How to take a sterile milk sample for culture and sensitivity

One of the tools available to dairy farmers in relation to managing and controlling mastitis is culture and sensitivity testing, Padraig O’Connor, Teagasc, tells us more.

This is where a milk sample from an infected quarter (clinical case of mastitis or a high somatic cell count quarter) is sent to a microbiology lab to identify the bacteria that’s causing the infection. The bacteria identified will indicate if it’s contagious or environmental and this will help guide the farmer and his/her vet to select and implement appropriate solutions.

Milk cultures are recommended where the number of clinical cases is high (more than five cases per 100 cows in the first month of lactation) or the somatic cell count (SCC) is rising. The lab can also check if the bacteria found in the sample is resistant or sensitive to a predetermined list of antibiotics. This is called sensitivity testing.

If the farmer has a herd mastitis problem, it is recommended that a minimum of 10 samples are taken to give a representative picture. Culture and sensitivity testing is also recommended prior to drying off the herd so that the farmer and his/her vet can select the most appropriate dry cow tubes for the cows  with infections. The price per sample will vary depending on which lab is used and the number of samples sent, but the average cost per sample is about €25.

Items needed to take a sterile milk sample are:

• Sterile sample bottle;
• Clean disposable gloves;
• Teat disinfectant;
• Disposable paper towel;
• Cotton wool and methylated spirits / surgical spirits or medicated wipes.

Procedure

1. Label the sterile bottle (date, farmer name, cow number, quarter);
2. Put on clean disposable gloves;
3. Spray or dip the teat with teat disinfectant;
4. Leave for 30 seconds and dry with a disposable paper towel;
5. Disinfect the teat end with cotton wool and methylated spirits / surgical spirits or medicated wipes. The teat end needs to be spotless;
6. Get the sterile bottle ready;
7. Draw the first three streams of milk onto the ground;
8. Open the lid and hold the bottle at an angle to avoid anything falling into it (3-4 cm from the end of the teat);
9. Only quarter fill the bottle;
10. Close the bottle ensuring the cap didn’t get contaminated during the process;
11. Store in a cool dry place until it is sent to the lab.

General points

• Use the Californian Mastitis Test (CMT) paddle to identify the quarter in a high SCC cow;
• Don’t sample a cow for culture and sensitivity if she has been treated with antibiotics in the previous four weeks;
• Milk samples can be frozen and stored for up to four months without any negative effect on most major mastitis pathogens;
• Dairy farmers in consultation with their vets can make informed decisions in relation to dry cow treatment and management, as a result of taking a number of samples for culture and sensitivity;
• Taking milk samples for culture and sensitivity will reduce antimicrobial resistance as the results will guide the farmer to use the most appropriate antibiotic in his/her herd.

Contamination of milk samples with environmental bacteria is a common problem during the process of taking the milk sample. It is critical that the milk sample bottle is sterile, that a clean pair of disposal gloves are used and that the cow’s teat is cleaned and sterilised properly when taking the sample. It is also recommended to take the milk sample from the infected quarter and not a composite sample from all four quarters.

Culture and sensitivity testing is one of the important building blocks that dairy farmers and their vets can use to manage and reduce mastitis and also reduce antimicrobial resistance.

Also read: Milking 10 times per week - could it work for you?