Are U.S. Dairy Farms Ready for a Drop in the SCC Legal Limit?

Introduction

With a potential reduction in the legal limit for bulk tank somatic cell counts looming in the future to improve milk quality, how well have U.S. producers adopted mastitis control technologies?

Mastitis remains a major livestock disease for U.S. dairy producers, with losses of approximately $2 billion/year. To comply with global quality standards, consumer demand, and exportation requirements, the dairy industry is striving to improve product quality. To make this a reality, it is likely that the legal limit for somatic cell counts (SCC) in raw milk will be reduced in the near future. Already, milk purchasers are requiring milk with lower SCC from their suppliers. In 2011, Kroger, for example, set their SCC limit to 250,000/ml, which is down from 350,000/ml a year ago.

In May of 2011, a proposal to lower the legal SCC limit from 750,000 to 400,000/ml sequentially over a three-year period was submitted to the National Committee on Interstate Milk Shipments (NCIMS). This proposal was turned down by a very narrow vote. However, similar proposals will be submitted to the NCIMS in 2013, and it is very likely that one will be approved, thereby lowering the legal SCC limit.

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Why lower the SCC legal limit?

The question becomes: “Why lower the limit? SCC are not a human health risk.” True, somatic cells per se, which are mainly white blood cells or leukocytes, are not detrimental to human health. But elevated bulk tank SCC are almost always associated with poor milk quality as a result of management deficiencies on the dairy farm. So it is important to improve milk quality for several reasons. The dairy industry wants to ensure:

Improved consumer confidence in the safety and wholesomeness of the U.S. milk supply and that milk is produced by healthy cows.
Harmonization of standards for international trade of milk and milk products.
Improved competitive position of the U.S. dairy industry in the global marketplace.
Reduced risk of antimicrobial residues.
Reduced risk of human bacterial pathogens and their toxins.
Greater profits through decreased subclinical and clinical mastitis and reduced SCC.

In addition, it must be emphasized that:

Elevated SCC indicate poor farm hygiene practices, improper sanitation, and mastitis, as well as the potential antibiotic residues.
High SCC are always associated with reduced milk yield.
Low SCC milk has a longer shelf life, better taste, and greater cheese yield.
Processors shipping to Europe must prove that each supply farm SCC is <400,000/ml.

How well would U.S. dairy farms comply with a limit of 400,000/ml?

Complying to a legal limit of 400,000/ml for the vast majority of U.S. dairymen would not be a problem, even if it was imposed now. In 2008, a U.S. Animal and Plant Health Inspection Service (APHIS) survey found that the average SCC was 245,000/ml, and at the time, about 90% of U.S. bulk tank SCC were below that level. The Dairy Herd Improvement Association (DHIA) also found that the SCC of herds on test decreased from 276,000/ml in 2007 to 228,000/ml in 2010. Moreover, a 2010 Hoard’s Dairyman survey showed that over 95% of U.S. bulk tanks had SCC of <400,000/ml. See figure below.

Thus, the average SCC of the vast majority of dairy farms is well below the proposed legal limit, and it appears that the 5% of farms that would have problems complying are those that are located in the southern states. Producers in this region will have to adopt stricter methods of mastitis control in their milking herds and dry cows, as well as in their heifers, to reduce the incidence of mastitis and successfully lower their bulk tank SCC.

How somatic cells in milk function

Before outlining mastitis control programs that have been adopted to manage intramammary infections, it is important to understand how somatic cells function and why they are so important to udder health. One major role of somatic cells (also known as leukocytes) in milk is to identify bacteria and kill them; otherwise, these organisms would grow unrestricted in the mammary gland and probably result in gangrenous mastitis. So these leukocytes are beneficial by destroying bacteria; however, leukocytes are also detrimental because high numbers of them are associated with decreased milk production and poor milk quality.

To understand this relationship, we need to review the anatomy of the udder and how milk is produced. Just as nutrients are transported from the bloodstream into mammary tissues to be converted into milk, leukocytes are also transported into milk as a surveillance mechanism to look for bacteria. In the typical uninfected mammary quarter, the leukocyte (or somatic cell) numbers remain less than 200,000/ml. Now, keep in mind that each mammary quarter is composed of millions of microscopic alveoli. Each alveolus, which is drawn out of proportion in the figure below, is a three-dimensional sphere composed of a layer of milk-producing cells that surrounds a lumen or collecting space, which drains milk to the gland and teat cisterns before exiting through the teat duct during milking.

