Mastitis Control on Large Dairy Herds
Lawrence K. Fox
College of Veterinary Medicine
Washington State University
Pullman, WA 99163
Introduction
The basic elements of mastitis control are basic to all herds. Herds with10 to 10,000
cows must adhere to basic mastitis control strategies. These basic strategies of mastitis
control can be simply reduced to the concept that mastitis pathogens must be kept away
from the teats. The factors that govern exposure to mastitis pathogens on a 10 cow herd
is surprisingly similarly to those factors on a 10,000 cow herd. However, managing those
factors, reducing that risk of exposure, can be very different on different sized farms.
Given the increasing interest in large herd dairy management, the thrust of this paper will
be a discussion of the basics of mastitis control, and the challenges of applying those
basics to the large dairy herd. Specifically, the challenges that face a large dairy
operation in identifying cows with contagious mastitis, how they manage the cow with
such a disease, and the challenges of applying milking time hygiene measures when cow
through-put in the parlor must be maximized, will be discussed in some detail. Also to be
discussed are the issues surrounding environmental mastitis on large, well managed
herds, with low herd average milk somatic cell counts.
The Pathogens and Problems Unique to Large Dairy Herds
Mastitis pathogens can be differentiated into those that cause contagious mastitis, and
those that are not contagious (Bramley et al., 1996). The major mastitis pathogens that
cause contagious mastitis are the Mycoplasma sp., Streptococcus agalactiae, and
Staphylococcus aureus. These pathogens are generally spread from cow to cow, causing
mastitis, during the milking time. The time of transmission for Streptococcus agalactiae
appears to be almost exclusively the milking period, as this is an obligate udder parasite
that is rarely found outside the mammary gland. Staphylococcus aureus and Mycoplasma
sp. can be found in other body sites of the cow, and also in the environment. Thus the
milking period is not the only time these later two pathogens are transmitted from cow to
cow in the mastitis infection route. However, milking time hygiene, that includes teat
dip, and coupled with dry cow therapy, are established control procedures that have led to
the reduced spread of contagious mastitis, and the elimination of Streptococcus
agalactiae mastitis in many herds.
The non-contagious mastitis pathogens fall into two subgroups, the environmental
pathogens and the opportunistic pathogens. The environmental mastitis pathogens are
primarily the coliforms and the environmental streptococci; principally Eshcerichia coli,
Klebsiella sp. and Streptococcus uberis and Streptococcus dysgalactiae, respectively.
The environmental pathogens as their name suggests reside in the environment, and are
often associated with fecal matter. The opportunistic pathogens are the staphylococci
other than Staphylococcus aureus, and are found on the skin, the teat skin in particular
with respect to mastitis. They will take the opportunity to cause mastitis and cause
intramammary infections sporadically (Hogan et al., 1987). The contagious and
environmental mastitis pathogens described will cause both clinical and sub-clinical
mastitis. However, the opportunists rarely are associated with clinical mastitis and thus
are mild in the inflammatory reaction they induce.
Contagious Mastitis
Many herds in the Pacific Northwestern US have good control of contagious mastitis,
where they have eliminated Streptococcus agalactiae, where Mycoplasma sp. mastitis is
not present, and where the prevalence of Staphylococcus aureus mastitis is less that 5%
of lactating cows (Hutton et al., 1990; Fox et al., 2003). Yet the large dairy herds are
probably more at risk for the spread of contagious mastitis for two reasons. First, these
large herds tend to be in a constant state of expansion, which requires that they purchase
animals from other dairies. Thus they run the risk of introducing new strains of
contagious mastitis pathogens. Our research has demonstrated that a new strain of a
contagious mastitis pathogen, in this case, Staphylococcus aureus, can transmit readily
through the lactating herd despite excellent milking time hygiene and mastitis control
procedures (Smith et al., 1998). Also, it is more difficult to detect individual cows with
specific pathogen intramammary infections in a large herd. Large herds tend to milk
strings or pens of cows rather than individual cows. With large numbers of animals its
can be difficult to determine which pens house the cows that might have a contagious
mastitis pathogen. Once the pen has been identified, then individual cows must be
sampled to identify those infected with contagious mastitis. The investigator must
systematically wade through a pen-full of cows to find those with contagious mastitis.
This task is compounded by the need to repeat this process for several pens on the dairy.
