Efficacy of an Iodine Backflush for Preventing New Intramammary Infections1

Efficacy of an Iodine Backflush for Preventing New Intramammary Infections I J. S. HOGAN, R. J, H A R M O N , B. E. LANGLOIS, R. W. HEMKEN, and W. L. CRIST Department of Animal Sciences University of Kentucky Lexington 40546-0215

ABSTRACT

physical condition of teat cup liners (8, 28). Bacteria on internal surfaces of milking clusters may be transmitted to teats of uninfected quarters at milking and give rise to new intramammary infections (5, 14, 17, 21, 23). Attempts to reduce this cow-to-cow spread of mastiffs pathogens have met with varying

Efficacy of an iodine backflush system for reducing new intramammary infection was tested in two 11-wk trials. Forty cows in each trial were paired by breed, age, stage of lactation, and intramammary infection status. Each pair was assigned randomly either to a group milked with clusters that were reverse flushed with water, 25 ppm iodine, water and air or to a group milked with clusters receiving no backflush treatment between cows. Backflushing clusters reduced infections caused by Corynebacteriurn boris and coagulasepositive staphylococci in both trials. However, backflushing clusters produced no clear advantage for reducing new infections with coagulase-negative staphylococci, Gram-negative bacilli, or streptococci (species other than Streptococcus agalactiae). No differences in somatic cell counts between experimental groups were observed. Teat cup liners and teat ends were swabbed after 120 and 1200 milkings/liner. Total microbial counts were significantly greater for liners that were not backflushed than from backflushed liners at each swabbing. However, no differences were significant between groups for mean teat end microbial counts in either trial.

Success.

INTRODUCTION

The microbial load of milking clusters is dependent on the number of microorganisms shed in milk from infected quarters, microflora of teat skin and lesions (6, 22), and

Received August 15, 1983. i The investigation reported in this paper (83-5-150) is in connection with a project of the K e n t u c k y Agricultural Experiment Station and is published with approval of the Director.

1984 J Dairy Sci 67:1850-1859

Earliest attempts to reduce microflora of milking clusters were by dipping teat cup liners in germicidal rinses. Although this method of disinfection reduced the number of bacteria, liners retained some bacterial contamination after clusters were dipped in disinfectant (19, 27, 28). Dodd et al. (7) reported circulating water at 74°C for 3 min or 85°(3 for 5 s through the milking cluster after milking Stapbylococcus aureus-infected cows reduced the number of bacteria on liners by 97 and 100%. However, disinfecting milking clusters by circulating 85°C water for 5 s through the units and applying postmilking disinfectant teat dips reduced intramammary infections by only 11 to 14% over teat dipping alone (7, 25). The need for special equipment, increases of milking routine time, and slight advantage gained in control of mastitis suggests that cluster disinfection between cows may be unfavorable in smaller herds. As the number of cows in a herd increases, any weakeness in a hygiene program affects more cows. The current trend in larger dairies is toward automation of hygiene practices to ensure minimal spread of organisms at milking. Devices that automatically disinfect the milking cluster with a backflush of germicidal solution have reduced the number of bacteria on teat cup liners by 99 to 100% (4, 10). Field surveys also have shown that backflushing the milking cluster between cows milked had an additional benefit to that of teat dipping in the control of intramammary infections (3). With current trend toward larger and more mechanized dairy farms, the role of automatic backflush systems for controlling the incidence

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IODINE BACKFLUSHAND INTRAMAMMARYINFECTIONS of mastiffs warrants further study under controlled conditions. This experiment was to investigate the effect of backflushing on the number of new intramammary infections, microbial numbers on teat ends and liners after milking, and somatic cell counts. M A T E R I A L S A N D METHODS Animal Selection

