1. Mastitis occurs when pathogens enter the udder through the teat canal and cause an infection. 2. The immune system responds by sending macrophages and neutrophils to identify and kill the pathogens. 3. High somatic cell counts indicate an intra-mammary infection, as somatic cells include immune cells that enter the milk to fight infection in the udder.

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How a Healthy Immune System Helps
Reduce Somatic Cell Count and Mastitis
1- Pathogens enter the udder through the streak canal
and create infections
2-Macrophages identify pathogens, engulf them, and
then recruit neutrophils to kill the pathogens using
signaling proteins called cytokines
3-Neutrophils roll along the blood vessel wall using an
adhesion protein called L-selectin. There are
approximately 200 billion neutrophils in an adult dairy
cow
4-Neutrophils then migrate through the blood vessel
when signaled by macrophages to kill the pathogens
5-Neutrophils destroy pathogens by physically engulfing
them by a process called phagocytosis and then kills
them through the use of enzymes and reactive oxygen
species (ROS) -

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Mammary Gland Defense Mechanisms

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The mammary gland is protected by a variety of defense
mechanisms that
can be separated into two distinct categories: innate
immunity and specific immunity. Innate immunity, also
known as nonspecific responsiveness, is the
predominant defense during the early stages of infection
Nonspecific responses are present or are activated
quickly at the site
of infection by numerous stimuli; however, they are not
augmented by
repeated exposure to the same insult. Nonspecific or
innate responses of the
mammary gland are mediated by the physical barrier of
the teat end,
macrophages, neutrophils, natural killer (NK) cells, and by
certain solublefactors
Immune Defenses Against Mastitis
The immune system is a highly specialized, coordinated
set of cells and tissues that have a primary role of body
surveillance for foreign antigens. Foreign antigens are any
macromolecule (protein, lipid, polysaccharide) or
microorganism (bacteria, virus, mold, protozoan) that does
not contain a special host-specific identification code
recognized as “self”. Many pathogenic organisms have
evolved exquisite mechanisms to evade the host immune
system and facilitate disease propagation. Staphylococcus
aureus can survive within phagocytic cells or become

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walled off within mammary tissue, thus evading immune
detection and preventing its elimination.
The immune system is often viewed solely as specialized
white blood cells (leukocytes) that either engulf and
destroy (e.g., phagocytosis) invading microorganisms
(cell-mediated immunity) or respond to vaccines to
produce antibody (humoral immunity). Over looked
components of the immune system are physical barriers
and non-specific immunity. The immune system can be
viewed as a three-tiered defense starting with physical
barriers and non- specific and specific immune responses.
Physical barriers and non-specific immune responses
comprise the innate or natural immunity. These immune
responses are not antigen specific, nor do they have any
memory response. Cell mediated and humoral immune
responses comprise active immunity and are antigen
specific and have memory.
Physical barriers of the udder are anatomic features of the
teat and associated structures that pose a physical
blockade to invading bacteria at the teat sphincter, the
point of entry. These anatomic features include the teat
skin, teat sphincter muscle and keratin plug. Teat skin that
has abrasions, cracks or is chapped increases contagious
bacteria colonization of the skin greatly increasing
bacterial numbers around the teat sphincter and thusly
increasing risk of bacterial penetration through the teat
duct. Following milking the teat duct is dilated, greatly

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increasing the risk of bacterial penetration. Contraction of
the teat sphincter takes time, which is why providing cows
fresh feed following milking is promoted. This practice
allows time for the teat sphincter to constrict, closing off
the teat opening, before cows return to their stalls and
have direct contact with the environment. The keratin plug
is produced by skin lining the teat duct. Keratin is gummy,
has bacteriostatic activity and completely occludes the teat
canal.
Other non-specific immune responses include phagocytic
cells (i.e., somatic cells), inflammatory response,
complement cascade and lactoferrin. Phagocytic cells of
various types are by in far the most important mediator of
mastitis infections. All though there are a number of cell
types, neutrophils and macrophages account for the
majority of phagocytic cells in mastitis infections.
Macrophages play multiple roles in coordinating activation
of the specific immune response. After engulfing a foreign
antigen, macrophages will present these on their cellular
surface to stimulate lymphocytes to respond.
The inflammatory response produces much of the signs
associated with clinical mastitis, heat, redness and
swelling of the udder. Inflammation is a response to
activated macrophages resulting in increased permeability
of blood vessels allowing fluids, minerals, proteins
(albumin and immunoglobulins) to move into the infection
site. Neutrophils are then attracted to the site and move

