Mastitis is the persistent, inflammatory reaction of the udder tissue. The disease a potentially fatal mammary gland infection, is the most common disease in dairy cattle. It is also the most costly to the dairy industry. Milk from cows suffering from mastitis has an increased somatic cell count.
Identification : This disease can be identified by abnormalities in the udder such as swelling, heat, redness, hardness, or pain (if it is clinical). Other indications of mastitis may be abnormalities in milk such as a watery appearance, flakes, or clots. When infected with sub-clinical mastitis, a cow does not show any visible signs of infection or abnormalities.
Clinical and sub-clinical mastitis
In clinical mastitis, the infected quarter often become swollen, sometimes painful to touch, and the milk is visibly altered by the presence of clots, flakes, or discolored serum and sometimes blood. In severe cases (acute mastitis), the cow shows signs of generalized reaction: fever, rapid pulse, loss of appetite and sharp decline in milk production.
In contrast, sub-clinical mastitis is subtle and more difficult to detect. The cow appears healthy, the udder does not show any signs of inflammation and the milk seems normal. However, micro-organisms and white blood cells (somatic cells) that fight infections are found in elevated numbers in the milk. The loss of milk and income due to clinical mastitis are readily apparent, milk production drops sharply and milk from cows treated with antibiotics must be discarded for three or four days.
Nonetheless, a lot more milk is lost due to sub-clinical mastitis because:
- The vast majority of mastitis cases are sub-clinical (on the average, for every clinical case, there are 20 to 40 sub-clinical cases);
- The reduction in milk production due to sub-clinical mastitis tends to persist for long periods of times and thus undermines the yield of infected cows.
Control of sub-clinical mastitis is more important than simply treating clinical cases because:
- The cows that have sub-clinical mastitis are reservoirs of organisms that lead to infection of other cows;
- Most clinical cases start as sub-clinical; thus, controlling sub-clinical mastitis is the best way to reduce the clinical cases.
Development of The Disease
Infections begin when microorganisms penetrate the teat canal and multiply in the mammary gland.
Invasion of the teat
The teat itself is the first line of defense against the penetration of bacteria into the udder. Normally, the sphincter muscle closes the teat canal tightly when the cow is not being milked. Invasion of the teat most often occurs during milking. Organisms present in the milk or at the teat end are propelled into the teat canal and cistern when there is admission of undesired air in the milking unit (slipping or squawking of the unit or removal of teat cup without first shutting off the vacuum). After milking, the teat canal remains dilated for one to two hours; however, the canal of a damaged teat may remain partially open permanently. Organisms from the environment (manure, bedding, etc.) or those found on injured skin at the tip of the teat may easily invade an open or partially open canal.
Establishment of infection and inflammation of the damaged area
Some bacteria may proceed into the udder by attaching and colonizing new tissue; others may move around via milk current produced by the cow’s movement. Bacteria first damages the tissues lining the large milk-collecting ducts. The bacteria may encounter leukocytes (white blood cells) present naturally in small numbers in the milk. These cells are the cow’s second line of defense because they can engulf and destroy bacteria. However, during this process, the leukocytes release substances that cause the movement of additional leukocytes from the blood into the milk. If bacteria are not entirely destroyed, they continue to multiply and begin to invade smaller ducts and alveolar areas.
Transmission of Various Types of Mastitis Organisms
In attempting to control different types of infections, it is important to consider the source and means of transmission of the disease. Organisms that cause mastitis live in different environments (manure, bedding, skin, etc.). General cleanliness of cows and their housing, as well as good management procedures especially at milking are effective ways of controlling the spread of mastitis.
Streptococcus agalactiae : Streptococcus agalactiae is the most common cause of sub-clinical infections but rarely causes severe illness (acute mastitis). This organism lives in the cow’s udder and survives only a short time outside the mammary gland. It spreads primarily during milking via the milking machine, contaminated operators’ hands, and materials (cloths) used to wash the udder. This organism may also infect the udder of a young calf if it is sucked by a second calf that has been fed contaminated milk. The infection can remain indefinitely in the heifer’s mammary gland.
Streptococcus agalactiae can be eradicated from a herd by appropriate treatment combined with good milking practices. However, it may easily spread again in a herd after the purchase of an infected animal.
Staphylococcus aureus : Staphylococcus aureus lives inside or outside the udder on the teat skin and causes many cases of both clinical and sub-clinical mastitis.
Treatment : Treatment is possible with long-acting antibiotics, but milk from such cows is not marketable until drug residues have left the cow’s system. Antibiotics may be systemic (injected into the body), or they may be forced upwards into the teat through the teat canal (intra-mammary infusion). Cows being treated may be marked with tape to alert dairy workers, and their milk is syphoned off and discarded. To determine whether the levels of antibiotic residuals are within regulatory requirements, special tests exist. Vaccinations for mastitis are available, but as they only reduce the severity of the condition, and cannot prevent re-occurring infections, they should be used in conjunction with a mastitis prevention program.
Control : Practices such as good nutrition, proper milking hygiene, and the culling of chronically infected cows can help. Ensuring that cows have clean, dry bedding decreases the risk of infection and transmission. Dairy workers should wear rubber gloves while milking, and machines should be cleaned regularly to decrease the incidence of transmission.
Prevention : A good milking routine is vital. This usually consists of applying a pre-milking teat dip or spray, such as an iodine spray, and wiping teats dry prior to milking. The milking machine is then applied. After milking, the teats can be cleaned again to remove any growth medium for bacteria. A post milking product such as iodine-propylene glycol dip is used as a disinfectant and a barrier between the open teat and the bacteria in the air. Mastitis can occur after milking because the teat holes close after 15 minutes if the animal sits in a dirty place with feces and urine.