Identification of contagious mastitis infected cows is important. Often herd managers
that are applying intensive mastitis control programs are using strict milking time
hygiene. Thus their only option to add further to mastitis control programs is through
segregation and preferential culling of cows with contagious mastitis. Studies on control
of Staphylococcus aureus mastitis through segregation of infected cows is mixed.
Segregation here is defined as housing cows with contagious mastitis in separate pens
and milking those cows after all other cows have been milked. In this way transmission
via fomites or other vectors of spread is minimized, both within the pen and during the
milking time. But data does support the application of segregation of cows with
Staphylococcus aureus mastitis. Our research in this area suggests that segregation of
infected animals can supplement a control program and arrest the spread of contagious
mastitis. However we concluded that all other recommended contagious mastitis control
procedures must be strictly in place before segregation would be an effective control
measure (Fox and Hancock, 1989). Other studies indicate that segregation should be
strictly enforced (Wilson et al., 1995; Middleton et al., 2001; Zecconi et al., 2003)..
Once a cow is identified as having Staphylococcus aureus and is segregated, she must be
kept away from the lactating herd until it is absolutely clear that she is free of this
disease. Most Staphylococcus aureus intramammary infections will not resolve, hence
the expression: “once a Staphylococcus aureus cow, always a Staphylococcus aureus
cow.” Thus the large majority of Staphylococcus aureus mastitis cows will never leave
the “Staph”, or segregated, string. They will be culled when they contribute negatively to
the herd, through less milk produced or through the production of inferior milk, a result
of increased somatic cell count. Whereas some producers will cull cows immediately
when they are confirmed to have a contagious intramammary infection, others will be
content to maintain a segregation string. Immediate culling of cows with contagious
mastitis has an advantage in cases when the contagious pathogen is most virulent and
causes a chronic clinical infection, or when the pathogen is most transmissible. Many
producers and herd veterinarians advocate the practice of immediate culling of cows with
mycoplasma mastitis. Thus these workers tend to have zero tolerance for this disease.
Mycoplasma can be highly transmissible and causes cessation of secretion in mammary
gland quarters that are infected. However, there was a report that some herd managers
have been able to segregate mycoplasma mastitis cows and control its spread (Wilson,
1992). Since Mycoplasma sp. will be carried in different body sites, elimination of cow
with infected mammary glands will not eliminate the potential threat. This would argue
for keeping and segregating cows with mycoplasma mastitis. However, since this
pathogen is completely refractory to antibiotic treatment, immediate culling might be
warranted. Cows with Streptococcus agalactiae mastitis can generally be treated. Given
that this pathogen is an obligate udder parasite and can be cured with antibiotics,
eradication is readily achievable when coupled with strict milking time hygiene.
Identification is critical to control of contagious mastitis on any dairy. But perhaps
identification of infected animals on large dairies is particularly critical since infected
cows have the opportunity to mingle with many other cows and can contaminate fomites
that will come in contact with an even greater number of cows than those that are in the
pen of the infected cow(s).
Non-contagious Mastitis Pathogens
The environmental pathogens are a constant threat to all dairies. Their ability to cause an
outbreak of mastitis is more a function of the weather, housing conditions, stocking
density of the barns, bedding type, and frequency which stalls and alleys are cleaned.
These factors can be controlled as well on large dairies as well as small dairies. Thus
environmental mastitis is not more of a risk on the large farm than the small one. Yet the
large farm may have the advantage in replacing hand labor to clean stalls and alleyways
with mechanized labor of tractors and water flushing systems. Keeping stalls (bedding
areas) and alley-ways clean and dry are perhaps the most important factors in the control
of environmental mastitis (Smith and Hogan, 1993). Intuitively housing facilities that
keep the cows and their bedding dry, not overcrowding cows, frequent removal of soiled
bedding and replacement with clean, dry bedding, are significant elements in the control
of environmental mastitis. Inorganic bedding appears to be a better choice than organic
types of bedding in minimizing environmental mastitis pathogen growth in the bedding
material. Choosing the bedding type, housing type and building structures that will
minimize the risk of environmental mastitis to the cow are a constant, and not likely to be
affected by herd size.
Mastitis caused by environmental agents generally occurs during the dry period and
peripartum period. Thus the greatest effect of these pathogen types is in early lactation,
and environmental mastitis incidence in general decreases as days in milk increases.
