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Department of Veterinary Science, The Pennsylvania State University, University Park 16802
Corresponding author: B. M. Jayarao; e-mail: bmj3{at}psu.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Key Words: bulk tank milk • somatic cell • bacterial count • milk quality
Abbreviation key: BTM = bulk tank milk, BTSCC = bulk tank somatic cell count, CC = coliform count, ES = environmental streptococci, LPC = laboratory pasteurization count, NC = noncoliform count, PIC = preliminary incubation count, SA = Staphylococcus aureus, SAG = Streptococcus agalactiae, SPC = standard plate count.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Successful milk quality assurance programs start with farm BTM free of antibiotic residues and with low somatic cell and bacterial counts, resulting in better quality products with longer shelflife (Boor et al., 1998; Ma et al., 2000; Reugg and Tabone, 2000). Many dairy producers also receive premiums from their milk cooperative for producing milk with low somatic cell and bacterial counts. Several guidelines have been proposed to interpret BTM milk bacterial counts (Bray and Shearer, 1996; Britt et al., 1997; Murphy, 1997; Jones and Sumner, 1999; Edmondson, 2000; Jayarao et al., 2001; Jayarao and Wolfgang, 2003). However, many of the guidelines are based on individual or collective experience, or extrapolations from other scientific studies. Further, many of the interpretive guidelines lack validation and provide little insight into the interrelationship between different groups of bacteria found in BTM. An extension and research study, conducted in Pennsylvania from April 2000 through March 2001, focused on BTM analysis. The findings of the milk quality survey were used to establish guidelines for interpreting BTM counts and also to understand the relationship between different bacterial groups that occur in BTM.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Dairy producers who opted to participate in the program answered a self-administered questionnaire. The questionnaire sought information on the following aspects of the dairy herd: 1) herd size, 2) milk production, 3) milking frequency, 4) milkings per tank pickup, 5) type of milking facility, 6) change in milking facility, 7) use of automatic milking detachers, 8) type(s) of bedding, 9) animals purchased, 10) residue violations in the past 6 mo, 11) milk quality premiums in the past 6 mo, 12) type of milk equipment cleaning system, 13) mastitis prevention and control practices, and 14) milking procedures. The questionnaire used in this study has been successfully used previously (Jayarao and Cassel, 1999). The responses to the questions were analyzed to determine if any of these practices were associated with bulk tank somatic cell count (BTSCC) or bacterial counts.
Collection and Processing of BTM
The county extension agent provided on-farm instruction on BTM collection and handling procedures as described by National Mastitis Council BTM sample collection and handling guidelines (NMC, 1999). Dairy producers collected the sample in the first and third week of each month for 2 mo (4 samples total). Sampling kits containing gloves, racks, tubes (50 mL sterile screw cap tubes), and labels were provided. Bulk tank milk samples were collected in sterile 50-mL screw-cap centrifuge tubes. Within 24 hr of collection, all milk samples were shipped on ice overnight to the laboratory. On receipt of the sample in the laboratory, only those samples that recorded a temperature of <7°C were processed. The BTM in the 50-mL centrifuge tube was mixed thoroughly several times, and 20 mL of the milk was transferred to a snap-cap vial containing a preservative and sent to the Dairy One Laboratory in State College, PA, for determination of BTSCC. The remainder of the milk sample was used for bacteriological analysis.
