PRODUCTION ANIMALS
avj_649
PRODUCTION ANIMALS
Subclinical mastitis and associated risk factors on dairy farms in
New South Wales
41..46
K Plozza,a JJ Lievaart,a* G Pottsb and HW Barkemac,d
Conclusion The prevalence of SCM in this cross-sectional study is
comparable or lower than reported in other studies from North
America and the European Union. The outcome provides a benchmark for the current focus of the NSW dairy industry on the management practices associated with a low prevalence of SCM, such as
wearing gloves, using paper towels and feeding cows directly after
milking.
overseas, using housed animals and often in climates that are very
different to those encountered in Australia. These differences are
important because SCM has the greatest effect on the somatic cell
count (SCC) of the bulk milk11,14 and often penalty systems are
enforced at the dairy processor level for high SCC in the bulk milk
(BMSCC). In Australia, the 2008 national average BMSCC was 2.17 ¥
105 cells/mL,15 with some dairy processors currently deeming milk to
be premium if <3 ¥ 105 cells/mL and substandard if >6 ¥ 105 cells/
mL.16 Levels of acceptable BMSCC are likely to be continually assessed
because international marketing bodies continually decrease their
allowable BMSCC range. The European Union (EU), for example,
currently regards milk or milk products with >4 ¥ 105 cells/mL to be
unfit for human consumption,17 which places pressure on Australia as
an exporting nation to continually decrease the acceptable BMSCC.
The management of clinical and subclinical mastitis largely focuses on
decreasing the presence and spread of contagious pathogens on the
farm, which is usually monitored by herd recording systems that allow
the individual cow’s SCC (ISCC) to be recorded regularly.18 As there
are little published data from within Australia regarding the current
prevalence of SCM and related risk factors, this study aimed to
increase the understanding and awareness of the current situation on
dairy farms in New South Wales (NSW), firstly by determining the
current prevalence of SCM and secondly by outlining the associated
risk factors.
Keywords dairy cattle; disease prevalence; management practices; subclinical mastitis
Materials and methods
Objective To determine the current prevalence of subclinical
mastitis (SCM) and associated risk factors on dairy farms in New
South Wales.
Methodology A survey was sent to 382 dairy farmers to acquire
information on the relevant risk factors associated with SCM.
Results The average herd prevalence of SCM among the 189
respondents (response rate 49.5%) was 29%. Farmers who had
herds with a low prevalence (<20% cows with individual somatic
cell count (ISCC) >2 ¥ 105 cells/mL) more frequently wore gloves
during milking (26% vs 62%), used individual paper towels for
udder preparation (16% vs 62%), fed cows directly after milking
(47% vs 87%) and more frequently treated cows with high ISCC
(69% vs 80%) than farmers who had herds with a high prevalence of
SCM (>30% cows with ISCC >2 ¥ 105 cells/mL). The latter more often
used selective dry cow therapy (52% vs 24%), compared with low
prevalence herds.
Abbreviations BMSCC, bulk milk somatic cell count; CI, confidence interval; DCT, dry cow treatment; EU, European Union; DHI,
dairy herd improvement; ISCC, individual somatic cell count; OR,
odds ratio; SCC, somatic cell count; SCM, subclinical mastitis
Aust Vet J 2011;89:41–46
doi: 10.1111/j.1751-0813.2010.00649.x
S
ubclinical mastitis (SCM) is of great economic importance to
dairy farmers because it results in reductions in milk yield and
undesirable changes in the milk’s composition,1–3 as well as
increased costs associated with control strategies.4 Previous studies
outlining recommended control strategies for SCM commonly focus
on risk factors associated with management,4–7 the cow8,9 and the
environment.10–13 However, the majority of studies are conducted
*Corresponding author: EH Graham Centre for Agricultural Innovation (an alliance
between Charles Sturt University & NSW DPI), Locked Bag 588, Wagga Wagga, New
South Wales 2678, Australia; [email protected]
a
School of Animal and Veterinary Sciences, Charles Sturt University, Wagga Wagga,
NSW, Australia
b
Dairy Express, Armidale, NSW, Australia
c
Department of Production Animal Health, Faculty of Veterinary Medicine, University
of Calgary, Alberta, Canada
d
Department of Reproduction, Obstetrics and Herd Health, Faculty of Veterinary
Medicine, Ghent University, Merelbeke, Belgium
© 2011 The Authors
Australian Veterinary Journal © 2011 Australian Veterinary Association
Data collection
Questionnaires and return address envelopes were distributed across
NSW to 382 farms that participated in dairy herd improvement (DHI)
production monitoring. Each farm was marked with an 8-digit code
that was used instead of farmer/property name. The questionnaire
included 26 questions related to dry cow management practices and
current protocols for dealing with high ISCC, such as milking procedure, therapy, heifer calving management, nutrition and mastitis management (Tables 1, 2). (The survey is available on request by emailing
the corresponding author.) Dichotomous questions were used
because of their simplicity to answer19 and the responses were coded
for statistical analysis. Each returned questionnaire was given a farm
number based on the farm’s address. Farmers who did not wish to be
identified were able to remove their address from the returned questionnaire, but these farmers were then removed from the risk factor
study. In addition, ISCC data from lactating cows between January
2006 and June 2009 were provided by a herd recording company,
Dairy Express (Armidale, NSW, Australia).
