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

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Individual Cow Testsincubated at 55 ° F for 18 hoursand then the bacterial count isdetermined using the SPCmethod.Veterinarians and others arefrequently interested in not onlythe number of bacteria presentin bulk tank milk, but also thevarious types of bacteria. A testthat is growing in use is bulktank milk culturing (BTMC). Incontrast to the SPC and PImethods, BTMC is not standardized,not quantitative, andexact methods vary amonglaboratories. A common practiceis to spread approximately .01ml of milk evenly over the entiresurface of an esculin blood agarplate. Some laboratories usemedia that is selective forgrowth of staphylococci, streptococci,coliforms, or mycoplasmain addition to the esculin bloodagar. The total growth on esculinblood agar approximates, but isnot identical to, the SPC. Thetest is primarily used to determinethe presence of contagiouspathogens in the dairy.The primary contagious pathogensare Staphylococcusaureus, Streptococcusagalactiae, and Mycoplasmaspp.Individual cow SCC are mostfrequently determined in DHIlaboratories, but independentlaboratories do offer this service.Cells are counted electronically,generally using aFossomatic machine. The DHI-SCC program generally reportsthe values as the Linear Score,but some report the value asthousands of cells (i.e. 100,000= 100). The scale for linearscore is from 0 to 9, where alinear score of 4 is equal to200,000 cells/ml and linearscore 6 is equal to 800,000cells/ml. The linear score ispreferred as it can be relateddirectly to production losses.Each increase of 1 in linearscore corresponds to a doublingof SCC and a milk loss of 400lbs/lactation (1.5 lbs/day) onsecond lactation or older animalsand 200 lbs/lactation (.75lbs/day) on first lactation animals.High SCC quarters can bedetermined at cow side usingthe California Mastitis Test(CMT). The CMT test is anestimate of the SCC.Veterinarians usually conductbacterial analysis of milksamples from individual cowsand quarters in their ownlaboratories or they use theservices of independent oruniversity laboratories. Theprimary purpose of culturingmilk from individual cows is todetermine the infection status ofthe cow or quarter and todetermine the specific pathogensinfecting the herd. Milksamples for culture must betaken with great care. Teatsmust be clean and dry and theteat end thoroughly scrubbedwith alcohol pads prior tocollection. Either composite milkfrom all four quarters or individualquarter samples can betested. The most accuratemethod is to culture individualquarter samples. Compositemilk samples are often used inan attempt to reduce the cost oftesting. However, compositemilk samples have limitedmicrobiological value and theydo not reveal which quartersare infected.6
Using the Information toManage Mastitis and Milk QualityThe production of quality milkbegins with the understandingthat mastitis must be controlledand sanitary conditions mustexist throughout the cow’senvironment. Bulk tank milktests for SCC, SPC, and PIprovide producers valuableinformation about the currentstatus of mastitis and sanitationin their herds. Coop/plantincentive programs have greatlyincreased producers’ awarenessof these quality tests. Realisticgoals are BTSCC of less than200,000 cells/ml and SPC andPI counts of less than 5,000bacteria/ml. Many dairymenconsistently produce milk withSCC less than 100,000 cells/mland less than 5,000 bacteria/ml.A question frequently asked iswhether or not SCC can get toolow. The SCC goals suggestedare in the range of countsassociated with uninfectedquarters and are not physiologicallyabnormal.Somatic cell counts are reducedby controlling mastitis and, morespecifically, subclinical mastitis.The BTSCC is a function of thepercentage of quarters infectedin the dairy herd. The contagiousmastitis pathogens, suchas S. aureus, S. agalactiae, andMycoplasma spp. are frequentlycauses of subclinical mastitis,high percentage of quartersinfected, and BTSCC of 500,000cells/ml or greater. On occasion,high SCC herds are found tohave a significant problem withthe environmental streptococci.The BTSCC is the first clue tothe amount of subclinical mastitisin the herd, and the DHI-SCCprogram can be used to identifythe high SCC cows within theherd. A general rule is that cowswith linear scores of 4 or greaterare very likely to be infected andthe goal should be to have 90%of the cows with linear scores of3 or less.Many producers try to makecomparisons between DHI-SCCherd means and BTSCC derivedfrom coops/plants. Directcomparisons are generally notvalid. Somatic cell counts arevery dynamic within individualcows for a given infection statusand the infection status of theherd is variable from day to day.Bulk tank milk SCC representsthe milk from cows actuallymilked into the bulk tank, whileDHI-SCC herd means mayinclude cows whose milk wasdiscarded. Samples may havebeen taken on different days,derived from a different numberof milkings, analyzed in differentlaboratories, and cells countedby different methods.The specific bacterial cause ofhigh SCC is important informationfor purposes of implementinga control procedure. Contagiouspathogens are reduced byteat dipping with efficaciousproducts, dry cow therapy, useof single service towels forudder preparation, and properlymaintained and functioningmilking machines. Environmentalmastitis is reduced by minimizingexposure to the environmentalpathogens in the environment(inorganic beddingmaterials), milking clean dryteats and udders, and minimizingliner slips during milking.Bulk tank milk cultures are aquick way to determine if thecontagious pathogens arepresent in the herd. The presenceof contagious pathogensin bulk tank milk almost alwaysindicates infected mammaryquarters in the herd. However,the absence of contagiouspathogens does not prove that7
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