Forty cows each in two l l - w k trials were paired by breed, age, stage of lactation, and intramammary infection status. Each pair was assigned randomly to a group milked with clusters automatically backflushed after removal or a group milked with clusters receiving no hackflush treatment between cows. To increase the number o f S t a p h y l o c o c c u s a u r e u s infections, two cows from each experimental group were inoculated in one quarter each by intramammary infusion with S. a u r e u s 305 (830 cfu) 5 days prior to Trial 1. Only one inoculated quarter in the backflush group became established by the start of Trial 1. One quarter of one cow in each group was inoculated with 8 cfu, and one quarter in each of two cows in both experimental groups was inoculated with 82 cfu S. a u r e u s 305 prior to Trial 2. Two of six inoculated quarters had established S. a u r e u s infections at the start of Trial 2, both in the control group. All established experimental infections persisted throughout each trial and were included in the numbers of infections that existed at the beginning of each trial (Tables 1 and 4). Intramammary infusions of challenge inocula were as outlined by Newbould and Neave (24). Trial 1 ran from November 22, 1982 to February 3, 1983. Trial 2 was from February 28, 1983 to May 13, 1983. Milking Procedures

The University of Kentucky dairy has two identical milking parlors operating on the same milking system equipped with automatic takeoffs and backflush. 2 Each is a double-two, sideopening parlor with four milking units. The

2HTO, Babson Bros. Co., Oakbrook, IL. aRapidyne, West Agro Chemical, Inc., Shawnee Mission, KS. 4Analytab Products, Plainview, NY.

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system was designed so the automatic backflush could be operative at any individual stall or combination of stalls. The backflush treatment group of cows was milked in one parlor with the backfiush system automatically initiated 20 s after machine removal. The five phase cycle incorporated in this backflush system was: 1) clear water r i n s e (.5 to .7 liters), 2) iodophor rinse (.5 to .7 liters; 25 ppm I), 3) iodine contact time (45 s), 4) clear water rinse (.5 to .7 liters) and 5) positive air dry (5 s). Available iodine concentration of the backflush rinse was determined as outlined by LaCroix and Wong (13). Standards were formulated by the concentrated iodophor sanitizer 3 utilized in the backflush system. Control cows were milked in the other parlor under the same regimen as the backflush group except clusters were not backflushed. Cows were milked in the same parlors in both trials with backflush treatments switched between parlors in the two trials. In the second trial, seven and five cows in the control and backflush groups were replaced because of stage of lactation. Udders and teats of all experimental cows were hand washed with disinfectant-free spraying water and dried with single service paper towels prior to milking. Milkers' hands were covered with smooth rubber gloves that were disinfected between cows by dipping first in a clear water rinse, then in an iodophor solution (100 ppm I). Disinfectant teat dips were not used throughout the two trials. Quarter Foremilk Samples

Quarter foremilk samples for bacterial isolation and somatic cell counts (SCC) were collected in 50-ml sterile screw cap tubes biweekly (18). Quarters yielding organisms from biweekly collected samples were resampled within 7 days. Foremilk samples also were collected from all quarters with signs of clinical mastiffs prior to treatment. All quarter samples were plated (.01 ml) on 5% sheep blood-esculin agar and incubated at 35°C for 48 h for intramammary infections (18). All isolates were Gram stained and tested for catalase activity. Tube coagulase and lysostaphin tests were conducted on all Grampositive, catalase-positive cocci to classify coagulase-positive and coagulase-negative staphylococci. Coagulase-positive staphylococci species were determined by the API 4 Staph-Ident Journal of Dairy Science Vol. 67, No. 8, 1984

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HOGAN ET AL.

system. Corynebacterium boris were identified by colony morphology on blood-esculin agar, cellular morphology, and Gram reaction. Gram-negative bacilli and Gram-positive, catalase-negative cocci were identified by the University of Kentucky Livestock Disease Diagnostic Laboratory. An intramammary infection was described as two or more consecutive quarter samples yielding the same organism or isolation of a mastitis pathogen from a quarter exhibiting clinical mastitis. After initial confirmation of an infection, any reoccurrence of the same organism in samples from the same quarter was judged as a previously established infection. In both trials, SCC were electronic on biweekly collected samples with a Coulter Milk Cell Counter. Results were SCC per milliliter of milk. Teat End and Liner Swabs