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from surrounding blood vessels. Complement proteins
also move into the inflamed area and promote
phagocytosis and killing of bacteria by neutrophils and
macrophages. Lactoferrin is a specialize protein
synthesized in the udder that binds iron making it
unavailable for bacterial growth, especially coliform
bacteria.
Lymphocytes are specialized leukocytes that are involved
in the active immune response, which includes cell
mediated and humoral immunity. T-lymphocytes
coordinate and stimulate the immune response as well
provide cytotoxic cells. B-lymphocytes are responsible for
the production of antibody (i.e., immunoglobulin).Both T
and B lymphocytes respond only to a very specific
antigen, thus the term specific immunity. When T and B
lymphocytes respond, in addition to generating clones of
their effector (active) cells, they produce memory cells.
These memory cells are retained for periods of time
allowing the animal to respond more immediately if the
same antigen is encountered. This is the function premise
behind vaccination protocols, generation of memory cells
to a specific pathogenic agent
Somatic Cell Count
What is a somatic cell count?
Somatic cell count (SCC) is the number of somatic cells
found in a millilitre of milk. Somatic cells (or “body” cells)

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are a mixture of milk-producing cells shed from the
udder tissue (about 2%) and cells from the immune
system (the other 98%), known as leukocytes (also called
white blood cells). Somatic cell counts are useful in
identifying intra-mammary infection in an individual cow
or herd. To understand a bit more about the cells and
why they are found in milk we need to understand the
function of leukocytes a bit better.
What is a leukocyte (or white blood cell) and
why are they important?
Leukocytes are the cells responsible for
identifying bacteria and killing them. Bacteria
and other harmful pathogens enter the body in
many different ways which is why these cells
circulate through the body and make their way
into different tissues. Just as nutrients are
transported from the bloodstream into the
udder to be converted into milk, leukocytes are
also transported into the udder as a
surveillance mechanism to look for bacteria
which is why SCC levels are never zero.

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Think of these cells as a night watchman at a
museum. Unless there is absolutely no chance
for crime, you will always have 1 night
watchman. If crime in the community
increases, you may have to hire a few more
guards. If one of the night watchmen sees
someone is breaking in and sounds the alarm,
you’ll have not only all of the night watchmen
running in, but also a large number of Gardaí
entering the museum to catch and get rid of
the thieves. The same thing happens with
leukocytes and infections. Leukocytes are
always circulating because we are always under
threat of infection. If one of the leukocytes on
routine surveillance runs into some bad
bacteria, they will signal to the rest of the
leukocytes and other immune cells that there is
a problem, so they rush to the area of infection-
hence the SCC in the milk will increase.

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What happens when leukocytes infiltrate the
udder during an infection?
When lots of leukocytes move to the udder
during infection, this results in an increase in
SCC and a decrease in milk yield. The udder is
made up of millions of groups of milk-producing
cells, collectively called alveoli. Within the
alveoli is the lumen where milk accumulates
until it can be transported through the milk
ducts and expressed through the teat. When an
animal has an infection in the udder, leukocytes
infiltrate the alveoli by breaking through the
tight junctions between and knocking out milk-
producing cells, to get to the area where the
bacteria grow. This damage to the milk-
producing cells, and to the junctions between
them, means the cow will produce less milk.
The determination of milk SCC is widely used to
moni¬tor udder health and, thus, milk quality.

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When combined with bacteriological culture
results, the factors of greatest importance can
be determined. When SCC are elevated, they
consist primarily of leukocytes or white blood
cells which include macrophages,
lym¬phocytes, and PMN. During inflammation,
the major increase in SCC is because of the
influx of PMN into milk. At this time, over 90%
of the cells may be PMN.
Milk from normal (i.e., uninfected) quarters
generally contain below 200,000 somatic
cells/ml. Many are less than 100,000. One study
estimated that 50% of unin¬fected cows have
SCC under 100,000/ml, and 80% have under
200,000. An elevation of SCC (above 300,000 )
is abnormal and an indication of inflammation
in the udder.

