by Sally Colby

Despite dairy farmers’ best efforts, mastitis can occur. Contagious mastitis can be especially difficult to manage and requires knowledge of the problem and appropriate management tools.

Dr. Adrian Barragan, Extension veterinarian and assistant research professor, Penn State, said mastitis is one of the most costly diseases in the dairy industry, with around $2 billion in losses annually, primarily from decreased milk production and quality. Cows with subclinical mastitis often leave the herd early, which becomes another expense, and treatment cost can be high, especially if treatment involves repeated veterinary intervention.

Mastitis is classified as either clinical or subclinical. Clinical mastitis involves visible changes in the milk and possibly other clinical signs. Cases can include changes in milk, drop in production, udder inflammation and in more severe cases, loss of appetite and fever. With subclinical mastitis, there’s no visual means to determine whether a cow has mastitis and must be tested for a diagnosis. Although there may not be changes in the milk, there are indications the cow has an infection – a somatic cell count (SCC). Cows with a SCC over 200,000 are considered to have subclinical mastitis.

“The key component (of subclinical mastitis) is we cannot see any changes in the milk or mammary gland or the behavior of the cow,” said Barragan. “Clinical forms can also have increased SCC, but other signs as well to indicate illness. Mastitis can also be classified based on the pathogen causing the disease. In that case, we classify as contagious or environmental mastitis. Contagious mastitis is transmitted from cow to cow through milking equipment or through the personnel milking the cows.”

The most common pathogens involved in contagious mastitis are Staphylococcus aureus, Streptococcus agalactiae and Mycoplasma bovis. S. aureus is a gram-positive bacterium and causes about 67% of mastitis cases in the U.S. Barragan said this pathogen can cause both clinical and subclinical mastitis, and usually causes a localized response confined to an infection of the udder.

Another contagious mastitis pathogen is S. agalactiae, a highly contagious, gram positive bacterium. Almost 40% of mastitis cases are caused by this pathogen. In addition to causing mastitis, Barragan said S. agalacticae is found in the cow’s GI tract. “On top of being transmitted through milking equipment and personnel,” he said, “there can be fecal-oral transmission, especially from animal to animal when they ingest contaminated drinking water.”

M. bovis is one of the least common pathogens that cause mastitis. It most often resides in the upper respiratory and urogenital tract rather than the udder. Mycoplasma can affect the mammary gland and also cause respiratory infections, metritis and other reproductive issues. Barragan said M. bovis is tricky because it doesn’t show up in traditional milk culturing, requires special media and doesn’t respond to antibiotic treatment. It’s most often introduced to a herd by outside animals. For this reason, he suggested monitoring for outbreaks after purchasing new animals.

Several diagnostic methods can be used to detect contagious mastitis. The California Mastitis Test (CMT) is an easy, fast, cowside test. “We’re not diagnosing the specific pathogens but identifying which quarters might be infected – which quarters have a higher concentration of somatic cells,” said Barragan.

Milk culturing can also diagnose contagious mastitis. Barragan suggested that when cows have three or more high tests, conduct the CMT to identify which quarter is infected followed by milk culture to identify the actual pathogen.

He advised producers to use caution when doing a milk culture, taking care to not contaminate the sample. The first step is to label the plate, then use a septic swab and spread the sample across the plate. The plate is incubated for about 24 hours to allow bacteria to grow.

Milking routine and personnel play a major role in managing contagious mastitis. “Two main components of the milking routine help us prevent mastitis: Decreasing pathogen exposure and protecting the udder’s natural defenses mechanisms,” said Barragan.

The goal of pre-dipping is to prevent the introduction of bacteria to the teats prior to milking. “Start with a clean towel to clean dirty teats – a dry wipe,” said Barragan. “Remove the layer of dirt before applying pre-dip solution. Ideally, immerse the entire teat in the product.” Effectiveness requires 20 – 30 seconds of teat contact with the solution. Milking routine protocols should be developed with this timing in mind.

Next, decrease bacteria exposure and aid in milk let down through stripping. “Stripping removes bacteria from the teat canal,” said Barragan. “It’s critical for finding mastitis infection as well. Strip, then clean hands between cows. If we identify a cow with mastitis, the recommended practice is to change gloves before milking the next cow or even another quarter to avoid passing the bacteria to another quarter on the same cow.”

Barragan advised stripping enough milk for stimulation as well as obtaining a sample to assess, and recommends not counting the initial strip. “Start with the second and count the next three or four,” he said. “If we don’t stimulate a cow well, we can damage the teat canal if the milker is put on before cow lets down.”

Bacteria that cause contagious mastitis are highly present in stripping, so this foremilk should be stripped into a cup to avoid contaminating bedding in tie-stalls or the cow’s legs, belly or udder. The front teats should be wiped first so the milker’s arm doesn’t carry bacteria to teats that were already wiped. Always use one or more clean towels per cow and don’t re-use a towel on a different cow.

The milking unit must be aligned properly on the udder at a 90º angle to prevent air entrance and subsequent vacuum loss during milking. Be sure any deficiencies are adjusted immediately to prevent bacteria on the skin from entering the milking unit.

Continued overmilking can damage the teat canal, primarily as a result of hyperkeratosis (rough teat ends due to excess keratin accumulation). This calloused surface harbors bacteria, so manual take-offs should be closely monitored to prevent overmilking.

Post-dipping prevents bacteria from entering the udder before the teat canal closes, which Barragan said occurs 30 to 45 minutes after milking. “Protect the teat canal while it’s open,” he said. “Cows go back to the pen, drink, eat and then go lay down where the udder can contact bacteria.”

Certain physiological features of a healthy teat can help prevent mastitis. A series of folded mucosa where the teat cistern and teat canal meet, known as Furstenberg’s rosette, helps prevent pathogens from entering the teat. A keratin plug forms between milkings to further protect the closed teat.

Mastitis cases rise during the dry-off period even in cows that have had no incidence of mastitis. Barragan said research shows the best dry-off treatment is antibiotic plus teat sealant. However, be aware that teat sealant may cause issues when the cow freshens resulting from residue.

Always consult with the herd veterinarian if mastitis cases are rising, if you suspect contagious mastitis and if milking equipment hasn’t been functioning properly.