After approximately 120 and 1200 milkings per liner, teat ends and liners were swabbed. Cows in both treatment groups were milked in random order with either the left or right front teat and corresponding liner being swabbed after first, third, and fifth cows were milked at each of the four units. The teat and linear selected for swabbing was dependent on the parlor side on which the cow was milked with the front quarter closest to the milker being swabbed. All swabs were taken after unit removal from the cow and after the backflush cycle was completed (for backflush group). Sterile cotton swabs were moistened in a buffered rinse solution containing .1% sodium thiosulphate immediately before swabbing (15). Teat ends were swabbed by four crossing motions over the teat end surface with equal pressure applied to each swabbing. Teat cup liners were swabbed by four swirls around the liner as the swab was brought from bottom to top. Special care was taken not to touch the swab to the linear mouthpiece. After teat end or liner was swabbed, swabs were placed into a screw cap test tube containing 4.0 ml of the rinse solution. Swab samples were placed on ice for transport from farm to laboratory.

s Fisher Scientific Co., Pittsburgh, PA. Journal of Dairy Science Vol. 67, No. 8, 1984

Bacterial enumeration was by our vigorously shaking the rinse solution for 20 s, performing decimal dilutions of each swab rinse solution in .1% peptone water, and pour-plating with APT agar. Plates were incubated at 35°C for 48 h, and colony forming units determined with an automated colony counter, s Statistical Analysis

Infection data were analyzed by binomial distribution for subset unequal numbers (9). Somatic cell counts and teat end and liner bacterial numbers were converted to tog10 for statistical analysis. Statistical analysis of somatic cell counts was by a random design in which the first sample period was a covariant. Bacterial counts were analyzed by a Student's t test (26). RESULTS Trial 1

Intramammary infections are in Table 1. Total number of new infections in quarters milked with backflushed clusters was 73 compared to 63 in quarters milked with untreated clusters (P>.05). Coagulase-negative staphylococci accounted for 68% of the total infections in the backflush group and 49% of infections in the control group. The difference in numbers of coagulase-negative staphylococci infections between groups (50 and 31 coagulasenegative staphylococci infections in backflush and control groups) was significant (P<.05). Quarters' milked with untreated clusters totaled 10 coagulase-positive staphylococci infections compared to 3 in quarters milked with backflushed clusters (P<.05). Cumulative new infections for coagulase-positive staphylococci are in Figure 1. Of the 10 coagulase-positive staphylococci infections in the control group, 8 were due to S. aureus, and 2 were due to S. byicus. The three coagulase-positive staphylococci infections in the backflush group were due to S. aureus. The number of new C. boris infections in cows milked with backflushed liners was 7 compared to 17 in the control group (P<.05). Cumulative new C. boris infections are in Figure 2. Although there were no significant differences (P>.05) in the number of streptococci or Gram-negative bacilli infections between groups, the difference in Gram-negative bacilli infections approached

IODINE BACKFLUSH AND 1NTRAMAMMARY INFECTIONS

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TABLE 1. Bacterial infections for backflush (BF) and control (C) groups at the beginning and during Trial 1. Number new infections during Trial 1

Number infections beginning Trial 1 Bacterial group Coagulase-negative staphylococci Coagulase-positive staphylococci

Corynebacterium boris Streptococci Gram-negative bacilli Total

BF

C

BF

C

11

10

50 a

31 b

4 2 3

3 2 2

3a 7a 6

10 b 17b 3

1

0

7c

2d

21

17

73

63

a'bNumbers in the same row differ (P<.05). C'dNumbers in the same row differ (P<.10).

