Contagious mastitis and organic milk production

Characterizes the three common contagious mastitis organisms. Defines bovine mastitis Lists 8 steps that organic herds can take to improve udder health

Bovine mastitis is the most costly disease for dairy producers, no matter whether they farm conventionally or organically. Losses stem from decreased production, loss of quality bonuses, increased labor and veterinary expense, discarded milk and potentially culling. There are a number of risk factors for mastitis present on all dairy farms. By minimizing these risks with certain preventive measures, economic losses from mastitis can be reduced.

In this section we will look at the most common causes of contagious mastitis in cattle, the risk factors for developing contagious mastitis, and some intervention strategies to reduce the risk and to decrease the prevalence of contagious mastitis. The National Mastitis Council has developed a ten step plan for improving udder health; many of these steps can be adapted to the organic farm and those are included as a part of this discussion. Briefly, they are:

  • Set farm specific udder health goals
  • Implement a plan to regularly monitor udder health
  • Follow proper milking procedures
  • Maintain milking equipment regularly
  • Evaluate dry cow management
  • Institute biosecurity and culling guidelines
  • Maintain a clean and comfortable environment
  • Prevent mastitis in youngstock

Mastitis: Definition

Mastitis is an inflammation of the mammary gland that is usually caused by bacteria that enter the gland through the teat end. Mastitis can rarely be caused by chemicals (infusion of a product into the gland) or from physical trauma (kicks or other injuries to the bag).

Mastitis may be subdivided into two categories, clinical and subclinical. Clinical mastitis is that which we see in the barn: swollen quarters, clots in the strip cup, abnormal appearing milk. Subclinical mastitis is not visible to the naked eye but microscopic cell counts are high and milk quality and productivity are decreased. Subclinical mastitis can be diagnosed through DHIA cell counting or on a California mastitis test. Subclinical mastitis is much more common than clinical mastitis: for every clinical case, there may be 15 to 40 subclinical cases.

We can also categorize mastitis by the bacteria that cause it: contagious or environmental. Contagious mastitis is the “cow-associated” form where infected cows are the reservoirs for bacteria and spread occurs cow to cow at milking time. As the name suggests, environmental mastitis comes from the cow’s world. The risk of mastitis is directly related to the number of organisms at the teat end. Infection by environmental organisms may occur throughout the cow’s day: at milking time from dirty equipment or between milkings from bedding, mud, and manure.

Contagious Mastitis:

The most common causes of contagious mastitis are:

  • Streptococcus agalactiae
  • Staphylococcus aureus
  • Mycoplasma spp.

We begin to suspect the presence of contagious mastitis in herds with gradually rising bulk milk cell counts. Initially, one cow may be infected. She will spread it to another cow; those two cows will then spread it to another two; those four will spread it to another four and so on throughout the herd

Bulk milk culture is a good first step for diagnosing the cause of the rising bulk milk cell counts or for screening herds prior to purchase into an existing herd. The sensitivity (ability to find a single positive cow in a bulk tank) varies with the organism we for which we are looking. A single bulk milk sample has about a 90% chance of finding a cow infected with Streptococcus agalactiae, a 70% chance of finding a cow infected with Staphylococcus aureus and a 50% chance of finding a cow infected with Mycoplasma spp. For this reason, multiple bulk tank samples are recommended for an accurate diagnosis (at least three samples, each a day a part).

Streptococcus agalactiae

Streptococcus agalactiae (S. ag) is a contagious gram-positive bacterium that lives only in the mammary gland and cannot survive outside of milk for very long. It is a common cause of contagious mastitis in the US although many herds in Canada and the European Union have eliminated it completely. Recent herd data from Quality Milk Production Services has indicated that the prevalence of S. ag in New York dairy herds is decreasing.

S. ag usually causes a subclinical mastitis and individual cow cell counts are frequently greater that 1,000,000 cells/ml. Millions of bacteria can also be shed into the milk so farms frequently have high bacteria counts as well as high somatic cell counts in bulk milk. Rarely does S. ag cause clinical mastitis. It is highly contagious, being spread cow to cow at milking time on inflations and other common items used during milking.

In conventional herds, antibiotic treatment is highly effective but this is not an option for organic herds. It is recommended that S. ag be eliminated before transition by treatment and that all replacements be carefully screened for these bacteria. In organic herds already infected with S. ag, all herdmates should be cultured and infected cows should be strictly segregated, identified permanently with legbands, and milked with a unit that is used just on them. It can be slowly eliminated from the herd by culling and prevention of further spread.

