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A study of mastitis bacteria and herdmanagement practices to identify their relationship to high somatic cell counts in bulk tank milk
Br. vet.J. (1995). 151, 17
A STUDY OF MASTITIS BACTERIA A N D H E R D MANAGEMENT PRACTICES T O IDENTIFY THEIR R E L A T I O N S H I P T O H I G H SOMATIC CELL C O U N T S IN BULK T A N K MILK
D. R. FENLON,* D. N. LOGUE, J. GUNN and J. WILSON*
*SAC Aberdeen, 581 King Street, Aberdeen, AB9 1UD and SAC Veterinary Services, Auchincruive, Ayr KA6 5AE, Scotland
SUMMARY Thirty dairy herds, selected to cover a wide range o f bulk tank somatic cell c o u n t (BTSCC) values, were used to study the relationship between the levels o f the principal species o f mastitis-causing bacteria, h e r d m a n a g e m e n t practices and the BTSCC. A good. correlation was f o u n d between the. n u m b e r o f mastitis streptococci (Strept0c0ccus agalactiae, S. dysgalactiae and S. ube~is) f o u n d in bulk tank milk and the BTSCC. Staphylococcus aureus was less significantly c o r r e l a t e d to BTSCC, but was o f increasing i m p o r t a n c e in b o r d e r l i n e BTSCC herds, where lower excretion levels into milk were unlikely to trigger hygiene penalties and so alert p r o d u c e r s to the p r e s e n c e o f a significant mastitis problem. High BTSCC herds had significantly lower yields a n d were less likely to use a post-milking teat dip or to have a regular p r o g r a m m e o f milking m a c h i n e m a i n t e n a n c e or automatic cluster removal. T h e s e herds also t e n d e d to buy in r e p l a c e m e n t s r a t h e r than b r e e d their own. Overall the m a n a g e m e n t o f high BTSCC herds showed less c o m m i t m e n t to i m p l e m e n t i n g mastitis control proc e d u r e s than herds with a consistently low BTSCC. Ki~w,o~'Ds: Mastitis; somatic cell counts; Staphylococcus aureus; Streptococcus spp; h e r d m a n a g e m e n t .
INTRODUCTION In a series o f Directives, 8 5 / 3 9 7 / E E C and 9 2 / 4 6 / E E C , the E u r o p e a n C o m m u n i t y has a d o p t e d the somatic cell c o u n t (SCC) as o n e o f the basic m e a s u r e m e n t s o f milk hygienic quality for i n t r a c o m m u n i t y trade (United Kingdom Dairy Facts and Figures, 1993). In o r d e r to e n c o u r a g e p r o d u c e r s to m e e t the EC standard o f < 400 000 cells ml -I in milk for h u m a n c o n s u m p t i o n , all the U n i t e d Kingdom milk 0007-1935/95/010017-09/$08.00/0
© 1995 Bailli~reTindall
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BRITISH VETERINARY JOURNAl., 151, 1
marketing boards (MMB) have incorporated bulk tank somatic cell cotmts (BTSCC) as a component of their milk quality payments. Premium and penalty payments are calculated on tile basis of the 3-month geometric mean of weekly BTSCC naeasurements. Together with the pre-existing hygienic payments based on a standard for total bacterial count (TBC), dairy herds are now facing increased financial pressure to produce milk of high hygienic quality. These two measures of milk hygienic quality are frequently adversely affected by the same factors. In particular a common cause of high TBC is the presence of large numbers of mastitis bacteria excreted into tile milk by subclinically infected cows, which also produce large numbers of somatic cells. A survey by Jeffrey and Wilson (1987) ill the Aberdeen and District Milk Marketing Board (ADMMB) region found that a preponderance of mastitis bacteria caused an elevation of TBC ill 43.8% of hulk milk samples which consequently incurred hygiene penalty. The majority of these mastitis bacteria were identified as streptococcal species (Jeffrey & Wilson, 1987). It was also recognized that the percentage of infected cows increased with BTSCC elevation (Jones et aL, 1984). In Scotland, Streptococcus agalactiae was found to be the most common subclinical mastitis infection causing high BTSCC (Logue et al., 1993). The objectives of the present study were frstly to confirm that many producers with high BTSCC also had difficulty in consistently meeting the TBC standard due to contamination of milk by mastitis bacteria, and secondly to deterntine the main bacterial 'contaminant' in milk in herds with BTSCC figures around the EC standard of 400 000 ml -~ which rarely incur hygiene penalties. Quantitative bacteriology using selective media enumerated the significant mastitis pathogens within the bacterial population of bulk tank milk samples. These data were combined with BTSCC and herd management information in an analysis of factors associated with elevated BTSCC.
MATERIALS A N D M E T H O D S
The 12-1nonth study began ill May 1990 and covered the period when producers were first informed that a payment penalty scheme based on SCC would be introduced by the ADMMB. Thirty producers were selected by tile Board on tile basis of their arithmetic mean BTSCC for the previous 12 months (June 1989-May 1990) to cover a range of cell counts, designated 'low' (<250 000 cells ml-I), 'borderline' (250-450 000 cells ml -j) and 'high' (>450 000 cells ml-I). At weekly intervals, later reduced to fortnightly, the latest routine bulk milk samples collected by the tanker drivel" at every collection fi-om the 30 study herds were taken from overnight refrigeration at 4°C at the Board's laboratory. The BTSCC figures used in this study were those determined in the appropriate week by ADMMB for the production of a rolling geometric mean. The bacteria identified and enumerated for tlle purpose of this study (Staphylococcus aureus, Streptococcus agalactiae, S. dysgalactiae and S. uberis) were recognized to be significant mastitis pathogens (Bramley & Dodd, 1984; Jeffrey & Wilson, 1987). Mastitis streptococci were counted on a specific streptococcal agar, using a pour plate technique on 10-fold dilutions in maximum recovery diluent
MASTITIS AND BULK TANK MILK SOMATIC CELL COUNTS
19
(MRD) (lab m Ltd) described by Wilson and Jeffrey (1987). Presumptive identification o f mastitis streptococci was based on colony m o r p h o l o g y and haemolytic pattern on blood agar; biochemical characteristics were established by sugar f e r m e n t a t i o n and catalase test. Aaatigenic-typing using 'Streptex' reagents (Wellcome Diagnostics Ltd) was used to confirm the identification. Staphylococci were isolated and e n u m e r a t e d by spreading 0.1 ml o f milk and a 10-fold dilution of the milk in MRD on Kranep agar (Oxoid Ltd) and identified by blood-agar colony m o r p h o l o g y and c o n f i r m e d as coagulase positive S. aureus using a 'Staphaurex' test kit (Wellcome Diagnostics Ltd) At the e n d o f the study the p r o d u c e r s whose bulk milk had been m o n i t o r e d were c o n t a c t e d by the Milk Board Regional Officer and h e r d m a n a g e m e n t information was collected by an interview questionnaire c o m p l e t e d on 29 o f the 30 farms. T h e h e r d m a n a g e m e n t information included n u m b e r and age o f lactating cows, culling and r e p l a c e m e n t policy, and calving pattern. T h e elements o f a mastitis control p r o g r a m m e used on the farm such as post-milking teat dipping, dry-cow therapy and automatic cluster removal (ACR) were also recorded.