In order for leukocytes to migrate from the bloodstream into milk to look for bacteria, they must cross the layer of milk-producing cells surrounding each alveolus and move into the lumen, and this is where they become detrimental to milk production. In the figure below, the circulating somatic cell (1) crosses the bloodstream (2) and migrates through the connective tissue toward the layer of milk-producing cells (3) that lines the alveolus so that it (the somatic cell) can eventually move into the alveolar lumen to look for bacteria (4).

But to access the lumen, one of the milk-producing cells is sacrificed to accommodate the migrating somatic cell. The milk-producing cell may be displaced or killed, or it may revert to a resting, unproductive stage for a period of time. In the figure below, the somatic cell (1) is exerting pressure on a milk-producing cell (2).

Through physical pressure exerted by the somatic cell (1), the milk-producing cell (2) may be displaced into the alveolar lumen and thus lost to future production, as depicted in the figure below. Consequently, as more somatic cells move through an alveolus, more milk-producing cells are lost or become unproductive, and milk yield decreases. Even at a SCC of 200,000/ml in milk from an uninfected udder, cows experience a small loss in yield of approximately 2.6 pounds per day. This is all due to the normal movement of somatic cells into milk to guard against any potential bacterial growth.

Now, consider the case in which the somatic cells cannot prevent bacterial growth and have to call in recruitments to fight the infection. In this scenario, the SCC is usually millions/ml; therefore, many milk-producing cells are lost due to somatic cell migration. Also, in the case of infection, the bacteria produce toxins that are harmful to milk-producing cells, which further reduces yield. Lastly, in the process of killing bacteria, the somatic cells release chemicals that are not only harmful to bacteria but are also deleterious to milk-producing cells, yet again reducing yield.

It must be kept in mind, however, that despite their harmful effect on milk production, somatic cells are very beneficial and absolutely necessary because they do prevent mammary gland infections……most of the time.

Managing mastitis to lower SCC: How well do U.S. dairy farms follow recommendations?

So what can be done to manage the level of mastitis in a herd and keep the SCC low and in compliance? The best way is through the prevention of this disease because once cows become infected, the cure rate with antibiotic therapy is low, except in the case of Strep. agalactiae. Culling may be the only option.

Over 40 years ago, the 5-Point Plan below was established to manage mastitis in lactating and dry cows, and it remains the core of all mastitis control programs today. This plan includes:

Teat dipping
Proper milking machine function
Treatment of clinical cases
Dry cow therapy, and
Culling chronic cows

Since that time, additional methods of control have been added such as vaccination, feed supplementation, predipping, use of teat seals, and even control programs for bred heifers. Thus, dairy producers have several tools at their disposal that have been around for many years to incorporate into a mastitis control program. Yet the extent to which U.S. dairy producers have adopted – or have failed to adopt – such control measures is surprising. During the last four years, two surveys have been conducted to monitor mastitis control practices followed on dairy farms. The National Animal Health Monitoring System (NAHMS) conducted one in 2007, and Hoard’s Dairyman conducted one in 2010. Both surveys looked at practices such as milking procedures, machine function, vaccination, antibiotic therapy, teat seals, segregation, fly control, and feed supplementation. Results were presented at the 2011 Annual American Dairy Science Association meeting in New Orleans and are summarized below.

Regarding milking procedures, forestripping is normally recommended because it flushes potential mastitis-causing bacteria from the teat canal, allows the operator to observe milk for early signs of clinical mastitis, and helps to promote milk let-down. Yet, only 60% of dairymen follow this practice, and 43% do it wrong. This latter group actually forestrips after predipping and drying, and by forestripping the sanitized and dried teat with potentially contaminated hands, bacteria are redeposited on teat surfaces, which can cause mastitis. By forestripping before predipping, bacteria already present on the teat skin as well as from milkers’ hands are removed by the predip. Some milkers prefer to predip first, and that is fine. By predipping first, forestripping may transfer bacteria from hands to teats, but these will be killed by the germicide in the predip.

The practice of predipping with an approved teat germicide is very effective in controlling the rate of new infections with environmental bacteria, which accumulate on teat surfaces between milkings. Predipping has been shown to be 40 to 50% efficacious in preventing new infections with coliforms, such as E. coli, and the environmental streps, such as Strep. uberis. However, only 75 to 80% of dairy farms use a predip. It is important that the germicide contact the entire barrel of the teat to contact as many bacteria as possible, at least that portion of the teat barrel that will come in contact with the teat cup inflation liner. The preferred method of applying predip is by immersion (dipping) into a cup of the germicide, and of the producers in the 2007 survey who predipped, about 50% used a predip cup and 20% used a sprayer.

