For many dairies, mastitis caused by the environmental species of bacteria, such as the coliforms and environmental Streptococci, is a serious problem. These may be related, at least in part, to the high stocking densities in many of the new facilities and cattle are routinely exposed to high concentrations of these bacteria. These organisms tend to produce a high incidence of clinical cases of mastitis.

Despite these changes, statistics also indicate Staph aureus mastitis remains a significant ongoing problem. Despite our knowledge of this pest, we still have to manage it because we haven't figured out how to eliminate it or effectively treat it. What do we know about this species that may enable us to do a better job of minimizing it?

Source of Staph aureus
Staph aureus, in terms of mastitis, is considered a bacterium that primarily originates from infected quarters of cows. It is contagious, meaning it basically lives in and on the cow and is transferred cow to cow by some mechanism such as contaminated liners or the milker's hands. Infected cows are the source.

Staph aureus-Skin Colonization
Staph aureus has developed a strong ability to attach to skin and similar tissue known as epithelial tissue. Wherever skin tissue is damaged such locations provide even better opportunities for Staph aureus to attach and colonize. These breaks may include physical injuries, cuts, chapping, viral sores, irritation due to machine or liner function, frost bite or any other condition that may cause skin problems. Given such conditions this organism invades and grows prolifically. These infected sites potentially form the basis of new udder infections. Maintaining good skin condition is critical in limiting this problem. Practices and tools need to be utilized that help in this regard.

The streak canal or teat end opening functions to help prevent bacteria entering the gland between milkings. It acts as a physical barrier when fully closed. It is also lined with keratin, a waxy material that tends to act as a sticky surface that can trap bacteria. There is some evidence this material may have germicidal capabilities and may be able to actually kill trapped bacteria, but this is not likely the primary means of defense. It is also thought that during milking, the rapid flow of milk through the streak canal actually erodes some of the keratin layer, taking trapped bacteria with it. Between milkings keratin is replaced. The key is not to erode too much of this layer or damage it somehow so that it loses its protective ability. 

Any time damage is done to this material and the interior of the streak canal, there is an increased risk of new infections. Stepped on teats or otherwise damaged teat ends will result in more problems for this reason. Faulty pulsation, prolonged overmilking or reaming out the keratin lining by use of long treatment tube cannulas can result in problems.

Infection Characteristics
Once Staph aureus gains a foothold inside the gland, if not promptly eliminated, it can lead to long-term problems. Initially the infection involves the milk collecting cisterns and ducts of the lower udder. As the infection lingers it tends to move deeper into the gland and colonizes milk-producing tissue and can permanently destroy this tissue. The body reacts by sending large quantities of somatic cells to infection sites and by forming scar tissue to wall off the infection. When this occurs, pockets of infection develop, trapped inside these scar tissue boundaries. The scar tissue makes it tough to deliver sufficient concentrations of antibiotic to eliminate the problem, and at this stage, the infections linger for extended periods with little possibility of cure. The scar tissue permanently replaces milk producing tissue.

Occasionally these infection sites flare up and release organisms that are shed into the raw milk and may be transferred to other cows. Some may also relocate to other areas within the same quarter, developing new infection sites and destroying more milk producing tissue.

The body also reacts by sending large numbers of white blood cells (somatic cells) to the infection site. Infections with Staph aureus tend to produce elevated SCC for prolonged periods. These bacteria are even known to be able to survive inside somatic cells so they are very formidable opponents.

Impact on Bacteria Count
Somewhat ironically these infections do not greatly impact the bacteria count (standard plate count) of raw milk. They are not normally shed in large numbers and may be shed sporadically. Infected cows may go extended periods with minimal bacteria shedding and then, as a flare-up occurs, there may be an increase. This characteristic makes bulk milk and individual cow sample results difficult to interpret.

Bulk tank test results for Staph aureus are not well correlated with the number of cows infected. A low Staph aureus count in the bulk tank does not necessarily mean few infected cows because of the characteristics of shedding.

