Diagnostic testing is a key component of any good mastitis control program. In fact, there are many types of tests that provide valuable information to help dairy producers achieve excellent milk quality.
Diagnostic tests like quarter milk culture identify the type of pathogen causing mastitis infections. Screening tests like bulk tank culture monitor the herd for the presence of contagious pathogens, and also provide information that can be used to monitor milking time hygiene. Other tests like bedding or towel cultures can identify potential risk factors for mastitis, or help monitor management of those areas on the dairy. There are many quality labs that provide mastitis screening and diagnostic testing, including local veterinary clinics and milk processors. The University of Minnesota Laboratory for Udder Health is a fully accredited diagnostic lab with highly trained technicians who specialize in mastitis and milk quality diagnostics.
No matter what lab you use, there are certain steps to make sure you get the most value out of your diagnostic testing. Asking certain questions before you submit samples to the lab can help you avoid common mistakes.
Select the right test
To select the best test for a particular situation, start with a specific question. This will help you select a test that best answers the question, and will help you select the right sample for that test. It is important to consider how you will use the test results. Complete this line of thought: "If the results are _____, I will _____." For example, if a mastitis culture is positive for Staph aureus, will you try to treat, do-not-breed and segregate, or cull? If the action is a cow-level action, you can select a cow-level test. A cow-level test would be culture of a composite milk sample, where milk from all four quarters of the cow is combined. However, composite samples can be difficult to interpret because it is very common to isolate multiple organisms. Unless one of the organisms is a contagious pathogen, it's impossible to say whether any of the organisms were truly causing infection or merely contaminants. Composite samples are really only useful for screening for contagious pathogens, when the result of the test would lead you to a cow-level action (like culling). In most cases, a quarter milk sample will give you the most useful, easy-to-interpret results. In addition, depending on your question, you may be able to select a specific screening test that costs less than a full mastitis culture if you only want to know if certain pathogens are present (like Staph aureus or Mycoplasma).
Collect the best sample
The key to this step is CLEAN. For any type of microbiological testing you are attempting to identify bacteria that originated in the sample. But bacteria are everywhere, so it's very easy to accidentally introduce bacteria from elsewhere (like your hands, udder skin, collection equipment, etc.) into the sample. When this happens, the results of the test are meaningless. Regardless of the sample type, be aware of possible sources of contamination and use clean or sterile sampling technique. For example, when collecting a towel for culture, how do you grab the towel out of the dryer? If your hand is dirty, you may be culturing bacteria from your hand. Use clean gloves, or grab the towel with the inside of a clean ziplock bag.
Another important factor to consider is whether or not the sample is truly representative of what you are trying to assess. For bedding cultures, we recommend collecting a sample from at least 10 different locations (back of 10 stalls, 10 spots in a bedded pack) into a clean bucket, mixing thoroughly and then submitting a smaller sample from the mix. For more instructions on how to collect the best sample for each test, including downloadable guides, visit vdl.umn.edu/udderhealth.
Understand the results
It is important to have some idea how you will interpret the test results before you submit samples to the lab. Know that there is some variation in how different labs report test results. When in doubt, call ahead and ask about the testing procedures, how and which results are reported, and if guidelines exist to help interpret the results. In some cases, interpreting results is fairly straightforward, such as a positive Mycoplasma culture. In other cases, there may not be any science-based guidelines for interpreting results, as is the case with bedding and towel cultures. With these tests, it is especially important to understand how you will use the test results ahead of time. For bedding culture, can you compare clean or unused bedding to bedding in stalls just before new bedding is added? What management action could you take to improve things if bacteria counts on bedding or towels are too high?
Make use of your herd veterinarian, or other knowledgeable dairy advisors who can help you design an appropriate testing strategy, select the right test, collect the best sample, and interpret test results. For more information, visit vdl.umn.edu/udderhealth, call or email us at 612-625-8787, firstname.lastname@example.org.