The use of single service paper towels or rewashable individual cloth towels is recommended to dry teats after predipping and prior to machine attachment. Only about 70 to 75% of producers follow this recommendation; among other practices, approximately 5 to 8% still use a common rag or sponge, or use paper/cloth towels on more than one cow, all of which are notorious for spreading contagious pathogens such as Staph. aureus, Strep. ag., and mycoplasma.

Postdipping immediately after machine removal is probably the single best method to prevent new infections with these contagious pathogens. As long as the germicide is applied to teats properly, immersion (dipping), spraying, or use of foams are equally effective. As with predipping, the germicide should contact the entire barrel of the teat surface that comes in contact with the teat cup liner. During milking, contagious bacteria from infected cows adhere to teat cup inflations. After milking, if the milking cluster is not sanitized, these bacteria are introduced to the next cow in line. More specifically, during the milking of this cow, these contagious pathogens are deposited on the teat surface as well as at the teat orifice. After milking, when the teat cups are removed, the bacteria can grow in the milk film deposited on teats, which provides a growth medium for bacteria at the teat end. And, because the teat canal remains dilated for up to 1 hour after milking, these bacteria are in an opportunistic position to gain access to the interior of the udder. However, by postdipping, the bacteria are eliminated before they have a chance to multiply and cause new infections. Surprisingly, 5 to 10% of dairymen do not postdip. Of those who do, about 78% dip by immersion (the preferred method of application) and 13% spray teats.

Because bacteria can readily adhere to the skin of milkers’ fingers and hands and because hands are used to prepare teats for milking, milkers can potentially spread contagious pathogens from teat to teat and from cow to cow. So it is highly recommended that latex or nitrile gloves be worn during milking, as bacteria are less able to stick to gloves and can be more easily washed off as needed. The survey showed that only 55% of milkers wear gloves; thus, there is a lot of room for improvement in reducing the spread of contagious mastitis-causing bacteria during milking.

Because these contagious pathogens are spread during the milking process, it is recommended that cows with such infections be segregated from the rest of the herd or milked last into a separate bucket. This is because the bacteria-laden milk from infected quarters contaminates teat cup inflations, which then transfer bacteria to uninfected cows. According to the surveys, approximately 60 to 70% of producers follow either one of these recommendations, so 30 to 40% do not milk last or segregate their mastitic cows, and bacteria from infected quarters are being spread to uninfected cows in the milking herd.

Because contaminated inflations serve as fomites for spreading infections among cows, backflushing units are sometimes installed to flush bacteria from inflations. If they work properly, they can reduce the spread of contagious pathogens. The survey showed that less than 7% of operations actually use backflushing systems; however, most mastitis experts agree that if a producer’s postdipping germicide is working effectively, then backflushing the unit is not necessary.

Automatic take-offs can also be installed to save labor and prevent overmilking, helping to maintain good teat end condition. In turn, healthy teats are less apt to harbor mastitis-causing bacteria. Surprisingly, less than half (45%) of operations have installed take-offs, which suggests that overmilking can occur on many dairies, resulting in impaired teat end condition.

Vaccination has been used to simulate the production of antibodies in the cow to prevent infections and reduce the severity of mastitis cases. Commercial vaccines are currently available for coliforms, Staph. aureus, mycoplasma, and salmonella. The surveys found that overall, only about 40 to 50% of dairymen use one of these vaccines. Of that percentage, only about one-third of producers use one of the three coliform vaccines. Vaccines directed against the coliforms have been proven to reduce clinical symptoms, reduce SCC, and prevent mortalities, so it is unfortunate that more dairymen do not incorporate coliform vaccination into their herd health programs. Vaccines directed against Staph. aureus and mycoplasma have only a small share of the market (less than 10%).

If mastitis cannot be prevented in the lactating herd using the above practices, then antibiotic therapy is used to cure existing infections. In 2010, about 53% of producers used some form of antibiotic to treat clinical cases. Most was in the form of infusing infected quarters (>90%) followed by injections into the muscle and intravenous injections. This percentage is down 10% from the previous five years and may reflect a cost-cutting mechanism, but this means that about 50% of producers may not be treating clinical mastitis cases, which will lead to increased bulk tank SCC.

