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Coagulase-Positive Staphylococcus Intramammary Infections in Primiparous Dairy Cows1
PHYSIOLOGY AND MANAGEMENT
Coagulase-Positive Staphylococcus Intramammary Infections in Primiparous Dairy Cows 1 J. R. ROBERSON, L. K. FOX, D. D. HANCOCK, and C. C. GAY Field Disease Investigation Unit Department of Veterinary Clinical Sciences Washington State University Pullman 99164-6610 T. E. BESSER Department of Veterinary Microbiology and Pa~holo~y Washington State University Pullman 99164-7040 ABSTRACT
prevalence of coagulase-positive staphylococcal IMI (9.2%; 40 of 436) at parturition than did primiparous cows from low herds (6.9%; 27 of 392), the difference was not significant. Of primiparous cows with coagulase-positive staphylococcal IMI at parturition, 43% had coagulase-positive staphylococcal IMI at least 2 mo after parturition. Primiparous cows with coagulase-positive staphylococcal IMI at parturition may represent significant reservoirs of infection to uninfected herdmates. (Key words: coagulase-positive staphylococci, Staphylococcus aureus, prevalence, mastitis)
Objectives were to determine the prevalence of coagulase-positive staphylococcal IMI in primiparous cows at first parturition, to contrast the differences in coagulase-positive staphylococcal IMI in primiparous cows at parturition in herds with high and low prevalences of coagulase-positive staphylococcal IMI in the lactating herd, and to determine the percentage of primiparous cows having persistent coagulase-positive staphylococcal IMI. Milk samples were collected aseptically from cows at the start and end of the study, at dry-off, and at parturition. Herds (n = 18) were split evenly into two categories: high (>10%) or low (<5%) prevalence of coagulase-positive staphylococcal IMI. At the start, the mean prevalence of coagulase-positive staphylococcal IMI in high prevalence herds was 30%, ranging from 13 to 65%, and in low prevalence herds was 2%, ranging from 0 to 5%. Overall the prevalence of coagulase-positive staphylococcal IMI in primiparous cows at parturition was 8.1 % (67 of 828), ranging from o to 27%. Although primiparous cows from high prevalence herds had a higher
Abbreviation key: CI = cumulative incidence; CNS = coagulase-negative staphylococci, CPS = coagulase-positive staphylococci, HCPS = high prevalence of CPS IMI herds, IR = incidence rate; LCPS = low prevalence of CPS IMI herds. INTRODUCTION
Received July 6, 1993. Accepted November 12, 1993. IThis study was supported in part by Grant lOA 3072 0858 from the Agricultural Research Unit, Washington State University, and Grant BC 2530 0123 from the Washington State Dairy Products Commission. 1994 J Dairy Sci 77:958-969
Staphylococcus aureus, the most prevalent and most costly of the major mastitis pathogens, can result in both clinical and subclinical mastitis (5). Although a great deal of progress in mastitis control has taken place over the years, S. aureus IMI may still occur in more than 80% of dairies (14, 33). However, the prevalence of S. aureus IMI has been reduced to less than 5% of cows in many herds (12, 26). Some herds claim complete eradication, but others, which practice all the standard milking time hygiene techniques and which have mean SCC in milk of less than 200,000
958
STAPHYLOCOCCUS MASTITIS IN HEIFERS
cells/ml, have been unable to eradicate S. aureus (19). One reason for the failure to eradicate S. aureus IMI may be that the control measures for mastitis do little to control the disease in prepartum primiparous cows. Thus, for those cows, S. aureus IMI at parturition may represent an impediment to eradication of S. aureus. Not only do these cows add to the prevalence of S. aureus IMI in the lactating herd, but they also can act as reservoirs of this contagious pathogen to uninfected herdmates. Although several studies have documented that primiparous cows may have S. aureus IMI prior to entering the lactating herd (3, 4, 30, 32), these studies were conducted with three or fewer herds and in one study with only 10 primiparous dairy cows (4). Therefore, both the prevalence of this infection in primiparous cows at parturition and the relationship between primiparous cows with S. aureus IMI at parturition and the prevalence of S. aureus IMI in the lactating herd are not well established. In a 1942 report, Schalm (27) concluded that feeding milk containing Streptococcus agalactiae to heifer calves and subsequent suckling among heifers would result in IMI by this major contagious pathogen at first parturition. Although never proven, the transmission pathway of S. aureus IMI in heifer calves is assumed to be the same as that for Strep. agalactiae (22). It is generally accepted that the infected mammary gland represents the major reservoir of S. aureus for dairy cows (1, 5, 9). Therefore, the major method of transferring S. aureus to heifer calves may be by feeding mastitic milk to heifer calves. Thus, herds with a high prevalence of S. aureus 1MI would expose many more heifer calves to this contagious pathogen than would herds with a low prevalence of S. aureus IMI. Therefore, we hypothesized that the prevalence of S. aureus IMI in primiparous cows at parturition from herds with a high prevalence of S. aureus IMI in the lactating herd is higher than the prevalence of S. aureus IMI in primiparous cows at parturition from herds with a low prevalence of S. aureus IMI. The purpose of this study was to contrast the prevalence of S. aureus IMI in primiparous cows at parturition in herds with high and low prevalences of S. aureus IMI among multiparous cows.
959
MATERIALS AND METHODS
Herd Selection
The nomenclature of coagulase-positive staphylococci (CPS) and S. aureus are often used interchangeably. Historically, staphylococci that gave a positive coagulase test reaction were considered to be S. aureus (6). Even though taxonomic studies have identified CPS species (Staphylococcus intermedius and some strains of S. hyicus) other than S. aureus (10, 18), for bovine mastitis, the terms are almost synonymous, because more than 95% of all CPS that are associated with IMI are S. aureus (15, 20). However, in this paper, the more accurate term, CPS, will be used to describe staphylococcal isolates aseptically collected from milk that caused a positive tube coagulase reaction. A heifer is defined as a female bovine that has not had her first parturition. A primiparous cow is defined as a female bovine that has had her first parturition and is in her first lactation. Herds were selected based on a past history of CPS IMI and a willingness to cooperate in the project. Based on the herd's historical CPS IMI prevalence, an effort was made to select an equal number of herds with high CPS (HCPS) and low CPS (LCPS) prevalence and to include herds from all major dairy areas of Washington State. Fourteen herds contained only Holstein cows, 1 contained only Jersey cows, I contained only Guernsey cows, 1 contained Holstein and Jersey cows, and 1 contained Holstein and Brown Swiss cows. Herd size ranged from 45 to 375 milking cows. Based on the initial results of milk culture for the herds, the herds having a CPS prevalence of 5% or less were assigned to the LCPS group. Herds having a CPS prevalence of greater than 10% were assigned to the HCPS group. Pertinent management practices and herd demographics are summarized in Table 1. One HCPS herd withdrew before the end of the study. Data from this herd were included for prevalence but not for incidence calculations. Sample Periods and Collection
Composite milk samples were collected aseptically (1) by the investigators from all Journal of Dairy Science Vol. 77. No.4, 1994
.... 0
'D 0\
c::
0
3
e!-
....o a
~.
Q en n
".
TABLE I. Demographics and heifer management practices of herds with high (HCPS) and low (LCPS) prevalence of coagulase-positive staphylococcal 1M!. HCPS Herds
::I
!i
< !2....,
;-.J
Z
0
!"
:g .f>.
Breed I Initial lactating herd size, no. Initial 12 mo mean, SCC x 1()3 Time calf with dam after birth, h Preweaned calf housing 3 Preweaned calf feeding" Milker feeds calves Time weaned prior to group penning, d Age first bred, mo Heifers grouped with dry cows 7 Heifers grouped with lactating cows 7 IDairy breeds: BS 2NK
= Not
B
G
I
J
K
M
N
0
Q
A
C
0
E
F
H
L
P
R
J
H
H
H
H
H
H
H
H-BS
H-J
G
H
H
H
H
H
H
H
131
104
133
39
200
135
323
152
88
137
88
119
76
166
299
241
329
118
270
360
562
301
214
584
370
704
426
341
176
142
164
164
217
157
270
173
12
12
24
2
NK2
24
12
24
6
8
12
24
12
24
4
5
24
72 ::Ill
B
I
I
NK
I
I
I
G
I
B
I
I
I
I
I
B
0
I
tl:l
ttl
W/S N
WfR Y
NA6 13
NA
0 15
Y
Y
Y
Y
NK
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
N
Y
N
N
N
Y
NK
Y
N
N
Y
N
Y
N
Y
N
N
N
N
Y
= Guernsey,
H
W Y
NK NK
W
Y
W N
W N
W NS
W N
WfR Y
NA
0 14
S Y
W N
W/S Y
W Y
W N
W N
W Y
W N
en
0
z tT1
= Brown
18
Swiss, G
0 14
NK NK
60
15
= Holstein,
0 14
and J
0 15
14
NA
15
3 14
0 15
0 14
10
10
14
13
= Jersey.
known. Herd dropped out before data was obtained.
3Preweaned housing: I
= Yes; N = no. = Not applicable
6NA
B
::Ill
4Preweaned feeding: W
sy
LCPS Herds
= individual hutch or pen, G = group pen, and B = calves subject to both individual and grouping = waste milk (colostrum, antibiotic, and mastitic), R = milk replacer, and S = saleable milk. if preweaned heifers already group penned.
7Grouped within 2 mo prior to parturition.
prior to weaning.