Data management and statistical analysis
Individual farm data sets were compiled in Excel (Microsoft 2007)
format and calculations for the percentage of cows with an ISCC >2 ¥
Australian Veterinary Journal Volume 89, No 1–2, January/February 2011
41
PRODUCTION ANIMALS
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Table 1. Management practices associated with the milking process for all farms and on farms with a low or high prevalence of cows with
subclinical mastitis
All farms
(n = 189)
ISCC prevalence
Lowa (n = 52)
Management practice
Wearing gloves
Prestripping prior to milking
Washing udder prior to milking
Paper towels for udder preparation
Wet cloth for udder preparation
Predipping/spraying
Post-dipping/spraying
Milking machine serviced (months)
Liners replaced (months)
Parlour type
Rotary
Herringbone
Walk-through
Use of automatic cluster removal
Monitoring
Checking individual SCC records
Using BMSCC as indicator of SCM
No. of herds submitting samples for culture
Recording treatment of clinical mastitis
Recording treatment of high SCC
Culling cows because of high SCC
Treatment
Treat high SCC cows
Use of DCT
None
Selective
Blanket
Prevention
Separate calving paddock for cows
Separate calving paddock for heifers
Purchase heifers
Cows fed directly after milking
Cows fed on a feed pad
Cows fed in the paddock
Vitamin E and selenium supplementation:
None
All cows
Lactating cows only
Dry cows only
Others
Highb (n = 62)
n
(%)
n
(%)
n
(%)
76
24
17
85
15
4
177
10.2
6.9
(40)
(13)
(9)
(45)
(8)
(2)
(94)
32
6
4
32
4
1
49
10.3
6.5
(62)
(11)
(8)
(62)
(8)
(3)
(95)
16
10
4
10
5
1
58
10.9
7.2
(26)c
(16)
(7)
(16)c
(8)
(2)
(94)
20
148
21
103
(11)
(78)
(11)
(55)
7
39
6
30
(14)
(75)
(11)
(57)
4
48
10
31
(7)
(77)
(16)
(50)
183
183
104
181
147
132
(97)
(97)
(55)
(96)
(78)
(70)
49
51
30
49
36
38
(95)
(97)
(58)
(94)
(69)
(72)
60
61
36
60
53
48
(97)
(98)
(58)
(97)
(85)d
(77)
121
(64)
42
(80)
33
(53)c
7
68
114
(4)
(36)
(60)
0
13
39
(0)
(24)
(76)
1
32
29
(2)
(52)c
(46)
152
24
24
133
58
96
(80)
(13)
(13)
(70)
(31)
(51)
45
8
6
45
18
34
(87)
(15)
(11)
(87)
(34)
(66)
49
9
10
29
16
46
(78)
(15)
(17)
(47)c
(26)
(74)
108
27
39
5
10
(57)
(14)
(21)
(3)
(5)
35
7
7
1
1
(67)
(14)
(14)
(3)
(3)
38
7
10
3
3
(62)
(12)
(17)
(5)
(5)
<20% of cows with ISCC >200,000 cells/mL; b>30% of cows with ISCC >200,000 cells/mL.