Probably the best time to treat mastitis is at drying off. Cure rates at this time are a lot higher than during lactation, and another benefit is that new infections are prevented. Thus, cows freshen free of mastitis with low SCC. Clearly, dry treating all four quarters of all cows at the end of lactation is one of the best mastitis management control practices that dairymen can adopt. Yet, only 75% of producers dry treat all of their cows. It is important to remember that without dry cow therapy, the new infection rate at calving is 8 to12% of quarters, which will increase the bulk tank SCC.

Another practice to prevent mastitis shortly after drying off is the use of teat seals. The sealing material, composed of bismuth and paraffin, is infused after the last milking, right after dry cow therapy is applied, and then removed at calving. Teat seals do not cure infections, and that is why they are used in conjunction with antibiotics, but they do prevent new cases of mastitis by between 50 and 90%. The seal formed in the teat duct and teat cistern actually serves as a physical barrier to bacterial penetration. In spite of this success rate, only a third of producers have adopted this control strategy.

According to the 2010 survey, almost one-third of producers use some type of yeast product as a probiotic to improve overall animal health and/or as a nutritional supplement, especially during times of stress, to increase milk production. Another possible benefit is reducing SCC. For example, one yeast-based product currently on the market has been shown to enhance the ability of certain white blood cells to identify and kill mastitis-causing bacteria, thereby potentially reducing the new infection rate. More research is needed before making general recommendation, but dietary supplementation with such products could be a practical strategy to adopt for reducing SCC in the future.

Although not covered by the surveys, herds suffering from a high bulk tank SCC most likely have Staph. aureus problems, and producers need to incorporate a control program for their heifers. The level of infection among bred heifers in such herds is quite high (up to 60%). New Staph. aureus infections can be reduced between 45 and 60% by vaccination, and up to 80% by using fly control. For heifers already infected, the use of dry cow therapy administered no sooner than 45 days before calving is almost 100% effective in curing such cases. Heifers that calve free of infection have lower SCC and higher milk production than those that freshen with Staph. aureus, which can then potentially be spread to the milking herd.

Summary

The results of these recent surveys were quite surprising. Because mastitis control experts have been highly recommending the basic 5-Point Plan for over 40 years, as well as additional control measures, the assumption has been that the vast majority of producers have adopted the recommendations. However, the summary below reveals that a lot of progress is still needed to convince dairymen that following recommended practices will reduce mastitis and lower herd SCC:

Recommended mastitis control practice	Percentage of dairymen NOT following recommendations
Fore-stripping	40%
Predipping	20-25%
Drying teats with paper or cloth towels	25-30%
Postdipping	5-10%
Wearing gloves	45%
Segregating or milking mastitic cows last	30-40%
Using automatic take-offs	55%
Vaccinating	50-60%
Treating clinical cases	50%
Dry cow therapy	25%
Teat seals	65%
Average percentage NOT following recs:	~40%

Unfortunately, only six in 10 U.S. dairy farms have actually adopted most of the mastitis control practices that have been stressed over the years by mastitis control experts. The good news is that only 5 to 10% of U.S. dairies have SCC greater than 400,000/ml. If that minority of producers could be convinced to successfully incorporate the practices listed above into their mastitis control programs, they would eventually reduce the incidence and duration of intramammary infections, and their herd SCC would automatically decrease. This would bring them closer in line with the most recent 2008 APHIS survey of 245,000/ml, which is well below the 400,000/ml legal limit proposed earlier this year (2011).

The vast majority of those with herd SCC above 400,000/ml are located in the southern region of the United States. For years, the excuse has been that the heat and humidity experienced during summer months make it impossible to lower SCC in the South. Heat and humidity DO NOT cause mastitis. These factors will increase the ability of mastitis-causing bacteria to grow and thrive in the cows’ environment; however, it is the management deficiencies on southern farms that allow these potential pathogens to actually cause infections. There are many well-managed operations in the South that consistently have SCC well under 400,000/ml throughout the year, so it can be done. Producers with herd SCC above 400,000/ml who want to remain in business in the next few years need to start managing their level of mastitis right NOW.

The traditional control measures based on the 5-Point Plan developed 40 years ago, as well as the newer control procedures, have been proven to work and have been adopted by those southern dairymen producing high-quality/low-SCC milk. Those struggling with milk quality need to emulate their successful neighbors in order to market a product of which they can be proud and one which can remain below the next legal limit whenever it is imposed.

Author Information

Stephen C. Nickerson, University of Georgia