4 14
60
15
-l
~
961
STAPHYLOCOCCUS MASTITIS IN HEIFERS
lactating cows once during summer 1988 and once during summer 1989 to determine the prevalence of CPS IMI at the beginning and end of the study for each herd. During the study, dairy personnel collected milk samples aseptically from all cows at parturition, dryoff, and culling. Once a primiparous cow was identified as having CPS IMI at parturition, subsequent milk samples were taken by dairy personnel at monthly intervals in order to determine whether CPS IMI persisted. Milk samples collected at parturition were obtained prior to first milking. Milk samples were stored frozen prior to bacteriological culture. Bacteriological Procedures
Milk samples were thawed at room temperature (30·C), and .05 ml were streaked on blood agar plates containing 5% bovine red blood cells. Plates were incubated at 37"C for 24 h, and presumptive identification of isolates was made by visual observation of colony characteristics. Plates were visually examined again at 48 h for presumptive identification of slower growing organisms. Classification of organisms was by recommended methods (6). Recovered organisms were classified as CPS, coagulase-negative staphylococci (eNS), Streptococcus species, coliforms, and other. The CPS, Streptococcus species, and coliforms were considered to be major mastitis pathogens. The CNS and others were considered to be minor mastitis pathogens. Diagnosis of IMI
Diagnosis of IMI was based on criteria established previously (16) that any CPS isolate was considered to be from an IMI. This method has been used for composite milk samples and considers the number of colonyforming units of a major or minor pathogen in relation to contaminants. A cow with a previous CPS IMI was considered to be free of this IMI after 2 consecutive milk samples were negative for CPS. A cow with a previous IMI caused by any organism other than CPS was considered to be free of this IMI after one negative milk sample. Some true IMI by environmental agents shed in low numbers will be missed by this method.
Statistical Analyses
Prevalence of IMI was summarized for the major mastitis pathogens and for CNS by period of sampling and by parity. Prevalence was calculated by dividing the number of cows with a particular mastitis pathogen by the number of cows sampled for a particular period. Both major and minor pathogens were included in prevalence calculations for the prevalence of 1M!. Wilcoxon rank sum was used to determine whether a significant difference in mean CPS prevalence at parturition existed in primiparous cows between the herd groups (8). Incidence rate (lR) in cow-months was determined for the major mastitis pathogens by parity (multiparous. primiparous, and both multiparous and primiparous) during the total period of study (12 mo), a standard lactation period (10 mo). and a standard dry period (2 mo). The IR was calculated for IMI (including both major and minor pathogens) by dividing the number of new IMI occurring during the specified period by the sum of cow-months in which cows were susceptible to a new IMI during the specified period. Multiple IMI were considered, and, thus. a current IMI did not preclude a cow from a second or more IMI. Cumulative incidence (CI) was determined for the major mastitis pathogens by parity during the total study period (t = 12 rna), lactation (t = 10 rna), and the dry period (t = 2 rna) by the following equation: CI = 1 - exp(-IR x t), where IR is the incidence rate, and t is number of mo. The percentage of contribution of new CPS IMI in the lactating herd was determined separately for primiparous cows with CPS IMI at parturition and for all other new cases of CPS IMI occurring during the study. RESULTS
The prevalence of CPS IMI in lactating cows at the beginning of the study and in primiparous cows at parturition is presented by herd in Table 2 and summarized by herd group in Table 3. The prevalence rates for LCPS primiparous cows ranged from 0 to 13%, and prevalence rates for HCPS primiparous cows ranged from 3.2 to 27% (Table 2). The prevalence of CPS IMI in all primiparous cows at parturition was 8.1 %. The difference in CPS IMI prevalence in primiparous cows at parturiJournal of Dairy Science Vol. 77, No.4, 1994
962
ROBERSON ET AL.
TABLE 2. Prevalence of coagulase-positive staphylococcus (CPS) IMI in 18 dairy herds. Initial herd prevalence Sampled l
Herds
% CPS IMI
CPS2 (no.)
LCPSs A C D E F H L P R HCPSs B G I J K M N
0
Q
Heifer at parturition Sampled 3
(%)
137 88 119 76 166 299 241 340 118
2 3 3 I 0 1 3 17 2
131 104 133 39 200 135 323 152 88
35 14 21 22 54 37 87 99 25
1.5 3.4 2.5 1.3 0 .3 1.2 5 1.7 27 13 16 56 27 27 27 65 28
% CPS IMI
CPS4 (no.)
38 II 30 21 67
III 28 50 36 24 56 77
II 31 51 97 58 31
(%) 2 0 2 2 9 6 2 2 2
5.3 0 6.7 9.5 13 5.4 7.1 4 5.6
3 2 7 3 2 8 8 6 1
13 3.6 9.1 27 6.5 16 82 10 3.2
INumber of lactating cows from which uncontaminated composite milk samples were obtained at the beginning of the study. 2Number of lactating cows at the beginning of the study that had CPS 1M!. 3Number of heifers from which uncontaminated composite milk samples were obtained at parturition. 4Number of heifers that had CPS IMI at parturition. sPercentage prevalence 5% or less are considered to be low CPS herds, and percentage prevalence greater than 10% are considered to be high herds.
tion between LCPS (6.9%) and HCPS (9.2%) was not significant (P > .05), but the difference in prevalence rates among lactating cows between the herd groups was significant (P < .01)
(Table 3). All but one herd had at least 1 primiparous cow with CPS IMI at parturition. The number of cases of CPS IMI in primiparous cows at parturition were con-
TABLE 3. Summary of prevalence of coagulase-positive staphylococcus (CPS) IMI by herd group in primiparous dairy cows at parturition and lactating cows at the initial and final sampling periods.
Primiparous cows at parturition Initial Final
LCPSI
HCPS2
(%)
(%)
6.9 2 2.6
(no./no.) 27/392' 3211573 b 411l550b
9.2 30 20
Overal13 (no./no.)
(%)
(nolno.)
40/436' 39411305' 22611130'
8.1 15 10
67/828 426/2878
,.bMean percentages of CPS IMI within rows without common superscripts are different (P < .01). lLCPS
= Low
prevalence of CPS 1M) herds.
2HCPS = High prevalence of CPS IMI herds. 3Mean prevalence of CPS IMI for all primiparous cows at parturition or all cows. Journal of Dairy Science Vol. 77, No.4, 1994
26712680
STAPHYLOCOCCUS MASTITIS IN HEIFERS
trasted with all new cases of CPS IMI occurring during the study from all primiparous or multiparous cows other than those from primiparous cows at parturition. In both LCPS (27 out of 1(0) and HCPS (40 out of 146) herd groups, 27% of total new CPS IMI were due to primiparous cows with CPS IMI at parturition. There were 1.25 times more primiparous cows entering the HCPS lactating herd than primiparous cows entering the LCPS herds. The contribution of CPS IMI by primiparous cows at parturition to the herd was recalculated to determine the effect of the proportion of primiparous cows entering the lactating herds. After recalculation, the contribution of primiparous cows with CPS IMI at parturition was 29% for the LCPS herd group and 32% for the HCPS herd group. Of the 67 primiparous cows that had CPS IMI at parturition, subsequent milk samples were collected from 58 (at monthly sampling intervals postpartum). Twenty-five of the 58 primiparous cows (43%) had a CPS IMI at the second sampling period (1 mo postpartum). The CPS IMI persisted to the end of the study in 20 of the 25 primiparous cows that had CPS 1MI at a postpartum sampling period. Nine of 23 (39%) of LCPS primiparous cows with CPS IMI at parturition had CPS IMI at subsequent samplings, and 16 of 35 (46%) of HCPS primiparous cows with CPS IMI at parturition had CPS IMI subsequent samplings. The prevalence of 1MI and the common mastitis pathogens determined by milk samples collected at the start of the study, at parturition, at dry-off. and at the end of the study are presented in Table 4. The prevalence of 1MI in primiparous cows at parturition (55%) was higher than the IMI prevalence at parturition (44%) for multiparous cows. For primiparous cows from both herd groups, the prevalence for IMI was higher at parturition than at any other time. Primiparous cows from LCPS herds were more likely to have IMI at parturition than were primiparous cows from HCPS herds. Primiparous cows from LCPS herds were almost twice as likely to have CPS IMI at parturition than were LCPS multiparous cows. Thirty-five percent of IMI in primiparous cows at parturition were due to major mastitis pathogens. The CPS were the most common major agents of mastitis isolated from the primiparous cows during all four sampling periods.
963
Incidence and CI rates for all IMI and for IMI by specific pathogens in cows during the entire study, during lactation, and during the dry period are presented in Tables 5, 6, and 7, respectively. In all time periods, the LCPS herd group had a higher IR than the HCPS herd group for total 1MI, CNS IMI, Streptococcus species IMI, and coliform IMI. The HCPS herd group had a higher IR than LCPS for CPS 1MI during total, lactation, and dry periods (Tables 5, 6, and 7). The IR of IMI during lactation for all cows in the LCPS herd group was 3.4 cases/lOO cow-months, and the rate for the HCPS herd group was 3 cases/I 00 cow-months (Table 6). The IR of 1MI occurring during the dry period in the LCPS herd group was 18 cases/IOO cow-months, but the IR for IMI in the HCPS herd group was 12 cases/l 00 cow-months (Table 7). DISCUSSION Prevalence of CPS in Primiparous Cows at Parturition
Primiparous cows from HCPS herds do not have a significantly (P > .2) higher prevalence of CPS IMI at parturition than do LCPS primiparous cows. Our results do not support the assumed mode of transmission of CPS IMI for prepartum heifers, feeding mastitic milk, and allowing suckling heifers. Additionally, the findings of two studies in which milk infected with S. aureus was fed to a group of heifer calves indicated that the control group of heifers, which had not received milk containing S. aureus, had essentially the same rate of S. aureus IMI at parturition (3, 7). In the present study, all but one herd routinely fed waste milk, which often included mastitic milk, to heifer calves. The only herd that did not routinely feed waste milk was herd C, which fed fresh milk at random from any milking cow, regardless of S. aureus IMI status. Although herd C practiced group penning of preweaned heifer calves and housed postweaned heifer calves with the lactating herd, herd C is the only herd in which no CPS IMI was isolated from milk samples from the 11 primiparous cows tested at parturition. The data suggest that the theorized risk factors, such as group penning of preweaned calves, mastitic milk feeding, and a high CPS IMI Journal of Dairy Science Vol. 77, No.4. 1994
...
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0
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3
e. 0 .... ~
~
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n
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TABLE 4. Prevalence of IMI by pathogen as determined from composite milk samples from primiparous and multiparous cows at four different time periods over I yr in 18 northwestern US dairies. Number and percentage of cows with IMI! Cows and sampling periods
Sampled
::>
~
< !2-.l
;J
~ ;~
:;; 'f
Total cows with IMI (no.)
All multiparous Initial J Parturition Dry-off Final 4 LCPS Multiparous Initial Parturition Dry-off Final
1804 1628 lOll
1667 1023 933 664
1001
CPS
CNS
STR
COL
(%)
(no.)
(%)
(no.)
