News & Advice

Milk samples – when should you send samples to the lab?

Mar 8, 2023 | Dairy, Milk Quality

Your vet has suggested you should take milk samples from cows with clinical mastitis or high somatic cell counts (i.e. subclinical mastitis), but do you know what happens to those samples once you drop them off at your local Anexa clinic?


Options for testing milk samples

These days, there are several options available for testing the milk samples you collect from your cows. Whether you use on-farm testing or drop samples into the clinic soon after collection or store samples in the freezer to test as a batch prior to your milk quality consult (benefiting from a 10% ‘batch’ discount on lab fees), each option provides different information and so answers a different question. It is important to think about what information you would like to know so you can decide which option is best for you.


Gold Standard

Lab-based tests, like those carried out at Anexa’s microbiology lab in Morrinsville are considered to be industry ‘gold standard’, and sometimes that’s exactly what’s needed.


The life cycle of a milk sample in the lab…

When you drop off one or more milk samples to your local Anexa clinic, the samples are sent to Anexa Morrinsville which is home to our state of the art microbiology laboratory. This lab is managed and operated by highly qualified microbiologists with years of training and experience.

The milk sample is initially ‘cultured’ (i.e. we try to grow any bacteria present in the milk) by ‘plating’ the milk onto agar plates; agar is a substance designed to enable the growth of bacteria. There are many different types of agar available, some designed to grow everything and some, called ‘selective media’, which are very specific to promote the growth of individual bacterium species.

Following ‘incubation’ where the agar plates are kept at optimum temperature and humidity for bacterial growth, the plates are ‘read’, meaning the microbiologists interpret the growth of colonies present on the plates. Depending on what is found to be growing at this stage, a report can be sent to the vet, or further testing may be required, taking an extra day or so, to differentiate and identify the bacteria growing from the milk sample. Antimicrobial sensitivity testing can be carried out on these bacteria to help determine if the bugs are resistant to the antibiotics commonly used to treat mastitis.


Identifying the weird and wonderful…

In New Zealand, Strep. uberis and Staph. aureus are the bacteria most commonly responsible for causing mastitis in dairy cows. However, there are a vast range of bugs that can cause mastitis. In-lab testing allows accurate identification of pretty much all the common and uncommon pathogens (disease causing bugs, including bacteria and fungi) that cause mastitis in ruminants……Staph. aureus, CNS, E. coli, Klebsiella, Serratia, Enterobacter, Proteus, Strep. dysgalactiae, Strep. agalactiae, Strep. uberis, Bacillus, Prototheca, Trueperella pyogenes, Nocardia, Corynebacterium, Yeasts, Aspergillus (and other fungi) to name but a few. It’s a very long list!

On-farm diagnostics can be really useful but there are some limitations in the bacteria they can identify, and some pathogens may be missed altogether. If you are seeing results from your on-farm diagnostics that don’t quite add up, then it may be useful to have a chat with your vet or drop off some aseptically collected milk samples to your local Anexa clinic to see if we can help make sense of what’s going on.



When milk testing is carried out in Anexa’s lab, our specialist microbiologists have the skills, experience and technology available to provide a meaningful report. When results are a little tricky, we know we can rely on the expertise developed over years of training and clinical experience to not only correctly identify the bugs listed above, but also to differentiate between mixed infections (more than one bug present at the same time) and contaminated samples (when bacteria from the cow’s skin or environment has polluted the sample). Furthermore, all milk samples are stored (frozen) for at least 4 weeks in the lab, allowing further testing to be carried out if required.


What other tests are done in the lab at Anexa?

A range of other tests are regularly carried out at Anexa’s internal lab. Milk, faecal, and blood samples from individual animals, pooled samples from groups of animals or bulk tank milk samples are routinely tested. Test such as antimicrobial sensitivity (from clinical mastitis samples), individual or bulk tank somatic cell count, Dairy Antibiogram Testing to monitor antimicrobial resistance profiles of bulk milk (ordered through your vet) as well as a full range of parasitology (faecal), haematology and biochemistry tests (blood samples) are just some of the tests that keep our lab staff busy!

If you’d like a refresher on how to collect a milk sample using an aseptic technique, review our guide How to collect a milk sample or send us a message to enquire about on-farm training here. If you want to know more about what goes on in our lab, just ask… we test well over 3000 milk samples per year (not even counting all the research milk samples we test), we are proud of what we do and are more than happy to chat to you about it.


Other Anexa resources that you might find helpful:

Mastitis is bugging farmers this season, but which bugs are to blame?


Milk Quality

My vet always talks about the Dairy Antibiogram. What is this and how can it be of use to me?

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