Bacteriological Analysis of BTM
The BTM samples were examined for standard plate count (SPC), preliminary incubation count (PIC), laboratory pasteurization count (LPC), CNS count, environmental streptococci (ES) count, coliform count, and gram-negative noncoliform (NC) count. Bacteriological tests for milk quality were done as described by the American Public Health Association (Marshall, 1992). The milk samples were mixed thoroughly by gently inverting the milk vial 20 to 25 times. One milliliter of milk was transferred to a sterile tube containing 9 mL of quarter-strength Ringer’s solution (Oxoid, Unipath Ltd., UK). The 10-fold diluted sample was vortexed at high speed for 15 s, and 50 µL was plated on selective and nonselective media using a spiroplater (Autoplate 4000, Spiral Biotech, Bethesda, MD). Plate count agar was used for enumeration of SPC, PIC, and LPC. The numbers of ES and Streptococcus agalactiae (SAG) in BTM samples were estimated using modified Edward’s agar supplemented with colistin sulfate and oxolinic acid (Sawant et al., 2002). MacConkey’s agar no. 3 (Oxoid) was used to determine coliform and noncoliform counts. Baird Parker’s agar (Difco) was used to determine the number of CNS and presence of Staphylococcus aureus (SA). Plates for enumeration of SPC, PIC, and LPC were incubated at 32°C for 48 h. Plates for enumeration of CNS, ES, coliform count (CC), and NC were incubated at 37°C for 48 h. The Autoplate 4000 user guide (Spiral Biotech) was used to enumerate bacterial counts.
Colonies suggestive of SAG from modified Edward’s agar supplemented with colistin sulfate and oxolinic acid were randomly selected and streaked on 5% sheep blood agar and incubated for 48 h at 37°C. All isolates were examined for gram’s reaction and catalase production, serotyped (Streptex, Oxoid), and identified using API 20 STREP (BioMe 'rieux, Hazelwood, MO) (Sawant et al., 2002). Colonies suggestive of SA from Baird Parker agar were randomly selected, streaked on 5% sheep blood agar, and incubated for 48 h at 37°C. The isolates were examined for hemolysis, catalase production, and coagulase production, and identified using API-STAPH (BioMerieux) (NMC, 1999).
Isolation of Mycoplasma was done as described by Gonzalez et al. (1995), with modifications. Briefly, 500 µL of BTM was pre-enriched in modified Hayflick’s broth and incubated for 48 h at 37°C in a moist 10% CO2 incubator. One hundred microliters of the pre-enriched broth was streaked on modified Hayflick’s agar and incubated for 7 d at 37°C in a moist 10% CO2 incubator. Mycoplasma colonies were viewed under a low-power microscope. Mycoplasma was differentiated from Acholesplasma laidlawdii using the digitonin inhibition test as described by Thurmond et al. (1989).
Data Analysis
A total of 149 dairy herds elected to participate in the study of which 4 herds opted not to participate during the course of the study. Of the 145 herds, 7 dairy herds were unable to provide information on farm management practices, and 12 dairy producers, on 2 consecutive occasions, supplied contaminated bulk tank milk, or the milk that was received for analysis had a temperature in excess of 7°C. A total of 126 dairy herds with complete data sets were used for data analysis.
Answers to the questionnaire were transferred to Microsoft Excel and grouped by their categorical response (e.g., yes, no). To estimate if a response had an influence on the mean BTSCC, SPC, PIC, LPC, SA, CNS, ES, CC, and NC counts for each group within a response were compared with the 3 categories (low, medium, high) within each bacterial count using one-way AN-OVA. A P-value of <0.05 was considered a significant association between the response and a category of the count. All statistical analyses were performed using JMP software version 4.0 (SAS Inst., Inc., Cary, NC). BTSCC and bacterial counts from the 4 BTM samples from each farm were transformed to log10 values. The log10 transformed BTSCC and bacterial counts (SPC, PIC, LPC, SA, CNS, ES, CC, and NC) from the 4 bulk tank samples from each farm were averaged and subjected to correlation coefficient analysis (SAS Inst. Inc.).