However, outbreaks can occur and the risk factors described will affect the frequency of
such outbreaks. On well managed herds, the environmental mastitis pathogens are the
predominant cause of mastitis. The coliforms are associated with most of the clinical
cases, and are generally of short duration, less than 2 weeks. The streptococcal
intramammary infections will be of longer duration and less frequently result in clinical
mastitis.
The opportunistic pathogens, the coagulase negative staphylococci, are not primarily
affected by herd size.. These skin bacteria can be best controlled by teat dip and dry cow
therapy. Postmilking teat disinfection has been demonstrated to effectively reduce, but
not eliminate, this type of mastitis. Dry cow therapy will likewise reduce opportunistic
mastitis incidence, but not eliminate it. The prevalence with this type of mastitis
increases in the late dry period and seems to peak at calving, decreasing thereafter. Thus
the cows immune system appears to be very capable of controlling this disease, which is
often mild in comparison to the other pathogens discussed.
Identification
Identification is critical in the control the contagious mastitis pathogens. Treatment of
Streptococcus agalactiae cows, and segregating and culling contagious mastitis cows,
can be very effective control measures when preventive efforts fail. But timely
verification of those cows with contagious mastitis must be made. In Washington state
many dairy producers rely on cultures of bulk tank milk as an initial screen to determine
if there are cows in their herds with contagious mastitis (Fox et al., 2003). Dairy
managers will routinely have the bulk tank milk cultured. Appearance of a contagious
pathogen in the bulk tank milk sample indicates that there is probably at least one cow
with a case of contagious mastitis in the herd. When a history of the contagious mastitis
pathogen counts in bulk tank milk is made, a sustained increase from the historical norm
might be evidence that transmission might be occurring with an increase in the number of
infected cows. In this case the dairy manager knows that transmission is occurring within
the herd and appropriate action must be taken. That appropriate action might be an
evaluation of the mastitis control procedures or the identification of cows with the disease
for segregation or culling. Identification can be made by sequentially sampling the
bulk tank milk after each string has been milked. The first appearance of a contagious
pathogen in a pen bulk milk sample will indicate its presence and likely association with
mastitis. Aseptically collected samples from cows suspected of having this disease when
cultured will identify the infected animals. Suspect cows would include those with high
somatic cell counts, signs of clinical mastitis, or a history of mastitis. If after the suspect
cows have been identified and removed from the pen, and the pen bulk tank milk sample
is still positive with the contagious pathogen, then efforts to refine the search for infected
animals must be made. The search can be refined by aseptically collecting milk from the
entire pen of cows to identify those infected. Once this pen’s bulk tank milk sample is
found free of contagious mastitis pathogens, then identification of cows in pens that are
milked subsequently can be made using a similar procedure. If there is zero tolerance for
contagious mastitis in each pen, then serial identification of infected cows must be made
for each pen. It is not hard to envision how larger herds will have a greater degree of
difficulty in identifying cows with contagious mastitis. Effects of numbers, dilution of
the pathogen in the bulk tank milk with non-infected cows, can impede identification.
But large dairy herds that establish a constant monitoring program can be very effective
in applying these “search and destroy” techniques for contagious mastitis. Early warning
signs, such as the first appearance of a contagious mastitis pathogen in a bulk tank
sample, or increasing number of that type of agent, will signal a response that might be
routine. With this approach contagious mastitis can be better controlled.
Bulk tank counts of the environmental pathogens are generally not as revealing. High
bulk tank milk counts of the environmental streptococci can be associated with mastitis.
Yet often such counts are indicative of contamination of the milk, either as it leaves the
cow or enters the bulk tank, but not a mastitis problem. High coliform counts in bulk
tank milk are similarly a result of contamination, and rarely due to mastitis. High counts
of opportunists in the bulk tank milk are also a result of contamination, generally a result
of poor milking preparation, such as milking cows while their udders are wet.
Control of MastitisMuch of the mastitis control procedures have been discussed. Yet some details have
been omitted and the challenges to large herds in applying some practices have not been
discussed. Most of the challenges of control rest with the contagious mastitis pathogens.