significance ( P < . 1 0 ) w i t h m o r e o c c u r r i n g in t h e b a c k f l u s h g r o u p . S t r e p t o c o c c i i n f e c t i o n s were w i t h species o t h e r t h a n S. agalactiae. N o d i f f e r e n c e s ( P > . 0 5 ) were o b s e r v e d in n u m b e r o f clinical i n f e c t i o n s (Table 2) b e t w e e n experim e n t a l groups. M e a n log10 c o u n t s for liner a n d t e a t end swabs are in T a b l e 3. Significantly m o r e organisms were r e c o v e r e d f r o m liners receiving no backflush treatment than from backflushed liners a f t e r 120 and 1200 m i l k i n g s p e r liner (P<.0001). Although total counts from backflushed liners were less t h a n t h o s e f r o m unt r e a t e d liners, o n l y 12.5% o f b a c k f l u s h e d liners yielded n o d e t e c t a b l e organisms. D i f f e r e n c e s in t e a t e n d m i c r o b i a l c o u n t s were n o t significant between the two groups (P>.05). G e o m e t r i c m e a n SCC for q u a r t e r s m i l k e d w i t h b a c k f l u s h e d clusters was 7 6 , 2 0 0 w i t h a first s a m p l e p e r i o d average o f 6 9 , 2 0 0 . S o m a t i c cell c o u n t s averaged 6 7 , 8 0 0 in q u a r t e r s m i l k e d w i t h u n t r e a t e d clusters. G e o m e t r i c m e a n SCC in t h e first s a m p l e p e r i o d for t h e c o n t r o l g r o u p was 5 7 , 5 0 0 . No significant d i f f e r e n c e s (P>.05) in SCC were f o u n d b e t w e e n e x p e r i m e n t a l g r o u p s at a n y sample period. S o m a t i c cell c o u n t s increased significantly ( P < . 0 5 ) in b o t h g r o u p s b e t w e e n wk 5 a n d 7 and also b e t w e e n w k 9 a n d 11.

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Figure 1. Cumulative new coagulase-positive staphylococci infections for backflush (BF) and control (C) groups in Trial 1. Journal of Dairy Science Vol. 67, No. 8, 1984

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HOGAN ET AL.

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Figure 2. Cumulative new Corynebacterium boris infections for backflush (BF) and control (C) groups in Trial 1.

Trial 2

Table 4 lists i n t r a m a m m a r y infections in Trial 2. Total new infections in quarters milked

with backflushed clusters was 42 c o m p a r e d to 78 in quarters milked with u n t r e a t e d clusters (P<.05). The percent of n e w infections caused by coagulase-negative staphylococci was 69% in the backflush group and 40% in the control group. These results were similiar to those in Trial 1. Quarters milked with control clusters totaled 31 coagulase-negative staphylococci infections c o m p a r e d to 29 in quarters milked with backflushed clusters (P>.OS). As in Trial 1, the difference in n u m b e r of C. boris infections in control and backflush groups (39 and 1 0 ) w a s significant (P<.05). The cumulative n u m b e r of C. boris infections for Trial 2 is in Figure 3. F o u r Gram-negative bacilli infections occurred in quarters milked with clusters receiving no disinfecting t r e a t m e n t c o m p a r e d with n o n e in quarters milked with backflushed clusters (P<,05). There were no differences (P>.05) in numbers of infections with streptococci or coagulase-positive staphylococci between experimental groups. Infections with coagulase-positive staphylococci were equally distributed b e t w e e n S. aureus and S. b y i c u s in the control group and due to S. aureus in the backflush group. As in Trial 1, all streptococci isoIated f r o m infected glands were species o t h e r than S. agalactiae. Only two clinical infections were detected in each e x p e r i m e n t a l group. Clinical infections in the control group were one S. aureus infection and one Gram-negative bacilli infection. Clinical infections in the backflush group were one S. aureus infection and one streptococci infection. Table 5 shows c o m b i n e d new infections for Trials 1 and 2. Both coagulase-positive staphylococci and C. bovis infections were signifi-

TABLE 2. Bacterial infections from clinical quarters for backflush (BF) and control (C) groups in Trial 1. Number new infections Bacterial group Coagulase-negative staphylococci Coagulase-positive staphylococci Streptococci Gram-negative bacilli No isolate Total

Journal of Dairy Science Vol. 67, No. 8, 1984

BF

C

0 1 4 3 4

1 5 3 2 1

12

13

IODINE BACKFLUSH AND INTRAMAMMARY INFECTIONS

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TABLE 3. Mean log10 bacterial counts from liner and teat end swabs for backflush (BF) and control (C) groups in Trial 1. Milkings/ liner

n

BF

120 1200

15 15

.903 a .778 a

3.851 b 3.954 b

.845 a

3.903 b

Mean

Avg. log10 cfuC/liner C

n

Avg. log10 cfu/teat BF C

15 14

4.683 4.643

4.869 4.740

4.662

4.806

a'bMeans in the same rows differ (P<.O001). C

Colony forming units.

