Automatic milking systems as a risk factor for intramammary infections caused by environmental
pathogens.
R. Peters-Sengers, student at the Faculty of Veterinary Medicine, Utrecht University. January, 2013.
Abstract
Intramammary infections (IMI) in cows are of great economic impact for the dairy industry. With the
increasing number of automatic milking systems (AMS) on dairy farms, it is important to know
whether AMS can influence IMI status of the cow. In this study, prevalence of IMI caused by
environmental and contagious pathogens were compared between herds with AMS and
conventional milking systems (CMS). Milk sample data was collected from 966 cows in 84 Dutch dairy
farms; 248 cows were milked by an AMS and 718 cows were milked conventionally. The quarter milk
samples were taken at two moments; at drying off and two weeks (+/- 7 days) after calving.
Information on management was obtained by means of a questionnaire. Bacteriological culture was
performed on the milk samples and the major mastitis-causing pathogens were divided in
environmental pathogens (S. uberis, S. dysgalactiae, E. Coli, Klebsiella) and contagious pathogens (S.
aureus, S. agalactiae and S. dysgalactiae). Influence of management factors as possible confounders
on the relationship between milking system and IMI were statistically analysed through a univariable
and multivariable model. A significant association was found between AMS and the occurrence of
IMI caused by environmental mastitis-causing pathogens (p<0.05). Parity showed to be a
confounder; multiparous cows were more at risk than heifers. For contagious pathogens, no
significant relation was shown. This study demonstrated that AMS milked cows are at higher risk of
IMI caused by environmental pathogens.
Introduction
Recently, there has been growing interest in automatic milking systems (AMS) in the dairy industry.
In 1992, the first AMS was installed in the Netherlands. Since 2000 AMS became a worldwide
accepted technology and at the end of 2009 more than 8000 AMS farms were found in over 25
countries (de Koning, 2010).
Although, in the Netherlands, herringbone and side-by-side milking parlours are still the most
frequently used milking parlours (50.5% and 15.8% respectively), robotic milking is rapidly increasing
over the last few years. In September 2012, statistics obtained by the Dutch foundation for quality
management and maintenance of milking installations (KOM, 2012), show that 15.1% of the Dutch
dairy farms are currently using AMS. Main reasons for farmers to switch from conventional milking
systems (CMS) to AMS are the increasing labour costs, relief from the daily milk routine and to
control milking frequency on an individual cow basis.
Although considerable research has been devoted to udder health of dairy cows when using an AMS,
little attention has been paid to the occurrence of intramammary infections (IMI) in cows milked by
an AMS. Studies have been performed on the alteration in SCC, frequency of treatment for clinical
mastitis, total bacterial plate count in milk samples and cleanliness of the teats just after introduction
of the AMS (Dohmen, Neijenhuis, & Hogeveen, 2010; Hammer, Morton, & Kerrisk, 2012; M. Hovinen,
Rasmussen, & Pyorala, 2009; M. Hovinen & Pyorala, 2011; Jacobs & Siegford, 2012; Klungel, Slaghuis,
& Hogeveen, 2000; Rasmussen, 2006). But, to our knowledge, no literature exists on the occurrence
of IMI caused by environmental and contagious pathogens when AMS and CMS are compared in a
settled situation.
Understanding the role of AMS in the occurrence of IMI on farms is important for prevention of IMI.
Subclinical and clinical mastitis develop from an infected udder and IMI in dairy herds have a
significant economic impact through production losses, discarded milk, veterinarian costs, labour
costs, drugs, culling, penalties, etc. (Huijps, Lam, & Hogeveen, 2008).
The aim of the present study was to test whether AMS is a risk factor for IMI caused by the major
mastitis pathogens, when compared with CMS. We will specifically study the effect of environmental
pathogens and contagious pathogens. This will enable us to better understand the mechanics by
which AMS affects IMI.