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COW
Healthy cows with good immune systems will
be able to fight off mastitis infections. Many
mastitis pathogens are eliminated by the cow's
own defense system. Assure that the diets are
balanced for Vitamin E and
Selenium. Immunization with J5 vaccine will

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not prevent infection but will decrease the
severity of clinical signs.
Diets - During lactation cows should receive
400-600 IU Vit E and .3 ppm of Selenium per
day. During the dry period, cows should get
1000 IU Vit E and .3 ppm Selenium per day.
J5 vaccine schedule - Immunization with E. coli
mutant (J5) is reported effective. Protection
extends to other coliform species. The protocol
we use is 3 injections, 1st at dry off, 2nd 3
weeks later, and the 3rd injection 2-4 weeks
after calving. Avoid giving immunization to
cows at calving and up to two weeks after
calving.
Much current research is focused on improving
the cow's immune system.
Anatomy of the Mammary Gland
Cross section of the udder infused with
dye. The front quarter is smaller than the rear

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quarters and is separated by a membrane
through which bacteria and drugs do not
diffuse. Each quarter is separate.
The entrance of the udder is known as the
streak canal or teat canal. it is surrounded by a
band of muscle tissue that keeps the canal
closed. The cavity within the teat is known as
the teat sinus. It is separated from the udder
cistern by a ring of tissue known as theannular
ring. Canals connect to the udder cistern like
the branches of a tree and terminate in tiny
circular areas known as alveoli which secrete
milk.

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A cubic inch of udder tissue contains millions
of alveoli. Each alveolus is richly fed by blood
vessels and surrounded by muscle fibers known
as myoepithelial cells. Oxytocin acts on these
cells to cause milk let down.

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Bacteria overcome or penetrate the teat
canal. This occurs by multiplication, propulsion
during milking and perhaps other
factors. These bacteria are not considered
motile.
The most important factor to keep in mind to
control mastitis is to keep bacteria away from
the teat end.
How Does Mastitis Result?
Mastitis results when bacteria pass through
the teat canal, overcome the defenses in milk

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and multiply. Organisms pass through the teat
canal in several ways:
1. Between milkings, organisms pass through
the teat canal by multiplying inside the canal.
2. During machine milking, organisms may
be propelled into through the teat canal into
the teat cistern and udder.
Defenses of the Cow:
1. Smooth muscle sphincter surrounding the
teat canal inhibits bacterial closure. Because
the teat canal lumen remains dilated for up to 2
hours after milking, feed cows after milking to
keep them on their feet.

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2. Keratin, a waxy substance derived from the
teat canal lining partially occludes the lumen of
the teat canal and inhibits bacterial
penetration. Only infuse the tip of
intramammary infusion cannulas into the teat
canal.
3. Somatic Cells are the most important
natural defense mechanism to
infection. Leukocytes (mostly PMN,
polymorphonuclear neutrophils) function by
phagocytosing and killing bacteria. They may
reach in the millions.
4. Antibodies and other soluble factors in
milk. They coat bacteria and enhance PMN
engulfment. They also interfere with bacterial
adhesion to tissues, reducing multiplication and
neutralizing toxins.
Establishment of Infection:

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1. The inherent virulence of the bacterial
species is often associated with is ability to
adhere to mammary epithelium and remain in
the gland during lactation when the udder is
periodically flushed. Strep ag and Staph aureus
adhere well. E. coli does not adhere well but
multiplies rapidly.
2. If bacteria are eliminated by leukocytes, the
infection is cleared!
3. Bacteria initially affect tissues lining the
large milking collecting ducts and
cisterns. They enter small ducts and alveolar
areas of the gland by multiplication and via milk
currents.
4. Bacteria produce toxins and irritants that
cause swelling and death of alveoli. This results
in the release of substances that increase blood
vessel permeability and attract PMN to the
affected area.

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Inflammation
The inflammatory response is due to the influx
of PMN, serum components and fluid. The first
change in milk during inflammation is increase
in blood proteins followed by massive
movement of PMN into the gland. These
changes are accompanied by edema, redness
and swelling of the udder and abnormal watery
secretions containing clots and red blood cells.
Tissue Response
PMN cross blood vessels and move through
tissue toward damaged tissue site. PMN
accumulate around alveoli and can release

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enzymes that cause the destruction of the
alveoli. So the presence of bacteria, toxins and
PMN may cause healthy alveoli to
involute. Tissue debris, bacteria and PMN clog
ducts. If the bacteria are eliminated,
inflammation subsides, clogged ducts are
opened and milk composition returns to normal
in several days. If the infection persists and
ducts remain clogged, milk accumulates in
alveoli exerting pressure on the alveoli. These
alveoli will involute or be destroyed depending
on how severe the infection. Destroyed alveoli
are permanently replaced by scar tissue
(fibrosis).