RESULTS
T h e mean counts o f mastitis streptococci and S. aureus from the samples collected d u r i n g the course o f the investigation were calculated. Statistical analysis o f the data sbowed a significant relationship (P < 0.001) between the level o f mastitiscausing bacteria in the bulk milk and the BTSCC. T h e mean level o f mastitis streptococci was m u c h greater (geometric mean 1469 cfu ml -l) in high BTSCC herds than in e i t h e r the b o r d e r l i n e (geometric mean 557 cfu ml -t) or low BTSCC g r o u p (geometric mean 114 cfu ml -I) (Table I). Figure 1 illustrates the linear regression analysis o f log~0 BTSCC on logt0 mean streptococci. T h e h e r d categories r e m a i n e d as discrete groups along the regression line; log BTSCC = 4.614 + 0.3498 log strep
Table I Relationship between bulk tank somatic cell count (BTSCC) category and m e a n level o f main mastiffs bacteria Held category (BTSCC range 1000 ml)
High (467-969) Borderline (274-443) Low (136-247) SEO (27 df)
No. of BTSCC Loglo(B7SCC) Log~, mean mastitis Logl, mean herds (x 1000 ml5 I) (geomeO~c mean streptococd ml -I Staphylococcus x l O00 m f I) (geometric aureus ml-I mean ml -I ) (geometric mean mr j)
11 8 11
648 370 188
5.800 (631) 5.558 (361) 5.269 (186) 0.0408
*Standard error of difference between two means.
3.167 (1469) 2.746 (557) 2.055 (114) 0.1845
2.458 (287) 2.321 (209) 1.602 (40) 0.1560
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BRITISH VETERINARYJOURNAl, 151, 1
indicating mastitis streptococcal c o u n t was highly correlated with BTSCC (r='= 0.827, P < 0.001) over the whole range of cell counts. Regression analysis o f the S. aurc'us data showed that tiffs mastitis p a t h o g e n c o u n t was not as highly correlated with the BTSCC (r='=0.686) (Figure 2). T h e regression equation log BTSCC = 4.880 + 0.3134 log staph showed a laighly significant relationship (P<0.001), tlaough not as g o o d as that for the mastitis streptococci. T h e high and b o r d e r l i n e BTSCC results are less distinctly separated on the S. a u r e u s scatter plot than on that for the streptococci. T h e addition o f S. a u r e u s to the mastitis streptococci in the regression analysis explains slightly m o r e o f the p e r c e n t a g e variance (71.2) c o m p a r e d to the streptococci alone (68.1). I n c o r p o r a t i n g the S. a u r e u s and mastitis streptococci separately into the same regression equation, log BTSCC = 4.530 + 0.2709 log strep 0.1391 log-stapla revealed that mastitis streptococci have a significantly greater effect on the cell c o u n t than staphylococci, c o n f i r m i n g the results in Table I. This highlights that the problem in high BTSCC herds is associated with mastitis streptococci. T h e presence o f S. a u r e u s in bulk milk was distributed m o r e evenly between high and b o r d e r l i n e herds. Figures 3 and 4 present the quantitative bacteriology data fi-om two different high BTSCC herds to illustrate the difference in the level and pattern of excretion o f S. a u r e u s and mastitis streptococci, respectively, in bulk milk samples. T h e level o f mastitis streptococci r e a c h e d in excess o f 250 000 cfu ml -t in bulk tank milk,
+ t
-~ 5.75 + ~t3
m
+
5.5
+
E 5.25F
5!
,
1.5 1.75
t
2.25
2:5
I
2.75
3
I
3.25
I
3.5
__1
3.75
Log(mean mastitis streptococci m1-1)
Fig. 1. Regression analysis showing relationship between hJg,, mean bulk tank somatic cell count and log,, mean bulk tank mastitis streptococcal count.
tX,b\STITIS AND BUI.K TANK MII.K SOMATIC CELl. COUNTS
21
w h e r e a s t h e m a x i m u m S. a'tctvus level was 1 7 0 0 0 c f l l m l -I. T h e p r e d o m i n a n t mastitis p a t h o g e n was i d e n t i f i e d in 31 o f 330 b u l k m i l k s a m p l e s f r o m h i g h B T S C C h e r d s w h e r e t h e total mastitis b a c t e r i a e x c e e d e d 10 000 cfu ml -j. S. agalactiae was t h e p r e d o m i n a n t p a t h o g e n in 13 s u c h s a m p l e s ; 12 w e r e m a i n l y S. ube~Ts, a n d S. dysgalactiae was t y p e d in a f u r t h e r six s a m p l e s . Aalalysis o f t h e h e r d m a n a g e m e n t d a t a f r o m t h e q u e s t i o n n a i r e , s u m m a r i z e d in T a b l e II, i n d i c a t e d t h a t t h e r e w e r e m a j o r d i f f e r e n c e s in m a n a g e m e n t p r a c t i c e s , p a r t i c u l a r l y b e t w e e n h i g h B T S C C h e r d s a n d t h e rest. H i g h B T S C C h e r d s t e n d e d to b e younge,-, with a l o w e r p r o p o r t i o n o f cows o l d e r t h a n fifth l a c t a t i o n . T h e s e h e r d s also b o u g h t - i n r e p l a c e m e n t s r a t h e r t h a n b r e d t h e i r o w n heifers. H i g h
+ 5.~ Z. E
5.6 ¢0 r.fJ [.-, m 5.4 I=
5.2
.-"
5"
4.8
~
_ J_ _t i i ~ r i i I 0 0.25 0.5 0.~/5 1 1.25 1.5 1.75 2 2.25 2.5 2._5 Log(mean S. aureus m1-1)
Fig. 2. Regression analysis showing relationship between log,~ mean bt, lk tank somatic cell count and IOgl0 mean bulk tank Slaphylococcus au~vus count. ( I ) , High; (+), borderline; (*), lOW.