Materials and methods
Study design and data collection
A cohort study was undertaken using data from a research project executed by the Dutch Udder
Health Centre (UGCN). The Dutch Udder Health Centre conducted a randomized trial in which 1640
cows from 98 dairy farms in the Netherlands were admitted. In this trial the use of dry cow therapy
was examined. To take part in this research project, farms had to have at least 40 cows, participate in
a 4-6 week DHI schedule and the farm type had to be conventional (not organic). Furthermore,
participation in the project was on a voluntary basis.
A minimum of 2 cows per farm were selected. Only seemingly healthy cows were included with a
somatic cell count < 250,000cells/mL for cows and < 150,000cells/mL for heifers as analysed by the
DHI program before drying off. Furthermore, cows were only admitted when they had four
functional quarters.
For the research done on dry cow therapy, every cow received dry cow therapy (supermastidol®) in
two out of four teats, the other two teats were left untouched. It was randomly selected whether the
dry cow therapy was used in either the left or the right side of the udder. Cows were followed up
from the day of drying off until 100 days in milk.
This study uses a subset of the data obtained from the study on dry cow therapy. This subset
comprised data collected from 1000 cows, of which 34 were excluded due to incomplete data. A
total of 966 cows from 84 Dutch dairy farms were used in this study.
Farms with an automatic milking system were compared with farms milking in a milking parlour.
Farms using a rotary milking parlour were not included in this study. In total, 248 cows from 19 AMS
farms and 718 cows from 65 CMS farms were included.
In the start period of the project, information on management factors was obtained through
questionnaires. These questionnaires were administered by a trained student or an employee from
the Dutch Udder Health Centre. It comprised five different topics: the dry period (strategy, way of
drying off, dry cow therapy, supplements, barn type and bedding material); calving (barn type and
bedding material of location of calving, cleaning of this location and milking around calving); lactating
cows <100 days in milk (treatment and medication in case of mastitis, pasture grazing, barn type and
bedding material); milking (hygiene, dipping, etc.) and milking machine; health status (vaccination,
occurrence of certain diseases e.g. milk fever, breed, fertilisation, etc.)
Milk samples and bacteriology
Aseptic quarter milk samples were collected at drying off and two weeks after calving (+/- 7 days).
Bacteriological culture was conducted on the samples, following the NMC guidelines (NMC, 1999). In
short, the bacteriological culture procedure was as follows; with a standardised inoculation loop,
0,01 mL of milk was inoculated on a 6% sheepblood agarplate and a streptococcus-selective Edward’s
agar (Biotrading, Mijdrecht, The Netherlands). After inoculation the plates were placed in an
incubator at a temperature of 37 ºC. First, a visual assessment was done after an incubation period of
24-48 hours. The visual assessment comprised assessment on growth, morphology and (type of)
haemolysis. Next, the Edward’s plates were further assessed under UV light. After this visual
assessment gram staining was done and, if needed, other supplementary tests were conducted,
depending on the bacteria found (i.e. coagulase test in case of staphylococci, Lancefield-typing in
case of streptococci, API 20E system in case of gram-negative bacteria).
Case definition
This research focuses on the major pathogens that can cause mastitis in the Netherlands, S. aureus,
S. agalactiae, S. uberis, S. dysgalactiae, E. coli and Klebsiella spp.. These pathogens were divided in
environmental pathogens (S. uberis, S. dysgalactiae, E. coli, Klebsiella spp.) and contagious pathogens
(S. aureus, S. agalactiae and S. dysgalactiae), with S. dysgalactiae having characteristics of both
environmental and contagious pathogens (Blowey & Edmondson, 2010; Zadoks & Fitzpatrick, 2009).
Analyses were performed at cow-level. An IMI-positive cow was defined as the establishment of
bacteria growth of one of the major mastitis-causing pathogens in at least one of the samples taken.
When milk samples were positive at two sampling moments this was seen as one positive case for
IMI in dry period and one positive case for IMI in lactation period. When milk samples taken at one
sampling moment were positive for both environmental pathogens (EP) and contagious pathogens
(CP), this was counted as one case for EP and one case for CP.