The BTSCC, SPC, PIC, LPC, SA, CNS, ES, CC, and NC counts were each classified as low, medium, or high. These 3 categories are the suggested interpretive criteria for monitoring BTM (see Table 2
). The average counts for each of the 3 groups were compared using the Tukey-Kramer (equal variance) or Dunnett’s T3 (unequal variance) procedures. These 2 procedures were used due to unequal sample sizes observed in the 3 categories of a given count. The Tukey-Kramer procedure performs all pair-wise comparisons, testing whether the 3 means are significantly different. The Dunnett’s T3 procedure performs all comparisons with a control category. In our study, the second category (medium) was used as the control because the sample mean falls in this category. P <0.05 was considered significant. Epi-info-2002 (Centers for Disease Control and Prevention, Atlanta, GA), a database and statistics system for epidemiology on microcomputers, was used for performing 
2-square tests and odds ratio analysis.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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). Nearly 71% of the farms had fewer than 100 lactating cattle, typical of farm families engaged in milking cows in Pennsylvania. Farm management practices changed as the herd size increased. This observation can be supported by change in the management practices such as 1) number of milkings per day, 2) type of milking facility, 3) use of automatic milking detachers, 4) type of cow bedding, 5) number of animals purchased, 6) milk equipment cleaning system, 7) mastitis prevention and control, and 8) milking practices. For the majority of the dairy herds, cows were milked twice a day (88%) in stanchion barns (61%) and/or parlors (39%). About 45% of dairy herds had automatic milking detachers. As the herd size increased, so did the use of automatic milking detachers. Sawdust was used as bedding on 44% of the farms surveyed. Nearly 5% of the respondents to the questionnaire indicated antibiotic residues in the last 6 mo. A majority of the dairy producers practiced dry cow treatment (88%), whereas 73% of the dairy producers who teat dipped their cows practiced both pre-and postdipping. Milking practices varied considerably within a given herd size and between the 4 herd size categories (Table 1). Significant differences were observed with respect to the type of bedding used (P 
0.000), antibiotic residues in bulk tank milk (P 0.043), type of milking equipment cleaning system (P 0.022), dry cow treatment (P 0.043), teat-dipping practices (P 0.031), stripping practices before milking (P 0.028), and towel type (P 0.011) (Table 1).
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). Bulk tank somatic cell counts were categorized into 3 groups (low, <200,000; medium, 200,000 to 400,000; and high, >400,000 cells/mL) (Table 3). Mean CNS count was significantly associated with mean BTSCC (Table 3). A BTM with a mean BTSCC >200,000 cells/mL was 5 times more likely to have high CNS (>500 cfu/mL) counts compared with BTM with BTSCC <200,000 cells/mL (Table 4). Dairy producers who received milk premiums had significantly lower BTSCC (291,300 cfu/mL) compared with the BTSCC (378,090 cells/mL) in BTM of those dairy producers who did not receive premiums [2 (p) = 3.27(0.0014)]. BTSCC was significantly lower when cows were milked using automatic milk detachers as compared with BTM from herds that milked cows without automatic milk detachers. The same observation was made with herds that teat dipped the cows with a dip cup instead of using a spray. Interestingly, BTSCC was significantly higher in herds that practiced fore-stripping before milking compared with BTM from herds that did not. Dairy farms that used sand as bedding had significantly lower BTSCC in their BTM compared with dairy producers who used organic bedding such as shavings, newspaper, and straw (Table 5).
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). SPC were categorized into 3 groups (low, <5000; medium, 5000 to 10,000; and high, >10,000 cells/mL) (Table 3). The mean PIC, LPC, CNS, ES, and NC counts were significantly different for the 3 SPC categories (low, medium, and high) (Table 3). BTM with a mean SPC >5000 cfu/mL were 9.5, 5, 6, 7, and 6 times more likely to have medium or high PIC, LPC, CNS, LPC, ES, and NC, respectively, compared with BTM with SPC <5,000 cfu/mL (Table 4). The SPC was significantly lower in BTM when cows were subjected to both pre-and postdipping. In contrast, BTM samples had significantly higher SPC when cows were sprayed with a teat dip instead of using a dip cup (Table 5).
