[Type here]

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) -

[Type here]

[Type here]

[Type here]

[Type here]

[Type here]

[Type here]

[Type here]

[Type here]

[Type here]

Mammary Gland Defense Mechanisms

[Type here]

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 soluble factors

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

[Type here]

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

[Type here]

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

[Type here]

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)

[Type here]

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.

[Type here]

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.

[Type here]

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.

[Type here]

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.

[Type here]

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

[Type here]

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

[Type here]

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.

[Type here]

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.

[Type here]

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

[Type here]

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.

[Type here]

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:

[Type here]

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.

[Type here]

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

[Type here]

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).