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Staph aureus Mastitis

March 2004 - by Billy Walker, Ph.D.

There have been several questions raised regarding Staph. aureus mastitis since Jersey Marketing Service (JMS) has required all milking cows sold privately or in public auction, be tested for that particular bacteria.

This article will help explain why there is much concern over Staphylococcus aureus mastitis and attempt to outline strategies for prevention and control of this potentially devastating form of mastitis.

Staphylococcus aureus or "Staph" is a bacterium that causes a wide range of mastitis syndromes from mild subclinical or chronic mastitis to potentially fatal gangrenous mastitis in dairy cattle.

Staph is found primarily on exposed skin surfaces such as teats, udder, nose, vagina and human hands. Milk from infected cows is another major source of Staph. It is important to realize that damaged skin, especially teat skin, can harbor Staph and thereby serve as a reservoir of bacteria that can cause mastitis.

The teat skin can be damaged by a variety of means: irritating chemicals; weather-related chapping; milking machine maladies such as vacuum fluctuations; fly bites; and infectious viral diseases like herpes mammilitis and cowpox.

Staph tends to spread from cow to cow during milking time. Common items that significantly contribute to the spread of Staph are milker’s hands, infected residual milk on teat cups, flies and single-use udder wiping towels that are not truly used on a single udder. Milker’s hands can harbor milk with Staph in it.

Residual milk serves as a source of Staph that can be back-jetted into a quarter if a liner slip or squawk occurs during milking. Flies, like hands, are a mechanical means of spreading Staph. Some species bite, thereby leaving a damaged teat surface where Staph can hide out.

Viral diseases cause direct teat skin damage in the form of blisters and ulcers. Again, damaged skin is a place where Staph can hang out and thrive.

The problem with Staph mastitis is that it tends to cause small abscesses and scarring deep in the udder tissue. From a treatment standpoint, abscesses are notoriously hard to get antibiotics into and, therefore, serve as a reservoir of infection that you cannot effectively treat. Staph can also evade the immune system by hiding out inside some immune system cells.

One typical scenario is an older cow that has mastitis and is treated by a tube or concoction to treat it. Even though it clears up, in a month or so, the same cow is back with mastitis again. The process repeats itself until the dairy producer gets tired of treating and she is culled.

Another typical scenario is increasing somatic cell count (SCC) with few cases of clinical mastitis being treated. The small abscesses stimulate the immune system to produce more cells to try to fight the infection, therefore the SCC increases. However, the problem is not cured because the abscesses remain so the SCC continues to remain high from the immune system’s continued efforts to cure the abscess.

There is one more scenario that doesn’t typically involve the formation of abscesses. That scenario is gangrenous mastitis. This typically occurs early in lactation and is due to a toxin produced by Staph that seems to mainly affect heifers. This type of mastitis is often quite debilitating and fatal. Survivors may physically lose a quarter or udder tissue and often must be culled.

Treatment of Staph cases is frequently unrewarding. Cure rates have been quoted from 20 percent to 60 percent, even with extended antibiotic therapy. The reason is those darned abscess again! However, there may be one notable exception. Heifers that have a positive culture for Staph may only be infected in the teat streak canal or teat cistern and not in the udder tissue itself. In these cases, cure rates may be increased if treatment prevents the Staph from infecting the udder tissue.

When the udder tissue is infected, as it is frequently with older cows, chronic infection usually ensues and treatment success plummets. Veterinarian should be consulted when planning treatment protocols for mastitis. Their expertise can help achieve the best cure rates possible at an economical price.

Control and prevention of Staph mastitis basically entails a three-sided approach. One side deals with prevention of spread to other animals. Another side is deciding what to do with animals already infected. The final side is the surveillance arm, which is why JMS has required testing. The three aspects work together to help ensure that the impact of any one case of Staph mastitis does not lead to a major herd problem.

Preventing the spread of Staph is of utmost importance in a control program.

Staph is generally spread from cow to cow during milking. A producer must hone in on a time period to place into effect a meaningful change.

Milkers should wear gloves to avoid spreading their own or the cow’s Staph to other cows. More importantly gloves provide a surface that is not amenable to bacterial growth and is easily cleanable.