Statistical analysis
To determine the statistical difference in prevalence between positive samples in CP and EP we
performed a chi-square test. Associations between EP or CP and AMS and associations between
potential confounders and AMS were assessed using univariable logistic regression in STATA (version:
STATA SE12). Variables with p<0.20 were selected for the multivariable analyses. Associations
between EP or CP and AMS correcting for potential confounders were tested in two multivariable
logistic regression models. A random herd-effect was included to account for the clustering of cows
within herds. The potential confounders that were tested are summarized in Table 3. A backward
selection was performed while adjusting for confounding (>25% change in coefficients was deemed
an indication of confounding). Variables were kept in the model when significant (p<0.05) or when
they were confounding the association between IMI and AMS/CMS. The best model was selected
based on Likelihood ratio test.
Results
Descriptive statistics of the prevalence of intramammary pathogens are listed in Table 1 and Table 2.
Table 1. Intramammary pathogens cultured from milk samples of 966 included dairy cows in 84
Dutch herds at two sampling moments.
EP1
CP2
Negative (%)
865 (89.5%)
887 (91.8%)
Positive (%)
101 (10.5%)
79 (8.2%)
Distribution of positive cases
D+, L-3
22 (21.8%)
19 (24.1%)
D-, L+
73 (72.3%)
50 (63.3%)
D+, L+
6 (5.9%)
10 (12.7%)
1
Environmental pathogens.
2
Contagious pathogens.
3
Positive cows are divided in dry period positive/negative (D+/-) and/or lactation period
positive/negative (D+/-).
The majority of cows were found to be negative on environmental and contagious pathogens in the
milk at drying off and at two weeks (+/- 7 days) after calving. A small difference can be seen between
the environmental and contagious pathogens, with slightly more negative cows for CP. This
difference was not significant (p=0.35). Within the positive cows, most of the cows were cultured
with IM pathogens during the lactation period only. The pattern in the distribution of positive cases is
similar for EP-infected and CP-infected cows.
Table 2. Overview of the intramammary pathogens found in milk samples of cows milked by either an
automatic milking system or a conventional system.
No of cows
Milking system
AMS
CMS
EP positive
101 (10.5%)
34 (13.7%)
67 (9.3%)
EP negative
865 (89.5%)
214 (86.3%)
651 (90.7%)
CP positive
79 (8.2%)
26 (10.5%)
53 (7.4%)
CP negative
887 (91.8%)
222 (89.5%)
665 (92.6%)
Total no of cows
966
248
718
The percentage of EP and CP positive cows milked with an AMS is higher than the percentage of cows
milked with a CMS.
Descriptive statistics for the two milking systems and the potential confounding factors are given in
Table 3.
Table 3. Descriptive statistics of possible confounding management factors on the use of AMS/CMS.
Management factors
Herd size
Pasture grazing
Overcrowded
Dry cow barntype
Barntype lactating cows
Bedding material dry cows
Bedding material lactating cows
Cleaning frequency of the dry cow boxes (no
of times/week)
Cleaning frequency of the lactating cows
boxes (no of times/week)
Treatment clinical mastitis (% treated with
antibiotics)
Treatment subclinical mastitis (% treated
with antibiotics)
Flushing of teat cup liners after milking
Average somatic cell count in herd (2011)
Parity
Hygiene score udder
AMS
102 [80-112]
134 (54.0%)
114 (46.0%)
220 (88.7%)
28 (11.3%)
0 (0.0%)
248 (100%)
0 (0.0%)
248 (100%)
0 (0.0%)
62 (25%)
0 (0.0%)
35 (14.1%)
151 (60.9%)
CMS
99 [78-122]
107 (14.9%)
611 (85.1%)
516 (72.9%)