Many large dairies are maximizing use of their milking parlors by milking cows nearly
around the clock. It is common to find dairies milking 23 hours a day, saving 1 hour for
cleanup. Cow throughput in the parlor is critical given the constraints of time. A dairy I
visited in the state of Oregon milks 5,000 cows using two 72 stall carousel (rotary)
parlors. Each carousel loads 1 cow every 7-8 seconds. Thus cows will have a stall
residence time on this carousel of 8.5-9.5 minutes per milking. Moreover, they generally
have less than 30 seconds to receive premilking preparation. Thus cleaning of teats,
palpation, and drying of teats must occur in this time. This type of pressure on cow
through put is not unusual on large dairies. With this type of pressure it is not unusual to
envision why short cuts in mastitis control are made. Yet the essence of control of
contagious mastitis occurs at milking time. Premilking preparation that includes single
service towels to clean udders, disinfectant in the wash solution to clean udders, use of
latex gloves worn by milkers, and drying of the teats after washing, can be requistes to
good control. Such control measures take time and may not be consistent managements
efforts to push cows through the parlor at 9 second intervals. Steps may be omitted or
procedures may be applied hastily in an effort to rush cows through. Automatic milking
unit cluster detachers are common on large farms, and thus over-milking is less of a risk.
Post –milking teat dip is an absolute requirement, and milking unit backflush is highly
recommended. A number of mastitis researchers view milking unit backflush as a nonessential component. Their claim suggests it weakly enters into the contagious mastitis
control scheme and with a large expense, is probably not cost effective. Our data (Smith
et al., 1998), and some from Florida, indicates that at least 1 liter of disinfectant solution
must flush through each milking unit teat cup to provide effective flush. Such volume
will reduce the risk that residual contagious mastitis pathogens will be left in the cup.
We estimated that a contagious mastitis prevalence of greater that 15% of cows would
warrant the cost of the installation of a milking unit backflush system. With such a high
prevalence of mastitis, the cost of installation could be balanced against the savings
generated through the reduction in the spread of contagious mastitis. However, it could
be argued that installation of a backflushing system is warranted even when the
prevalence of contagious mastitis is lower, perhaps much lower, than 15%. In all cases
such installation could be viewed as “insurance” against an outbreak of contagious
mastitis. In herds where cow through-put in the parlor is of high importance, and milkers
are likely to take shortcuts in milking time hygiene techniques, backflushing milking
units might serve as a “back-up” to prevent the spread of contagious mastitis.
Conclusion
The postmilking mastitis control rountines can be automated, but the pre-milking routines
are generally best applied manually. The use of pre-milking sprinkler pens was quite
prevalent 20 years ago. However, most of those systems have been removed when it was
discovered that excess application of water to wash cow udders has been associated with
an increase in the bacterial counts in milk and an increased risk of environmental
mastitis. Thus milkers routinely have the responsibility of washing and drying udders.
Pre-dip, use of a disinfectant solution with the active ingredient concentration of a post
dip, has been demonstrated to help control environmental mastitis (Pankey et. al, 1992).
Use of terry-cloth towels has replaced paper towels on many farms. These towels are
more absorbent and provide a larger cleaning surface. Moreover, including costs of
equipment to clean the towels between cow milkings, the cloth towels are less expensive
to use than the paper towels. Additionally important for large herds is the elimination of
the need to dispose of paper towels. A 3000 cow herd, milking three times per day,
would need to dispose nearly 10,000 towels per day.
Use of vaccines to control mastitis has had mixed results. There have been many
attempts to develop Staphylococcus aureus mastitis vaccines. Although recently there is
a report suggesting positive results (Leitner et al., 2003), the experience of many over the
years suggests that results are rather variable (Tyler et al., 1993). Thus use of a
Staphylococcus aureus mastitis vaccine should still be evaluated on herd by herd basis.
The use of coliform vaccines, using antigens prepared from R mutant clones has had the
most success (Tyler et al., 1993). Yet even with coliform vaccines, the incidence of
coliform mastitis remains constant, but the severity is reduced. Given that clinical form
of coliform mastitis can be quite acute, success of vaccines in reducing the severity of the
clinical signs is significant.
In summary, the basics of mastitis control are still the isolation of the teat end from
mastitis pathogens. Such isolation is as important on a small as it is on a large dairy.
Large herds must take care to apply rigorous milking time hygiene mastitis control
strategies and mastitis identification techniques. There is a tendency on large herds for
operators to move cows through the parlor quickly which leaves less time to apply such
techniques. Problems in identifying cows with contagious mastitis are more difficult
when they are mixed with 100s, if not 1000s of cows, rather than 10s of cows.
Environmental mastitis control might be facilitated on large herds that can take advantage
of the economies of scale and use more automated systems to clean barns and pens.
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