20 000
15 000 i
E
lO 000
5000
0
4
8
12 16 20 24 28 32 36 40 44 Week
48
52
Fig. S. Tile numbers of Stap@lococcus aureus in bulk tank milk over the pe,'iod o1' study fi-om the herd with the highest mean level of S. aureus in the bulk milk.
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BRITISH VETERINARYJOURNAL, 151, 1
30O 250 i
"~ 20C •~ 15(;
~ 100 ~ 5o 4
8
12
16 20 24 28 32 36 40 44 48 Week
52
Fig. 4. The numbers of mastitis st,'eptococci in bulk tank milk over the period of study from a herd with the highest mean level of Streptococcus agalactiae in the bulk milk.
Table II Relationship between bulk tank somatic cell count (BTSCC) and some management parameters
7~3,peof herd Mean BTSCC (×1000) Mean annual yield Size of herd (mean) Buy in replacements Post-milk teat dip/spray Possess ACR* Yearly machine test Percent herd >fifth lactation (average/herd) Mean culling rate Main reason for culling
High
Borderline
Low
648 5391 60-250 (106) 9/11 5/11 5/11 5/11 11%
370 6657 32-140 (90) 2/7 6/7 3/7 5/7 21%
188 6207 80-231 (133) 2/11 11/I 1 9/| 1 9/11 23%
17.8% Mastitis related
20.9% Infertility/low yield
16.3% Infe,tility/old age
*ACR, automatic cluster removal. B T S C C h e r d s were less likely to use a post-milking teat dip o r to have a r e g u l a r p r o g r a m n a e o f m i l k i n g m a c h i n e m a i n t e n a n c e . H i g h a n d b o r d e r l i n e h e r d s were less likely to have a u t o m a t i c cluster removal. A l t h o u g h the c u l l i n g rates were similar for all herds, mastitis-related causes were m o s t f r e q u e n t l y cited as the r e a s o n for culling in h i g h B T S C C h e r d s c o m p a r e d to b o r d e r l i n e a n d low B T S C C herds, w h e r e old age a n d infertility were c l a i m e d as the p r i n c i p a l causes.
DISCUSSI(~ Jeffrey & Wilson (1987) r e p o r t e d t h a t 43% o f T B C h y g i e n e failures in the A D M M B r e g i o n were d u e to the p r e d o m i n a n t - p r e s e n c e o f mastitis bacteria, 9 0 %
MASTITIS AND BULK TANK MILK SOMATIC CELL COUNTS
23
o f which were mastitis Streptococci spp. ( S. agalactiae, S. dysgalactiae or S. uberis). A single cow with clinical mastitis may excrete mastitis streptococci at >10 000 000 cfu ml -t (Cousins & Bramley, 1981), which can potentially increase the TBC o f the bulk milk in a 100 cow h e r d by 100 000 cfu ml q. In this study, high levels o f mastitis streptococci were f o u n d in high BTSCC herds, 31 bulk samples w e r e identified in which mastitis streptococci were present in n u m b e r s > 10 000 cfu ml -~. Since persistent subclinical infection is a c o m m o n c o n s e q u e n c e o f infection by mastitis streptococci, infected cow(s) may remain u n d e t e c t e d . A high TBC may provide the only evidence o f such subclinical infection with mastitis streptococci, particularly when associated with a high BTSCC. S. aureus n u m b e r s were considerably lower than those o f mastitis streptococci, c o n f i r m i n g the findings o f Cousins and Bramley (1981) that S. aureus were excreted from an infected u d d e r in lower n u m b e r s than streptococci. Jeffrey and Wilson (1987) f o u n d S. aureus to be the p r e d o m i n a n t bacterium in only 3.6% o f TBC failures. This is in contrast to the prevalence o f the organism in individual cow milk samples from whole h e r d bacteriological surveys. In a non-quantitative analysis o f results of h e r d tests ill the ADMMB region between 1974-1990 u n d e r taken on herds with a clinical mastitis problem, S. aureus was the most c o m m o n isolate, a c c o u n t i n g for 65% o f all significant mastitis bacteria and was present in 16% o f all samples tested (Logue et al., 1992). Nevertheless in the present study, a significant level of S. aureus infection was d e t e c t e d in bulk milk samples from high and b o r d e r l i n e BTSCC herds, and can be a significant cause o f elevated BTSCCs in b o r d e r l i n e herds, where the TBC o f the bulk milk remains consistently below the penalty levels o f the Milk Boards. H u t t o n el al. (1989) observed that managers o f low SCC herds were m o r e likely to attend meetings, pay m o r e attention to details and have a greater awareness o f mastitis control practices. In Scotland, Logue et al. (1993) n o t e d that farms in the Scottish Milk Marketing Board area which r e c o r d e d milk yields had lower BTSCC figures than those that did not and were three times less likely to incur an SCC penalty. T h e y suggested that awareness and c o m m i t m e n t were t h e r e f o r e very i m p o r t a n t in mastitis control. T h e results o f o u r questionnaire also suggest that these factors c o n t r i b u t e to the low BTSCC in some herds. In a review of the effect o f the milking m a c h i n e and mastitis (IDF, 1987) overmilking a p p e a r e d not to be a significant cause o f new teat infection. However, in a study o f high (460 000 cells ml -I) and low (175 000 cells ml -t) BTS~2C herds, H u t t o n et al. (1989) r e p o r t e d atttomatic cluster removal (ACR) was m o r e fi-equently