Collecting Milk Samples

Updated 9/8/2017

Collecting Milk Samples

Quarter Milk Samples

A quarter is seldom infected with more than one organism at the same time. However, milk samples may become contaminated with bacteria from the environment, the hands of the person collecting the sample, teat skin or the teat canal. When a sample is contaminated (3 or more different organisms isolated), it is impossible to differentiate which organisms are causing true mastitis infection, and which are contaminants. When 3 or more organisms are found in a milk sample submitted for Full Mastitis Culture, the lab will report the result as “Contaminated”. For screening tests (Staph/Strep/Mycoplasma) the lab will still report the pathogen of interest, if found. However, the presence of excessive contamination may make it impossible to find the pathogen of interest, in which case the lab will report “Inconclusive due to overgrowth”. Screening tests include the following:

  • Mycoplasma only
  • Prototheca only
  • Staphylococcus and Mycoplasma
  • Staphylococcus and Streptococcus
  • Staphylococcus only
  • Streptococcus only

Quarter or Composite Milk Samples?

Quarter milk samples are preferred for Full Mastitis Cultures. The large proportion of composite milk samples submitted for full mastitis culture will grow 3 or more organisms and be reported as “Contaminated.”

Composite milk samples may be submitted for screening cultures (Staph/Strep/Mycoplasma) to reduce the overall cost of testing compared to submitting individual quarter samples. However, the ability to detect an infection in a single quarter will be reduced when using composite samples due to dilution of the infected milk with milk from healthy quarters. You may also want to consider what action will be taken based on the culture result. For example, if you will treat a cow based on culture results of a composite sample, how will you determine which quarter(s) need to be treated?

Proper sample collection technique is very important to avoid sample contamination and erroneous culture results. Please review the following guide before collecting samples to help minimize sample contamination:

Bulk Tank Milk Samples

Please follow these steps carefully to avoid contamination:

  • Collect samples 3-5 days in a row. Five day sampling procedures achieve more accuracy than single day sampling for contagious pathogens such as Staph aureus. Agitate the tank well before sampling.
  • Use a clean dipper or sterile syringe to draw the sample from the top of the tank. If using a syringe, collect approximately 5-10cc. Empty the syringe immediately into a clean milk collection vial. Fill the tube one-half full (Remember, milk expands when frozen).
  • Please do not send milk samples in a syringe as the plunger frequently comes out during transit and introduces contamination.
  • Place immediately in the coldest part of the freezer. Do not delay! Extensive delays will allow bacteria to grow giving erroneous results

IMPORTANT: Do not collect samples from the outlet valve unless absolutely necessary because samples collected in this manner often will be contaminated. If you must collect from the outlet valve, allow 1-2 gallons of milk to run before collecting the sample (you can catch the excess milk cleanly and return it to the tank).