Let’s face it, latex or nitrile gloves are an inexpensive insurance policy! When they get dirty or after they have been in contact with a known Staph infected cow, throw them away.

Now is the time to review proper milking procedures with your employees. They must have a technically acceptable routine that should include pre-dipping, forestripping, wiping, machine attachment and proper adjustment and post dipping.

There has been a mixture of love and hate comments when backflushing is mentioned. However, this author still believes in the efficacy of those systems if they are properly maintained and the backflush solutions they use are kept in the proper concentrations. It makes sense to rid the teat cup of all the residual milk you can because it serves as a source of Staph and it only takes one liner slip to back jet that milk into another quarter!

Infected cows present a challenge for all dairy producers, especially when taken in relation to the size of the farm.

In California where the average dairy size is about 750 cows, 12 cows lost to Staph mastitis may not be too much of a financial strain. However, in Virginia, where the average herd size is about 120 cows, that can be 10 percent or more of a producer’s milk check.

A herd owner basically has three options for dealing with infected cows. First is just to cull them. Individual economic situations dictate whether or not this can affordably be done.

Second, infected cows can be segregated into a separate pen. They can be milked as the last group before washing the milking system. If there is a breakdown in milking procedures or inadvertently spread Staph from cow to cow, it is not such a big deal. A proper milking routine should always be executed, so sloppy procedures are not encouraged when working with the Staph pen. Sloppy practices can carry over into the uninfected pens.

The system will be cleaned immediately after this pen is milked. Therefore, any residual infected milk should be disposed of properly. Cows tend to unintentionally drift from pen to pen sometimes, so one must be careful to identify Staph infected cows clearly so they may be returned to their segregation pen.

The third and least desirable option is to positively identify Staph infected cows, keep them with the rest of the herd, but clean the individual claw and equipment after milking them and before milking the next cow. This is a doable option but is very risky and only should be attempted when the dairyman has 100 percent confidence that milking procedures are followed strictly without failure.

Surveillance is a hard concept to grasp. It involves spending money on culturing and testing time after time, often with little or no problematic bacteria found. Many think of it as a waste of money since there is little or no problem. Most of those who have gone through a major mastitis outbreak realize the value they are getting for their money. It must be thought of as a good thing and a cheap insurance policy.

A good Staph surveillance program includes culturing bulk tanks as often as possible (weekly is great). All fresh cows and all cows with clinical mastitis before treatment are cultured. Finally, all cows with persistent high SCC are cultured. A persistent high SCC would be defined as two or more consecutive tests with greater than 200,000-300,000 SCC or a linear score of more than 4.

Another method of surveillance is to monitor SCC or CMT scores at dry off and soon after calving. Persistently high scores may help identify Staph carriers because dry treatment may not be effective against walled off Staph infections. Culturing must not be given up on, because Staph is characteristically an intermittent shedder (remember the abscesses?). It may be quite difficult to find in cows with elevated SCC, but no clinical mastitis.

Staph mastitis is a multifaceted problem to handle. Staph does not rear its ugly head the same way each time. It can vary from severe life threatening mastitis to only elevated SCC’s. It is characteristically resistant to treatment because of abscess formation. It is easily spread at milking time and sometimes hard to culture and identify in chronically shedding cows. Treatment is frequently unrewarding. Culling is the best option, but segregation will work. Surveillance is the key to keeping a few cases of Staph from turning into a full blown outbreak. It would be wise to consult the herd veterinarian when faced with a Staph mastitis problem because he/she can provide a multitude of expertise and experience to help.

The aforementioned information was derived from a few sources besides my own personal thoughts. All of these websites provide excellent sources of cattle health and mastitis related information.

First is the University of California Extension Service at the Veterinary Medicine Teaching and Research Center (VMTRC) of the University of California at Davis (www.vmtrc.ucdavis.edu).

Secondly, and most dear to my heart, is the Virginia Cooperative Extension Service and the dairy extension staff (www.ext.vt.edu).

Thirdly, the National Mastitis Council (www.nmconline.org) is an all around milk quality resource.