(%)
(no.)
(%)
(no.)
721 7W 425 585
40 42 35
327 219 157 212
18 13 16 13
259 311 174 285
14 19 17 17
65 87 43 46
4 5 4 3
29 85 10 31
2 5 1 2
286 361 225 291
28 39 34 29
17 32 22 28
2 3 3 3
171 202 \33 191
17 22 20 19
50 60
5 6 6 4
17 56 8 21
2 6
44
37 40
(%)
I
2
;l:l
0
I;l:I
HCPS Multiparous Initial Parturition Dry-off Final
781 695 347 666
435 348 200 294
56 50 58
310 187 135
44
184
27 39 28
All primiparous Initial Parturition Dry-off Final
956 828 614 864
326 452 225 3W
34 55 37 36
140 67 57 46
LCPS Primiparous Initial Parturition Dry-off Final
511 392 361 492
146 229 176
29 58 31 36
HCPS Primiparous Initial Parturition Dry-off Final
445 436 253 372
180 223 114 133
40 51 45 36
III
2 4 2
12 29 2
2 4
94
15 27 6 6
I
10
2
t'l1 -I
15 8 9 5
219 319 142 243
23 39 23 28
15 61 7 12
2 7
II
1 6
f!:
15 27 12 12
3 7 3 2
114 171 80 146
22
7 31 5
1\ I
8
5 2
10
2
3
I
125 40 45 34
28 9 18
8 30 2
2 7
5 27
6
I
2
I
I I
9
88 IW 41
105
148 62 97
44
22 30 24 34 25 26
I I
1
ITotal number of cows with IMI includes any mastitis pathogen; prevalence is the number of cows with an IMI divided by the number of cows sampled.
2(:ps = Coagulase-positive staphylococci. CNS
= coagulase-negative
31nitial milk sampling occurred during summer 1988. 4Final milk sampling occurred during summer 1989.
t'l1
11 16 12 14
40
staphylococci, STR = Streptococcus, and COL
= coliforms.
47 9 4 6 20
I
I
I
I
;l:l CIl
0
Z
965
STAPHYLOCOCCUS MASTITIS IN HEIFERS
prevalence in the adult herd, may not necessarily translate into a high CPS IMI prevalence in primiparous cows at parturition. Additional evidence for this is provided by herds G and Q (Tables I and 2), both of which group penned preweaned heifers, fed waste milk, and were HCPS herds, but the prevalence of CPS IMI in primiparous cows at parturition in these two herds was the second and third lowest of the 18 herds studied. Conversely, herds F and H (Tables I and 2) had average or higher prevalences of CPS IMI in primiparous cows at parturition even though heifers were individually penned until weaning and the lactating herd prevalence of CPS IMI was less than 1%. These examples strongly suggest the need for a thorough epidemiologic study at CPS IMI in heifers. The prevalence of CPS IMI in primiparous cows at parturition in 18 northwestern US dairies was 8.1 %. The actual prevalence for S. aureus IMI would be lower because two other CPS species. Staphylococcus hyicus and Staphylococcus intermedius, were included in
the calculation of CPS IMI. The actual prevalence of S. aureus IMI at parturition for both groups, for LCPS primiparous cows, and for HCPS primiparous cows was 6.1, 4.1, and 7.8%, respectively (Roberson et aI., 1993, unpublished data). This prevalence contrasts with estimates of 37, 2.6, and 9.6% from studies in Louisiana (30), Vermont (25), and Sweden (21). In the current study, 55% of primiparous cows had IMI at parturition, and 15% of these were CPS IMI. Studies involving single herds or a limited number of primiparous cows from multiple herds have reported that the S. aureus IMI prevalence of primiparous cows at parturition ranged from 0 to 51 % (3, 7, 9, 24, 32). In this study. a range of 0 to 27% of primiparous cows had CPS IMI at parturition, which may reflect diverse heifer management factors and general dairy herd management in the 18 herds studied. Additionally, with the exception of one herd, at least I primiparous cow per herd had CPS IMI at parturition, indicating that even primiparous cows in herds with good mastitis control are not exempt from this disease.
TABLE 5. Overall incidence rate and overall cumulative incidence of mastitis pathogens isolated from milk samples collected from cows in 17 northwestern US dairies. Cumulative incidence 2
Incidence rate l Herd groups
IMP
CPS3
CNS4
STR5
COL6
IMI
CPS
CNS
STR
COL
All cows LCPS7 (n = 9)8 HCpS9 (n 8) 17) Both (n
4.5 3.7 4.1
.5 1.6 1
2.8 2.1 2.5
.7 .3 .5
.6 .3 .5
42 35 39
5.8 17 II
28 22 25
8.5 3.5 6.2
7 3.5 5.3
Primiparous cows LCPS (n 9) 8) HCPS (n Both (n 17)
4.5 3.5 4.1
.8 1.3 1
3 2.3 2.6
.7 .4 .6
.6 .2 .4
41 34 38
8.5 13 11
29 23 26
7.9 4.5 6.3
6.3 2.4 4.5
Multiparous cows 9) LCPS (n HCPS (n 8) Both (n 17)
4.7 4.1 4.4
.4 2 1.2
2.7 2.2 2.5
.8 .3 .6
.5 .3 .4
43 38 41
4.8 20 12
29 22 26
9.1 3.7 6.5
6.2 4 5.2
=
=
= = =
= = =
IIncidence rate = Number new cases occurring during entire study/cow-months at risk during study. Figures in the table are expressed as the number of new cases per 100 cow-months. 2Cumulative incidence
=1 -
exp(-IR x t), where IR
= incidence
rate, and t
= 12
mo.
3CPS = Coagulase-positive staphylococci. 4CNS = Coagulase-negative staphylococci. 5STR = Streptococcus species. 6(:OL = Coliforms. 1LCPS = Low prevalence (:!>5%) CPS 1M! herds. 8Numbers in parentheses represent the number of herds per group. 9HCPS = High prevalence (>10%) CPS 1M! herds. Journal of Dairy Science Vol. 77, No.4, 1994
966
ROBERSON ET AL.
As indicated in Table 4, the overall most common major mastitis pathogen isolated from primiparous cows at parturition was CPS. This result is in contrast to results from the Vermont study (25) in which Streptococcus species and coliforms were more common. Reasons for these contrasting results are unknown. The most common major mastitis pathogens isolated from primiparous cows at parturition in the LCPS herds were the Streptococcus species at 7.9%, a result that is similar to the 7.6% identified in the Vermont study (25). The most common major pathogen for the HCPS primiparous cows was the CPS, which agrees with results of the Louisiana study (30). The most common mastitis pathogens in the present study were identified as CNS at 39%, which is in close agreement with three previous studies (24, 25, 30). The actual prevalence differences between the major mastitis pathogens in primiparous cows at parturition in this study are relatively minor.
Persistence of IMI
Subsequent milk samples were collected at monthly intervals after parturition from 58 of 67 primiparous cows with CPS 1MI at parturition. Twenty-five of the 58 primiparous cows subsequently sampled were positive for CPS 1MI, and 20 of these primiparous cows had CPS 1MI at the end of the study. However, differentiation between persistent CPS 1MI and new CPS 1MI was not performed in this study. Teat canal infections that are eliminated with the onset of milking or spontaneous cure are offered as reasons for the 33 primiparous cows that appeared to clear CPS IM1 postpartum. Persistency of S. aureus 1M1 in primiparous cows was 83% in a California study (29). Reasons for the large discrepancy between the current study and the California study are not known. Nonetheless, nearly 50% of all CPS 1MI in primiparous cows at parturition appeared to persist for at least 1 mo and would therefore present a risk of transmission to uninfected herdmates.
TABLE 6. Lactational incidence rate and cumulative incidence of mastitis pathogens isolated from milk samples collected from cows in 17 northwestern US dairies. Cumulative incidence 2
Incidence rate I Herd groups
IMI
CPS3
CNS4
STRs
All cows LCPS7 (n = 9)8 HCPS9 (n = 8) Both (n = 17)
3.4 3 3.2
.5 1.6 1
2.2 1.8 2
.5 .2 .4
Primiparous cows LCPS (n = 9) HCPS (n = 8) Both (n = 17)
3.4 2.9 3.1
.7 1.3
.9
2.5 2.1 23
.4 .2 .3
Multiparous cows LCPS (n = 9) HCPS (n = 8) Both (n = 17)
3.4 3.4 3.4
3 1.9 1.1
2.2 1.8 2
.5 .2 .4
COL6
IMI
CPS
CNS
STR
COL
.3 .1 .2
29 26 27
4.4 14 8.9
20 16 18
5.1 1.6 3.4
3
3
28 25 27
6.2 11 8.3
22 18 20
3.7 1.9 2.9
3.1 .4 1.8
28 28 28
3.3 17 9.6
19 16 17
5 4.5 4.7
3 1.9 2.5
0 .2 .3 .2
.3
1.3
2.2
Iincidence rate = Number of new cases occuning during lactation/cow-months at risk during lactation. Figures are eltpressed as the number of new cases per 100 cow-months. 2Cumulative incidence 3CPS
=1 -
= Coagulase-positive
eltp(-IR x t), where IR = incidence rate, and t = 10 mo.
staphylococci.
4CNS = Coagulase-negative staphylococci. sSTR = Streptococcus species. tiCOL = Coliforms. 7LCPS
= Low
prevalence (~5%) CPS IMI herds.