Preliminary Incubation Count
The mean PIC for an 8-wk period ranged from 500–139,750 cfu/mL with a mean PIC of 8740 cfu/mL. As observed with SPC, herd size did not influence the mean PIC of BTM. Nearly 50% of the BTM milk samples had a PIC of <12,500 cfu/mL. In addition to SPC, PIC had correlation coefficients >0.5 for LPC (0.501) and ES (0.533) (Table 2). The mean SPC, LPC, CNS, and ES counts were significantly different for the 3 PIC categories (low, medium, and high) (Table 3). The BTM with a mean PIC >10,000 cfu/mL were 3, 3, 4, and 4 times more likely to have medium or high LPC, CNS, ES, and NC, respectively, compared with BTM with PIC <10,000 cfu/mL (Table 4). As observed with SPC, BTM had significantly higher PIC when cows were sprayed with a teat dip instead of using a dip cup (Table 5).
Laboratory Pasteurization Count
The mean LPC for an 8-wk period was 125 cfu/mL. The herd size did not influence the LPC of BTM. Approximately 10% of BTM samples had a LPC <30 cfu/mL, whereas 90% of the BTM samples had a LPC of <1240 cfu/mL (Table 2). The mean SPC, PIC, CNS, and ES counts were significantly different for the 3 LPC categories (low, medium, and high) (Table 3). The BTM with a mean LPC >100 cfu/mL were 4 and 3 times more likely to have medium or high CNS and ES, respectively, compared with BTM with LPC <100 cfu/mL (Table 4). None of the management practices had any significant effect on LPC in BTM (Table 5).
Coagulase-Negative Staphylococci
The mean CNS for an 8-wk period ranged from 60 to 15,180 cfu/mL with a mean CNS of 650 cfu/mL. As observed with other bacterial counts, herd size did not have any significant effect on CNS count of BTM. Fifty percent of BTM samples had CNS counts of <700 cfu/mL (Table 2). The mean BTSCC, SPC, PIC, LPC, and ES counts were significantly different for the 3 CNS categories (Table 3). The BTM samples with a mean CNS >500 cfu/mL were 6 times more likely to have medium or high ES compared with BTM with CNS <500 cfu/mL (Table 4). The CNS counts were significantly lower in BTM when cows were milked using automatic milk detachers compared with BTM from cows that were milked without the use of automatic milk detachers (Table 5).
Environmental Streptococci
The mean ES for an 8-wk period was 820 cfu/mL. The herd size did not have a significant effect on ES count of BTM. Fifty percent of BTM samples had ES counts of <900 cfu/mL. Ten and 90% of the BTM samples had ES counts of <210 and <3100 cfu/mL, respectively. Paired correlation analyses between ES and CNS had a correlation coefficient of 0.503 (Table 2). The mean SPC, PIC, LPC, and CNS counts were significantly different for the 3 ES categories (Table 3). The ES count was significantly lower in BTM when cows were pre-and postdipped (Table 5).
Coliform Count
The mean CC for an 8-wk period was 70 cfu/mL. A significant association was observed between CC and herd size. As the herd size increased, so did the CC of BTM. About 50% of BTM samples had CC <60 cfu/mL (Table 2). There was no significant difference in the mean of all of the bacterial counts for the 3 CC categories (low, medium, and high) (Table 3).
Gram-Negative Noncoliform Bacteria
For the 126 dairy herds, the mean NC count was 200 cfu/mL. As observed with other bacterial counts, herd size did not have any significant effect on an NC count of BTM. About 50% of the BTM samples had an NC count of <230 cfu/mL (Table 2). The mean SPC and PIC were significantly different for the 3 NC categories (low, medium, and high) (Table 3). The BTM samples with a mean NC >200 cfu/mL were 6 and 4 times more likely to have medium or high SPC and PIC counts compared with BTM with NC <200 cfu/mL (Table 4).