Staphylococcus aureus

Staphylococcus aureus (S. aureus) is the most common type of contagious mastitis in dairy herds around the world. At least 90% of dairy herds have at least one infected cow. It lives within the udder and also externally on the cow; especially high numbers are found on teat skin. These bacteria gain access to the udder at milking time and can also infect the udder when the teat skin is damaged (stepped on teats, frostbite, and chapping).

Mastitis caused by S. aureus can be both subclinical and clinical. Clinical signs may range from a few flakes in the milk to gangrene and sudden death, often depending on the strain of S. aureus that infects the herd.

When S.aureus infects the udder, it produces toxins that damage the milk producing tissues. White blood cells enter the gland in an attempt to kill bacteria and somatic cell counts rise. As infection continues, scar tissue develops and bacteria become walled off in abscesses. Cows with long-standing infections often have udders with hard quarters or ‘knots’. Because of this ability of S. aureus to survive within white blood cells and abscesses, it can ‘hide’ from the cow’s immune system and may not always be detectable by a single culture. Even when antibiotic therapy is permitted, it is often ineffective against S. aureus. This disease is better controlled through prevention of further spread. Spontaneous cures are possible. Cows that are most likely to cure are those with only one quarter infected, young animals, cows with low cell counts (<1,000,000 cells/ml) and cows with recent infections.

The key to managing S.aureus within a herd is prevention of additional spread to uninfected cows. Once identified in the bulk tank, cows with individual SCC’s greater than 300,000/ml, clinical mastitis and/or CMT-positive quarters should be cultured. Because of the stress of transition and the increased likelihood for shedding bacteria at that time, it is recommended that all cows be cultured at freshening in herds that are trying to eliminate and control S.aureus. Heifers that were raised on unpasteurized waste milk can be guaranteed free of infection and should also be cultured as they freshen. As with S. ag, infected cows should be permanently identified and milked last, usually for as long as they remain in the herd. They should be put on the potential cull list and removed from the herd when it becomes economically feasible.

Good milking machine maintenance to avoid vacuum fluctuations, liner slips and teat end damage is essential to preventing S.aureus mastitis. Post milking teat dipping is also an effective means of preventing new infections. Since S.aureus can live in the cracks and crevices of milkers’ hands, gloves should be worn at milking time to reduce further spread.

New methods of ‘fingerprinting’ strains of S.aureus in herds are being developed and may increase our potential to control this mastitis. By knowing more about the strain infecting particular herds and its biology and preferred reservoirs, we may be able to decrease the potential for new infections.

Mycoplasma spp.

Mycoplasmas are highly contagious organisms that cause a variety of diseases in many species: cattle, humans, cats, etc. They can live as a normal inhabitant of the respiratory and reproductive tracts of cattle without being noticed. However, during times of stress they may become capable of producing disease and spread through the bloodstream to the udder, causing mastitis. It is most commonly spread cow to cow at milking time or via the respiratory tract.

Signs of this contagious mastitis are variable. There is usually a drastic decrease in milk production; one or more quarters may be infected per cow. This mastitis does not usually make the cow sick but milk may appear ‘sandy” in the strip cup. Cows may also have pneumonia or swollen joints while calves that are infected may develop pneumonia, head tilts, and droopy ears. Frequently this mastitis is introduced into a herd through the purchase of replacement cattle.

Routine culture methods do not identify Mycoplasma spp. so special techniques must be requested. Milk should be cultured on a special media in a decreased oxygen environment for a longer period than the standard 48 hours. Cows that are infected should be immediately culled from the herd and waste milk from infected animals should not be fed to calves. Multiple bulk tank cultures from the herd of origin are recommended when replacements are purchased.