77 (10.9%)
115 (16.2%)
663 (92.3%)
55 (7.7%)
716 (99.7%)
2 (0.3%)
4 (5.9%)
44 (6.1%)
193 (26.9%)
439 (61.1%)
9 (3.6%)
19 (7.7%)
44 (17.7%)
176 (71.0%)
0 (0.0%)
30 (4.2%)
203 (28.3%)
485 (67.6%)
1 [0-1]
1 [1-2]
median [iqr]
2 [2-3]
2 [2-2.5]
median [iqr]
100 [100-100]
100 [100-100]
median [iqr]
20 [0-50]
5 [0-50]
1 (no)
2 (yes, after high SCC
cows)
3 (yes, after cows
treated with AB)
4 (yes, after every
cow)
median [iqr]
heifer
multipari
clean
dirty
0 (0.0%)
0 (0.0%)
128 (17.8%)
358 (49.9%)
0 (0.0%)
135 (18.8%)
248 (100%)
97 (13.5%)
212 [171-276]
89 (35.9%)
159 (64.1%)
214 (87.0%)
32 (13.0%)
193 [151-227]
268 (37.3%)
450 (62.7%)
562 (78.7%)
152 (21.3%)
median [iqr]
no
yes
1 (< 5%)
2 (5-<10%)
3 (>10%)
1 (ligbox)
2 (grupstal)
1 (ligbox)
2 (grupstal)
no litter or sand
manure
straw
sawdust or wood
shavings
no litter or sand
manure
straw
sawdust or wood
shavings
median [iqr]
Univariable and multivariable analyses
The results of the univariable and multivariable analyses for environmental and contagious
pathogens are given in Tables 4 and 5. The univariable analyses of environmental pathogens (Table 4)
show that risk factors are the milking system, bedding material for dry cows, percentage of
treatment of subclinical mastitis and parity. Straw bedding material for dry cows is not significant
(p=0.47). In the multivariable analyses, parity is significantly associated with AMS (p<0.05), but it is
not a strong confounder, as the coefficient changes with less than 25% (OR of AMS changes from
1.54 into 1.57). Table 5 shows that risk factors (p<0.20) for contagious pathogens are the milking
system, bedding material for the lactating cows (likelihood ratio test = 0.16), percentage of
treatment of subclinical mastitis and flushing of teat cup liners after milking (likelihood ratio test =
0.12). Only the percentage of treatment of subclinical mastitis becomes significant in the
multivariable analysis (p<0.05).
Table 4. Results of the univariable and multivariable analyses of environmental pathogens, based on
966 cows in 84 Dutch dairy herds.
Milking system
Herd size
Pasture grazing
Overcrowded
Dry cow barntype
Barntype lactating cows
Bedding material dry cows
Bedding material lactating cows
Cleaning frequency of the dry cow boxes (no of
times/week)
Cleaning frequency of the lactating cows boxes
(no of times/week)
Treatment clinical mastitis (% treated with
antibiotics)
Treatment subclinical mastitis (% treated with
antibiotics)
Flushing of teat cup liners after milking
Average somatic cell count in herd (2011)
Parity
Hygiene score udder
CMS
AMS
no
yes
1 (< 5%)
2 (5-<10%)
3 (>10%)
1 (ligbox)
2 (grupstal)
1 (ligbox)
2 (grupstal)
no litter or sand
manure
straw
sawdust or wood shavings
no litter or sand
manure
straw
sawdust or wood shavings
1 (no)
2 (yes, after high SCC cows)
3 (yes, after cows treated
with AB)
4 (yes, after every cow)
heifer
multipari
clean
dirty
univariable analysis
multivariable analysis
OR (95%CI)
pvalue
OR (95%CI)
pvalue
1.5 (0.99-2.4)
1.0 (1.0-1.0)
0.053
0.59
1.6 (1.0-2.5)
0.045
1.1 (0.7-1.9)
0.53
1.4 (0.8-2.5)
0.6 (0.3-1.3)
1.2 (0.5-3.0)
0.26
0.23
0.73
1.0 (0.4-2.8)
0.6 (0.3-1.2)
0.8 (0.4-1.4)
0.98
0.15
0.38
0.5 (0.1-2.9)
0.4 (0.1-1.8)
0.41 (0.1-2.0)
0.9 (0.7-1.1)
0.43
0.22
0.28
0.30
1.0 (0.8-1.2)
0.85
1.0 (1.0-1.0)
0.31
1.0 (1.0-1.0)
0.11
1.0 (1.0-1.0)
0.081
1.1 (0.5-2.1)
0.9 (0.4-2.2)
0.83
0.89
1.3 (0.7-2.6)
1.5 (0.8-3.0)
0.44
0.21
1.7 (1.1-2.7)
0.03
1.7 (1.1-2.7)
0.026
0.8 (0.5-1.5)
0.53
Table 5. Results of the univariate and multivariate analyses of contagious pathogens, based on 966
cows in 84 Dutch dairy herds.