f o u n d in the low SCC group. T h e findings o f the c u r r e n t study support this observation and the use o f ACR has been shown significantly to r e d u c e BTSCC (Logan, 1993). This may be because with ACR the vacuum is shut off before the cluster is r e m o v e d r e d u c i n g irregular vacuum fluctuations within the m a c h i n e and r e d u c i n g the risk o f backflow o f milk on the teat with the conseq u e n t risk o f p e n e t r a t i o n o f tile teat duct (Kingwill et aL, 1977). Also, the presence o f ACR was associated with better m a i n t a i n e d milking e q u i p m e n t . This is significant since p e n e t r a t i o n o f the teat by mastitis bacteria is m o r e likely to o c c u r in a poorly functioning milking m a c h i n e in which there is excessive vacuum fluctuation or an i n c o r r e c t pulsation rate. T h e high BTSCC h e r d milking machines had p o o r e r testing records than those used for o t h e r h e r d groups. T h e excessively
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BRITISH VETERINARYJOURNAl,, 151, 1
high vacuum level in a poorly f u n c t i o n i n g milking m a c h i n e may result in teat damage. T e a t lesions are susceptible to colonization by as few as 100 cfu S. aurezts (Bramley el aL, 1979) and this bacterinm (and also S. dvsgalactiae) can readily penetrate the teat canal to establish an u d d e r infection. It is well d o c u m e n t e d that the o p e n teat orifice makes the u d d e r m o r e susceptible to infection by p a t h o g e n s after milking than before milking (Kingwill et aL, 1977) and that post-milking teat dipping reduces infection. In tile high BTSCC herds, the absence o f post-milking teat dipping allowed c o n t i n u e d cross-infection. A f u r t h e r significant difference between the high BTSCC herds and those with lower cell counts was a lower n u m b e r o f cows above fifth lactation. Age is known to result in h i g h e r individual cow somatic cell counts (ICSCC), however in uninfected quarters there would a p p e a r to be little or no age effect ( G u n n et aL, in p r e p a r a t i o n ) . Thus by i n f e r e n c e the rise is due to increased prevalence o f subclinical mastitis. This c o u p l e d with the fact that m o r e low BTSCC herds b r e e d their own replacements and are less likely to i n t r o d u c e infection from a n o t h e r h e r d suggests that there is an interrelationship between a n u m b e r o f different factors. However the effects o f such culling on mastitis will be limited if mastitis hygiene control measures are not put in place (Natzke & Everett, 1975). T h e lower overall yield was particularly noticeable in the high BTSCC g r o u p and may have been d u e to a c o m b i n a t i o n of clinical and subclinical mastitis c o u p l e d with the lower numbers o f high yielding o r d e r animals. T h e r e are limits to the information which can be gained from the e x a m i n a t i o n of bulk milk for mastitis organisms. ' E n v i r o n m e n t a l ' mastitis organisms such as Esche~vlchia coli originating from the u d d e r c a n n o t be differentiated fi'om those arising from faecal contamination. However, the organisms in the present study were shown by Veterinary Investigation C e n t r e data (VIDA, MAFF) to be responsible for almost half the clinical cases and virtually all the subclinical cases o f mastitis in Scotland. It is possible to identify the p r e d o m i n a n t bacterial species causing the TBC failure and provide an advance warning of a streptococcal mastitis problem, particularly S. agalactiae. It is also possible to d e t e r m i n e if there is an underlying problem. Gonzalez et al. (1986) f o u n d that bulk milk levels o f S. agalactiae in excess o f 4000 cfu ml -j gave a m o d e r a t e l y high correlation with at least 7% o f the h e r d shedding the bacterium; their study was less conclusive for S. au.reus. This study has shown that a m e a n level o f streptococcal mastitis bacteria in the milk of 1000 cfu ml -~ (log,, 3.0) suggest a definite streptococcal mastitis problem. This will often cause sporadic high TBC in the bulk milk and potential hygiene failures. D,qaen a h e r d has a BTSCC which remains obstinately a r o u n d 400 000 ml -~, with few if any TBC failures, the p r e s e n c e o f S. aureus in the bulk milk at levels o f 100 cfu ml -j (logj~ 9.0) is indicative o f a p r o b l e m in the herd. In summary high BTSCC herds generally failed to i m p l e m e n t standard mastitis control p r o c e d u r e s fldly. This was well illustrated by the herds with consistent low BTSCC which practised such a policy. ACKNOWLEDGEMENTS
T h e authors would like to thank Mike Franklin o f the Scottish Agricultural Statistics Service for the statistical analysis and T e r r i e Stewart for technical assistance
MASTITIS AND BUI.K TANK MII.K SOMATIC CEI.L COUNTS
25
in this p r o j e c t . T h e w o r k was f u n d e d with g r a n t s f r o m t h e t h r e e S c o t t i s h Milk M a r keting Boards and the Scottish Office Agricultural and Fisheries Department.