8Numbers in parentheses represent the number of herds per group. 9HCPS = High prevalence (>10%) CPS IMI herds. Journal of Dairy Science Vol. 77, No.4, 1994
967
STAPHYLOCOCCUS MASTITIS IN HEIFERS
Primiparous Cow IMI at Parturition
In this study, 55% of primiparous cows had an IMI at parturition. This figure is similar to that determined in other studies (24, 25). Primiparous cows from the LCPS had a slightly higher prevalence of IMI at parturition than did primiparous cows from the HCPS herds. Major mastitis pathogens caused IMI in approximately 21 % of primiparous cows. Approximately 18 and 35% of primiparous cows were infected with major pathogens at parturition in Vermont and Massachusetts studies (24, 25). Prevalence of CPS IMI in Cows
The prevalence of CPS IMI in this study, as determined by composite milk sampling of all lactating cows, was 15% for initial and 10% for final sampling periods. The lower prevalence rate at the final period may reflect increased awareness and may be due to preferential culling (15), particularly among the HCPS
herds. In support, the prevalence of CPS IMI increased slightly, although not significantly, from 2% at the initial sampling to 3% at the final sampling in the LCPS herds, but the prevalence for CPS IMI in the HCPS herds decreased from 32 to 20% during the same time period. The overall prevalence rates for CPS IMI in lactating cows in this study are lower than the 19% prevalence of S. aureus IMI determined in 44 northwestern dairies in 1982 (14), but considerably higher than the 2.4% overall prevalence of CPS IMI reported for 50 Ohio dairies (2). The overall prevalence of CPS IMI reported in the Ohio study is quite similar to the 2 % CPS IMI prevalence of the LCPS herds. No herds were able to eradicate S. aureus mastitis during the course of this study even though bacteriological results were provided and several herds employed standard mastitis control recommendations of milking time hygiene, dry cow therapy, and selective culling of cows with CPS IMI. These findings suggest that use of current mastitis control
TABLE 7. The dry period incidence rate and cumulative incidence of intramammary infections in cows in from 17 northwestern US dairies. Cumulative incidence2
Incidence rate! Herd groups
IMI
CPS3
CNS4
STRs
COL6
IMI
CPS
CNS
STR
COL
All cows LCPS7 (n = 8)8 HCpS9 (n = 7) Both (n = 15)
18 12 14
.9 2.3 1.6
10 7.4 8.9
33 1.3 2.4
3.7 1.8 2.8
30 21 26
1.8 4.5 3.1
18 13 16
6.3 2.6 4.6
7 3.5 5.4
Primiparous cows LCPS (n = 8) HCPS (n = 9) Both (n = 15)
15 9.5 13
1.1 1.8 1.4
7.5 4.6 6.2
3.5 2.6 3
3.6 1.5 2.6
26 17 22
2.1 3.4 2.7
14 8.5 11
6.5 4.6 5.6
6.7 2.9 4.9
Multiparous cows LCPS (n = 8) HCPS (n 9) Both (n = 15)
19 13 16
.8 3 1.8
12 8.5 11
3.1 .7 2
3.6 1.8 2.7
31 22 27
1.6 5.5 3.4
21 15 18
6 1.4 3.9
6.8 3.4 5.2
=
=
llncidence rate Number of new cases occurring during dry period/cow-months at risk during dry period. Figures are expressed as the number of new cases per 100 cow-months. 2Cumulative incidence
=1 -
exp(-IR x t), where IR
= dry
period incidence rate, and t
=2
mo.
3CPS = Coagulase-positive staphylococci. 4CNS
= Coagulase-negative
sSTR
= Streptococcus
staphylococci.
species.
6COL = Coliforms. 7LCPS = Low prevalence
(~5%)
CPS IMI herds.
8Numbers in parentheses represent the number of herds per group. 9HCPS = High prevalence (>10%) CPS IMI herds. Journal of Dairy Science Vol. 77, No.4, 1994
968
ROBERSON ET AL.
recommendation may control and reduce S. aureus IMI but do not readily lead to eradication. The continuing influx of primiparous cows with CPS IMI may be an important impediment to eradication. Incidence Rates and Cumulative Incidence Rates
Incidence and CI rates were calculated for three time periods (Tables 5 to 7). Because milk samples were normally collected only four times throughout the year for anyone cow, the IR we report are probably lower than the actual IR for IMI by environmental organisms, which are thought to be of shorter duration (17). This result may be especially true for coliforms, which have been identified as the most common pathogens of clinical mastitis in herds with low rolling mean SCC (13, 28). The overall CI for IMI in this study was 39 casesl 100 head at risk, which is comparable with the 36.8 casesllOO head at risk reported in a previous study (31). In the current study, CNS were the most common agents of new IMI, and the CPS were second; overall IR was I new easel 100 cow-months. The LCPS primiparous group had higher overall, lactational, and dry period IR of IMI, CNS, Streptococcus species, and coliforms compared with those of the HCPS primiparous group. Conversely, the HCPS primiparous group had the highest IR of CPS IMI at all time periods. Comparisons of multiparous cows between LCPS and HCPS groups were almost identical to those of the primiparous groups except that IR for all IMI occurring during lactation was identical. These data may suggest that cows of HCPS herds with CPS IMI are less susceptible to new IMI because fewer quarters are at risk to new IMI. This suggestion is supported by lower rates of environmental IMI for HCPS than for LCPS cows and is consistent with previous findings (13). The data also indicate very little difference in the overall IMI IR between the 2 herd groups during lactation. This result suggests that, using mastitis control procedures, producers have the ability to alter the infectious agents of mastitis from contagious to noncontagious organisms, yet the rate of new IMI changes little. Also of interest, IR for CNS, Streptococcus species, and coliforms were several times higher during the dry Journal of Dairy Science Vol. 77. No.4. 1994
period than during lactation for both herd groups. However, the IR for CPS IMI was only slightly higher during the dry period than during the lactation period for both herd groups despite the fact that all herds were using dry cow intramammary therapy. Occurrence of CPS IMI arising over the dry period is not a new finding (23, 34). The higher IR during the dry period as opposed to lactation for CPS IMI is similar to results of two earlier reports (11, 23). Additionally, results from an Israeli study indicated that 31 % of cows that were not infected with S. aureus IMI prior to drying off developed S. aureus IMI during the dry period even though all received dry cow intramarnmary therapy at dry-off (34). The transmission of new CPS IMI occurring during the dry period is not understood but may have a similar epidemiology to CPS IMI occurring in prepartum heifers. CONCLUSIONS
There is little difference in the prevalence of CPS IMI at parturition between primiparous cows of HCPS herds and primiparous cows of LCPS herds. Thus, primiparous cows with CPS IMI at parturition are significant reservoirs of this pathogen to uninfected herdmates in LCPS herds as well as HCPS herds. Primiparous cows with CPS IMI at parturition represent nearly one-third of the new CPS IMI cases. The prevalence of CPS IMI in the adult herd does not necessarily parallel the prevalence of CPS IMI in primiparous cows. Therefore, mastitis control procedures need to include methods to control or to prevent this disease in heifers. The assumed risk factors, group penning preweaned heifers and feeding mastitic milk, were not associated with an increased risk to heifers. The data from this study do not support the theory that the infected mammary gland is the reservoir of S. aureus for primiparous cow IMI at first parturition. Thus, the sources and modes of transmission of this disease need to be determined. ACKNOWLEDGMENTS
The authors express sincere appreciation to Dot Newkirk, Mark McFrederick, Jeff Hoffman, Sarah Christian, and Jean Cypher for technical assistance and to the cooperating
STAPHYLOCOCCUS MASTITIS IN HEIFERS
dairy producers for their participation and assistance. REFERENCES I Barnes-Pallesen, F. D.. P. Blackmer, A. Britten. R. B. Bushnell. D. M. Van Damme, and F. Welcome. 1987. Laboratory and Field Handbook on Bovine Mastitis. Natl. Mastitis Counc.• Arlington. VA. 2 Bartlett. P. C.• G. Y. Miller. S. E. Lance, and L. E. Heider. 1992. Clinical mastitis and intramammary infections on Ohio dairy farms. Prev. Vet. Med. 12:59. 3 Barto, P. B., L. J. Bush, and G. D. Adams. 1982. Feeding milk containing SUlphylococcus aureus to calves. J. Dairy Sci. 65:271. 4 Boddie. R. L.. S. C. Nickerson. W. E. Owens, and J. L. Wans. 1987. Udder microflora in nonlactating heifers. Agri-Practice 2:22. 5 Bramley. A. J.• and F. H. Dodd. 1984. Reviews of the progress of dairy science: mastitis control-progress and prospects. 1. Dairy Res. 51:481. 6 Brown, R. W., D. A. Barnum. D. E. Jasper. J. S. McDonald, and W. D. Schultze. 1981. Microbiological Procedures for Use in the Diagnosis of Bovine Mastitis. Natl. Mastitis Counc., Arlington, VA. 7 Bushnell, R. B. 1989. Pasteurization of milk and colostrum fed to dairy calves. Page 211 in Proc. Eastern States Vet. Conf., Gainesville. FL. Publ. by Eastern States Vet. Assoc., Gainesville, FL. 8 CRC Handbook of Tables for Probability and Statistics, Version 2 Edition. 1985. CRC Press. Inc., Boca Raton, FL. 9 Davidson, I. 1961. Observations on the pathogenic staphylococci in a dairy herd during a period of six years. Res. Vet. Sci. 2:22. 10 Devriese. L. A.• V. Hajek, P. Oeding, S. A. Meyer, and K. H. Schliefer. 1978. Staphylococcus hyicus (Sompolinsky 1953) comb. nov. and Staphylococcus hyicus subsp. chromogenes subsp. nov. Int. J. Syst. Bacteriol. 28:482. 11 Eberhart, R. 1.. and J. M. Buckalew. 1977. Intramammary infections in a dairy herd with a low incidence of Streptococcus agalactiae and Staphylococcus aureus infections. J. Am. Vet. Med. Assoc. 171:630. 12 Erskine, R. 1.. R. J. Eberhart, L. J. Hutchinson. and S. B. Spencer. 1987. Herd management and prevalence of mastitis in dairy herds with high and low somatic cell counts. J. Am. Vet. Med. Assoc. 190:1411. 13 Erskine. R. J., R. J.Eberhart, L. J. Hutchinson, S. B. Spencer. and M. A. Campbell. 1988. Incidence and types of clinical mastitis in dairy herds with high and low somatic cell counts. J. Am. Vet. Med. Assoc. 192: 761. 14 Auharty. D. H., and D. Wright. 1982. Mobile lab hit the road to help northwest dairymen. Hoard's Dairyman 127:1100. 15 Fox. L. K., and D. D. Hancock. 1989. Effect of segregation on prevention of intramammary infections by Staphylococcus aureus. J. Dairy Sci. 72:540. 16 Fox, L. K., D. D. Hancock, C. W. Weems, W. Toma, and E. Chang. 1987. The effect of intramammary antibiotic therapy at calving on udder health traits. J. Dairy Sci. 70:1696. 17 Gonzalez, R. N., D. E. Jasper, T. B. Farver, R. B.
