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Mastitis, an inflammation of the mammary gland, is almost always the result of bacterial infection.

Characteristics of Contagious Bacteria

Contagious Bacteria

Simply stated, the term contagious mastitis means that the infection can be transferred from an infected quarter to a non-infected quarter. This may occur either between quarters on the same cow or from one cow to another. Streptococcus agalactiae (Strep ag) and Staphylococcus aureus (Staph aureus) are the two major species of contagious bacteria commonly found in the Midwest.

Several characteristics of Strep ag are very important to understanding attempts to control it in a herd. First, Strep ag is able to live and multiply only when it is in the cow's udder. Once in the udder, Strep ag grows on the tissue surface. These characteristics, and it's sensitivity to penicillin, mean that Strep ag can usually be eliminated from the udder by the proper use of antibiotics.

Staph aureus is also very contagious. However, in addition to living in the udder like Strep ag, Staph aureus may also be found colonizing on teat skin surfaces. This is especially true when the teat skin has been damaged. Staph aureus does not grow on udder tissue surfaces where it would readily be exposed to antibiotics; instead, it grows deep within the connective tissue where it will form small abcesses. Furthermore, Staph aureus tends to develop resistance to antibiotics. These characteristics make Staph aureus more difficult than Strep ag to control and/or eliminate in a dairy herd.

Length of Infection:

Once established, contagious bacteria may live in the udder for the entire life of the cow, unless there is effective intervention. The end result is lowered milk production because bacterial growth in the udder causes damage to milk secreting tissue.

Signs of Contagious Mastitis:

Infection by contagious bacteria usually results in subclinical mastitis, i.e. the milk is visibly normal. Occasionally, there may be mild flare-ups, and on rare occasions, certain strains of Staph aureus may result in gangrenous mastitis.

Somatic Cells:

Somatic cells in milk are primarily white blood cells (leukocytes). When bacteria infect a quarter, the cow's immune system responds to fight the infection by increasing the number of somatic cells present in the milk. Because infections caused by contagious bacteria are usually of long duration, milk somatic cell counts in cows infected with these organisms are usually elevated for long periods of time. In fact, the most frequent finding in herds where the somatic cell count exceeds legal standards is a high proportion of quarters infected with contagious bacteria.

Control of Contagious Mastitis

In the 1960s, research indicated that dipping of teats in a germacide after milking was effective in reducing the rate of new mastitis infections by 50 to 90 percent. In addition, researchers demonstrated that the length of an infection could be decreased by intramammary infusion of antibiotics at drying-off time. Such drug therapy has also been shown to reduce the rate of new infections that occur shortly after drying-off.

Postmilking Teat Dipping with a Germacide:

Properly done, immersion of teats after milking in an effective germacide is the most important means available to prevent new mastitis infections caused by contagious bacteria. Apply the dip to at least the lower half of the teat immediately after the milker unit has been removed. Dipping is usually preferable to spraying because, in most cases, better teat-end coverage is achieved. Other points to consider concerning postmilking teat dipping include:

  1. Dip-containers should be cleaned daily.
  2. All products labelled as teat dips may not be equally effective. At the present time the FDA does not require that teat dips be tested for effectiveness prior to marketing. The best assurance of product effectiveness may be obtained by testing under Protocols B or C of the National Mastitis Council (NMC). These procedures (Protocol B and C) determine how effective the product is in reducing the number of new cases of mastitis in cows.
  3. Avoid teat skin irritation. Chapping or "burning" teat skin can actually allow Staph aureus and other bacteria to colonize teats and result in additional new mastitis infections. Mixing products, use of damaged products, or injudicious dipping in extremely cold weather may result in damaged teat skin.

Dry Cow Therapy:

Immediately after the last milking of each lactation, each quarter of a cow should be treated with a single tube of a specifically formulated commercial dry cow product. A suggested procedure is:

  1. Immerse teats in postmilking dip
  2. Thoroughly clean teat-ends with alcohol and cotton
  3. Carefully infuse each quarter
  4. Immerse teats in postmilking dip
  5. Rinse hands in udder sanitizer and dry between cows.

Caution: Follow label directions to avoid antibiotic residue in meat and milk.

Although postmilking teat dipping in a germacide and dry cow therapy are the principal methods of contagious mastitis control, several additional management procedures merit attention.

Fomites - Contagious bacteria may spread from infected to non-infected quarters by way of contaminated milk droplets. Any object that aids in this transfer of contaminated milk droplets is called a fomite. Fomites considered important in spreading contagious bacteria include:

  1. Use of a common udder cloth or sponge among cows. A single-service, individual paper towel is recommended for preparation of each cow.
  2. The hands of the person milking. Care should be taken to avoid the transfer of milk droplets on hands.
  3. Milker-units and teat-cup liners. Contagious mastitis bacteria can be spread by the milking equipment. Equipment manufacturer's have designed some products which may be helpful in preventing this from occurring.

Milking Order - Because contagious bacteria are spread from cow-to-cow primarily during milking, it makes good sense to milk non-infected cows first. Although bacteriologic culturing of milk is necessary to determine whether an animal truly is infected with contagious bacteria, an initial milking order could be used in the following sequence: first calf heifers, cows with low somatic cell counts, cows with higher somatic cell counts, and finally cows with clinical mastitis.

Purchased animals - Studies show that about 5 percent of the herds free of Strep ag become reinfected each year. This reinfection results primarily from purchasing infected animals. If animals are purchased, it is advisable to isolate them and bacteriologically culture their milk before placing them in the regular milking string.

Calves - Calves that have been fed milk containing Strep ag can transfer the bacteria to other calves that they may suckle. These suckled animals can then freshen with blind quarters or Strep ag infected udders. This may be controlled by simply preventing contact among pre-weaned calves.

Herd Monitoring - A regularly conducted program of individual cow somatic cell counting (or CMT) is an excellent method of monitoring herds for probable contagious mastitis infections. Whether high somatic cell count cows are the result of contagious or environmental infections can only be determined by bacteriologic culturing of milk. Somatic cell counting and culturing of milk are suggested for all herds, regardless of bulk somatic cell count.

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This page was last updated on November 16, 2002