REFERENCES B~..x~lt.l.:v,A..J. & DOl)D, F. H. (1984). Reviews of the progress of Dairy Science: Mastitis control progress and prospects.Journal of Dais), Science51,481-512. Bit.x~tl.l..v,A.J., I4ayc;,J. S., Htcx;s, T. M. & Nt':-xV~:R,F. K. (1979). Colonisation of the bovine teat duct following inoculation with Staphylococcus aureus and Esche~chia coli. BTqtish Veterinao, Journa1135, 149-62. (,¢~t's1.~s, C. M. & BP~.X~u.rv,A.J. (1981). T h e microbiology of raw milk. In Dahy Microbiology, Volume I: The Microbiolo©, of Milk, pp. 119-63 ed. R. K. Robinson. London: Applied Science Publishers I.td. G()NZ.-\I.I:S,R. N.,JAsI'ER, D. E., BL'SHNI"I.I.,R. B. & FORVER,T. B. (1986). Relationship between mastitis pathogen m e m b e r s in bulk tank milk and bovine u d d e r infections in California dairy herds.Journal of the American Vele14nmyMedicine Assodation 189, 442-5. Ht"t-rox, C. T., Fox, L. K. & H.-x×cocK, D. D. (1989). Mastitis control practices: Differences between herds with high and low cell counts.Jou~zal of Dai~y Science73, 1135-43. lYn.:RY.-\TIO.x..~t.DAtRV Fl:.m:.~-vrio× (1987). Machine Milking and Mastitis. Bulletin of the International Dairy Federation (No. 215). .IV.VVRI.:V,D. C. & WH.sox, J. (1987). Effect of mastitis-related bacteria on total bacterial cotmt of bulk nailk supplies. Journal of the Society of Dahy Technology 40, 23-6. .lo×l.:s, G. M., PIc-\Rsox, R. E., CL.~t~ot'~;H,G. A. & Hl.=-~Lo,C. W. (.1984). Relationships between somatic cell counts and milk production.Jom~al of Dahy Science67, 1823-31. I~×~;WH.L, R. G., DoDl~, F. H. & Nzavt.:, F. K. (1977). Machine milking and mastitis. In Machine Milking, ed. C. C. Thiel & F. H. Dodd, pp. 231-85. Reading: NIRD Shinfield. I.oc;ax, E. (1993). I n t r o d u c i n g a National Mastitis Control Programme. In Proceedings of the B~4tish Mastitis Conference 1993, pp. 50-61. Genus: AFRC Institute for Animal Health: Ciba. l.~Jc;t'l.:, D. N., Gc×x,J. & Fl~M.o×, D. (1992). Subclinical mastitis in p r o b l e m herds in Scotland. Animal Production 54, 479 (Abstr.). l.¢x;tt-, D. N., GUNN,J. & FENI.ON,D. (1993). Results of our a p p r o a c h to mastitis control in Scotland. In Proceedings of the British Mastitis Conference 1993, pp. 33-49. Genus: AFRC Institute for Animal Heahh: Ciba. N.vrz~l.:, R. P. & Evl.:Rt.:r3,R. W. (1975). T h e elimination of mastitis by culling. In Proceedings of Seminar on Mastilis Control. IDF BuUetin Docltment 85, pp. 303-310. United Kingdom Dait), Facts and Figures (1993). pp. 82-85. Federation of United Fdngdom Milk Marketing Boards, T h o m a s Ditton, Surrey. WH.soy,J. & Jl.:fTRl':v,D. C. (1987). A selective naedium for mastitis streptococci iri milk.Journal r?flhe Socie(v of Dai~y Technology40, 27-9.
(Ara~)ledfor puldicalion 12 Auffusl 1994)
A STUDY OF MASTITIS BACTERIA A N D H E R D MANAGEMENT PRACTICES T O IDENTIFY THEIR R E L A T I O N S H I P T O H I G H SOMATIC CELL C O U N T S IN BULK T A N K MILK
D. R. FENLON,* D. N. LOGUE, J. GUNN and J. WILSON*
*SAC Aberdeen, 581 King Street, Aberdeen, AB9 1UD and SAC Veterinary Services, Auchincruive, Ayr KA6 5AE, Scotland
SUMMARY Thirty dairy herds, selected to cover a wide range o f bulk tank somatic cell c o u n t (BTSCC) values, were used to study the relationship between the levels o f the principal species o f mastitis-causing bacteria, h e r d m a n a g e m e n t practices and the BTSCC. A good. correlation was f o u n d between the. n u m b e r o f mastitis streptococci (Strept0c0ccus agalactiae, S. dysgalactiae and S. ube~is) f o u n d in bulk tank milk and the BTSCC. Staphylococcus aureus was less significantly c o r r e l a t e d to BTSCC, but was o f increasing i m p o r t a n c e in b o r d e r l i n e BTSCC herds, where lower excretion levels into milk were unlikely to trigger hygiene penalties and so alert p r o d u c e r s to the p r e s e n c e o f a significant mastitis problem. High BTSCC herds had significantly lower yields a n d were less likely to use a post-milking teat dip or to have a regular p r o g r a m m e o f milking m a c h i n e m a i n t e n a n c e or automatic cluster removal. T h e s e herds also t e n d e d to buy in r e p l a c e m e n t s r a t h e r than b r e e d their own. Overall the m a n a g e m e n t o f high BTSCC herds showed less c o m m i t m e n t to i m p l e m e n t i n g mastitis control proc e d u r e s than herds with a consistently low BTSCC. Ki~w,o~'Ds: Mastitis; somatic cell counts; Staphylococcus aureus; Streptococcus spp; h e r d m a n a g e m e n t .
INTRODUCTION In a series o f Directives, 8 5 / 3 9 7 / E E C and 9 2 / 4 6 / E E C , the E u r o p e a n C o m m u n i t y has a d o p t e d the somatic cell c o u n t (SCC) as o n e o f the basic m e a s u r e m e n t s o f milk hygienic quality for i n t r a c o m m u n i t y trade (United Kingdom Dairy Facts and Figures, 1993). In o r d e r to e n c o u r a g e p r o d u c e r s to m e e t the EC standard o f < 400 000 cells ml -I in milk for h u m a n c o n s u m p t i o n , all the U n i t e d Kingdom milk 0007-1935/95/010017-09/$08.00/0
© 1995 Bailli~reTindall
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BRITISH VETERINARY JOURNAl., 151, 1
marketing boards (MMB) have incorporated bulk tank somatic cell cotmts (BTSCC) as a component of their milk quality payments. Premium and penalty payments are calculated on tile basis of the 3-month geometric mean of weekly BTSCC naeasurements. Together with the pre-existing hygienic payments based on a standard for total bacterial count (TBC), dairy herds are now facing increased financial pressure to produce milk of high hygienic quality. These two measures of milk hygienic quality are frequently adversely affected by the same factors. In particular a common cause of high TBC is the presence of large numbers of mastitis bacteria excreted into tile milk by subclinically infected cows, which also produce large numbers of somatic cells. A survey by Jeffrey and Wilson (1987) ill the Aberdeen and District Milk Marketing Board (ADMMB) region found that a preponderance of mastitis bacteria caused an elevation of TBC ill 43.8% of hulk milk samples which consequently incurred hygiene penalty. The majority of these mastitis bacteria were identified as streptococcal species (Jeffrey & Wilson, 1987). It was also recognized that the percentage of infected cows increased with BTSCC elevation (Jones et aL, 1984). In Scotland, Streptococcus agalactiae was found to be the most common subclinical mastitis infection causing high BTSCC (Logue et al., 1993). The objectives of the present study were frstly to confirm that many producers with high BTSCC also had difficulty in consistently meeting the TBC standard due to contamination of milk by mastitis bacteria, and secondly to deterntine the main bacterial 'contaminant' in milk in herds with BTSCC figures around the EC standard of 400 000 ml -~ which rarely incur hygiene penalties. Quantitative bacteriology using selective media enumerated the significant mastitis pathogens within the bacterial population of bulk tank milk samples. These data were combined with BTSCC and herd management information in an analysis of factors associated with elevated BTSCC.