candy (P<.05) reduced by backflushing of clusters. A l t h o u g h t h e r e were m o r e ( P < . 0 5 ) n e w coagulase-negative s t a p h y l o c o c c i i n f e c t i o n s in t h e b a c k f l u s h group, this d i f f e r e n c e was p r i m a r i l y d u e to results o f Trial I (Table 1). New s t r e p t o c o c c i a n d G r a m - n e g a t i v e bacilli i n f e c t i o n s were n o t significantly ( P > . 0 5 ) a f f e c t e d b y b a c k f l u s h i n g o f clusters. T a b l e 6 lists t h e m e a n logl0 c o u n t s f r o m tiner a n d t e a t e n d swabs. A s in Trial 1, a signific a n t l y g r e a t e r n u m b e r o f organisms were recovered f r o m liners receiving n o b a c k f l u s h t r e a t m e n t t h a n f r o m b a c k f l u s h e d liners after b o t h 120 and 1 2 0 0 m i l k i n g s p e r liner ( P < . 0 0 0 1 ) . However, no organisms were d e t e c t e d f r o m o n l y 16.7% of b a c k f l u s h e d liners sampled. T h e t o t a l m i c r o b i a l liner c o u n t for control liners was g r e a t e r a f t e r 1 2 0 0 m i l k i n g s

t h a n a f t e r 120 m i l k i n g s ( P < . 0 5 ) . T o t a l c o u n t per t e a t end f r o m t e a t s m i l k e d w i t h u n t r e a t e d liners was significantly greater t h a n t h o s e f r o m t e a t s m i l k e d w i t h b a c k f l u s h e d liners after 120 milkings per liner ( P < . 0 5 ) . Also, m e a n t o t a l c o u n t was greater after 1 2 0 0 milkings t h a n a f t e r 120 m i l k i n g s f r o m t e a t e n d s m i l k e d w i t h b a c k f l u s h e d liners ( P < . 0 5 ) . No o t h e r d i f f e r e n c e s b e t w e e n t e a t e n d m i c r o b i a l c o u n t s were significant (P>.05). G e o m e t r i c m e a n SCC for q u a r t e r s m i l k e d w i t h b a c k f l u s h e d clusters was 1 1 1 , 0 0 0 , w i t h a first sample p e r i o d average o f 6 2 , 0 0 0 . S o m a t i c cell c o u n t s d u r i n g t h e trial averaged 1 2 1 , 0 0 0 for ,quarters m i l k e d w i t h u n t r e a t e d clusters. M e a n SCC in t h e first sample p e r i o d for t h e c o n t r o l g r o u p was 6 8 , 0 0 0 . T h e r e were n o significant d i f f e r e n c e s b e t w e e n e x p e r i m e n t a l

TABLE 4. Bacterial infections for backflush (BF) and control (C) groups at the beginning and during Trial 2. Number new infections during Trial 2

Number infections beginning Trial 2 Bacterial group Coagulase-negative staphylococci Coagulase-positive staphylococci Corynebacterium bovis

Streptococci Gram-negative bacilli Total

BF

C

BF

C

10

10

29

31

6 10 1

5 8 2

1 10 a 2

4 39 b 0

0

0

0a

4b

27

25

42 a

78 b

a'bNumbers in the same row differ (P(.05). Journal of Dairy Science Vol. 67, No. 8, 1984

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HOGAN ET AL.

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greater (P<.05) than mean SCC for the first sample period and wk 3 and 7 in both experimental groups.

