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Living with Staphylococcus Aureus
by Richard L. Wallace


  • Staph cows should be identified and separated from uninfected cows, then Staph cows should be milked last. "Once a Staph cow always a Staph cow!" Anonymous
  • Antibiotic treatment of lactating cows infected with S. aureus is unrewarding. Requesting antibiotic sensitiv- ities from mastitis laboratories is fruitless.
  • Post-milking teat dipping with a germicidal product containing skin conditioners and complete dry cow therapy of all quarters is essential when trying to control S. aureus.
  • First calf heifers can be an unexpected reservoir of S. aureus contamination for the lactating herd. Do not feed mastitic milk to heifer calves.
  • Relatively low BTSCC can be achieved in S. aureus herds only when excellent management procedures are practiced.


Staphylococcus aureus has long been recognized to be a cause of contagious mastitis. This organism can spread from cow to cow during the milking process and remain in a herd indefinitely due to a chronic carrier state. Economic losses due to elevated somatic cell counts (SCC), decreased milk production, higher treatment costs, and excessive discarded milk are commonplace in herds infected with S. aureus. Implementation of post-milking teat dipping and complete dry cow therapy has controlled this disease in most herds. Yet, losses due to S. aureus still occur, even in herds which faithfully practice these two key management procedures.

Dairy producers considering expansion may inadvertently introduce S. aureus into their herds. Initially, the organism may not generate significant losses. Eventually, as more cows develop intramammary infections (IMI), the somatic cell count starts to climb, clinical cases emerge, and the losses begin to pile up. This paper will offer some insights into diagnosing, eliminating, preventing, and living with S. aureus in your herd.


Detection at the bulk tank

The first signs of a herd problem with S. aureus may appear in the bulk tank. Bulk tank somatic cell counts (BTSCC) in excess of 400,000 could indicate a problem with contagious mastitis pathogens. Elevations in BTSCC are generally dependent on the number of cows and quarters infected with S. aureus. Very well managed herds can maintain relatively low BTSCC even though many cows are affected. Bulk tank bacteria counts do not tend to increase as more cows become infected with S. aureus. Graphing BTSCC and bacteria counts can indicate changing conditions and offer a warning sign to management.

Monthly bulk tank culturing has proven useful in monitoring udder health, particularly with regard to contagious pathogens (Staphylococcus aureus, Streptococcus agalactia and Mycoplasma bovis). The sensitivity of a single bulk tank culture for contagious organisms is fairly low, especially when the herd prevalence of contagious mastitis is low as well. In other words, often one bulk tank sample will be culture negative for S. aureus even though a herd may have cows infected with this organism. Bulk tank cultures for contagious organisms is highly specific (94%). So, it is rare that a bulk tank culture will be positive when in reality no cows in the herd have contagious mastitis.

Multiple sampling will improve the sensitivity of bulk tank culturing, particularly with intermittently shedding organisms like S. aureus. Serial testing can be performed by aseptically collecting an agitated bulk tank sample in a sterile container. This procedure can be repeated every other day when the bulk tank contains four milkings. The samples can be frozen immediately after they are obtained and delivered to the testing facility once each month.

Detection of individual cows

Although S. aureus can cause a severe, gangrenous mastitis which can lead to death in cows, greater than 80% of all S. aureus IMI are subclinical. Usually there will be no elevation in body temperature and the only visible sign will be the presence of garget in the fore-milk. Many producers are inclined to treat these quarters, leading to discarded milk and potential antibiotic residues. After treatment, these cases usually recur in two to three weeks and the treatment/discarded milk cycle is repeated. As much as 88% of the losses incurred when treating cases of mastitis are due to dumped milk and decreased milk production.

Cows with one or more quarters infected with S. aureus tend to have elevated SCC. Keep in mind that one high score may not be indicative of a chronic infection. Cows with multiple SCC above 300,000 or multiple somatic cell scores (SCS) above 4.0 are most likely to be infected. The monthly Dairy Herd Improvement Association (DHIA) somatic cell data is generated from a composite milk sample. Average SCC when one quarter is infected is 500,000/ml. When two or three quarters are infected, the average SCC can reach 700,000 or 1,500,000/ml, respectively. Generally these cows can be detected with the California Mastitis Test (CMT), but monthly (DHIA) data can be more revealing, and requires less time to obtain.

Milk cultures are the best method to determine if clinical and subclinical mastitis is due to S. aureus. Selecting cows to culture can be a involved process. With the use of individual cow SCC, the procedure can be streamlined. All cows with several counts above 300,000 should be sampled. Once cows are selected, the CMT paddle can be used to determine which quarter(s) to culture. Combining positive CMT quarters into one composite vial may be acceptable, but individual quarter samples are preferred over pooled milk samples. Even under the best conditions, many composite samples become contaminated. Contaminated milk samples are impossible to interpret and a waste of resources.

A small (3-5 ml), sterile quarter sample is preferable to a voluminous contaminated composite. Teat ends should be thoroughly scrubbed with an alcohol pad. The fore-milk should be discarded and a mid-stream milk sample obtained in a sterile container. Milk cultures should be immediately chilled to prevent overgrowth of environmental bacteria. If microbiologic procedures are to be delayed, the samples should be frozen. A veterinarian's assistance in obtaining high quality samples may be warranted.