Eight Steps to Better Udder Health

1. Set farm specific udder health goals

These will vary from farm to farm depending on the challenge facing the farm, the certifying agency, and producer philosophy. Realistic, achievable goals for the majority of farms are:

  • 0% Streptococcus agalactiae and 0% Mycoplasma
  • <5% Staphylococcus aureus
  • Bulk tank SCC < 200,000 cells/ml
  • Fewer than 5% new infections each month
  • 5-7% or less chronic infections
  • 2-3% clinical mastitis

2. Implement a plan to regularly monitor udder health

This can most easily be done with regular cell count testing through the Dairy Herd Improvement Association but effective monitoring with a strip cup, visual observation, and a CMT test can also achieve goals. Once a month evaluate the udder health of all lactating animals. Cows with high cell counts on two consecutive DHIA tests or two monthly CMT tests should be cultured so they can be milked last, monitored, managed with an alternative therapy, or culled. Clinical mastitis cases should also be cultured so that intervention opportunities in the environment can instituted. Semi-annual meetings with your certifier, veterinarian, and nutritionist to evaluate the current mastitis control program and review udder health goals are advised.

3. Proper milking procedures

A proper milking routine should be instituted at all milkings and be followed by all milkers. Cows crave consistency so having a standardized procedure will reduce stress and enhance milk letdown and milkout. Components of a good milking procedure are:

  • Udder wash/Predip: Visibly dirty teats should be washed or predipped and wiped clean before units are attached. Single use towels should be used on each cow: Do not share towels between cows as this will spread mastitis.
  • Forestripping: Removing 3-4 squirt of milk from each teat prior to milking helps stimulate the milk letdown response. Cows will milk out faster and on-time of the milking unit will be decreased so that damage to teat ends will be minimized. Forestripping also allows you to identify clinical mastitis earlier and also removes milk in the teat end that is higher in bacteria and somatic cells. Forestripping should be done into a cup or into the gutter so that bedding and hands do not become contaminated with milk.
  • Gloves: Gloves (either nitrile or latex) should be worn at milking time to reduce the risk of spreading mastitis on hands. Gloved hands are more easily disinfected between cows or when contaminated with milk or manure than bare skin
  • Milking order: Cows infected with contagious mastitis should be identified permanently (legbands) and milked last or with a unit designated just for them at each and every milking.
  • Overmilking/machine stripping: It is not necessary to get every drop of milk out of the cow’s udder. Leaving the units on the cow when milk flow is low or pulling down on units will damage teat ends: keratin that develops at the teat end and has some natural antibacterial properties will be removed. Overmilking will also cause eversion of the teat sphincter and development of scar tissue which leads to multiple problems. The teat sphincter and its ability to close between milkings is the first line of defense against mastitis. Scars and cracks that damage teat ends are more likely to be colonized by S. aureus and will increase the risk of mastitis.
  • Attention to post milking teat dipping: Attention to the selection and application of post milking teat dip is of utmost importance in herds with contagious mastitis. If an uninfected cow is unknowingly milked after an infected cow, contagious bacteria are deposited on the milk film of the teats. There they will multiply and move toward the teat end in an attempt to infect. Applying post milking teat dip to at least 2/3 of each teat will kill bacteria before they have a chance to multiply and spread on the milk film. Teat dips should contain at least 10% teat skin conditioners to maintain udder health and prevent chapping and cracking.

4. Maintain milking equipment regularly

The mechanical milking system (including dynamic testing) should be evaluated by a qualified individual at least twice yearly. Vacuum fluctuations, due to liner slips, flooding, inadequate vacuum reserve, and poorly functioning regulators, are likely to force bacteria up the teat end during milking and increase the risk of mastitis.

Rubber milking liners should be replaced every 800 cow milkings or every 60 days, whichever comes first. Rubber used longer than this deteriorates and will develop microscopic cracks that hold mastitis bacteria even through the wash cycle. Other rubber parts (hoses, tubes, gaskets) should be inspected regularly and replaced when worn.

5. Evaluate dry cow management

Although dry cow antibiotic therapy is not permitted in organic dairy production, this is still an important time for udder health improvement and rejuvenation of milking tissue. Energy intake should be reduced one week prior to dry off so that milk production is decreased. At dry off, the udder should be milked out one final time and the teat ends dipped with post milking teat dip twice daily for two weeks after dry off. It takes about two weeks for the keratin plug to seal the teat end and prevent the development of new infections during the dry period. Until those plugs form, we need to do our best to sanitize the teat end and prevent contamination by keeping the cow in a clean dry environment. As the cow dries up, changes occur in the mammary secretions that kill bacteria (increases in lactoferrin and alpha-lactoglobulin, increases in antibody levels) and help resolve existing infections. Nutrition of the dry cow is also important especially as she reaches the prefresh period. At this time, selenium and vitamin E levels become important for support of the immune and reproductive systems and to ensure the birth of a vigorous calf. The National Research Council recommends that 0.3 ppm per head per day of selenium and at least 1500 IU of vitamin E per head per day be fed. For continued udder health benefits, supplementation of selenium and vitamin E is recommended throughout lactation.