Contagious Mastitis Pathogens
The bulk tank milk that tested positive for contagious mastitis pathogens was categorized as low frequency (1 of 4 samples positive), medium frequency (2 of 4 samples positive), or high frequency (3 or 4 samples positive) (Table 6). Based on the analysis of 4 milk samples from each bulk tank, SA was detected in 39 of 126 (31%) bulk tanks. It was observed that 17, 8, and 6% of the BTM samples had low, medium, and high isolation rates of SA, respectively. As the frequency of sampling increased from 2 to 4 samples, the number of bulk tanks with SA also increased. The mean BTSCC count was significantly associated with the frequency of isolation of SA (Table 6). Streptococcus agalactiae was detected at least once in 13 of 126 (10%) bulk tanks (Table 6). Of the BTM samples, 3, 5, and 2% had low, medium, and high isolation rates of SAG, respectively. As seen with SA, with increased frequency of sampling, the number of BTM samples with SAG was also observed (Table 6). Mycoplasma was isolated from 3 of 39 (7.5%) BTM samples examined.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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).
Based on the 1996 to 1997 BTSCC data collected from dairy herds from 49 states, Pennsylvania ranked 20th, with a state average of 331,000 cells/mL of milk (Norman et al., 2000). Paired correlation analyses between BTSCC and bacterial counts showed low correlations (Table 1). Van Schaik et al. (2002), studying the trends of somatic cell counts in New York State during 1999 to 2000, observed that the average BTSCC was 363,000 cells/mL. The findings of their study suggest that larger farms had lower BTSCC and plate loop count, but had more antibiotic residue violations.
An individual cow somatic cell count of <200,000 cells/mL is typical of an uninfected udder (Laevens et al., 1997). A similar guideline was used by Van Schaik et al. (2002) for evaluating trends in somatic cell counts in New York. In our study, 15% of the BTM had SCC <200,000 cells/mL. This suggests that lowering BTSCC is still a challenge for many dairy producers in Pennsylvania. It was observed that the mean CNS count was significantly associated with mean BTSCC. Coagulase-negative staphylococci are frequently isolated from milk samples and are a significant cause of mild inflammation and elevated cell counts. The CNS generally produce a mild elevation of milk SCC, but if the cows have chronic mastitis, the SCC can elevate to millions (Sears and McCarthy, 2003).
The SPC provides an estimate of the total number of aerobic bacteria present in raw milk. This test is required by the FDA and state regulatory agencies. In our study, 50% of BTM samples had SPC <4120 cfu/mL. Boor et al. (1998) found that 50% of dairy producers in New York routinely produced milk with SPC <10,000 cfu/mL. The Virginia Department of Agriculture and Consumer Services reported that 59% of BTM samples had SPC <5000 cfu/mL, and 76% had <10,000 for the period of December 1997 to November 1998 (Jones and Sumner, 1999).
These observations suggest that SPC is strongly influenced by specific groups of organisms. If SPC increases, counts of specific groups of bacteria (thermoduric, psychrotrophic, and environmental mastitis pathogens) should be examined. With an increase in the SPC, the composition of the bacterial microflora changes considerably. In most cases, an increase in the SPC correlates with unsanitary conditions associated with unclean udders before milking, poor teat and teat-end sanitation, cleaning and sanitation milking equipment, and cooling of milk (Chambers, 2002).
Panes et al. (1979) reported a correlation of 0.65 between thermoduric and SPC when the geometric means were compared for 12 monthly BTM samples from about 350 individual farms. When the numerical means for individual samples were compared, the correlation coefficient between psychrotrophic counts and SPC was 0.66. Boor et al. (1998) observed a correlation of 0.66 between SPC and PIC, whereas Peeler et al. (1989) observed a correlation of 0.71 between SPC and PIC. Based on the findings in our study and those reported by Boor et al. (1998) and Peeler et al. (1989), it can be inferred that correlation coefficients between counts lack predictive value.
The PIC is used as an indicator of the number of psychrotrophic bacteria in raw milk. Milk with high PIC can influence the keeping quality of raw milk and reflect sanitation practices (Jones and Sumner, 1999; Jayarao et al., 2001). The association between PIC and SPC, CNS, LPC, NC, and ES counts can be explained based on the observations of Cousin (1982) and Chambers (2002). The most commonly occurring psychrotrophs in raw milk are the gram-negative bacteria (CC and NC), of which Pseudomonas spp. account for nearly 50% (Cousin, 1982). Gram-positive species belonging to the genera Enterococcus and Streptococcus have been reported to occur with psychrotrophic bacterial flora of raw milk. The psychrotrophic bacteria may account for 10 to 50% of the SPC (Chambers, 2002).