Antibiotic Resistance
Staph aureus may develop resistance to antibiotics approved and frequently used to treat mastitis. Frequently they are resistant to penicillin based products which, as a group, are the most commonly used products for treating mastitis infections. Veterinarians may do sensitivity testing to determine which antibiotics are most likely to work, but frequently what should work, based on lab testing and actual field results, does not correspond.

Managing Staph aureus Spread
Staph aureus remains a significant problem on many dairies. Minimizing problems requires a strategy that limits the number of cows infected, as well as the duration of infection and the infected cows' contact with the remainder of the herd. Finally, the strategy must minimize the possibility of cow to cow transfer.

Limiting the number of cows infected requires consideration of several factors. First, within the existing herd, cows infected with Staph aureus have to be identified by proper milk sampling and culturing by a qualified laboratory. A percentage of infected cows often are older, chronically infected and repeat offenders. Every herd tends to have them. Treatment during lactation tends to be ineffective. Dry cow treatment offers a better chance of curing the problem because of the higher antibiotic dosage of treatment products and the cessation of milking. If this fails they are candidates for culling. Younger, Staph aureus infected animals may be segregated from the main herd, treated at dry-off and then be tested again after calving in the subsequent lactation to determine if they are still carrying the organism. If still positive they may have to be culled.

Caution has to be used if new cattle are purchased and brought onto the farm. Bringing in infected cows can result in the total pool of infected animals suddenly increasing and risking the status of the existing herd. Implement a program that minimizes this possibility.

Monitor freshening heifers. Studies have indicated heifers frequently have Staph aureus infections at the time of calving. Flies or some other vector may be involved in the transmission of the disease. Regardless, it is common for this to happen and it needs to be carefully monitored.

During severe cold, frost-bite is a risk, especially with animals that are recently fresh and may have swollen teats and udders. It is tempting to stop dipping during severe cold. Unfortunately, if teats are not dipped for a few days they soon develop a Staph aureus buildup which may lead to problems. A better alternative is to dip teats, allow approximately one minute contact time and then blot dry the residual teat dip before releasing the cows.

Monitor teat sores that may be viral in origin and if possible, isolate cows that develop such sores. These tend to be more common as temperatures drop. Staph aureus quickly infects these sores, worsening and prolonging the problem. There is no quick cure for virus caused teat lesions but it is critical they be managed to minimize the duration and severity. Again, a quality teat dip with high levels of emollient is required. The emollients assist healing while the active ingredient must penetrate and kill all the bacteria in the sores. Typically iodine teat dips, that provide high levels of free iodine, are effective in penetrating and killing bacteria in the lesions.

Cow to Cow Transfer
Cow to cow transfer at milking is the major means of spread. Staph aureus contaminated milk, from an infected quarter, can remain on the liner surface and be transferred to the next animal milked. It is estimated that bacteria from an infected cow may be present on liner surfaces for several cows milked thereafter. This is virtually impossible to prevent if infected cows are milked unless an automated cluster sanitizing system (backflush) is installed that cleans the liner surfaces between cows. It is possible to do and may be a consideration in certain herds.

Teat Dipping
The most relied upon tool, to limit spread, is the post milking application of teat dip. This practice does not prevent the transfer from one cow to the next but it does the next best thing. It sanitizes teats by killing the majority of bacteria present on the teat end immediately after milking. Essentially the teat and teat end is rid of bacteria reducing the chances of new infections. Use dips that are backed by data indicating effectiveness in controlling Staph aureus infections. 

For maximum benefit, teat dips and teat dipping require certain characteristics. First, the product used has to be capable of killing all the bacteria it contacts in short order. Properly formulated iodine based teat dips are capable of rapidly killing all growing bacteria very quickly on contact.

Secondly the product must contact all surfaces of the teat that have been in contact with the liner and milk. Generally at least two-thirds of the teat length needs to be coated with teat dip. This can be done with a dip cup or a spray device. Dipping with a dip cup is preferred because there is less likelihood of missing areas of the teat due to improper spraying procedures. Spraying generally uses significantly more dip in order to get full coverage.

Staph aureus is an old opponent that still is a significant problem on many dairies. Minimizing it by implementing the management programs described is the best strategy until new tools become available.