969
Bushnell, and C. E. Franti. 1988. Prevalence of udder infections and mastitis in 50 California dairy herds. 1. Am. Vet. Med. Assoc. 193:323. 18 Hajek, V. 1976. Staphylococcus intermedius, a new species isolated from animals. Int. J. Syst. Bacteriol. 26:401. 19 Hutton, C. H., L. K. Fox. and D. D. Hancock. 1990. Mastitis control practices: differences between herds with high and low milk somatic cell counts. J. Dairy Sci. 73:1135. 20Jasper. D. E.• F. Infante, and J. D. Dellinger. 1985. Relationships among the results of coagulase, staphylococcal toxin, and therrnonuclease tests on staphylococci from cow milk. 1. Clin. Microbiol. 21: 582. 21 Jonsson. P.• S. Olsson. A. Olofson. C. Falth. O. Holmberg. and H. Funke. 1991. Bacteriological investigations of clinical mastitis in heifers in Sweden. J. Dairy Res. 58:179. 22 McDonald, J. S. 1982. Experimental infection of the bovine mammary glands during the dry period. Page 112 in Proc. 21st Annu. Mtg. Natl. Mastitis Counc., Louisville. KY. Natl. Mastitis Counc.• Arlington, V A. 23 Neave, F. K.• F. H. Dodd, and E. Henriques. 1950. Udder infections in the dry period. J. Dairy Res. 17: 37. 24 Oliver, S. P., and B. A. Mitchell. 1983. Intramammary infections in primigravid heifers near parturition. J. Dairy Sci. 66:1180. 25 Pankey, 1. W., P. A. Drechsler. and E. E. Wildman. 1991. Mastitis prevalence in primigravid heifers at parturition. J. Dairy Sci. 74: 1550. 26 Saperstein, G.• L. S. Hinckley. and J. E. Post. 1988. Taking the team approach to solving staphylococcal mastitis infection. Vet. Med. 83:939. 27 Schalm. O. W. 1942. Streptococcus agalactiae in the udders of heifers at parturition traced to suckling among calves. Cornell Vet. 32:49. 28 Schukken. Y. H., F. J. Grommers, D. van de Geer. and A. Brand. 1989. Incidence of clinical mastitis on farms with low somatic cell counts in bulk milk. Vet. Rec. 125:60. 29 Schultze, W. D. 1985. Control of new intramammary infection at calving by prepartum teat dipping. J. Dairy Sci. 68:2094. 30 Trinidad. P., S. C. Nickerson. and T. K. Alley. 1990. Prevalence of intramammary infection and teat canal colonization in unbred and primigravid dairy heifers. J. Dairy Sci. 73:107. 31 Weigler, B. 1.. D. W. Hird, W. M. Sischo. J. C. Holmes, C. Danaye-Elmi. C. W. Palmer, and W. W. Utterback. 1990. Veterinary and nonveterinary costs of disease in 29 California dairies participating in the national animal health monitoring system from 1988 to 1989. 1. Am. Vet. Med. Assoc. 196:1945. 32 White. F.• and I. McDonald. 1961. Some observation on an outbreak of staphylococcal mastitis in a dairy herd. J. Compo Pathol. 71:159. 33 Wilson, C. D.• and M. S. Richards. 1980. A survey of mastitis in the British dairy herd. Vet. Rec. 106:431. 34 Ziv, G.• M. Storper. and A. Saran. 1990. Incidence of new S. aureus infections during the dry period in antibiotic-treated and non-treated infected and noninfected cows. Int. Symp. Bovine Mastitis, Sept. 13-16. Indianapolis, IN. Nat!. Mastitis Counc., Arlington, VA. Journal of Dairy Science Vol. 77. No.4, 1994
Coagulase-Positive Staphylococcus Intramammary Infections in Primiparous Dairy Cows 1 J. R. ROBERSON, L. K. FOX, D. D. HANCOCK, and C. C. GAY Field Disease Investigation Unit Department of Veterinary Clinical Sciences Washington State University Pullman 99164-6610 T. E. BESSER Department of Veterinary Microbiology and Pa~holo~y Washington State University Pullman 99164-7040 ABSTRACT
prevalence of coagulase-positive staphylococcal IMI (9.2%; 40 of 436) at parturition than did primiparous cows from low herds (6.9%; 27 of 392), the difference was not significant. Of primiparous cows with coagulase-positive staphylococcal IMI at parturition, 43% had coagulase-positive staphylococcal IMI at least 2 mo after parturition. Primiparous cows with coagulase-positive staphylococcal IMI at parturition may represent significant reservoirs of infection to uninfected herdmates. (Key words: coagulase-positive staphylococci, Staphylococcus aureus, prevalence, mastitis)
Objectives were to determine the prevalence of coagulase-positive staphylococcal IMI in primiparous cows at first parturition, to contrast the differences in coagulase-positive staphylococcal IMI in primiparous cows at parturition in herds with high and low prevalences of coagulase-positive staphylococcal IMI in the lactating herd, and to determine the percentage of primiparous cows having persistent coagulase-positive staphylococcal IMI. Milk samples were collected aseptically from cows at the start and end of the study, at dry-off, and at parturition. Herds (n = 18) were split evenly into two categories: high (>10%) or low (<5%) prevalence of coagulase-positive staphylococcal IMI. At the start, the mean prevalence of coagulase-positive staphylococcal IMI in high prevalence herds was 30%, ranging from 13 to 65%, and in low prevalence herds was 2%, ranging from 0 to 5%. Overall the prevalence of coagulase-positive staphylococcal IMI in primiparous cows at parturition was 8.1 % (67 of 828), ranging from o to 27%. Although primiparous cows from high prevalence herds had a higher
Abbreviation key: CI = cumulative incidence; CNS = coagulase-negative staphylococci, CPS = coagulase-positive staphylococci, HCPS = high prevalence of CPS IMI herds, IR = incidence rate; LCPS = low prevalence of CPS IMI herds. INTRODUCTION
Received July 6, 1993. Accepted November 12, 1993. IThis study was supported in part by Grant lOA 3072 0858 from the Agricultural Research Unit, Washington State University, and Grant BC 2530 0123 from the Washington State Dairy Products Commission. 1994 J Dairy Sci 77:958-969
Staphylococcus aureus, the most prevalent and most costly of the major mastitis pathogens, can result in both clinical and subclinical mastitis (5). Although a great deal of progress in mastitis control has taken place over the years, S. aureus IMI may still occur in more than 80% of dairies (14, 33). However, the prevalence of S. aureus IMI has been reduced to less than 5% of cows in many herds (12, 26). Some herds claim complete eradication, but others, which practice all the standard milking time hygiene techniques and which have mean SCC in milk of less than 200,000
958
STAPHYLOCOCCUS MASTITIS IN HEIFERS
cells/ml, have been unable to eradicate S. aureus (19). One reason for the failure to eradicate S. aureus IMI may be that the control measures for mastitis do little to control the disease in prepartum primiparous cows. Thus, for those cows, S. aureus IMI at parturition may represent an impediment to eradication of S. aureus. Not only do these cows add to the prevalence of S. aureus IMI in the lactating herd, but they also can act as reservoirs of this contagious pathogen to uninfected herdmates. Although several studies have documented that primiparous cows may have S. aureus IMI prior to entering the lactating herd (3, 4, 30, 32), these studies were conducted with three or fewer herds and in one study with only 10 primiparous dairy cows (4). Therefore, both the prevalence of this infection in primiparous cows at parturition and the relationship between primiparous cows with S. aureus IMI at parturition and the prevalence of S. aureus IMI in the lactating herd are not well established. In a 1942 report, Schalm (27) concluded that feeding milk containing Streptococcus agalactiae to heifer calves and subsequent suckling among heifers would result in IMI by this major contagious pathogen at first parturition. Although never proven, the transmission pathway of S. aureus IMI in heifer calves is assumed to be the same as that for Strep. agalactiae (22). It is generally accepted that the infected mammary gland represents the major reservoir of S. aureus for dairy cows (1, 5, 9). Therefore, the major method of transferring S. aureus to heifer calves may be by feeding mastitic milk to heifer calves. Thus, herds with a high prevalence of S. aureus 1MI would expose many more heifer calves to this contagious pathogen than would herds with a low prevalence of S. aureus IMI. Therefore, we hypothesized that the prevalence of S. aureus IMI in primiparous cows at parturition from herds with a high prevalence of S. aureus IMI in the lactating herd is higher than the prevalence of S. aureus IMI in primiparous cows at parturition from herds with a low prevalence of S. aureus IMI. The purpose of this study was to contrast the prevalence of S. aureus IMI in primiparous cows at parturition in herds with high and low prevalences of S. aureus IMI among multiparous cows.
959
MATERIALS AND METHODS
Herd Selection
The nomenclature of coagulase-positive staphylococci (CPS) and S. aureus are often used interchangeably. Historically, staphylococci that gave a positive coagulase test reaction were considered to be S. aureus (6). Even though taxonomic studies have identified CPS species (Staphylococcus intermedius and some strains of S. hyicus) other than S. aureus (10, 18), for bovine mastitis, the terms are almost synonymous, because more than 95% of all CPS that are associated with IMI are S. aureus (15, 20). However, in this paper, the more accurate term, CPS, will be used to describe staphylococcal isolates aseptically collected from milk that caused a positive tube coagulase reaction. A heifer is defined as a female bovine that has not had her first parturition. A primiparous cow is defined as a female bovine that has had her first parturition and is in her first lactation. Herds were selected based on a past history of CPS IMI and a willingness to cooperate in the project. Based on the herd's historical CPS IMI prevalence, an effort was made to select an equal number of herds with high CPS (HCPS) and low CPS (LCPS) prevalence and to include herds from all major dairy areas of Washington State. Fourteen herds contained only Holstein cows, 1 contained only Jersey cows, I contained only Guernsey cows, 1 contained Holstein and Jersey cows, and 1 contained Holstein and Brown Swiss cows. Herd size ranged from 45 to 375 milking cows. Based on the initial results of milk culture for the herds, the herds having a CPS prevalence of 5% or less were assigned to the LCPS group. Herds having a CPS prevalence of greater than 10% were assigned to the HCPS group. Pertinent management practices and herd demographics are summarized in Table 1. One HCPS herd withdrew before the end of the study. Data from this herd were included for prevalence but not for incidence calculations. Sample Periods and Collection
Composite milk samples were collected aseptically (1) by the investigators from all Journal of Dairy Science Vol. 77. No.4, 1994
.... 0
'D 0\
c::
0
3
e!-
....o a
~.