A clean comfortable environment is important to prevent new environmental infections and to prevent teat injuries. When cows are housed inside in winter, stalls should be scraped frequently and sufficient amounts of clean, dry bedding should be available. Pasture environments for dry cows in the summer are generally preferred but wet areas around ponds and streams should be fenced off so that they do not become muddy loafing areas.

6. Institute biosecurity and culling guidelines

Bringing new animals into an established herd is a risk for introducing many diseases (BVD and Johne’s to name a few) besides mastitis into your herd. Spending a little money up front to ensure the cows entering are healthy may save thousands in disease eradication dollars. As mentioned before, multiple bulk tank samples should be cultured for both the usual mastitis bacteria and Mycoplasma. If this is not possible, new additions should be milked separately and cultured as soon as possible once they are added to the farm. Although heifers are generally thought to be safe to introduce, they may also be infected and warrant similar segregation and testing protocols.

As contagious mastitis cows are identified, strategic culling methods should be used to slowly reduce the prevalence in your herd. Milk these cows separately. Place cows infected with contagious mastitis on the ‘Do Not Breed’ list. Remove as soon as possible cows with continued high cell counts or scarred and abscessed bags. For herds with a high prevalence of Staph aureus cows that need to reduce bulk milk cell counts as soon as possible, infected quarters should be identified by culture and dried off to prevent that milk from entering the bulk tank and food supply.

7. Environment

As mentioned previously, a clean, dry comfortable environment is essential to good mastitis control. Properly sized, maintained and well-bedded stalls will prevent teat damage that may lead to S. aureus mastitis.

Consider segregating contagious mastitis cows to one area of the barn so that infected milk that leaks onto bedding does not become a reservoir of potential problems for clean cows.

Since flies have been shown to spread mastitis, in particular S. aureus and A. pyogenes, good fly control is essential to the prevention of mastitis.

8. Youngstock and replacement

Replacement animals are the future of the herd and are a key element in maintaining low bulk milk SCC in herds battling contagious mastitis. If we can keep this group clean and healthy, the farm’s financial future is ensured.

Contagious mastitis can be spread to youngstock via two ways when mastitic milk is fed to calves:

a. Cross-suckling: Bacteria from mastitic milk can live in the tonsils and other immune tissues of the mouth for up to two weeks post weaning. If calves are housed together during this period and cross-suckle, contagious mastitis bacteria can be introduced into the juvenile udder and subsequently that heifer may freshen with contagious mastitis.

Also, if calves are allowed to roam in the barn, they may nurse from a cow infected with contagious mastitis and then suckle from a non-infected cow thereby spreading mastitis.

b. Flies: Even in herds where cross-suckling is not a problem, flies feeding from milk buckets and then on skin and hair, may also spread contagious mastitis bacteria to the juvenile udder. Again, fly control is important.

In herds that have problems with contagious mastitis and are feeding waste milk, a number of preventive measures are recommended.

  1. Separate calves while on milk and for two weeks after weaning. This is best accomplished via calf hutches but can also be accomplished by tying calves apart.
  2. Good fly control during the summer months.
  3. If possible, pasteurize milk before feeding to calves. Although this does not ‘sterilize’ the milk, it will drastically reduce the number of infective organisms.
  4. In herds with a known contagious mastitis problem, culture first calf heifers as they freshen. This allows you to identify infected cows early and segregate them. Alternative therapies which enhance the immune system may have more of an effect if Infections are caught early.
  5. Since flies can transmit contagious mastitis and contagious mastitis can also be transmitted in bedding and by other environmental surfaces, pregnant heifers should not be house with dry cows.


There is no single magic bullet to solving contagious mastitis problems in any herd but involves time and a commitment to resolving problems. Each of these steps is designed to work in concert with the others and provide a comprehensive preventive and management program. Organic herds have been successful in reducing and eliminating contagious mastitis through these steps.

Related Links:

NMC Online Guide to Contagious Mastitis

NMC list of 10 steps towards mastitis prevention

Mastitis and its control

Bovine Mastitis and Milking Management

Cornell University

Cornell University

Cornell University's mission is to facilitate New York State economic development by increasing the profitability and competitiveness of its dairy industry.

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