Eliminating existing IMI

Antibiotic therapy of clinical mastitis caused by S. aureus during lactation has been unrewarding. It is often difficult to get the antibiotic to the invading organisms, and many times the selected antibiotic is ineffective. Staphylococcus aureus produces an enzyme that breaks down the lining of the milk ducts and allows the bacteria to invade deep within the udder. The cow attempts to wall off the infection into small abscesses. Approved antibiotics are unable to penetrate these micro-abscesses. These cows become chronic carriers and potential reservoirs for infection of herd mates. Early detection and treatment has the greatest success rate for lactational therapy. Treating cows within the first 30 days of infection may offer an 80-90% cure rate. Every month treatment is delayed, the chance of a cure drops by 20%. Someone once declared, "Once a Staph cow, always a Staph cow!" It is best to respect this statement. Dry cow therapy is not always effective at curing existing infections, particularly those caused by S. aureus. Although, the overall cure rate for S. aureus by infusion with a dry-cow, antibiotic preparation may approach 50%. All quarters of all cows should be medicated with an approved product at dry-off. There is some evidence that repeat infusions after three weeks may improve the success rate. This is an extra-label use of a dry cow preparation and should only be used within the context of a valid veterinarian/client/patient relationship. Currently, there are no systemic antibiotics that are effective against S. aureus approved for use in dry, dairy cows.

One sure method to eliminate S. aureus IMI is to remove the cow from the herd, ie. cull her. Several studies have been performed evaluating the economics of culling cows chronically infected with S. aureus. The prevalence of IMI within a herd will play a major role in the decision process. In herds with very few cows infected, culling chronic cows should receive high priority. This practice will help the prevention strategies to be discussed next. When several cows in a herd are infected (greater than 5%), the value of the milk may be more important to cash flow and control measures should be considered a higher priority.

Preventing new IMI

Milking-time procedures can greatly influence the spread of S. aureus to uninfected cows. Contamination -- milkers' hands, common wash rags, and milking clusters -- with milk from infected cows is the primary means of transmission. Care should be exercised when fore-stripping cows so that milk from infected quarters does not contaminate the milkers' hands or environment. Wearing latex gloves during milking can reduce the potential for spread. When soiled, gloves are much easier to sanitize than the skin on your hands.

Common wash rags or sponges used to prepare udders for milking are often laden with microorganisms. Staphylococcus aureus can easily be spread from cow to cow with a common rag even when sanitizer is used in the udder wash. Single service cloth towels or disposable paper towels will eliminate this method of transmission. During the milking process, milk within the inflation covers the teat skin. If the milk contains S. aureus organisms, all contact surfaces are potentially contaminated. Post-milking teat dipping with an approved, germicidal product will help reduce the number of viable organisms remaining on the teat skin after the milking unit is removed.

Worn out inflations will contain microscopic cracks in the rubber and S. aureus bacteria can remain in those recesses. Stepping up the replacement schedule for inflations will help. Back-flushing of the milking cluster can reduce the number of S. aureus organisms within the inflations. Automatic back-flushing units can be installed in most parlors, but are expensive. Manual systems have proven ineffective. Improper milking machine function, to a lesser extent, contributes to new IMI with S. aureus. Excessive trauma to the teat ends and reverse impacts of contaminated milk droplets are the primary means of transmission by the milking units.

Good teat skin condition is vital to controlling S. aureus. This bacteria grows very well under scabs and on irritated teat skin. Higher incidence of S. aureus mastitis can be expected in winter months when teat skin becomes chapped. Irritating teat dips, excessive contamination by urine and feces, as well as improperly functioning milking machines can all cause teat end and teat skin damage. Staphylococcus aureus only needs a tiny break in the teat skin to invade and begin multiplication. Selecting a teat dip with five to ten percent glycerine or some other skin conditioner will help tremendously. When the wind chill is below freezing and cows are kept in cold housing, post-milking teat dipping should be postponed. Some producers blot the teat ends with a clean paper towel prior to turning the cows out into a cold environment.

Segregation of infected cows or using separate milking clusters on infected cows is a viable option for herds not choosing to cull. Smaller herds can designate one or two milking clusters as "Staph" units. These claws should be clearly marked and only used on infected cows. Another option is to milk the uninfected cows first and the "Staph" cows last. This method relies on the post-milking sanitation procedures to effectively remove potential contamina- tion. Larger herds can create a "Staph" milking string. Even though these cows are housed in the same free stall barn, they can be separated at milking time and milked last. Dry treated cows which were infected in the previous lactation can be milked in between the two groups. Once the new infection status is determined, the cow can become a member of the proper group.

Heifers and new herd additions can be potential sources for introduction of S. aureus into uninfected herds. When heifers are fed dump milk contaminated with S. aureus, they become seeded with the organism. At first calving, these mastitic milk-fed heifers are more prone to acquiring IMI with S. aureus. Some will eliminate the infection on their own, while others will become chronically infected. Early treatment may save some of these animals. Heifers, as a reservoir of S. aureus are more of a problem in herds with a high prevalence of chronically infected cows. All new herd additions including heifers should be cultured within 30 days of entering the lactating herd. Animals testing positive should be immediately segregated. Milk culturing should be repeated to determine if the infections have become chronic. Somatic cell count data can help determine the chronicity of the infections.


The best way to live with S. aureus in your herd is to identify infected cows and prevent the exposure of uninfected cows to contaminated milk. Elimination of existing infections is best achieved with appropriate lactational therapy, complete dry cow therapy and culling chronically infected cows. New infections can be prevented by proper milking time procedures, post-milking teat dipping, maintaining excellent teat skin condition, and segregating uninfected cows from infected cows at milking time.

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