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From dairy lab to veterinary lab: The do’s and don’ts of mastitis diagnostics

Justine Britten for Progressive Dairy Published on 07 October 2021

The use of a laboratory to identify and diagnose mastitis has been in practice for decades by veterinary practitioners focused on animal health in dairy production animals. As such, it remains one of the most powerful tools to this day in achieving excellent udder health.

The fundamental reasons for using a milk quality laboratory service have changed very little: Knowledge of causative organisms allows data-driven management decisions with troubleshooting high bacteria counts, overall awareness and monitoring of udder health, as well as antibiotic treatment, control and prevention of contagious mastitis. This, in turn, creates better milk quality by decreasing overall mastitis and lowering bacteria and somatic cell counts (SCCs).

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However, the landscape has changed in the last 50 years, and what was once conducted entirely by veterinary practitioners now may be done as an on-farm lab, by a professional lab (veterinary or academic) or some combination of the above. Not all laboratories are the same in their testing capacity, which can lead to confusion for producers regarding what type of lab they should be utilizing. I think an important first step is to clarify the primary differences between lab types.

  • Professional laboratory. May be private veterinary practice or university diagnostic lab. Nearly always operated by veterinarian or advanced scientific degree personnel. The advantage of a professional milk quality lab generally includes highly sensitive and specific methods, accurate results and increased scope of service to include environmental and bulk tank testing. However, these services do come at a cost that is likely higher than on-farm culture, as labs may not always be located nearby, requiring shipping of samples.

  • On-farm dairy lab. On-farm culture is typically limited to bi- or tri-plate culture of clinical mastitis quarters for treatment decisions. Large dairies located in remote areas may choose to expand the scope of their on-farm testing capabilities and pursue additional training for dedicated personnel. The clear advantages of on-farm culture are faster turnaround times and controlling costs, but these benefits come with the risk of inaccurate results due to inadequate training, lack of supervision and limited culture capability.

Another important distinction to understand is the difference between traditional microbiological culture methods and newer molecular methods, such as PCR or MALDI-ToF. The use of these advanced methods has been revolutionary in terms of understanding, controlling and preventing spread of certain mastitis pathogens in certain circumstances. However, they are not appropriate for all testing approaches, i.e., individual cow testing versus bulk tanks versus environmental or specialty. This can quickly lead to confusion when speaking with producers about different methods, especially if the explanations become filled with a lot of scientific jargon and terminology.

The better question to ask may be: What are you trying to achieve on the farm? Common objectives are to improve milk quality by lowering bacteria and cell counts, controlling and preventing contagious mastitis, decreasing clinical mastitis, identifying and decreasing subclinical mastitis, selective use of intramammary antibiotics, improving animal health and, of course, the resulting economic benefits. I find it helpful to break down these objectives into categories and run through a few key questions:

  • What is the primary goal (identify, control, eradicate)?
  • What are the target organisms or group of organisms?
  • What is the source and/or point of transmission?
  • Can you achieve these goals with culture only?
  • Can this be done on-farm or is a professional mastitis lab required?

A final thought to consider is what I call the “$5 microbiology” question. The dairy industry and agriculture in general operate on thin margins, much smaller than that of human or even small-animal medicine so, consequentially, testing schemes need to be as accurate as possible while still being affordable. In the world of milk cultures, a single culture is often nominally in the $5 range. While this is affordable for most producers, there are limitations to what level of identification you can achieve in that price span. So when is it safe to spend less and use a “screening” type diagnostic method, and when is it important to take the extra steps? Pulling together the answers for all these questions will help both the veterinarian/consultant and the producer put together the appropriate testing plan for the dairy.

