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Q: I got a no-growth result on a clinical mastitis diagnostic test?? What the...

A: Getting a no-growth result from a clinical mastitis diagnostic test is pretty common (around 10-20% is normal in clinical trials). Here are four possible explanations, the reasons behind them, and some tips on steps you might want to take.

1. It's just not bacterial:

Mastitis is technically defined as inflammation of the udder. While the inflammation is usually caused by a bacterial infection, but not always. Possible non-bacterial causes could be a physical injury such as a knock to the udder, or a non-bacterial pathogen (such as a yeast).

2. Already on the mend:

The cows immune system is pretty good at responding and eliminating some types of gram-negative coliform bacteria such as E.coli. It is possible you might have spotted the back end of an infection that the immune system has already cleared, and that the visible symptoms remaining are the residual effects.

3. Transport issue: If you're not culturing on-farm, then make sure your samples are transported refrigerated, and tested promptly. The bacteria in your sample need to be alive to be cultured! If you are using a transport service offered by your testing lab, then a failure here is probably unlikely.

4. Technical issue: Whatever method you use for culturing your samples, regular checking of equipment and processes is important. If you use Mastatest, our system can detect an issue with your Lapbox, and we'll contact you to let you know. If you're using manual plating methods, regular re-training and quality control measures are essential to ensure you are getting accurate results. If your equipment has been tested, and your using it correctly, then failure here is also unlikely.

How do I manage a no-growth result?

Most veterinarians recommend a watch-and-wait approach for mild-moderate clinical mastitis that reports as no-growth. Since the test indicates a non-bacterial cause, antibiotics are not indicated. You might want to consider offering supportive therapies.

If you use Mastatest, your test result email will outline your vet's recommended management strategy

If symptoms don't improve, or the animals condition declines, then prompt re-testing and vet review are important. 

In Summary:

The most likely reason for you no-growth result, is that there is not any active bacterial infection due to a non-bacterial cause of inflammation, or the infection having already been cleared. Your vet will probably recommend a watch and wait strategy, with re-testing should the original symptoms not subside.