Laboratory pasteurization count determines the number of thermoduric bacteria present in raw milk. Bulk tank milk with an LPC count <200 cfu/mL is considered normal, while a count of <10 cfu/mL indicates excellent equipment hygiene (Ruegg and Reinemann, 2002). In our study, 50% of the dairy producers had a LPC count of <130 cfu/mL. High counts of thermoduric bacteria (>200 cfu/mL) have been associated with herds with poor milking hygiene, unclean equipment, improper sanitizing practices, and milkstone deposits (Murphy, 1997).
Some of the bacterial species that belong to the genera Micrococci and Bacilli have survived heating at 63°C for 30 min. Enterococcus faecalis, lactobacilli, and some corynebacteria are heat resistant, surviving at 60°C for about 20 min. A very small percentage (<1%) can survive heating at 63°C for 30 min (Chambers, 2002). The association between LPC and gram-positive cocci (CNS and ES) suggests that CNS and ES could contribute to the LPC count in BTM.
The CNS, ES, CC, and NC are collectively termed environmental mastitis pathogens. These organisms gain access to bulk tank milk, not only from intramammary infections, but also from nonspecific contamination from cow skin surface, bedding, manure, and water. The presence of these organisms in BTM may relate to the general level of environmental and milking hygiene in the herd (Godkin and Leslie, 1993). An increase in their numbers in BTM is suggestive of problems related to stall management, udder hygiene, and milking practices (Jayarao and Wolfgang, 2003). Coagulase-negative staphylococci are opportunistic pathogens and form a part of the resident bacterial flora on teat skin. When provided with a favorable opportunity to colonize the teat end or teat canal, they grow to considerable numbers and enter the gland to produce mastitis (Sears and McCarthy, 2003).
Catalase-negative, gram-positive cocci belong to a large heterogeneous group of organisms. Members of the genera Streptococcus, Enterococcus, Lactococcus, and Aerococcus have been isolated from BTM (Jayarao et al., 2001). With the exception of Streptococcus agalactiae, these organisms isolated from milk are collectively termed streptococci or streptococci-like organisms (Jayarao et al., 2001) or environmental streptococci (Hillerton and Berry, 2003). Environmental streptococci are widely distributed in the cow’s environment, including on teat ends, teat skin, bedding, and feces. Bedding materials with high moisture and organic content can serve as reservoirs for ES (Hillerton and Berry, 2003). ES can gain access to the mammary gland through the teat canal and induce changes in the mammary tissue. Amongst the environmental pathogens, S. uberis have been shown to increase SPC in BTM (Hayes et al., 2001). In our study, dairy producers who practiced pre-and postmilking teat dipping had significantly lower ES in their BTM compared with dairy producers who did not practice teat dipping. The use of pre-and postmilking teat dipping has been widely advocated for prevention of ES intramammary infections (NMC, 1996).
Presence of coliform bacteria in BTM milk is suggestive of fecal contamination. Coliforms include Escherichia coli, Klebsiella spp., Enterobacter spp., and Citrobacter spp. These environmental organisms are frequently isolated from BTM. Escherichia coli in particular has been shown to elevate bacterial numbers in BTM (Hayes et al., 2001). In our study, we observed a lack of a relationship between CC and other counts. Chambers (2002) reported that although coliforms are the predominant bacteria in cow bedding, CC on teat skin do not exceed 100 cfu/mL. Another potential source of coliforms and other problem organisms in bulk tank milk is the water used for cleaning the milking equipment. Potable water within the dairy production environment can be contaminated by the farm storage tank, rodent and bird droppings, insects, dust, and dirty buckets and hoses (Chambers, 2002).