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Figure 3. Cumulative new Corynebacterium bovis infectious for backflush (BF) and control (C) groups in Trial 2.

groups at any sample period (P>.05). With the e x c e p t i o n o f wk 7, an increasing trend for SCC was observed in b o t h groups. Mean SCC per quarter in wk 5, 9, and 11 was significantly

The objective o f utilizing an iodine backflush system is to reduce the n u m b e r of microorganisms transferred to the teat end of uninfected quarters via the milking machine. This e x p e r i m e n t was designed so that experimental groups were exposed to similiar natural bacterial challenges with the e x c e p t i o n that one group was milked with clusters backflushed with a 25 ppm iodine solution after each cow was milked whereas a control group was milked with clusters receiving no disinfecting t r e a t m e n t b e t w e e n cows milked. Milking procedures were uniform b e t w e e n experimental groups including milkers wearing rubber gloves that were disinfected before handling udders and teats to reduce the transfer of organisms f r o m hands to uninfected quarters (20). Because the University of K e n t u c k y dairy herd maintains a low incidence of infection with major pathogens, teats were not dipped in a disinfectant after milking to enhance infection rates. A l t h o u g h m a n a g e m e n t practices were the same for both experimental groups, noticeable differences b e t w e e n groups in traffic areas were observed. Cows in the backflush group in Trial 1 and the control group in Trial 2 travelled a long lane and tended to have dirtier udders than the other groups. It is n o t k n o w n if this influenced numbers of infections by streptococci and Gram-negative bacilli. Results of bacterial infections indicate that backflushing milking units with an i o d o p h o r

TABLE 5. Combined new bacterial infections for backflush (BF) and control (C) groups in Trials 1 and 2. New infections Bacterial group Coagulase-negative staphylococci Coagulase-positive staphylococci Corynebacterium boris Streptococci (non-ag) Gram-negative bacilli Total a'bNumbers in the same row differ (P<.05). Journal of Dairy Science Vol. 67, No. 8, 1984

BF 79 a 4a 17 a 8

7 115 a

C 62 b 14 b 56 b 3 6 141 b

IODINE BACKFLUSHAND INTRAMAMMARYINFECTIONS

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TABLE 6. Mean log10 bacterial counts from liner and teat end swabs for backflush (BF) and control (C) groups in Trial 2. Milkings/ liner

n

BF

120 1200

15 15

.520a .814 a

3.887be 4.693 bf

.667a

4.273 b

Mean

Avg. log10 cfug/liner C

Avg. log j0 cfu/teat C

n

BF

15 15

3.734ce 5.142f

4.830 d 5,001

4.438

4.915

a'bMeans in the same row differ (P<.O001). C'dMeans in the same row differ (P<.05). e'fMeans in the same column differ (P<.05). gcolony forming units.

solution reduces the spread of C. boris and coagulase-positive staphylococci infections. Differences in C. boris infection numbers between cows milked with backflushed clusters (7 new infections in Trial 1 and 10 new infections in Trial 2) and cows milked with untreated clusters (17 and 39 new infections in Trials 1 and 2) were significant in both trials. The increase of C. boris infections in cows milked with clusters that were not disinfected was similiar to increases of Corynebacterium boris infections reported by Bramley (1) and Waterman et al. (29) in herds not teat dipping. Results of this experiment and others (1, 29) indicate that C. bovis is a contagious mastiffs organism susceptible to hygiene practices. Although differences were not significant in Trial 2, a greater number of coagulase-positive staphylococci infections were observed in control groups than in backflush groups. Although small numbers of new infections were observed, there was greater than a 70% reduction of coagulase-positive staphylococci infections in cows milked with backflushed clusters compared to cows milked with clusters receiving no backflush treatment. The limited spread of these organisms may be attributed in part to the few coagulase-positive staphylococci infections in experimental groups prior to the trials. This experiment should be repeated with a higher natural challenge or with an experimental challenge of coagulase-positive staphylococci. Coagulase-positive staphylococci species isolated from infected glands were S. aureus and S. hyicus in the control group and S. aureus in the backflush group.