MATERIALS A N D M E T H O D S
The 12-1nonth study began ill May 1990 and covered the period when producers were first informed that a payment penalty scheme based on SCC would be introduced by the ADMMB. Thirty producers were selected by tile Board on tile basis of their arithmetic mean BTSCC for the previous 12 months (June 1989-May 1990) to cover a range of cell counts, designated 'low' (<250 000 cells ml-I), 'borderline' (250-450 000 cells ml -j) and 'high' (>450 000 cells ml-I). At weekly intervals, later reduced to fortnightly, the latest routine bulk milk samples collected by the tanker drivel" at every collection fi-om the 30 study herds were taken from overnight refrigeration at 4°C at the Board's laboratory. The BTSCC figures used in this study were those determined in the appropriate week by ADMMB for the production of a rolling geometric mean. The bacteria identified and enumerated for tlle purpose of this study (Staphylococcus aureus, Streptococcus agalactiae, S. dysgalactiae and S. uberis) were recognized to be significant mastitis pathogens (Bramley & Dodd, 1984; Jeffrey & Wilson, 1987). Mastitis streptococci were counted on a specific streptococcal agar, using a pour plate technique on 10-fold dilutions in maximum recovery diluent
MASTITIS AND BULK TANK MILK SOMATIC CELL COUNTS
19
(MRD) (lab m Ltd) described by Wilson and Jeffrey (1987). Presumptive identification o f mastitis streptococci was based on colony m o r p h o l o g y and haemolytic pattern on blood agar; biochemical characteristics were established by sugar f e r m e n t a t i o n and catalase test. Aaatigenic-typing using 'Streptex' reagents (Wellcome Diagnostics Ltd) was used to confirm the identification. Staphylococci were isolated and e n u m e r a t e d by spreading 0.1 ml o f milk and a 10-fold dilution of the milk in MRD on Kranep agar (Oxoid Ltd) and identified by blood-agar colony m o r p h o l o g y and c o n f i r m e d as coagulase positive S. aureus using a 'Staphaurex' test kit (Wellcome Diagnostics Ltd) At the e n d o f the study the p r o d u c e r s whose bulk milk had been m o n i t o r e d were c o n t a c t e d by the Milk Board Regional Officer and h e r d m a n a g e m e n t information was collected by an interview questionnaire c o m p l e t e d on 29 o f the 30 farms. T h e h e r d m a n a g e m e n t information included n u m b e r and age o f lactating cows, culling and r e p l a c e m e n t policy, and calving pattern. T h e elements o f a mastitis control p r o g r a m m e used on the farm such as post-milking teat dipping, dry-cow therapy and automatic cluster removal (ACR) were also recorded.
RESULTS
T h e mean counts o f mastitis streptococci and S. aureus from the samples collected d u r i n g the course o f the investigation were calculated. Statistical analysis o f the data sbowed a significant relationship (P < 0.001) between the level o f mastitiscausing bacteria in the bulk milk and the BTSCC. T h e mean level o f mastitis streptococci was m u c h greater (geometric mean 1469 cfu ml -l) in high BTSCC herds than in e i t h e r the b o r d e r l i n e (geometric mean 557 cfu ml -t) or low BTSCC g r o u p (geometric mean 114 cfu ml -I) (Table I). Figure 1 illustrates the linear regression analysis o f log~0 BTSCC on logt0 mean streptococci. T h e h e r d categories r e m a i n e d as discrete groups along the regression line; log BTSCC = 4.614 + 0.3498 log strep
Table I Relationship between bulk tank somatic cell count (BTSCC) category and m e a n level o f main mastiffs bacteria Held category (BTSCC range 1000 ml)
High (467-969) Borderline (274-443) Low (136-247) SEO (27 df)
No. of BTSCC Loglo(B7SCC) Log~, mean mastitis Logl, mean herds (x 1000 ml5 I) (geomeO~c mean streptococd ml -I Staphylococcus x l O00 m f I) (geometric aureus ml-I mean ml -I ) (geometric mean mr j)
11 8 11
648 370 188
5.800 (631) 5.558 (361) 5.269 (186) 0.0408
*Standard error of difference between two means.
3.167 (1469) 2.746 (557) 2.055 (114) 0.1845
2.458 (287) 2.321 (209) 1.602 (40) 0.1560
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BRITISH VETERINARYJOURNAl, 151, 1
indicating mastitis streptococcal c o u n t was highly correlated with BTSCC (r='= 0.827, P < 0.001) over the whole range of cell counts. Regression analysis o f the S. aurc'us data showed that tiffs mastitis p a t h o g e n c o u n t was not as highly correlated with the BTSCC (r='=0.686) (Figure 2). T h e regression equation log BTSCC = 4.880 + 0.3134 log staph showed a laighly significant relationship (P<0.001), tlaough not as g o o d as that for the mastitis streptococci. T h e high and b o r d e r l i n e BTSCC results are less distinctly separated on the S. a u r e u s scatter plot than on that for the streptococci. T h e addition o f S. a u r e u s to the mastitis streptococci in the regression analysis explains slightly m o r e o f the p e r c e n t a g e variance (71.2) c o m p a r e d to the streptococci alone (68.1). I n c o r p o r a t i n g the S. a u r e u s and mastitis streptococci separately into the same regression equation, log BTSCC = 4.530 + 0.2709 log strep 0.1391 log-stapla revealed that mastitis streptococci have a significantly greater effect on the cell c o u n t than staphylococci, c o n f i r m i n g the results in Table I. This highlights that the problem in high BTSCC herds is associated with mastitis streptococci. T h e presence o f S. a u r e u s in bulk milk was distributed m o r e evenly between high and b o r d e r l i n e herds. Figures 3 and 4 present the quantitative bacteriology data fi-om two different high BTSCC herds to illustrate the difference in the level and pattern of excretion o f S. a u r e u s and mastitis streptococci, respectively, in bulk milk samples. T h e level o f mastitis streptococci r e a c h e d in excess o f 250 000 cfu ml -t in bulk tank milk,
+ t
-~ 5.75 + ~t3
m
+
5.5
+
E 5.25F
5!
