Mastitis Control Programs: Milk Quality Evaluation Tools for Dairy ...

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Testing Milk From Individual CowsThe primary purpose for testingmilk from individual cows/quarters is to evaluate mastitis.None of the tests applied to milkfrom individual cows are usedby regulatory agencies or coop/plant quality premium programs.Individual cow tests are simplyinformational and the nextlogical step for troubleshootingherd mastitis problems.The most widely used programfor determining the SCC of milkfrom individual cows is the DairyHerd Improvement (DHI)-SCCprogram. Some independentmastitis testing laboratoriesoffer a similar service, andsome milk procurement agencieswill test milk samples fromindividual cows for SCC. TheDHI-SCC program determinessomatic cells in composite milkat monthly intervals. These datacan be used to determine whichcows are likely to be infected,but they cannot be used todetermine which of the fourmammary quarters are infectedor the type of pathogen causingthe infection. The CaliforniaMastitis Test (CMT) is the mostwidely used cow-side test forestimating the SCC of individualquarters.Somatic cell counts measureinflammation and are an indirectmeasure of the presence orabsence of infection. The trueinfection status of a cow/quartercan only be determined bymicrobiological analysis (culturing)of aseptically taken milksamples. Milk samples to betested can be either compositesamples (an equal volume ofmilk from each quarter is drawninto the same tube) or individualquarter samples. This type oftesting determines the presenceor absence of pathogens andthe type of pathogen. Resultsfrom multiple samples takenover time are more reliableindicators of infection statusthan results from a singlesample. When culturing a singlemilk sample, four types ofresults are possible: 1) thesample yields the correct result;2) a pathogen is isolated, butthe quarter is truly not infected– a false positive; 3) nothinggrows (no isolation), but thequarter is truly infected – a falsenegative; and 4) the sample iscontaminated, three or morepathogen types grow and theresults are impossible to interpret– contaminated sample.The number of samples yieldingincorrect results will vary between10 and 20% of milksamples when routinely samplingcows. The percent incorrectsamples can be as high as30 to 50% if samples are onlyfrom clinical quarters.4
Conducting Milk Quality TestsBulk Tank Milk TestsRegulatory tests are conductedin laboratories operated by statedepartments of health orequivalent agencies or inlaboratories approved by theseagencies. The PMO indicatesthat milk from all herds will betested at least four times in anysix-month period. Somatic cellcounts are determined almostexclusively by electronic countingmethods using a series ofmilk samples with varying cellconcentrations. Cell numbers inthe standards are determinedby the direct microscopicmethod and the cell count isreferred to as the direct microscopicsomatic cell count(DMSCC). The DMSCC methodis considered the standardmethod to which all other cellcounting methods are compared.Even with calibratedmachines there is approximately15% error in any single numbergenerated. A 15% error meansthat a cell count of 100,000cells/ml could really be anywherebetween 85,000 and115,000 cells/ml.The standard plate countmethod accurately determinesthe total number of bacteriapresent in a milk sample, but itdoes not determine the type ofbacteria present. To run theSPC, a precise volume of milk isadded to a given quantity ofmedia and the plate incubatedfor 24 hours. The number ofcolonies is accurately countedand expressed as bacteria/ml ofmilk. Other names often used todescribe the SPC are “loopcount” and “bacteria count.”Coop/plant quality premiumtests are generally conducted inlaboratories operated by thecoop or plant. Some tests maybe carried out in independentlaboratories, and some coops/plants may utilize the test dataderived by regulatory agencies.Most coop/plant quality premiumprograms determine SCCby electronic means and generallyuse a Fossomatic or equivalentmachine. The Fossomaticcounts the number of DNAparticles or somatic cell nuclei.Proper calibration of the machinesis essential. There is nosingle “standard milk sample” bywhich all machines are standardized.Most laboratoriesfrequently test the accuracy oftheir machines by comparingthe cell count generated to thevalue derived by the DMSCCmethod.Most coops/plants determinethe number of bacteria in milkusing both the SPC and PImethods. The value of the PIcount is that is gives a betterindication of the shelf life of fluidmilk. To determine the PI countthe raw milk sample previouslyused to determine the SPC is5
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