Gram-negative noncoliform bacteria belonging to 15 different genera have been isolated from BTM. In particular, organisms belonging to the genus Pseudomonas are most frequently isolated from raw milk (Jayarao and Wang, 1999). With the exception of Serratia spp. and Pseudomonas aeruginosa, other noncoliforms bacteria are less frequently isolated from cases of subclinical and clinical mastitis. Noncoliforms in general can serve as indicators of bacterial milk quality but are poor indicators of herd udder health. Gram-negative NC bacteria, such as Pseudomonas and Serratia, can cause mastitis. They can also grow at low temperatures (4 to 22°C) and colonize the stainless steel surface of the milking system (Cousin, 1982; Jayarao et al., 2001). Failure to cool milk to 4°C or improper sanitation can cause the NC count in bulk tank milk to increase dramatically and result in poor quality milk (Cousin, 1982). The association between NC, SPC, and PIC suggests that NC can influence SPC and, in particular, PIC in BTM. The fact that a large part of the PIC microflora consists of NC sustains this observation.
Responses to the 14 questions on the questionnaire were analyzed to determine if any responses/farm management practices were associated with BTSCC or bacterial counts. Fenlon et al. (1995) showed that herds with high BTSCC had significantly lower milk yield and were less likely to use postmilking teat dip, periodically perform maintenance of the milking system, and use automatic cluster removal. A study conducted in Washington State showed that the dairy farms that produced milk with low SCC milked high-producing cows first and clinical cows last, had automatic milking detachers, attended to cow bedding, and disinfected teat ends prior to intramammary antibiotic treatment (Hutton et al., 1990). The results of our study corroborate the findings reported by Hutton et al. (1990) and Fenlon et al. (1995). The CNS counts were significantly lower in the BTM of those dairy producers who used automatic milking detachers. This observation may not relate directly to the use of automatic milking detachers but to fewer cows with subclinical mastitis.
Hogan et al. (1989) conducted an extensive study that monitored for 1 yr bedding materials on 9 commercial dairies. Their findings revealed that inorganic bedding, such as sand and crushed limestone, had significantly lower moisture content, coliforms, gram-negative bacterial count, Klebsiella spp., and streptococcal counts compared with organic bedding materials such as chopped straw and sawdust. In a subsequent study, Hogan et al. (1990) showed that bacterial counts in bedding did not differ considerably, and using chopped newspaper over pelleted corn cobs or wood shavings did not reduce exposure to teat environmental mastitis pathogens. In our study, dairy herds that used sand as bedding had significantly lower BTSCC compared with dairy herds that used organic bedding. Based on the reports of Hogan et al. (1989, 1990), it can be inferred that the low BTSCC observed in our study could be due to the fact that sand as a bedding material may not be conducive to mastitis pathogens growth and intramammary infections resulting in elevated BTSCC.
The practice of pre-and postmilking teat dipping is one of the critical components of a mastitis prevention and control program in a dairy herd. Teat dipping, or disinfecting of the teat, is now a universally accepted practice for reducing the bacterial population around the teat end, thus decreasing the risk of intramammary infection. Thorough cleaning and drying of teats immediately before milking lowers bacterial numbers as well as coliform and Staphylococcus spp. counts and decreases milk sediment (Galton et al., 1984; Pankey, 1989).
Bacterial contamination of the teat and teat end occurs between milkings, when the teat comes in contact with bedding, soil, water, and dung. The number and type of bacteria present on the teat end and teat skin can vary considerably and is affected by season, grazing, and bedding type. Milk from cows with teats soiled with dung have coliform counts as high as 106 cfu/mL (Chambers et al., 2002). Milk samples from mastitis-free cows obtained without washing the teats had an average 7000 cfu/mL, whereas milk from cows with teats dipped and dried with paper towels had an average SPC of 1500 cfu/mL (McKinnon et al., 1988). Hogan et al. (1987) observed that application of germicidal teat dips reduced the incidence of CNS infection and selectively altered both the prevalence and distribution of Staphylococcus spp. intramammary infections. Based on the findings of our study, it can be inferred that pre-and postmilking teat dipping has the positive effect of reducing not only the number of environmental mastitis pathogens (CNS, ES), but also the number of thermoduric (LPC) and psychrotrophic (PIC) bacteria in the BTM.
