In veterinary school, students are taught that there are three types of cases for many diseases. Those cases that will cure, no matter what we do; those cases that will never cure, no matter what we do; and those cases we can reasonably improve the outcome of with medical intervention.
When it comes to mastitis, it is very difficult for the dairy industry to make the case that we are using antibiotics responsibly when many dairies never even attempt to determine the bacterial cause of the disease and into which of the three types of cases an individual cow falls.
Sending samples to milk quality labs can play a role, but if the goal is to make cow-level decisions on which cases we can reasonably improve the outcome on, on-farm culture often gives us a faster result at the same or less cost.
Giving some thought to process flow before setting up your lab, no matter how basic you intend it to be, will help your dairy be successful. The process starts with milking technicians identifying a cow with mastitis in the parlor.
Will the milking technicians take the sample in the parlor at that time? On one hand, this is probably the simplest strategy. On the other hand, it is very difficult to train multiple milking technicians on how to obtain clean milk samples, let alone sterile samples.
Another option is to divert those cows to a separate pen and let the herdsman evaluate the cow and take the sample at the next milking.
This strategy has the advantage of requiring training for only a limited number of herd staff on sterile milk sampling technique, which often leads to more accurate results. The downside is that more cows may be diverted from their home pen.
Common components of the on-farm lab
-
Refrigerator and freezer: Refrigerators are used to hold unused culture media plates and milk samples not yet plated. Having a dedicated refrigerator, with enough space and adequate cooling capacity, is critical. Dual-use refrigerators in the lab area should be frowned upon.
The more times per day the refrigerator door is opened, the higher the chance for contamination and temperature swings. Do not allow employees to place food items such as lunches and drinks in these refrigerators, as there is ample opportunity for food-borne disease as well as contaminated culture media.
Freezers are used to hold samples after plating until the cow leaves the hospital pen in case the dairy needs to send them to the reference lab for more advanced diagnostics.
-
Incubator: Incubators range in price from about $35 to more than $500, depending on the size and features. For dairies with less than 1,000 cows, typically a standard egg incubator from your local farm store will suffice.
Incubators must have an adjustable temperature control. All incubators should have a digital thermometer inside the chamber that is viewable from the outside to monitor that temperature stays consistent and within acceptable parameters.
-
Method to inoculate the media: Inoculation means taking the milk sample and applying it to the plate. For on-farm culture, the main method I see is to use sterile cotton swabs.
Most off-farm labs utilize sterile disposable culture loops. The advantage of loops is that they plate a precise volume of milk onto the media depending on the size of the loop. Either method is acceptable.
-
Area requirements: The area must have plenty of counter-top workspace that is easily sanitized. Ideally, the area should also have limited people traffic, especially during plating of samples to minimize contamination.
-
Plates/media: There are many suppliers and many media variations that need to be considered. Several labs throughout the country, including some veterinary schools, offer culture media for sale to dairies.
While price should be evaluated, the emphasis at the farm level should be on quality, consistency and ease of ordering. Be sure to order no more plates than you anticipate using before the expiration date on the plates.
Plating strategies
The most basic plating strategy for on-farm culture is the bi-plate system. Bi-plates use Factor and MacConkey agar, each poured into one half of the plate. Bi-plates are designed for basic differentiation of no-growth, gram-negative and gram-positive bacteria.
Gram-positive bacteria are those that grow solely on the bright red Factor media, while gram-negative bacteria are those that grow solely on the clear pink MacConkey agar.
Another popular plating strategy is to utilize the tri-plate system, which adds a third media known as mTKT media, which is very dark red in color and is selective for bacteria that are in the Streptococcus family.
Dairies that have historically had a high percentage of gram-positive mastitis cases or high levels of non-ag Streptococci on bulk tank cultures may benefit from utilizing tri-plates.
Additionally, because the industry recognizes that mastitis infections caused by Streptococcus are often hard to cure, and extended therapy may be required to achieve acceptable cure rates, dairies may see value by being able to identify these mastitis cases for separate extended protocols in some cases.
Any plates with clear zones around the colonies (hemolysis), results that are confusing or from quarters that repeatedly result in no-growth are great candidates to be sent to a reference lab. Plates that are confusing or that have odd growth can potentially be shown to the herd veterinarian or compared against culture manuals to get a confirmation.
If there is growth that appears to have similar size, shape and distribution on the plate on both the Factor and the MacConkey, these may be cases that also benefit from speciation at a reference lab.
Samples from cows with repeated consecutive no-growth results or those that do not respond to therapy are also candidates for further diagnostics.
Herds should be advised to continue doing bulk tank culture surveillance to detect the presence of pathogens like Mycoplasma that will not grow with traditional on-farm culture methods.
Both bi-plates and tri-plates are intended for culturing individual-quarter samples only. Individual-quarter infections tend to be caused by a single type of bacteria. Plates that grow multiple visually different bacterial species should be considered contaminated samples and are not diagnostic.
More specifics on interpreting results and milk culturing specifics are available from the University of Minnesota, College of Veterinary Medicine, Lab for Udder Health website in a free downloadable “Minnesota Easy Culture System User Guide.”
The reality is that we are in the midst of an age of increased scrutiny of our dairy practices. The calls to make more responsible use of our antibiotic resources are not going to go away.
It is not acceptable or defendable anymore to say that your dairy chooses to treat all cows or no cows out of convenience. Part of managing a dairy is being committed to doing what is necessary for each and every cow in your care.
In order to meet these challenges, the dairy industry will need more diagnostics to improve the drug use decisions for each individual cow. On-farm culture can be a key piece of helping to meet that challenge. PD
-
Brandon Treichler
- Quality Control Veterinarian
- Select Milk Producers
- Email Brandon Treichler