Q en n
".
TABLE I. Demographics and heifer management practices of herds with high (HCPS) and low (LCPS) prevalence of coagulase-positive staphylococcal 1M!. HCPS Herds
::I
!i
< !2....,
;-.J
Z
0
!"
:g .f>.
Breed I Initial lactating herd size, no. Initial 12 mo mean, SCC x 1()3 Time calf with dam after birth, h Preweaned calf housing 3 Preweaned calf feeding" Milker feeds calves Time weaned prior to group penning, d Age first bred, mo Heifers grouped with dry cows 7 Heifers grouped with lactating cows 7 IDairy breeds: BS 2NK
= Not
B
G
I
J
K
M
N
0
Q
A
C
0
E
F
H
L
P
R
J
H
H
H
H
H
H
H
H-BS
H-J
G
H
H
H
H
H
H
H
131
104
133
39
200
135
323
152
88
137
88
119
76
166
299
241
329
118
270
360
562
301
214
584
370
704
426
341
176
142
164
164
217
157
270
173
12
12
24
2
NK2
24
12
24
6
8
12
24
12
24
4
5
24
72 ::Ill
B
I
I
NK
I
I
I
G
I
B
I
I
I
I
I
B
0
I
tl:l
ttl
W/S N
WfR Y
NA6 13
NA
0 15
Y
Y
Y
Y
NK
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
N
Y
N
N
N
Y
NK
Y
N
N
Y
N
Y
N
Y
N
N
N
N
Y
= Guernsey,
H
W Y
NK NK
W
Y
W N
W N
W NS
W N
WfR Y
NA
0 14
S Y
W N
W/S Y
W Y
W N
W N
W Y
W N
en
0
z tT1
= Brown
18
Swiss, G
0 14
NK NK
60
15
= Holstein,
0 14
and J
0 15
14
NA
15
3 14
0 15
0 14
10
10
14
13
= Jersey.
known. Herd dropped out before data was obtained.
3Preweaned housing: I
= Yes; N = no. = Not applicable
6NA
B
::Ill
4Preweaned feeding: W
sy
LCPS Herds
= individual hutch or pen, G = group pen, and B = calves subject to both individual and grouping = waste milk (colostrum, antibiotic, and mastitic), R = milk replacer, and S = saleable milk. if preweaned heifers already group penned.
7Grouped within 2 mo prior to parturition.
prior to weaning.
4 14
60
15
-l
~
961
STAPHYLOCOCCUS MASTITIS IN HEIFERS
lactating cows once during summer 1988 and once during summer 1989 to determine the prevalence of CPS IMI at the beginning and end of the study for each herd. During the study, dairy personnel collected milk samples aseptically from all cows at parturition, dryoff, and culling. Once a primiparous cow was identified as having CPS IMI at parturition, subsequent milk samples were taken by dairy personnel at monthly intervals in order to determine whether CPS IMI persisted. Milk samples collected at parturition were obtained prior to first milking. Milk samples were stored frozen prior to bacteriological culture. Bacteriological Procedures
Milk samples were thawed at room temperature (30·C), and .05 ml were streaked on blood agar plates containing 5% bovine red blood cells. Plates were incubated at 37"C for 24 h, and presumptive identification of isolates was made by visual observation of colony characteristics. Plates were visually examined again at 48 h for presumptive identification of slower growing organisms. Classification of organisms was by recommended methods (6). Recovered organisms were classified as CPS, coagulase-negative staphylococci (eNS), Streptococcus species, coliforms, and other. The CPS, Streptococcus species, and coliforms were considered to be major mastitis pathogens. The CNS and others were considered to be minor mastitis pathogens. Diagnosis of IMI
Diagnosis of IMI was based on criteria established previously (16) that any CPS isolate was considered to be from an IMI. This method has been used for composite milk samples and considers the number of colonyforming units of a major or minor pathogen in relation to contaminants. A cow with a previous CPS IMI was considered to be free of this IMI after 2 consecutive milk samples were negative for CPS. A cow with a previous IMI caused by any organism other than CPS was considered to be free of this IMI after one negative milk sample. Some true IMI by environmental agents shed in low numbers will be missed by this method.
Statistical Analyses
Prevalence of IMI was summarized for the major mastitis pathogens and for CNS by period of sampling and by parity. Prevalence was calculated by dividing the number of cows with a particular mastitis pathogen by the number of cows sampled for a particular period. Both major and minor pathogens were included in prevalence calculations for the prevalence of 1M!. Wilcoxon rank sum was used to determine whether a significant difference in mean CPS prevalence at parturition existed in primiparous cows between the herd groups (8). Incidence rate (lR) in cow-months was determined for the major mastitis pathogens by parity (multiparous. primiparous, and both multiparous and primiparous) during the total period of study (12 mo), a standard lactation period (10 mo). and a standard dry period (2 mo). The IR was calculated for IMI (including both major and minor pathogens) by dividing the number of new IMI occurring during the specified period by the sum of cow-months in which cows were susceptible to a new IMI during the specified period. Multiple IMI were considered, and, thus. a current IMI did not preclude a cow from a second or more IMI. Cumulative incidence (CI) was determined for the major mastitis pathogens by parity during the total study period (t = 12 rna), lactation (t = 10 rna), and the dry period (t = 2 rna) by the following equation: CI = 1 - exp(-IR x t), where IR is the incidence rate, and t is number of mo. The percentage of contribution of new CPS IMI in the lactating herd was determined separately for primiparous cows with CPS IMI at parturition and for all other new cases of CPS IMI occurring during the study. RESULTS
The prevalence of CPS IMI in lactating cows at the beginning of the study and in primiparous cows at parturition is presented by herd in Table 2 and summarized by herd group in Table 3. The prevalence rates for LCPS primiparous cows ranged from 0 to 13%, and prevalence rates for HCPS primiparous cows ranged from 3.2 to 27% (Table 2). The prevalence of CPS IMI in all primiparous cows at parturition was 8.1 %. The difference in CPS IMI prevalence in primiparous cows at parturiJournal of Dairy Science Vol. 77, No.4, 1994
962
ROBERSON ET AL.
TABLE 2. Prevalence of coagulase-positive staphylococcus (CPS) IMI in 18 dairy herds. Initial herd prevalence Sampled l
Herds
% CPS IMI
CPS2 (no.)
LCPSs A C D E F H L P R HCPSs B G I J K M N
0
Q
Heifer at parturition Sampled 3
(%)
137 88 119 76 166 299 241 340 118
2 3 3 I 0 1 3 17 2
131 104 133 39 200 135 323 152 88
35 14 21 22 54 37 87 99 25
1.5 3.4 2.5 1.3 0 .3 1.2 5 1.7 27 13 16 56 27 27 27 65 28
% CPS IMI
CPS4 (no.)
38 II 30 21 67
III 28 50 36 24 56 77
II 31 51 97 58 31
(%) 2 0 2 2 9 6 2 2 2
5.3 0 6.7 9.5 13 5.4 7.1 4 5.6
3 2 7 3 2 8 8 6 1
13 3.6 9.1 27 6.5 16 82 10 3.2
INumber of lactating cows from which uncontaminated composite milk samples were obtained at the beginning of the study. 2Number of lactating cows at the beginning of the study that had CPS 1M!. 3Number of heifers from which uncontaminated composite milk samples were obtained at parturition. 4Number of heifers that had CPS IMI at parturition. sPercentage prevalence 5% or less are considered to be low CPS herds, and percentage prevalence greater than 10% are considered to be high herds.
tion between LCPS (6.9%) and HCPS (9.2%) was not significant (P > .05), but the difference in prevalence rates among lactating cows between the herd groups was significant (P < .01)
(Table 3). All but one herd had at least 1 primiparous cow with CPS IMI at parturition. The number of cases of CPS IMI in primiparous cows at parturition were con-
TABLE 3. Summary of prevalence of coagulase-positive staphylococcus (CPS) IMI by herd group in primiparous dairy cows at parturition and lactating cows at the initial and final sampling periods.
Primiparous cows at parturition Initial Final
LCPSI
HCPS2
(%)
(%)
6.9 2 2.6
(no./no.) 27/392' 3211573 b 411l550b
9.2 30 20
Overal13 (no./no.)
(%)
(nolno.)
40/436' 39411305' 22611130'
8.1 15 10
67/828 426/2878
,.bMean percentages of CPS IMI within rows without common superscripts are different (P < .01). lLCPS
= Low
prevalence of CPS 1M) herds.
2HCPS = High prevalence of CPS IMI herds. 3Mean prevalence of CPS IMI for all primiparous cows at parturition or all cows. Journal of Dairy Science Vol. 77, No.4, 1994
26712680
STAPHYLOCOCCUS MASTITIS IN HEIFERS
trasted with all new cases of CPS IMI occurring during the study from all primiparous or multiparous cows other than those from primiparous cows at parturition. In both LCPS (27 out of 1(0) and HCPS (40 out of 146) herd groups, 27% of total new CPS IMI were due to primiparous cows with CPS IMI at parturition. There were 1.25 times more primiparous cows entering the HCPS lactating herd than primiparous cows entering the LCPS herds. The contribution of CPS IMI by primiparous cows at parturition to the herd was recalculated to determine the effect of the proportion of primiparous cows entering the lactating herds. After recalculation, the contribution of primiparous cows with CPS IMI at parturition was 29% for the LCPS herd group and 32% for the HCPS herd group. Of the 67 primiparous cows that had CPS IMI at parturition, subsequent milk samples were collected from 58 (at monthly sampling intervals postpartum). Twenty-five of the 58 primiparous cows (43%) had a CPS IMI at the second sampling period (1 mo postpartum). The CPS IMI persisted to the end of the study in 20 of the 25 primiparous cows that had CPS 1MI at a postpartum sampling period. Nine of 23 (39%) of LCPS primiparous cows with CPS IMI at parturition had CPS IMI at subsequent samplings, and 16 of 35 (46%) of HCPS primiparous cows with CPS IMI at parturition had CPS IMI subsequent samplings. The prevalence of 1MI and the common mastitis pathogens determined by milk samples collected at the start of the study, at parturition, at dry-off. and at the end of the study are presented in Table 4. The prevalence of 1MI in primiparous cows at parturition (55%) was higher than the IMI prevalence at parturition (44%) for multiparous cows. For primiparous cows from both herd groups, the prevalence for IMI was higher at parturition than at any other time. Primiparous cows from LCPS herds were more likely to have IMI at parturition than were primiparous cows from HCPS herds. Primiparous cows from LCPS herds were almost twice as likely to have CPS IMI at parturition than were LCPS multiparous cows. Thirty-five percent of IMI in primiparous cows at parturition were due to major mastitis pathogens. The CPS were the most common major agents of mastitis isolated from the primiparous cows during all four sampling periods.