In our study we observed that dairy herds that applied teat dip using a dipcup had significantly lower BTSCC, SPC, and PIC in their BTM compared with dairy herds who sprayed the teats with a teat dip. Effective application of a teat dip is achieved when all areas touched by the milking machine are covered in the dip. Applying the dip using a spray is less likely to achieve maximum teat end and teat skin coverage compared with using a dipcup (NMC, 1996). Incomplete coverage of the teat skin could result in areas of teat skin where bacteria could survive and grow between milkings and could result in intramammary infections or contaminate bulk tank milk (NMC, 1996; Edmondson, 2002).
The frequency of isolation (number of positive samples out of 4) of SA, SAG, and Mycoplasma was used to monitor contagious mastitis pathogens (Table 6). This approach to interpreting SA, SAG, and Mycoplasma in bulk tank milk comes from several reports that suggest that the number of contagious organisms in bulk tank milk provides little or no clear evidence of the severity of the contagious mastitis problem present in the herd. This could be due to several factors; for example, latently infected cows may not be shedding or shedding intermittently, the dilution effect of milk in the bulk tank may cause the organisms to go undetected, and culture techniques may not work for that particular organism (Farnsworth, 1993; Godkin and Leslie, 1993; Kirk and Lauerman, 1994; Fenlon et al., 1995; Ruegg and Reinemann, 2002).
The isolation rates of SA and SAG were significantly associated with BTSCC in BTM (Table 5). Fenlon et al. (1995) showed a significant correlation between number of S. agalactiae, S. dysgalactiae, and S. uberis in BTM. Staphylococcus aureus was less significantly correlated to BTSCC. However of herds that had cows with SA infection, BTSCC between 250,000 and 400,000 (borderline) was a good indicator of SA infection. Greer and Pearson (1973) observed that herds with a higher BTSCC had a higher frequency of isolation of SAG.
In our study, Mycoplasma was detected in 7.5% of the BTM samples. Kirk et al. (1997) conducted a study to determine the prevalence of Mycoplasma spp. in herds that were members of a milk cooperative. They reported that Mycoplasma-positive samples ranged from 1.8 to 5.8% for all species of Mycoplasma, and 1.2 to 3.1% for Mycoplasma spp. known to be mastitis pathogens. In their study, M. bovis was the most commonly isolated species, and that the distribution of Mycoplasma spp. varied by year, season, and herd. They recommended that BTM samples should be routinely examined for Mycoplasma, and all isolates should be speciated.
With an increase in the frequency of sampling, an increase in the number of bulk tanks with SA and SAG was observed. Detection of SA and SAG in successive BTM samples taken from the same herd over a period of time is a good indicator that cows with SA and SAG infection are present in the herd. Based on the findings of our study, it can be inferred that several BTM samples must be examined before interpreting the findings of BTM analysis. These observations clearly provide evidence to support the recommendations of earlier reports that suggest that frequent sampling of BTM is needed to determine the true estimate of contagious mastitis pathogens (Farnsworth, 1993; Godkin and Leslie, 1993; Ruegg and Reinemann, 2002).
In conclusion, this study found that changes in BTSCC do reflect on the CNS count and frequency of isolation of SA and SAG from BTM. There was a low correlation between bacterial counts. Use of automatic milking detachers, teat dipping practices, pre-and post-milking teat disinfecting, and bedding type, influenced BTSCC and bacterial counts in BTM. Categorization of counts (low, medium, and high) can be used as guidelines for monitoring BTSCC and bacterial counts. The categorization of BTSCC and bacterial counts proposed in this study can be used for monitoring herd udder health and milk quality.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Received for publication June 5, 2003. Accepted for publication June 21, 2004.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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