,
1.5 1.75
t
2.25
2:5
I
2.75
3
I
3.25
I
3.5
__1
3.75
Log(mean mastitis streptococci m1-1)
Fig. 1. Regression analysis showing relationship between hJg,, mean bulk tank somatic cell count and log,, mean bulk tank mastitis streptococcal count.
tX,b\STITIS AND BUI.K TANK MII.K SOMATIC CELl. COUNTS
21
w h e r e a s t h e m a x i m u m S. a'tctvus level was 1 7 0 0 0 c f l l m l -I. T h e p r e d o m i n a n t mastitis p a t h o g e n was i d e n t i f i e d in 31 o f 330 b u l k m i l k s a m p l e s f r o m h i g h B T S C C h e r d s w h e r e t h e total mastitis b a c t e r i a e x c e e d e d 10 000 cfu ml -j. S. agalactiae was t h e p r e d o m i n a n t p a t h o g e n in 13 s u c h s a m p l e s ; 12 w e r e m a i n l y S. ube~Ts, a n d S. dysgalactiae was t y p e d in a f u r t h e r six s a m p l e s . Aalalysis o f t h e h e r d m a n a g e m e n t d a t a f r o m t h e q u e s t i o n n a i r e , s u m m a r i z e d in T a b l e II, i n d i c a t e d t h a t t h e r e w e r e m a j o r d i f f e r e n c e s in m a n a g e m e n t p r a c t i c e s , p a r t i c u l a r l y b e t w e e n h i g h B T S C C h e r d s a n d t h e rest. H i g h B T S C C h e r d s t e n d e d to b e younge,-, with a l o w e r p r o p o r t i o n o f cows o l d e r t h a n fifth l a c t a t i o n . T h e s e h e r d s also b o u g h t - i n r e p l a c e m e n t s r a t h e r t h a n b r e d t h e i r o w n heifers. H i g h
+ 5.~ Z. E
5.6 ¢0 r.fJ [.-, m 5.4 I=
5.2
.-"
5"
4.8
~
_ J_ _t i i ~ r i i I 0 0.25 0.5 0.~/5 1 1.25 1.5 1.75 2 2.25 2.5 2._5 Log(mean S. aureus m1-1)
Fig. 2. Regression analysis showing relationship between log,~ mean bt, lk tank somatic cell count and IOgl0 mean bulk tank Slaphylococcus au~vus count. ( I ) , High; (+), borderline; (*), lOW.
20 000
15 000 i
E
lO 000
5000
0
4
8
12 16 20 24 28 32 36 40 44 Week
48
52
Fig. S. Tile numbers of Stap@lococcus aureus in bulk tank milk over the pe,'iod o1' study fi-om the herd with the highest mean level of S. aureus in the bulk milk.
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BRITISH VETERINARYJOURNAL, 151, 1
30O 250 i
"~ 20C •~ 15(;
~ 100 ~ 5o 4
8
12
16 20 24 28 32 36 40 44 48 Week
52
Fig. 4. The numbers of mastitis st,'eptococci in bulk tank milk over the period of study from a herd with the highest mean level of Streptococcus agalactiae in the bulk milk.
Table II Relationship between bulk tank somatic cell count (BTSCC) and some management parameters
7~3,peof herd Mean BTSCC (×1000) Mean annual yield Size of herd (mean) Buy in replacements Post-milk teat dip/spray Possess ACR* Yearly machine test Percent herd >fifth lactation (average/herd) Mean culling rate Main reason for culling
High
Borderline
Low
648 5391 60-250 (106) 9/11 5/11 5/11 5/11 11%
370 6657 32-140 (90) 2/7 6/7 3/7 5/7 21%
188 6207 80-231 (133) 2/11 11/I 1 9/| 1 9/11 23%
17.8% Mastitis related
20.9% Infertility/low yield
16.3% Infe,tility/old age
*ACR, automatic cluster removal. B T S C C h e r d s were less likely to use a post-milking teat dip o r to have a r e g u l a r p r o g r a m n a e o f m i l k i n g m a c h i n e m a i n t e n a n c e . H i g h a n d b o r d e r l i n e h e r d s were less likely to have a u t o m a t i c cluster removal. A l t h o u g h the c u l l i n g rates were similar for all herds, mastitis-related causes were m o s t f r e q u e n t l y cited as the r e a s o n for culling in h i g h B T S C C h e r d s c o m p a r e d to b o r d e r l i n e a n d low B T S C C herds, w h e r e old age a n d infertility were c l a i m e d as the p r i n c i p a l causes.
DISCUSSI(~ Jeffrey & Wilson (1987) r e p o r t e d t h a t 43% o f T B C h y g i e n e failures in the A D M M B r e g i o n were d u e to the p r e d o m i n a n t - p r e s e n c e o f mastitis bacteria, 9 0 %
MASTITIS AND BULK TANK MILK SOMATIC CELL COUNTS
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o f which were mastitis Streptococci spp. ( S. agalactiae, S. dysgalactiae or S. uberis). A single cow with clinical mastitis may excrete mastitis streptococci at >10 000 000 cfu ml -t (Cousins & Bramley, 1981), which can potentially increase the TBC o f the bulk milk in a 100 cow h e r d by 100 000 cfu ml q. In this study, high levels o f mastitis streptococci were f o u n d in high BTSCC herds, 31 bulk samples w e r e identified in which mastitis streptococci were present in n u m b e r s > 10 000 cfu ml -~. Since persistent subclinical infection is a c o m m o n c o n s e q u e n c e o f infection by mastitis streptococci, infected cow(s) may remain u n d e t e c t e d . A high TBC may provide the only evidence o f such subclinical infection with mastitis streptococci, particularly when associated with a high BTSCC. S. aureus n u m b e r s were considerably lower than those o f mastitis streptococci, c o n f i r m i n g the findings o f Cousins and Bramley (1981) that S. aureus were excreted from an infected u d d e r in lower n u m b e r s than streptococci. Jeffrey and Wilson (1987) f o u n d S. aureus to be the p r e d o m i n a n t bacterium in only 3.6% o f TBC failures. This is in contrast to the prevalence o f the organism in individual cow milk samples from whole h e r d bacteriological surveys. In a non-quantitative analysis o f results of h e r d tests ill the ADMMB region between 1974-1990 u n d e r taken on herds with a clinical mastitis problem, S. aureus was the most c o m m o n isolate, a c c o u n t i n g for 65% o f all significant mastitis bacteria and was present in 16% o f all samples tested (Logue et al., 1992). Nevertheless in the present study, a significant level of S. aureus infection was d e t e c t e d in bulk milk samples from high and b o r d e r l i n e BTSCC herds, and can be a significant cause o f elevated BTSCCs in b o r d e r l i n e herds, where the TBC o f the bulk milk remains consistently below the penalty levels o f the Milk Boards. H u t t o n el al. (1989) observed that managers o f low SCC herds were m o r e likely to attend meetings, pay m o r e attention to details and have a greater awareness