Mycoplasma PCR

October 2018 Edition

Mycoplasma PCR

Now Available at Ag Health Labs!

The History…

PCR is a technology we have been evaluating the past couple years as it has entered the world of Mycoplasma diagnostics. As with many new technologies, there are commonly many hurdles to overcome in the development process and we waited until the PCR methods were working consistently and cost effectively before we began offering the service at Ag Health Labs.

Many dairymen have become familiar with Mycoplasma PCR as Udder Health Systems has developed the method and began running PCR on all bulk tank samples in the NDA Blue Dot program. There are many advantages to PCR , the primary one being the speed of the test. PCR can be completed in a matter of hours as compared to the days it takes to grow Mycoplasma in culture. However, no test is perfect and there are advantages to both PCR and culture. Therefore, we continue to use a culture base program with PCR added to the program where it’s beneficial.

How it works…

PCR, or Polymerase Chain Reaction, is a method of multiplying the DNA of an organism, such as Mycoplasma, to a level where we can detect it if the DNA of that organism is present. Depending on the PCR method used, we also have the ability to identify which strain of Myco is present. Mycoplasma bovis being the most common and aggressive strain in both mastitis and pneumonia. As described in a previous announcement, we have the ability to use PCR to differentiate Mycoplasma from a common environmental contaminant called Achoeloplasma. And of course, PCR is a fast test to perform.

One misconception with Myco PCR is it is more sensitive than culture. However, this is not always true, and it’s better described as different than culture. They both have their place. PCR does have the ability to detect very low levels of Myco DNA, including the presence of DNA from dead organisms. This has occasionally caused frustration or confusion when Blue Dot bulk tanks have been PCR positive. Then we fail to find a positive bulk tank or string upon culture which requires a live organism to grow.

Occasionally, PCR will fail to find Mycoplasma which culture does detect. One reason for a negative PCR is the presence of inflammatory biochemicals in mastitis cases or fresh cows which inhibit the DNA replication in the PCR process, resulting in a negative result. Mycoplasma can also be diluted to a level that PCR fails to detect.

Case example…

We recently experienced a Myco workup where a Tank was PCR positive, 1 String was PCR positive, 1 pool of 10 cows was PCR positive, and 1 cow was PCR positive (and 3+++ positive on culture 2 days later). However, because we cultured all cows in the positive string, a second cow was identified as a low positive on culture at 7 days which was part of a pool of 10 that was PCR negative. She was a low shedding cow which was further diluted by the pool of ten. This resulted in a negative PCR, but a positive on individual culture.

The value of PCR …

In the above outbreak, the tank was detected by PCR on day 1. On day 2 the strings were sampled and PCR tested. On day 3, the cows in the positive string were sampled and then PCR tested in pools (groups) of 10. On the same day, a positive pool was identified, and the 10 individual cows were then PCR tested. The positive shedding cow was identified and removed from the herd. On day 10, a second low shedding (1+) cow was found by culture on a 7 day myco culture reading. This cow was not shedding enough Mycoplasma organisms to trigger a positive PCR result in a pool of 10 cows. Had this process been performed by culture alone, it would have taken between 13 and 24 days to find both positive cows. Allowing positive cows to linger in a pen just results in further transmission and additional cows becoming positive and being culled.

The downside to adding PCR to an outbreak workup is cost. Tanks and strings cost $40 per PCR test and individual cow samples cost $30 per PCR test. Therefore, it can cost around $3.00 per cow tested to add PCR, or in this example, around $600 total. However, if the speed of detection prevents even 1 additional cow from being infected it is well worth the investment. Of course, the option is yours to take the slower traditional culture route if you so choose.

We are happy to discuss the new testing options, their methodology, and what would be best in your unique situation. In general, we continue to use culture for Myco screening of fresh cows, mastitis cows, and individual cows in a positive string. We typically are using PCR for String samples, pools of 10 individuals from a positive string, and individuals from a positive pool. This can be adjusted to fit your personal situation and preferences.

Thank you for continuing to make Ag Health Labs an integral part of your herd health team!

Dr. Fred