P < 0.05; dP ⱖ 0.05 and P < 0.10.
BMSCC, bulk milk somatic cell count; DCT, dry cow therapy; ISCC, individual cow SCC; SCC, somatic cell count; SCM, subclinical mastitis.
a
c
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Australian Veterinary Journal Volume 89, No 1–2, January/February 2011
© 2011 The Authors
Australian Veterinary Journal © 2011 Australian Veterinary Association
PRODUCTION ANIMALS
Table 2. Number of farms that identified specific pathogens from samples submitted for culture
ISCC prevalence
Lowa (n = 30)
Streptococcus agalactiae
Staphylococcus aureus
Escherichia coli
Arcanobacterium pyogenes
Pseudomonas spp.
Mycoplasma spp.
Streptococcus uberis
Coagulase-negative staphylococci
PRODUCTION ANIMALS
All farms
(n = 104)
Highb (n = 36)
n
(%)
n
(%)
n
(%)
35
62
29
2
9
1
56
7
(34)
(60)
(28)
(2)
(9)
(1)
(54)
(7)
14
15
11
1
4
1
12
3
(47)
(50)
(37)
(3)
(13)
(3)
(40)
(10)
11
18
4
0
4
0
14
2
(31)
(50)
(11)c
(0)
(11)
(0)
(39)
(6)
<20% of cows with ISCC >200,000 cells/mL; b>30% of cows with ISCC >200,000 cells/mL.
P < 0.05.
ISCC, individual cow somatic cell count.
a
c
105 cells/mL were completed on an individual farm basis for each
month that herd recording was conducted on the farm. Subclinical
mastitis was defined as ISCC >2 ¥ 105 cells/mL, which is regarded as
the cut-off value for an intramammary infection.20,21 At the farm level,
a high prevalence of SCM was defined as having on average >30% of
cows with SCM during the last 42 months of herd recordings, and a
low prevalence was defined as on average <20% of cows with SCM
during that same period. These cut-off values of low and high (20%
and 30%) prevalence of SCM were based on the lowest and highest
quartile of the total population.
The results of the questionnaire were analysed in two steps. Firstly, a
descriptive overview of all the results of the returned questionnaires
was completed and a comparison of variables between high and low
prevalence farms was performed using an ANOVA for continuous
variables and Chi-square test on contingency tables for categorical
variables (Table 1). Secondly, all variables that differed significantly
between the high and low prevalence farms (P < 0.10) were included
into a multivariable logistic regression model and backwards stepwise
elimination procedure of variables was performed to determine significant management risk factors associated with a difference between
the high and low prevalence farms.
Only variables significant at P < 0.05 in the likelihood ratio test were
retained in the final model. All analyses used SPSS for Windows
(version 17.0, Chicago, IL, USA).
Results
Of the 382 questionnaires that were distributed, 189 (49.5%) were
completed, including addresses, and returned. All farmers who
returned the questionnaire had a seasonal calving herd with a DHI
test-day interval varying between 4 and 6 weeks. The average herd
prevalence of SCM during the last 42 months between January 2006
and June 2009 was 28.9% (95% CI 26.2–28.9), with the minimum and
maximum average values being 11.3% and 42.8%, respectively
© 2011 The Authors
Australian Veterinary Journal © 2011 Australian Veterinary Association
(Figures 1, 2). In total, 52 and 62 farms fitted the low (average <20% of
cows) and high (average >30% of cows) SCM prevalence categories,
respectively (Figure 1). The mean herd size for farms grouped into the
low and high SCM categories was 172 and 154 cows, respectively. The
prevalence of SCM on the farms in the low prevalence category was on
average 16.1% (95% CI 15.1–17.0), whereas on farms in the high
category the average prevalence was 36.5% (95% CI 36.2–39.8).
