Mastitis testing: PCR or bacteriology?

Only by identifying the cause of a mastitis outbreak can the problem be controlled, making milk testing a valuable tool for all dairy farmers.

But to formulate the best control strategy, all producers should be involved in discussions with their vet or lab about the shortfalls of any mastitis test, says Quality Milk Management Services (QMMS) vet Andrew Bradley.

To help you make an informed choice, Dr Bradley, vet Andy Biggs, the Vale Vet Group and NML’s director, Ben Bartlett, outline the key areas for consideration:

What do the testing strategies involve?

Polymerase Chain Reaction (PCR) – a molecular genetic technique which identifies short sequences of DNA in a sample.

Bacteriology – a bacterial culture method involving the growth of bacteria on an agar plate.

PCR comparison

How should PCR be used?

“PCR allows more rapid diagnosis and the ability to use normal milk recording samples for mastitis testing, while removing the issue of no growths often seen in bacteriology.

Reliability is also second to none because the test works with DNA, which is either present or not.

With more and more cows testing positive for Staph aureus and the penicillin resistant gene, PCR will also allow more targeted use of antibiotics.

PCR has the ability to detect any number of pathogens, but to keep costs down, the current test identifies 95% of pathogens in the GB herd.

It is rare to find a herd with a mastitis problem not in this 95%, but PCR can act as a filter – when a farm knows it has a problem and it’s not picked up in PCR, further investigation can be carried out.

PCR is not an answer on its own, but forms another addition to the mastitis tool box.

Using PCR on bulk milk for surveillance testing is very useful. However, using PCR purely on bulk milk is not the answer. This should be the first point of call, followed by individual cow testing.

On farmers with a history of unique causes of mastitis, focused bacteriology may be more appropriate to identify pathogens not covered in PCR.

Using PCR is a balance of speed and accuracy against added cost. Any test should be carried out in conjunction with a vet to ensure results are interpreted correctly.

“It’s important for farmers and vets to understand how to interpret results and get full value from PCR. As uptake increases, the cost will go down.”

Andrew Bradley, QMMS:

Thoughts on PCR and Bacteriology

“Bacteriology provides the opportunity to detect a wide range of causes of mastitis – we have identified 60 different mastitis-causing bacteria in our lab, with 150 potential different mastitis causing-pathogens.

And although a high incidence of no growths has been a frustration for some farmers undertaking bacteriology, this is often a result of the test being carried out incorrectly, rather than a fault of the test itself.

I would expect a no growth rate of less than 20%, but some labs are reporting more than 50% – in some cases PCR may be better than poorly done bacteriology.

The worry with PCR is we currently don’t know how to fully interpret results correctly – just because a pathogen is present does not mean it is the cause of the problem.

And this could be particularly risky when farmers are carrying out their own interpretation of mastitis results – as with all testing, it is important to always consult your vet about results to ensure the best strategy of treatment.

How should PCR be used?

“I struggle to see the wider role of PCR testing under its current constraints. In my view, PCR is an adjunct to bacteriology, not a replacement.

Bacteriology should be the first test, PCR can then be used as a more specialist test to determine the nature of no growths when these are flagged up.

Starting with PCR is unlikely to be the most cost-effective route to understanding the pathogens currently causing problems on farm.

And because PCR only detects a few pathogens, there is concern about getting no result, or more worryingly, detecting low-level pathogens and formulating a misdiagnosis.”

Andy Biggs, The Vale Vet Group:

Thoughts on PCR and Bacteriology

“One of the pros of PCR is the speed, but is it speedy enough? Either way, both test results come back after a clinical case of mastitis would have been treated. An ideal test would give you a result, cow-side in the parlour.

The price of PCR may also encourage producers to test a smaller sample of cows for mastitis patterns – a dangerous strategy.

If you test samples from three cows, two could have bug A and one bug B. But when you test 15 cows, you could have 10 with bug B and only five with bug A. So by testing a small proportion, you could easily build up an inaccurate picture of problems on-farm.

PCR also has the ability to identify dead bugs, but how clinically important is doing so?

The worry is that by identifying dead or small amounts of pathogen through PCR, you could come to the wrong conclusion as to the cause of the problem – just because you find them doesn’t mean they are important.

Also, because PCR only identifies 11 pathogens, when contamination is outside this range, it will be difficult to tell whether the bug is from the environment or the cow.”

How should PCR be used?

“PCR could become useful in surveillance bulk milk testing for milk buyers. In such a case, buyers are interested in whether milk contains a more than ideal level of faecal bugs, rather than the cause of the problem itself.

However, the problem comes when using PCR to establish patterns of infection on farm. To get the most accurate picture of mastitis, a number of tests must be carried out – this is where PCR becomes costly.

All farmers are limited by budget so why chose PCR as the first testing strategy. Farmers need to work with their vets to decide the best testing strategy for them.”