible reasons for culture-negative results with traditional culturing: a low concentration of bacteria in the milk sample, bacteria requiring a specialized media for growth, the presence of inhibitors in the milk sample which can reduce the viability of the bacteria or the improper transportation of the milk samples to the laboratory.
Each of these is a reason for a potential report of a culture-negative result and need to be taken into account.
In order for an accurate determination of any bacteria in non-preserved milk samples, the milk samples have to be kept cool (on ice), refrigerated or frozen during transport to the testing laboratory. The requirement for the use of non-preserved milk samples can be a significant hurdle for the laboratory and dairyman in receiving accurate results.
This requirement adds additional cost and time as factors in the overall expense of traditional culturing compared to polymerase chain reaction (PCR) mastitis assays that can utilize both non-preserved and bronopol preserved milk samples.
Not only does the use of a PCR assay for the detection of bacteria negate the need for special handling of the milk samples – it also provides objective, specific results within a few hours after the sample has been received at the testing laboratory.
PCR-based assays are a fast, convenient and accurate laboratory test for the identification of bacteria responsible for bovine mastitis, but better herd management has been achieved through its use.
As I like to say to dairymen when talking about this assay: “Almost every farmer has a pickup truck and a manure spreader. Each of these tools is designed for a specific job and they do that job well. The truck is great and the spreader is great, but they do not normally work together.
But they can, and when they do, they are much more powerful and effective. So what can you do instead of treating the right mastitis with the wrong drug? Well, first you need to find out what you’re fighting and then work with your herd veterinarian.”
By knowing what is causing the mastitis on their farm, the dairyman and their herd veterinarian are able to use the appropriate antimicrobial drugs. Armed with this knowledge, they may see a decrease in the total treatment duration or a reduction in the use of unnecessary broad-spectrum antimicrobials.
One of the most common questions I receive is about the cost of the PCR test compared to the traditional culture test. The answer to that question is very simple and logical, when properly explained.
The PCR test does cost the dairyman a little more than twice as much as a traditional culture, but when you factor in the cost of repeat testing of culture-negative milk samples, the additional cost of handling and transporting non-preserved milk samples, along with the speed and accuracy of the PCR test, the cost is really equal between the two test types.
There have been a few recent articles about the use of other PCR-based assays (Nucleic acid sequence based amplification [NASBA] and single strand conformation polymorphism [SSCP]) and how their costs are significantly higher and, therefore, not competitive with traditional bacteria culture methods.
One reason for the increased costs of these other PCR methods is primarily due to them being used exclusively in research settings, without commercial applications. PD
Click here to email an editor for references which have been omitted due to space.
Lancaster DHIA has been using PCR-based asssays on milk samples since October 2010.
TOP RIGHT: High says effective milk sample testing and mastitis treatment protocols require traditional and PCR, or DNA-based, tests. Each is designed for a specific task, but they are most effective when integrated together. Photo illustration by PD staff