Milking system
Herd size
Pasture grazing
Overcrowded
Dry cow barntype
Barntype lactating cows
Bedding material dry cows
Bedding material lactating cows
Cleaning frequency of the dry cow boxes (no of
times/week)
Cleaning frequency of the lactating cows boxes
(no of times/week)
Treatment clinical mastitis (% treated with
antibiotics)
Treatment subclinical mastitis (% treated with
antibiotics)
Flushing of teat cup liners after milking
Average somatic cell count in herd (2011)
Parity
Hygiene score udder
CMS
AMS
no
yes
1 (< 5%)
2 (5-<10%)
3 (>10%)
1 (ligbox)
2 (grupstal)
1 (ligbox)
2 (grupstal)
no litter or sand
manure
straw
sawdust or wood shavings
no litter or sand
manure
straw
sawdust or wood shavings
1 (no)
2 (yes, after high SCC cows)
3 (yes, after cows treated
with AB)
4 (yes, after every cow)
heifer
multipari
clean
dirty
univariate analysis
multivariate analysis
OR (95%CI)
pvalue
OR (95%CI)
pvalue
1.5 (0.9-2.4)
1.0 (1.0-1.0)
0.13
0.71
1.3 (0.5-3.4)
0.587
1.0 (0.6-1.7)
0.94
1.3 (0.7-2.6)
0.7 (0.3-1.7)
0.40
0.47
0.6 (0.3-1.3)
0.21
0.7 (0.2-2.6)
0.7 (0.3-1.6)
0.9 (0.4-1.8)
0.58
0.42
0.70
0.1 (0.0-0.9)
0.3 (0.1-1.7)
0.3 (0.1-1.6)
1.0 (0.8-1.4)
0.04
0.18
0.16
0.83
0.1 (0.0-1.8)
0.7 (0.1-3.9)
0.5 (0.1-2.9)
0.128
0.642
0.455
1.0 (0.8-1.3)
0.89
1.0 (1.0-1.0)
0.68
1.0 (1.0-1.0)
0.15
1.0 (1.0-1.0)
0.039
2.2 (0.9-5.9)
1.8 (0.6-5.4)
0.10
0.32
2.5 (0.9-7.0)
1.7 (0.6-5.4)
0.073
0.342
2.8 (1.1-7.3)
1.1 (0.5-2.3)
0.04
0.74
2.7 (0.9-2.9)
0.089
1.1 (0.7-1.9)
0.59
1.2 (0.7-2.1)
0.58
Discussion
This study shows that AMS is a risk factor for IMI caused by environmental pathogens when
compared to CMS. To our knowledge, there have been no comparative studies on the occurrence of
environmental and contagious mastitis pathogens in which robotic milking is compared with
conventional milking. Numerous studies, reviewed in this discussion, have been performed on the
effect of AMS on SCC, treatment of clinical mastitis and the cleaning process of the teats on AMS
farms. Conclusions on these aspects could give an impression of the IMI status on AMS farms.
Nevertheless, existing literature mostly seem to review the transition of CMS to AMS or the use of
AMS just after installation, whereas the change in milking system will always be accompanied by
other changes such as management of the farmer. It must be noted that dairy farms submitted in
this study had already switched to AMS for an unknown period of time.
Zecconi et al. (2004) published a study on the spread of contagious pathogens in Italian AMS herds.