963
Incidence and CI rates for all IMI and for IMI by specific pathogens in cows during the entire study, during lactation, and during the dry period are presented in Tables 5, 6, and 7, respectively. In all time periods, the LCPS herd group had a higher IR than the HCPS herd group for total 1MI, CNS IMI, Streptococcus species IMI, and coliform IMI. The HCPS herd group had a higher IR than LCPS for CPS 1MI during total, lactation, and dry periods (Tables 5, 6, and 7). The IR of IMI during lactation for all cows in the LCPS herd group was 3.4 cases/lOO cow-months, and the rate for the HCPS herd group was 3 cases/I 00 cow-months (Table 6). The IR of 1MI occurring during the dry period in the LCPS herd group was 18 cases/IOO cow-months, but the IR for IMI in the HCPS herd group was 12 cases/l 00 cow-months (Table 7). DISCUSSION Prevalence of CPS in Primiparous Cows at Parturition
Primiparous cows from HCPS herds do not have a significantly (P > .2) higher prevalence of CPS IMI at parturition than do LCPS primiparous cows. Our results do not support the assumed mode of transmission of CPS IMI for prepartum heifers, feeding mastitic milk, and allowing suckling heifers. Additionally, the findings of two studies in which milk infected with S. aureus was fed to a group of heifer calves indicated that the control group of heifers, which had not received milk containing S. aureus, had essentially the same rate of S. aureus IMI at parturition (3, 7). In the present study, all but one herd routinely fed waste milk, which often included mastitic milk, to heifer calves. The only herd that did not routinely feed waste milk was herd C, which fed fresh milk at random from any milking cow, regardless of S. aureus IMI status. Although herd C practiced group penning of preweaned heifer calves and housed postweaned heifer calves with the lactating herd, herd C is the only herd in which no CPS IMI was isolated from milk samples from the 11 primiparous cows tested at parturition. The data suggest that the theorized risk factors, such as group penning of preweaned calves, mastitic milk feeding, and a high CPS IMI Journal of Dairy Science Vol. 77, No.4. 1994
...
\/)
0
~
c
3
e. 0 .... ~
~
CIl
n
ii'
TABLE 4. Prevalence of IMI by pathogen as determined from composite milk samples from primiparous and multiparous cows at four different time periods over I yr in 18 northwestern US dairies. Number and percentage of cows with IMI! Cows and sampling periods
Sampled
::>
~
< !2-.l
;J
~ ;~
:;; 'f
Total cows with IMI (no.)
All multiparous Initial J Parturition Dry-off Final 4 LCPS Multiparous Initial Parturition Dry-off Final
1804 1628 lOll
1667 1023 933 664
1001
CPS
CNS
STR
COL
(%)
(no.)
(%)
(no.)
(%)
(no.)
(%)
(no.)
721 7W 425 585
40 42 35
327 219 157 212
18 13 16 13
259 311 174 285
14 19 17 17
65 87 43 46
4 5 4 3
29 85 10 31
2 5 1 2
286 361 225 291
28 39 34 29
17 32 22 28
2 3 3 3
171 202 \33 191
17 22 20 19
50 60
5 6 6 4
17 56 8 21
2 6
44
37 40
(%)
I
2
;l:l
0
I;l:I
HCPS Multiparous Initial Parturition Dry-off Final
781 695 347 666
435 348 200 294
56 50 58
310 187 135
44
184
27 39 28
All primiparous Initial Parturition Dry-off Final
956 828 614 864
326 452 225 3W
34 55 37 36
140 67 57 46
LCPS Primiparous Initial Parturition Dry-off Final
511 392 361 492
146 229 176
29 58 31 36
HCPS Primiparous Initial Parturition Dry-off Final
445 436 253 372
180 223 114 133
40 51 45 36
III
2 4 2
12 29 2
2 4
94
15 27 6 6
I
10
2
t'l1 -I
15 8 9 5
219 319 142 243
23 39 23 28
15 61 7 12
2 7
II
1 6
f!:
15 27 12 12
3 7 3 2
114 171 80 146
22
7 31 5
1\ I
8
5 2
10
2
3
I
125 40 45 34
28 9 18
8 30 2
2 7
5 27
6
I
2
I
I I
9
88 IW 41
105
148 62 97
44
22 30 24 34 25 26
I I
1
ITotal number of cows with IMI includes any mastitis pathogen; prevalence is the number of cows with an IMI divided by the number of cows sampled.
2(:ps = Coagulase-positive staphylococci. CNS
= coagulase-negative
31nitial milk sampling occurred during summer 1988. 4Final milk sampling occurred during summer 1989.
t'l1
11 16 12 14
40
staphylococci, STR = Streptococcus, and COL
= coliforms.
47 9 4 6 20
I
I
I
I
;l:l CIl
0
Z
965
STAPHYLOCOCCUS MASTITIS IN HEIFERS
prevalence in the adult herd, may not necessarily translate into a high CPS IMI prevalence in primiparous cows at parturition. Additional evidence for this is provided by herds G and Q (Tables I and 2), both of which group penned preweaned heifers, fed waste milk, and were HCPS herds, but the prevalence of CPS IMI in primiparous cows at parturition in these two herds was the second and third lowest of the 18 herds studied. Conversely, herds F and H (Tables I and 2) had average or higher prevalences of CPS IMI in primiparous cows at parturition even though heifers were individually penned until weaning and the lactating herd prevalence of CPS IMI was less than 1%. These examples strongly suggest the need for a thorough epidemiologic study at CPS IMI in heifers. The prevalence of CPS IMI in primiparous cows at parturition in 18 northwestern US dairies was 8.1 %. The actual prevalence for S. aureus IMI would be lower because two other CPS species. Staphylococcus hyicus and Staphylococcus intermedius, were included in
the calculation of CPS IMI. The actual prevalence of S. aureus IMI at parturition for both groups, for LCPS primiparous cows, and for HCPS primiparous cows was 6.1, 4.1, and 7.8%, respectively (Roberson et aI., 1993, unpublished data). This prevalence contrasts with estimates of 37, 2.6, and 9.6% from studies in Louisiana (30), Vermont (25), and Sweden (21). In the current study, 55% of primiparous cows had IMI at parturition, and 15% of these were CPS IMI. Studies involving single herds or a limited number of primiparous cows from multiple herds have reported that the S. aureus IMI prevalence of primiparous cows at parturition ranged from 0 to 51 % (3, 7, 9, 24, 32). In this study. a range of 0 to 27% of primiparous cows had CPS IMI at parturition, which may reflect diverse heifer management factors and general dairy herd management in the 18 herds studied. Additionally, with the exception of one herd, at least I primiparous cow per herd had CPS IMI at parturition, indicating that even primiparous cows in herds with good mastitis control are not exempt from this disease.
TABLE 5. Overall incidence rate and overall cumulative incidence of mastitis pathogens isolated from milk samples collected from cows in 17 northwestern US dairies. Cumulative incidence 2
Incidence rate l Herd groups
IMP
CPS3
CNS4
STR5
COL6
IMI
CPS
CNS
STR
COL
All cows LCPS7 (n = 9)8 HCpS9 (n 8) 17) Both (n
4.5 3.7 4.1
.5 1.6 1
2.8 2.1 2.5
.7 .3 .5
.6 .3 .5
42 35 39
5.8 17 II
28 22 25
8.5 3.5 6.2
7 3.5 5.3
Primiparous cows LCPS (n 9) 8) HCPS (n Both (n 17)
4.5 3.5 4.1
.8 1.3 1
3 2.3 2.6
.7 .4 .6
.6 .2 .4
41 34 38
8.5 13 11
29 23 26
7.9 4.5 6.3
6.3 2.4 4.5
Multiparous cows 9) LCPS (n HCPS (n 8) Both (n 17)
4.7 4.1 4.4
.4 2 1.2
2.7 2.2 2.5
.8 .3 .6
.5 .3 .4
43 38 41
4.8 20 12
29 22 26
9.1 3.7 6.5
6.2 4 5.2
=
=
= = =
= = =
IIncidence rate = Number new cases occurring during entire study/cow-months at risk during study. Figures in the table are expressed as the number of new cases per 100 cow-months. 2Cumulative incidence
=1 -
exp(-IR x t), where IR
= incidence
rate, and t
= 12
mo.
3CPS = Coagulase-positive staphylococci. 4CNS = Coagulase-negative staphylococci. 5STR = Streptococcus species. 6(:OL = Coliforms. 1LCPS = Low prevalence (:!>5%) CPS 1M! herds. 8Numbers in parentheses represent the number of herds per group. 9HCPS = High prevalence (>10%) CPS 1M! herds. Journal of Dairy Science Vol. 77, No.4, 1994
966
ROBERSON ET AL.
As indicated in Table 4, the overall most common major mastitis pathogen isolated from primiparous cows at parturition was CPS. This result is in contrast to results from the Vermont study (25) in which Streptococcus species and coliforms were more common. Reasons for these contrasting results are unknown. The most common major mastitis pathogens isolated from primiparous cows at parturition in the LCPS herds were the Streptococcus species at 7.9%, a result that is similar to the 7.6% identified in the Vermont study (25). The most common major pathogen for the HCPS primiparous cows was the CPS, which agrees with results of the Louisiana study (30). The most common mastitis pathogens in the present study were identified as CNS at 39%, which is in close agreement with three previous studies (24, 25, 30). The actual prevalence differences between the major mastitis pathogens in primiparous cows at parturition in this study are relatively minor.