Backflushing milking clusters showed little efficacy in reducing coagulase-negative staphylococci infections. Infections with this bacterial group appear to occur principally during the intermilking period with spread of infections during milking being minimal. The high incidence of coagulase-negative staphylococci infections is an interesting aspect of this experiment. The number of infections increased rapidly in both trials approximately 1 mo after teat dipping was discontinued. Gram-negative bacilli and streptococci infections also were not affected greatly by backflushing of clusters. Results of this experiment support the findings of Jasper and Dellinger (11) and King (12) that infections with these organisms are principally a result of exposure to the organisms between milkings with transfer of Gram-negative bacilli and streptococci infections (other than S. agalactiae) at milking being secondary. Environmental factors appeared to have a greater influence on infection rates with these two bacterial groups than did backflushing. The efficiency of the backflush system in reducing the microbial load of teat cup liners under natural challenge was similiar to the 99% efficiency reported by Busnell et al. (4) and Jasper and Bushnell (10) when liners were contaminated with an experimental challenge. The number of backflushed liners from which no organisms could be recovered was less than expected, with Gram-positive, sporeforming bacilli being the bacterial group most commonly isolated. The reason for the prevalence of Gram-positive bacilli on backflushed liners is unclear, but McDonald (16) also reported the Journal of Dairy Science Vol. 67, No. 8, 1984

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p r e s e n c e o f G r a m - p o s i t i v e bacilli o n liners a f t e r p a s t e u r i z i n g t e a t cups for 30 s at 90°C. B u r r o w s et al. (2) s t a t e d t h a t t h i s g r o u p o f b a c t e r i a can be isolated f r o m soil and d u s t a n d is resistant to t h e germicidal e f f e c t of h e a t a n d d i s i n f e c t a n t s w h i l e in t h e s p o r e state. A t r e n d t o w a r d increased m i c r o b i a l n u m b e r s o n liners was f o u n d as milkings p e r liner increased in clusters receiving n o b a c k f l u s h t r e a t m e n t (8, 28). This t r e n d was n o t in b a c k f l u s h e d liners. Backflushing clusters p r o d u c e d n o clear a d v a n t a g e in reducing the number of organisms recovered f r o m t e a t ends. Because t h e n u m b e r o f organisms t r a n s f e r r e d b y b a c k f l u s h e d clusters was decreased significantly, t h e bacterial load on the teat end after milking appeared to be most d e p e n d e n t u p o n c o n t a m i n a t i o n f r o m sources other than the milking unit. No d i f f e r e n c e s were significant in SCC b e t w e e n e x p e r i m e n t a l g r o u p s in e i t h e r trial. However, SCC generally increased t h r o u g h o u t b o t h trial p e r i o d s for b o t h q u a r t e r s m i l k e d w i t h b a c k f l u s h e d clusters and q u a r t e r s m i l k e d w i t h clusters receiving n o b a c k f l u s h t r e a t m e n t . This t r e n d was a s s u m e d to be caused b y t h e high incidence of intramammary infections with s e c o n d a r y p a t h o g e n s in b o t h e x p e r i m e n t a l groups. In s u m m a r y , t h e b a c k f l u s h s y s t e m r e d u c e d c o n t a g i o u s spread of m a s t i t i s p a t h o g e n s via t h e m i l k i n g cluster. As e x p e c t e d , n u m b e r s of i n f e c t i o n s w i t h o r g a n i s m s n o t c o m m o n l y spread f r o m cow t o cow at m i l k i n g were n o t a f f e c t e d b y b a c k f l u s h i n g . Efficacy o f this b a c k f l u s h s y s t e m was t e s t e d in a h e r d n o t utilizing a d i s i n f e c t a n t t e a t dip after milking. F u r t h e r studies are n e e d e d to d e t e r m i n e t h e effectiveness of b a c k f l u s h i n g clusters as p a r t o f a t o t a l milking hygiene program including disinfectant t e a t dipping. ACKNOWLEDGMENTS

T h e a u t h o r s are grateful to B a b s o n Bros. Co., O a k b r o o k , IL for e q u i p m e n t supplied g e n e r o u s l y for this study. T h e e x c e l l e n t t e c h nical assistance o f Bernice S m i t h and K a b b y A k e r s also is a c k n o w l e d g e d gratefully. REFERENCES

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Journal of Dairy Science Vol. 67, No. 8, 1984