Herds with and without S. aureus infections were compared. In a herd where S. aureus was
considered a problem, 3.4% of the cows were infected with S. aureus at calving. At the end of the
study this percentage increased to 66.7%. This extreme increase did not occur in the herd where S.
aureus was not considered a problem. According to the authors, the progressive increase in IMI was
a consequence of the spread of infections during milking. They conclude that introduction of an AMS
in herds does not negatively impact the prevalence of IMI and SCC, as long as the initial cow health
status and overall herd management are good. Nevertheless, only two herds were included in this
study and it only comprised heifers. Moreover, no information was given about the frequency of
replacing teat cup liners, which could be an important factor in the spread S. aureus (de Koning,
Slaghuis, & Van der Vorst, 2004).
Klungel, Slaghuis, & Hogeveen (2000) compared bulk milk total bacterial plate count (TPC) from 28
Dutch AMS farms with CMS farms. Although the mean TPC only slightly increased after introduction
of the AMS, the number of measurements exceeding 100,000 cfu/mL was clearly higher in AMS
farms than in CMS farms. This could be due to the milking machine, but yet other factors may have
contributed to the increase, i.e. elevated mean SCC on farms, hygiene scores, etc. Certainly not only
IMI status of the cows affects bacterial growth in bulk milk. Milk may remain in the milk equipment
for a longer time and when cows kick a lot, there is chance that dirt gets vacuumed by the AMS as
there will be no-one to visually check on it.
Multiple investigations have been done on SCC in AMS dairy herds. Several studies found an increase
in SCC after the transition from conventional milking to automatic milking (Dohmen, Neijenhuis, &
Hogeveen, 2010; Hovinen & Pyorala, 2011; Klungel, Slaghuis, & Hogeveen, 2000; Poelarends et al.,
2004; Rasmussen, 2006). Klungel, Slaghuis, & Hogeveen (2000) observed that SCC was already higher
on AMS farms, prior to the installation of the new milking machine, indicating that the increase in
SCC might not be (totally) due to the AMS. These findings were supported by Poelarends et al. (2004)
and Hovinen, Rasmussen, & Pyorala (2009). Nevertheless, in the latter study the SCC on AMS farms
remained on a higher level after introduction compared with CMS farms.
Hovinen, Rasmussen, & Pyorala (2009) reported that after the introduction of an AMS, the
proportion of high-SCC cows was higher than milking in a conventional way before introduction,
indicating more new infections. The authors suggest an apparent adaptation period as the
proportion of high-SCC cows appeared to stabilize close to the original values toward the end of the
study. Rasmussen (2006) also speculated about an adaptation period, when comparing the
percentage of cows with elevated SCC in the year before and after installation of AMS. The increase
did not depend on the year of introduction or on the AMS brand. The percentage of high-SCC cows
was detectable throughout the first four years with AMS.
Hovinen, Rasmussen, & Pyorala (2009) noted that first-parity cows had significantly fewer recorded
treatments after the introduction of the AMS than older cows. Poelarends et al. (2004) also reported
that there was a significant effect of AMS on high-SCC cows in second and third parity cows. Heifers
did not show a significant increase. We found that multiparous cows have more potential risk on
environmental pathogens in comparison with the first-parity cows when using an AMS. However, the
comparison of difference in parity for cows between AMS and CMS on environmental pathogens was
beyond the scope of this research. These findings raise the question of why first-parity cows have
decreased frequency of treatments for clinical mastitis and decreased IMI caused by environmental
pathogens. The reason is yet unknown, but it is believed that cows that were used to conventional
milking had more difficulties in getting used to the AMS. However, in this study AMS farms did not
suffer from an adaptation period as the farmers already used the AMS for a longer period.
In this study it was observed that only IMI caused by environmental pathogens were significantly
associated with the use of an AMS. Risk factors exist for environmental pathogens to cause IMI. With
an AMS there is no visual check per cow when cleaning the teats, which means no differentiation is
made between dirty or clean teats. Knappstein et al. (2004) found a reduction of 99% of artificially
contaminated teats in CMS, whereas the reduction in AMS had a large variance (50-85%)¸ depending
on the brand. Brushes show to be more effective than cups when contamination of the teats was
higher. Hovinen et al. (2005) supports this finding and conclusion. Dohmen et al. (2010) found a
positive correlation between hygiene score of the cow and the percentage of new high-SCC cows per
year. Dirty teats and dirty thighs before milking were positively correlated to the annual average herd
SCC and the annual average new high-SCC cows. These studies indicate that the cleaning process may
explain the observed increased rate of infection with environmental pathogens. Contagious
pathogens, however, are expected to be prevented from spreading from cow to cow by flushing of
the teat cup liners after every milking.