Persistence of IMI
Subsequent milk samples were collected at monthly intervals after parturition from 58 of 67 primiparous cows with CPS 1MI at parturition. Twenty-five of the 58 primiparous cows subsequently sampled were positive for CPS 1MI, and 20 of these primiparous cows had CPS 1MI at the end of the study. However, differentiation between persistent CPS 1MI and new CPS 1MI was not performed in this study. Teat canal infections that are eliminated with the onset of milking or spontaneous cure are offered as reasons for the 33 primiparous cows that appeared to clear CPS IM1 postpartum. Persistency of S. aureus 1M1 in primiparous cows was 83% in a California study (29). Reasons for the large discrepancy between the current study and the California study are not known. Nonetheless, nearly 50% of all CPS 1MI in primiparous cows at parturition appeared to persist for at least 1 mo and would therefore present a risk of transmission to uninfected herdmates.
TABLE 6. Lactational incidence rate and cumulative incidence of mastitis pathogens isolated from milk samples collected from cows in 17 northwestern US dairies. Cumulative incidence 2
Incidence rate I Herd groups
IMI
CPS3
CNS4
STRs
All cows LCPS7 (n = 9)8 HCPS9 (n = 8) Both (n = 17)
3.4 3 3.2
.5 1.6 1
2.2 1.8 2
.5 .2 .4
Primiparous cows LCPS (n = 9) HCPS (n = 8) Both (n = 17)
3.4 2.9 3.1
.7 1.3
.9
2.5 2.1 23
.4 .2 .3
Multiparous cows LCPS (n = 9) HCPS (n = 8) Both (n = 17)
3.4 3.4 3.4
3 1.9 1.1
2.2 1.8 2
.5 .2 .4
COL6
IMI
CPS
CNS
STR
COL
.3 .1 .2
29 26 27
4.4 14 8.9
20 16 18
5.1 1.6 3.4
3
3
28 25 27
6.2 11 8.3
22 18 20
3.7 1.9 2.9
3.1 .4 1.8
28 28 28
3.3 17 9.6
19 16 17
5 4.5 4.7
3 1.9 2.5
0 .2 .3 .2
.3
1.3
2.2
Iincidence rate = Number of new cases occuning during lactation/cow-months at risk during lactation. Figures are eltpressed as the number of new cases per 100 cow-months. 2Cumulative incidence 3CPS
=1 -
= Coagulase-positive
eltp(-IR x t), where IR = incidence rate, and t = 10 mo.
staphylococci.
4CNS = Coagulase-negative staphylococci. sSTR = Streptococcus species. tiCOL = Coliforms. 7LCPS
= Low
prevalence (~5%) CPS IMI herds.
8Numbers in parentheses represent the number of herds per group. 9HCPS = High prevalence (>10%) CPS IMI herds. Journal of Dairy Science Vol. 77, No.4, 1994
967
STAPHYLOCOCCUS MASTITIS IN HEIFERS
Primiparous Cow IMI at Parturition
In this study, 55% of primiparous cows had an IMI at parturition. This figure is similar to that determined in other studies (24, 25). Primiparous cows from the LCPS had a slightly higher prevalence of IMI at parturition than did primiparous cows from the HCPS herds. Major mastitis pathogens caused IMI in approximately 21 % of primiparous cows. Approximately 18 and 35% of primiparous cows were infected with major pathogens at parturition in Vermont and Massachusetts studies (24, 25). Prevalence of CPS IMI in Cows
The prevalence of CPS IMI in this study, as determined by composite milk sampling of all lactating cows, was 15% for initial and 10% for final sampling periods. The lower prevalence rate at the final period may reflect increased awareness and may be due to preferential culling (15), particularly among the HCPS
herds. In support, the prevalence of CPS IMI increased slightly, although not significantly, from 2% at the initial sampling to 3% at the final sampling in the LCPS herds, but the prevalence for CPS IMI in the HCPS herds decreased from 32 to 20% during the same time period. The overall prevalence rates for CPS IMI in lactating cows in this study are lower than the 19% prevalence of S. aureus IMI determined in 44 northwestern dairies in 1982 (14), but considerably higher than the 2.4% overall prevalence of CPS IMI reported for 50 Ohio dairies (2). The overall prevalence of CPS IMI reported in the Ohio study is quite similar to the 2 % CPS IMI prevalence of the LCPS herds. No herds were able to eradicate S. aureus mastitis during the course of this study even though bacteriological results were provided and several herds employed standard mastitis control recommendations of milking time hygiene, dry cow therapy, and selective culling of cows with CPS IMI. These findings suggest that use of current mastitis control
TABLE 7. The dry period incidence rate and cumulative incidence of intramammary infections in cows in from 17 northwestern US dairies. Cumulative incidence2
Incidence rate! Herd groups
IMI
CPS3
CNS4
STRs
COL6
IMI
CPS
CNS
STR
COL
All cows LCPS7 (n = 8)8 HCpS9 (n = 7) Both (n = 15)
18 12 14
.9 2.3 1.6
10 7.4 8.9
33 1.3 2.4
3.7 1.8 2.8
30 21 26
1.8 4.5 3.1
18 13 16
6.3 2.6 4.6
7 3.5 5.4
Primiparous cows LCPS (n = 8) HCPS (n = 9) Both (n = 15)
15 9.5 13
1.1 1.8 1.4
7.5 4.6 6.2
3.5 2.6 3
3.6 1.5 2.6
26 17 22
2.1 3.4 2.7
14 8.5 11
6.5 4.6 5.6
6.7 2.9 4.9
Multiparous cows LCPS (n = 8) HCPS (n 9) Both (n = 15)
19 13 16
.8 3 1.8
12 8.5 11
3.1 .7 2
3.6 1.8 2.7
31 22 27
1.6 5.5 3.4
21 15 18
6 1.4 3.9
6.8 3.4 5.2
=
=
llncidence rate Number of new cases occurring during dry period/cow-months at risk during dry period. Figures are expressed as the number of new cases per 100 cow-months. 2Cumulative incidence
=1 -
exp(-IR x t), where IR
= dry
period incidence rate, and t
=2
mo.
3CPS = Coagulase-positive staphylococci. 4CNS
= Coagulase-negative
sSTR
= Streptococcus
staphylococci.
species.
6COL = Coliforms. 7LCPS = Low prevalence
(~5%)
CPS IMI herds.
8Numbers in parentheses represent the number of herds per group. 9HCPS = High prevalence (>10%) CPS IMI herds. Journal of Dairy Science Vol. 77, No.4, 1994
968
ROBERSON ET AL.
recommendation may control and reduce S. aureus IMI but do not readily lead to eradication. The continuing influx of primiparous cows with CPS IMI may be an important impediment to eradication. Incidence Rates and Cumulative Incidence Rates
Incidence and CI rates were calculated for three time periods (Tables 5 to 7). Because milk samples were normally collected only four times throughout the year for anyone cow, the IR we report are probably lower than the actual IR for IMI by environmental organisms, which are thought to be of shorter duration (17). This result may be especially true for coliforms, which have been identified as the most common pathogens of clinical mastitis in herds with low rolling mean SCC (13, 28). The overall CI for IMI in this study was 39 casesl 100 head at risk, which is comparable with the 36.8 casesllOO head at risk reported in a previous study (31). In the current study, CNS were the most common agents of new IMI, and the CPS were second; overall IR was I new easel 100 cow-months. The LCPS primiparous group had higher overall, lactational, and dry period IR of IMI, CNS, Streptococcus species, and coliforms compared with those of the HCPS primiparous group. Conversely, the HCPS primiparous group had the highest IR of CPS IMI at all time periods. Comparisons of multiparous cows between LCPS and HCPS groups were almost identical to those of the primiparous groups except that IR for all IMI occurring during lactation was identical. These data may suggest that cows of HCPS herds with CPS IMI are less susceptible to new IMI because fewer quarters are at risk to new IMI. This suggestion is supported by lower rates of environmental IMI for HCPS than for LCPS cows and is consistent with previous findings (13). The data also indicate very little difference in the overall IMI IR between the 2 herd groups during lactation. This result suggests that, using mastitis control procedures, producers have the ability to alter the infectious agents of mastitis from contagious to noncontagious organisms, yet the rate of new IMI changes little. Also of interest, IR for CNS, Streptococcus species, and coliforms were several times higher during the dry Journal of Dairy Science Vol. 77. No.4. 1994
period than during lactation for both herd groups. However, the IR for CPS IMI was only slightly higher during the dry period than during the lactation period for both herd groups despite the fact that all herds were using dry cow intramammary therapy. Occurrence of CPS IMI arising over the dry period is not a new finding (23, 34). The higher IR during the dry period as opposed to lactation for CPS IMI is similar to results of two earlier reports (11, 23). Additionally, results from an Israeli study indicated that 31 % of cows that were not infected with S. aureus IMI prior to drying off developed S. aureus IMI during the dry period even though all received dry cow intramarnmary therapy at dry-off (34). The transmission of new CPS IMI occurring during the dry period is not understood but may have a similar epidemiology to CPS IMI occurring in prepartum heifers. CONCLUSIONS
There is little difference in the prevalence of CPS IMI at parturition between primiparous cows of HCPS herds and primiparous cows of LCPS herds. Thus, primiparous cows with CPS IMI at parturition are significant reservoirs of this pathogen to uninfected herdmates in LCPS herds as well as HCPS herds. Primiparous cows with CPS IMI at parturition represent nearly one-third of the new CPS IMI cases. The prevalence of CPS IMI in the adult herd does not necessarily parallel the prevalence of CPS IMI in primiparous cows. Therefore, mastitis control procedures need to include methods to control or to prevent this disease in heifers. The assumed risk factors, group penning preweaned heifers and feeding mastitic milk, were not associated with an increased risk to heifers. The data from this study do not support the theory that the infected mammary gland is the reservoir of S. aureus for primiparous cow IMI at first parturition. Thus, the sources and modes of transmission of this disease need to be determined. ACKNOWLEDGMENTS
The authors express sincere appreciation to Dot Newkirk, Mark McFrederick, Jeff Hoffman, Sarah Christian, and Jean Cypher for technical assistance and to the cooperating
STAPHYLOCOCCUS MASTITIS IN HEIFERS
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