Management-related risk factors
The management categorical explanatory variables at P < 0.10 and
the herd-level category of prevalence of SCM are presented in
Table 1. Post-milking teat disinfection was practised on nearly all
farms, but pre-milking teat disinfection was practised on only 4 (2%)
farms. Blanket dry cow treatment (DCT) was used on 60% of the
farms and 36% practised selective DCT. Of the 26 variables outlined
in the questionnaire, 6 management factors differed significantly
(P < 0.10) in the initial screening between the two groups (Table 1).
All 6 variables were included in the multivariable logistic regression
model and 5 were considered significant (P < 0.05) between the two
SCM prevalence groups. Results of the multivariate analysis of management practices (Table 3) indicated that farmers who wore gloves
(odds ratio (OR) 0.24, 95% CI 0.07–0.83, P = 0.02), used paper towels
for udder preparation (OR 0.20, 95% CI 0.06–0.72, P = 0.01) and fed
cows directly after milking (OR 0.14, 95% CI 0.03–0.58, P = 0.01)
had herds with a lower prevalence of SCM (<20%), compared with
farmers who did not. Farms using selective DCT (OR 4.0, 95% CI
1.16–14.06, P = 0.03) and those that recorded the treatment of high
SCC (OR 5.9, 95% CI 1.35–25.92, P = 0.02) had a higher (>30%)
prevalence of SCM, compared with farms where these practices were
not used.
Discussion
The outcome of this cross-sectional study of 189 dairy farms provides
a benchmark for the NSW dairy industry regarding the prevalence
Australian Veterinary Journal Volume 89, No 1–2, January/February 2011
43
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Average proportion of cows with and individual SCC >200,000 cells/mL
PRODUCTION ANIMALS
45
40
35
30
25
20
15
10
5
0
1
11
21
31
41
51
61
71
81
91
101
111
121
131
141
151
161
171
181
Farm number
Figure 1. Average proportion of cows with an individual somatic cell count (SCC) >200,000 cells/mL over the 42-month period, calculated for all 189
farms included in the study.
50
farms low proportion of cows (200,000 cells/mL) N= 52
45
farms high proportion of cows (200,000 cells/mL) N= 62
% cows with SCC >200,000 cells/mL
all farms N = 189
40
35
30
25
20
15
10
5
39934
39873
39814
39753
39692
39630
39569
39508
39448
39387
39326
39264
39203
39142
39083
39022
38961
38899
38838
38777
38718
0
Figure 2. Prevalence of subclinical mastitis for all farms and for farms with a low or high proportion of cows with subclinical mastitis. SCC, somatic
cell count.
and associated risk factors of SCM. In the design of this study
there were two potential biases: the selection of the farms and the
data available from these farms. All farms selected for this study
participated in regular DHI monitoring and their management
44
Australian Veterinary Journal Volume 89, No 1–2, January/February 2011
practices might differ from those of farmers not participating in DHI
monitoring. It is also possible that the significant management practices reported in this study are biased by the unavailability of data
from individual cows for analysis.
© 2011 The Authors
Australian Veterinary Journal © 2011 Australian Veterinary Association
PRODUCTION ANIMALS
b
Constant
Wearing gloves
Using paper towels for udder preparation
Selective DCT
Feed cows post milking
Record treatment of high SCC
1.11
-1.42
-1.59
1.39
-2.00
1.78
SE
Sig.
0.84
0.63
0.65
0.64
0.74
0.75
0.19
0.02
0.01
0.03
0.01
0.02
OR
–
0.24
0.20
4.03
0.14
5.92
PRODUCTION ANIMALS
Table 3. Outcome of the final multivariate logistic regression model between farms with a low (n = 52) or high (n = 62) proportion of cows with
subclinical mastitis
95% CI for OR
Lower
Upper
–
0.07
0.06
1.16
0.03
1.35
–
0.83
0.72
14.06
0.58
25.92
CI, confidence interval; DCT, dry cow therapy; OR, odds ratio; SCC, somatic cell count.
However, although we acknowledge that cow-level factors may result
in some change in the magnitude of the reported effects and should be
included in subsequent analyses, it is unlikely that the inclusion of
these hierarchical variables will result in negation of the overall effect
of the farm-level management practices. Despite this, it is important to
collect that information for a comparison of management practices
with other studies under different circumstances, such as in the EU or
North America where there is indoor housing of cows and a different
climate. In those countries the management practices examined in
the current study might have a different result or the ones that are
important for an indoor system might not be applicable for Australian
conditions.