Another risk factor for environmental pathogens to cause IMI in AMS cows is that disinfection of the
teats after milking is not always as precise as when this would be done manually. Dohmen et al.
(2010) found a positive correlation between teats that were not fully covered with teat disinfection
spray after milking and SCC.
Furthermore, with a higher milking frequency by voluntarily milking (AMS), the duration of the teat
canals being open will be longer and therefore a higher risk of negative pressure drawing the bacteria
into the teat canal exists. Especially when cows lie down in their boxes the risk for environmental
pathogens to enter the teat canal and cause infection is greater. Milk leakage also causes the teat
canal to be open and therefore is another risk of exposure to environmental pathogens. Persson
Waller et al. (2004) observed the difference in milk leakage between AMS and CMS over a period of
two years. The authors demonstrated a significant higher incidence of cows leaking milk with an
AMS. A large proportion of the cows that were leaking milk within four hours after milking was due
to disturbances of the machine. Although the SCC increased numerically, it was not significantly
higher in cows with milk leakage.
Contagious pathogens were not shown to be significantly associated with the use of an AMS in this
study. An advantage of the use of an AMS is that milking is quarter-based, which could limit the risk
for interquarter contamination with mastitis-causing pathogens within a cow (Hovinen & Pyorala,
2011; Hovinen, Rasmussen, & Pyorala, 2009). Furthermore, most of the AMS have teat cup liners that
are flushed with steam after every milking, which will kill bacteria that might otherwise have infected
the next cow during milking. Most CMS do not flush the liners with hot water after every milking
(Table 3). Another explanation for the finding that contagious pathogens were not associated with an
AMS is that the proportion of cows with IMI caused by contagious pathogens was somewhat smaller
than the proportion of cows with IMI caused by environmental pathogens. This could have caused
the sample size to be too small to show significant effects. More research has to be done to explore
this.
This study included only seemingly healthy cows with a low SCC. Consequently, the group of cows
used for this investigation is not fully representative for the cows in the field. Also, every cow
received dry cow therapy in two out of four teats. This is not expected to alter the outcomes of this
study, as every cow received the same treatment. However, in the majority of cows from Dutch dairy
herds dry cow therapy is a standard procedure in the prevention of the development of mastitis in
the dry period. As only half of the cows submitted in this study received dry cow therapy, it is
possible that more cows developed an IMI when compared with the average on Dutch dairy farms.
This could have influenced the statistical power and thus the significant effect of AMS on IMI.
Data was collected over a period of about two years, in which trained students participated. Per
farm, about five different trainees had visited to sample the cows. Although the students were well
trained and instructed, this might have affected the outcomes.
Conclusion
The fraction of cows with IMI caused by environmental mastitis-causing pathogens was significantly
higher in AMS cows compared with cows milked in a conventional way. Furthermore, the
multivariable analyses show that multiparous cows are more at risk than heifers. No relation was
found between the use of an AMS and contagious pathogens. To draw accurate conclusions on this
latter association, a larger sample size will be needed.
For future research we recommend analysing the comparison of difference in parity for cows
between AMS and CMS on environmental pathogens. Additionally, it would be interesting to analyse
the concerning IMI-causing bacteria by means of a PCR. With these results, one might be able to
draw conclusions on the spread of certain bacteria within a dairy farm.
Acknowledgements
I gratefully thank the Dutch Udder Health Centre for the opportunity to execute this study and the
farmers for the permission to use their data. My gratitude also goes to I. Den Uijl, veterinary
epidemiologist at Animal Health Services, for her statistical assistance. I also wish to extend my
gratitude to my supervisors, G. Koop (UU, faculty of Veterinary Medicine) and C. Scherpenzeel
(UGCN, Animal Health Services) for their guidance throughout this project.
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