o f mastitis control practices. In Scotland, Logue et al. (1993) n o t e d that farms in the Scottish Milk Marketing Board area which r e c o r d e d milk yields had lower BTSCC figures than those that did not and were three times less likely to incur an SCC penalty. T h e y suggested that awareness and c o m m i t m e n t were t h e r e f o r e very i m p o r t a n t in mastitis control. T h e results o f o u r questionnaire also suggest that these factors c o n t r i b u t e to the low BTSCC in some herds. In a review of the effect o f the milking m a c h i n e and mastitis (IDF, 1987) overmilking a p p e a r e d not to be a significant cause o f new teat infection. However, in a study o f high (460 000 cells ml -I) and low (175 000 cells ml -t) BTS~2C herds, H u t t o n et al. (1989) r e p o r t e d atttomatic cluster removal (ACR) was m o r e fi-equently f o u n d in the low SCC group. T h e findings o f the c u r r e n t study support this observation and the use o f ACR has been shown significantly to r e d u c e BTSCC (Logan, 1993). This may be because with ACR the vacuum is shut off before the cluster is r e m o v e d r e d u c i n g irregular vacuum fluctuations within the m a c h i n e and r e d u c i n g the risk o f backflow o f milk on the teat with the conseq u e n t risk o f p e n e t r a t i o n o f tile teat duct (Kingwill et aL, 1977). Also, the presence o f ACR was associated with better m a i n t a i n e d milking e q u i p m e n t . This is significant since p e n e t r a t i o n o f the teat by mastitis bacteria is m o r e likely to o c c u r in a poorly functioning milking m a c h i n e in which there is excessive vacuum fluctuation or an i n c o r r e c t pulsation rate. T h e high BTSCC h e r d milking machines had p o o r e r testing records than those used for o t h e r h e r d groups. T h e excessively
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BRITISH VETERINARYJOURNAl,, 151, 1
high vacuum level in a poorly f u n c t i o n i n g milking m a c h i n e may result in teat damage. T e a t lesions are susceptible to colonization by as few as 100 cfu S. aurezts (Bramley el aL, 1979) and this bacterinm (and also S. dvsgalactiae) can readily penetrate the teat canal to establish an u d d e r infection. It is well d o c u m e n t e d that the o p e n teat orifice makes the u d d e r m o r e susceptible to infection by p a t h o g e n s after milking than before milking (Kingwill et aL, 1977) and that post-milking teat dipping reduces infection. In tile high BTSCC herds, the absence o f post-milking teat dipping allowed c o n t i n u e d cross-infection. A f u r t h e r significant difference between the high BTSCC herds and those with lower cell counts was a lower n u m b e r o f cows above fifth lactation. Age is known to result in h i g h e r individual cow somatic cell counts (ICSCC), however in uninfected quarters there would a p p e a r to be little or no age effect ( G u n n et aL, in p r e p a r a t i o n ) . Thus by i n f e r e n c e the rise is due to increased prevalence o f subclinical mastitis. This c o u p l e d with the fact that m o r e low BTSCC herds b r e e d their own replacements and are less likely to i n t r o d u c e infection from a n o t h e r h e r d suggests that there is an interrelationship between a n u m b e r o f different factors. However the effects o f such culling on mastitis will be limited if mastitis hygiene control measures are not put in place (Natzke & Everett, 1975). T h e lower overall yield was particularly noticeable in the high BTSCC g r o u p and may have been d u e to a c o m b i n a t i o n of clinical and subclinical mastitis c o u p l e d with the lower numbers o f high yielding o r d e r animals. T h e r e are limits to the information which can be gained from the e x a m i n a t i o n of bulk milk for mastitis organisms. ' E n v i r o n m e n t a l ' mastitis organisms such as Esche~vlchia coli originating from the u d d e r c a n n o t be differentiated fi'om those arising from faecal contamination. However, the organisms in the present study were shown by Veterinary Investigation C e n t r e data (VIDA, MAFF) to be responsible for almost half the clinical cases and virtually all the subclinical cases o f mastitis in Scotland. It is possible to identify the p r e d o m i n a n t bacterial species causing the TBC failure and provide an advance warning of a streptococcal mastitis problem, particularly S. agalactiae. It is also possible to d e t e r m i n e if there is an underlying problem. Gonzalez et al. (1986) f o u n d that bulk milk levels o f S. agalactiae in excess o f 4000 cfu ml -j gave a m o d e r a t e l y high correlation with at least 7% o f the h e r d shedding the bacterium; their study was less conclusive for S. au.reus. This study has shown that a m e a n level o f streptococcal mastitis bacteria in the milk of 1000 cfu ml -~ (log,, 3.0) suggest a definite streptococcal mastitis problem. This will often cause sporadic high TBC in the bulk milk and potential hygiene failures. D,qaen a h e r d has a BTSCC which remains obstinately a r o u n d 400 000 ml -~, with few if any TBC failures, the p r e s e n c e o f S. aureus in the bulk milk at levels o f 100 cfu ml -j (logj~ 9.0) is indicative o f a p r o b l e m in the herd. In summary high BTSCC herds generally failed to i m p l e m e n t standard mastitis control p r o c e d u r e s fldly. This was well illustrated by the herds with consistent low BTSCC which practised such a policy. ACKNOWLEDGEMENTS
T h e authors would like to thank Mike Franklin o f the Scottish Agricultural Statistics Service for the statistical analysis and T e r r i e Stewart for technical assistance
MASTITIS AND BUI.K TANK MII.K SOMATIC CEI.L COUNTS
25
in this p r o j e c t . T h e w o r k was f u n d e d with g r a n t s f r o m t h e t h r e e S c o t t i s h Milk M a r keting Boards and the Scottish Office Agricultural and Fisheries Department.
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(Ara~)ledfor puldicalion 12 Auffusl 1994)