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 the annular 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.
Defence mechanisms of the streak canal or teat canal
1. Smooth muscle sphincter surrounding the teat canal inhibits bacterial closure. Because the teat canal lumen can remain dilated for up to 2 hours after milking
2. Keratin, a waxy substance derived from the teat canal lining partially occludes the lumen of the teat canal and inhibits bacterial penetration
3. Somatic Cells are the most important natural defence mechanism to infection. Leukocytes (mostly PMN, polymorph nuclear 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.
So how does your cow establish an infection?
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. `���E