The average prevalence of SCM on the NSW farms was 29%, with an
extensive range between 11% and 43%, and this result is comparable to
studies from the USA (31%)22 and the Netherlands (23% and 29%),23,24
but lower than in other studies from Belgium (40%)25 and Switzerland
(35%).26 Of the specific milking practices investigated in our study,
only the use of gloves and paper towels for pre-milking treatment was
different between the SCM prevalence groups. It was unexpected that
only 40% of NSW dairy farmers used gloves during milking, given that
it is widely recommended to reduce the spread of contagious pathogens during milking, and it is no surprise that the use of this management practice has been significantly associated with low SCM
prevalence.27 To further emphasise the implications of poor hygiene,
the use of single paper towels was also associated with the low prevalence category in the present study. Drying the teats prior to milking
is optimal for preventing the spread of contagious pathogens,28–30
because wet teats increase the risk of liner slip, as well as being a means
for bacteria to migrate to the tip of the teat.31 Using udder towels for
more than one cow has been associated with a higher prevalence of
contagious pathogens32 and can also increase transmission of contagious pathogens during milking.
Feeding of cows after milking was more common on the low SCM
farms in our study; however, there was no difference between feeding
on a feed pad or within the paddock, suggesting that the practice itself
is the important aspect. Feeding the cows directly after milking has
been advocated for mastitis prevention, because it discourages the
cows from lying down while their teat ducts remain patent and thus
vulnerable to environmental pathogens.32 Locking cows in a feed-line
© 2011 The Authors
Australian Veterinary Journal © 2011 Australian Veterinary Association
is associated with herds showing low SCC,33 and feed availability has a
direct effect on the length of time that cows remain standing.34
Blanket DCT, a cornerstone of the prevention of new intramammary
infections, was practised on 60% of the NSW dairy farms (Table 1),
significantly lower than on Canadian35 or Dutch7 dairy farms, where
72% and 75% of farms, respectively, used blanket DCT. The adoption
of this important management practice was even lower on the present
high SCM prevalence farms, consistent with previous studies performed worldwide.7,10,35 Our result was also confirmed in a metaanalysis by Halasa et al. who found that blanket DCT showed more
protection when compared with selective DCT, but that the effect was
reversed, with selective DCT associated with greater protection than
blanket DCT when the selection criterion was at the quarter level (as
opposed to the cow level) only.36 Similarly, no difference was found
between the two DCT practices in another Australian study.37 Our
results showed that farmers with high SCM herds more often
recorded the treatments of cows with high SCC, which was a surprising finding, as more of the low prevalence farmers treated their high
SCC cows. This finding may therefore represent a limitation of our
study, which has been discussed in other published articles regarding
the use of questionnaires.19 It is postulated that the significance of the
question regarding ‘recording of treatments of high SCC’ may be
invalid, because of respondents either misreading the question or
confusing SCM treatment recording with clinical mastitis treatment
recording. This seems likely because there was no significant difference between the two categories regarding the treatments for high
SCC. Regardless of this, the use of questionnaires in dairy management studies has been reported to be a valid and repeatable method of
data collection.18 It is important to conduct regional and national
mastitis prevalence studies on a regular basis, because the prevalence
of SCM and the adoption of management practices change over time
and therefore mastitis control programs and the focus of extension
efforts can be adjusted accordingly.
Conclusions
The average prevalence of SCM of 29% on the 189 NSW dairy farms
was comparable to or less than the prevalence in various other
studies conducted worldwide. Preventive management practices
Australian Veterinary Journal Volume 89, No 1–2, January/February 2011
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PRODUCTION ANIMALS
such as wearing gloves, using paper towels and feeding cows after
milking were associated with a low prevalence (<20%) of SCM.
Selective DCT was associated with a high prevalence (>30%) of SCM
and although seen as a useful for decreasing the use of antibiotics,
on many farms this type of DCT failed to prevent new intramammary infections.
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(Accepted for publication 20 June 2010)
© 2011 The Authors
Australian Veterinary Journal © 2011 Australian Veterinary Association