  • Contagious mastitis. The target contagious mastitis organisms are Staphylococcus aureus, mycoplasma, Streptococcus agalactiae and prototheca. The primary source and point of transmission are within the udders and from cow to cow at milking time, respectively. While all four of these organisms can be cultured, low shedding rates and low specificity (false positives) present risks. Secondary biochemical tests are needed to increase detection, and accuracy and molecular confirmation of species is required for mycoplasma and S. agalactiae. While an advanced on-farm culture program may be capable of implementing the cultures, molecular confirmations will necessitate a professional lab. So what are the do’s and don’ts?

o Do use secondary biochemical tests in detection protocols.

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o Do always use controls when running secondary tests.

o Do use molecular confirmation methods on low-specificity cultures.

o Don’t report results without confirmations.

o Don’t assume you can “eyeball it.”

  • Environmental mastitis. Target organisms in this group are frequently gram-negatives and include coliforms (klebsiella and E. coli), environmental streptococci and strep-like organisms (SLO), pseudomonas, pasteurella, serratia and bacillus. These bacteria may be found virtually anywhere in the cows’ environment: bedding, water, towels, swabs, milking machines and even sanitizers.

    Most environmental bacteria from milk samples can be easily grown and identified to an appropriate level using culture and secondary biochemical tests only; however, culturing environmental samples themselves (bedding, towels, water, etc.) is more complicated and requires enhancements, dilutions, selective agars and specific protocols. On-farm labs are capable of differentiating gram-negatives grown from milk samples but are not equipped to process these other types of samples. The specialty media, trained analyst labor and often necessary speciation required to generate accurate results from environmental samples will absolutely exceed the $5 microbiology rule. However, in a troubleshooting situation, receiving or generating inaccurate results may be more damaging than not doing any testing at all. So, again, what are the do’s and don’ts?

o Do use secondary biochemical tests to guide diagnosis.

o Do use selective agars.

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o Do speciate when necessary.

o Do use an experienced milk quality or specialty lab.

o Don’t skip steps.

o Don’t take the “cheapest” option.

  • Bulk tank cultures. A good comprehensive bulk tank culture can ideally detect all the bacteria present in a bulk tank milk sample, with enhanced detection of contagious mastitis pathogens. Bulk tank cultures may be done quantitatively for processor quality counts or qualitatively to culture for mastitis organisms. When executed appropriately, it is one of the most useful udder health monitoring tools available.

    This complex level of detection requires a “panel” of selective agars and enhancement methods, which is beyond the scope of an on-farm lab and not practicable in most scenarios. Bulk tank cultures done by a professional lab is an excellent way to “audit” the efficacy of how an on-farm lab is performing, while simultaneously offering an overall “snapshot” of udder health at the herd level. These snapshots can be used to create a baseline history when bulk tank cultures are done routinely. Like environmental sample testing, bulk tank cultures will exceed the $5 price point but will provide useless or even misleading data if shortcuts are taken. What are the key points to keep in mind?

o Do use a sophisticated milk quality lab.

o Do run monthly bulk tank cultures.

o Do use selective agars.

o Do confirmations on mycoplasma and S. agalactiae colonies.

o Don’t use molecular methods on a single drop of milk.

o Don’t go for the “cheapest” option.

o Don’t quit doing them.

Operating a mastitis laboratory is a challenging endeavor. Regardless of whether you use a simple on-farm culture program for treatment decisions or a large professional milk quality lab, the most difficult responsibility will be training, skilled labor and quality management. While identifying the agent of the problem can direct you to the solution, bad information can be worse than no information.

It is an important responsibility of the practitioner to know their strengths and limitations in scope and to the producer to make an informed decision to the best of their ability. The adage of “you don’t know what you don’t know” is a good one to keep in mind. Ask the right questions and don’t be reluctant to be admit when you don’t have all the answers. end mark

Britten earned her bachelor’s degree in animal science from Washington State University and doctorate degree in animal, dairy and veterinary sciences from Utah State University. Her duties at Udder Health Systems include managing research and development projects, milk quality consultation and laboratory operations, and providing technical support for the company’s laboratories.

Justine Britten
  • Justine Britten

  • Udder Health Systems
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