Selective Dry Cow Treatment (SDCT)– based on evidences

After 40 years of giving the entire herd blanket dry cow therapy (DCT), some dairy farms in Israel have transitioned to “selective drying-off” (SDCT) at the start of the dry period. This change is a welcome one since it once again emphasizes the recognition of the necessity of minimizing the use of antibiotics, without of course harming the health and well-being of the animals.

Years ago, in Northern Europe and America, a number of countries issued guidelines for reducing the use of antibiotics. This change came due to the increased diagnosis in human medicine of antimicrobial resistance to antibiotic (AMR) and out of a concern that this phenomenon might also appear in the veterinary world.

In Denmark, for example, the use of blanket DCT was banned 20 years ago. Was this act correct? Veterinarians and dairy farmers in Denmark reported that the infectious bacteria S. agalactiae, which had not been diagnosed prior to that time, began to “raise its head” and the rate of its having infected milk recently reached 5% of all diagnoses. It was additionally found that the savings in the use of antibiotics during drying-off led concurrently to increases in antibiotic use during the lactation period and in the incidence of clinical infections. In Holland, in contrast, the ban took effect in 2012, and a 50% decrease in the use of antibiotics was imposed on dairy farmers at the start of “drying off” and also during the lactation period. The Somatic Cell Count (SCC) level in this country before the ban became applicable was slightly above 200,000 cells per milliliter of milk. Five years after the ban on using antibiotics, the SCC score dropped to 170,000 cells/mL and the use of antibiotics decreased by more than 60%. The success of the move in Holland led researchers to the conclusion that the transition process must be closely accompanied by monitoring of udder health, improvement in the general interface, tailored feeding and being meticulous about a hygienic milking routine. The two differing results in the two countries prove that one cannot change one parameter in an equation (ceasing blanket antibiotic therapy) without at the same time changing the management and behavioral aspect in the herd and adapting it to the new situation.

Another new factor that needs to be taken into account is the option of using a teat sealant as a replacement to or in combination with an antibiotic. Introducing the use of an internal teat sealant has been found to be very effective in protection against new infection in the drying-off period (Friedman et al, 2017). This option provides the dairy farmer an additional capacity for treating some cows without antibiotics, especially those with a high risk of a new infection during “drying-off” (with teat defects). In the United States, in the state of Wisconsin, dairy farmers decided they would use a teat sealant to treat all the dry cows regardless of preliminary antibiotic therapy. Antibiotic treatment is given in these locations to all cows having an SCC score above 200,000 cells/mL of milk and/or who had a clinical infection during the most recent lactation period. As part of this program, they conduct a regular comparative follow-up of the SCC score before and immediately after calving for immediate identification of improvement/exacerbation trends in the level of infection in the udder during the dry period.

Other researchers found that the teats do not completely close over the first six weeks of the dry period. This is a high-risk cause of new infection during the dry period, mainly from gram-negative bacteria able to penetrate the teat canal despite preliminary antibiotic treatment, which is not effective against this group of bacteria. Based on these data, use of an internal teat sealant should be favorably considered in addition to – or without – antibiotic treatment, and all this in accordance with the microbial findings and teat condition diagnosed in the udder. A decision to treat only with a teat sealant and without antibiotics should only be taken when it is clear from the laboratory results that every udder quarter is clean of pathogenic infections and the sealant insertion process is conducted under the strictest conditions of hygiene.

As is known, every herd has its own ‘truth’, and the dairy farmer must take this into account while relying on the database that makes up this ‘truth’. The wider the database, the more accurate the professional decision made by the practitioner/ dairy farmer will be. The database should include: a history of clinical and sub-clinical udder infections in the most recent lactation period, diagnosis of pathogenic in the udder, SCC scores according to milk inspections at the herd and individual cow level, California mastitis test (CMT) results, sensitivity test results, the presence of defects at the teats end, and a known history of milk production.

The selective dry-off method requires of the dairy farmer a lot more time and attention for the collection of preliminary data and to perform additional testing in the laboratory and in the cowshed. Every dairy farmer must ask himself whether he is willing to transition to a method that based on variable findings or to continue the “easy” way and to ignore the demands of the consumers and of the milk industry. One should keep in mind that environmental conditions have changed, productivity is higher, the population of bacteria and their sensitivity to antibiotics is also changing and, we must act wisely. The appearance of antibiotic-resistant bacteria in dairy farming may harm safety in the dairy industry, and the health of dairy farmers and consumers alike.

Every dairy farmer must determine for himself whom measures/tools he can most skillfully use in diagnosing infected udder quarters while simultaneously identifying those that do not need antibiotic treatment at dry-off.

In the United States, many dairy farmers (especially those with giant cowsheds) perform preliminary identification of bacterial groups (gram-positive, gram-negative, staph. species) by using On-Farm Cultures (OFC). The diagnostic procedure is performed in the cowshed after milk has been sampled, swabbed onto a culture plate and placed into an incubator. Results can be interpreted by the dairy farmer 24 hours later according to changes in the color of the culture plate. Based on this result, the dairy farmer decides whether to treat and how to do so. In Israel, there is no great advantage to this method since it is possible to send milk samples directly to the laboratory at no cost and receive an accurate answer on the pathogenic cause (not only to which group it belongs) within 72 hours, including sensitivity test results. Optimal application of selective drying-off requires a preliminary sample one week prior to the dry date and, based on identification of the pathogen at the quarter level, a correct decision can then be made that corresponds to the actual infection and the sensitivity of the pathogen to the antibiotic.

Dairy farmers who choose to be assisted only by performing a CMT and/or relying only on an SCC score(DHI), must at the very least also be assisted by the clinical infection history in the most recent milking period in order to make a decision that is more closely aligned with the current intramammary infection. As a result of the relatively low sensitivity of the two tests mentioned above (see below), and in the absence of sensitivity test results, the success in truly identifying all the infected cows/quarters and in providing the optimal treatment, are not in themselves sufficient and cows could be overlooked in terms of receiving/not-receiving treatment.

The level of SCC in the bulk milk tank at the milk inspection indicates the rate of infected cows/quarters. Therefore, farms with a high SCC score (above 300,000) are probably less likely to be suited to selective treatment at dry-off in comparison with farms with an SCC score below this rate (see below). It should also be remembered that the level of SCC in a herd itself fluctuates over the course of the months of the year and this data should be calculated anew each month as a forecasting tool for the cow population requiring antibiotic treatment.

There are several reasons for the non-success of selective treatment. As stated, this treatment must be supported and based on a wide mix of data and any attempt to shorten the path and rely on a small set or on a single finding, such as the most recent SCC score and/or a CMT test alone, reduce the rates of success and can even end in failure.

Tests such as CMT and SCC scores at the cow level, indicate the existence of an infectious process but do not clearly indicate the presence of the pathogenic factor or its absence at the moment of the test itself. The sensitivity of these tests is not very high, and they need enforcement through additional tests to confirm the result where the gold standard of which is laboratory identification of the pathogenic factor (Table 1).

Table No. 1. Summaries of the sensitivities of tests for the identification of intra-mammary infections that are not accompanied by laboratory testing

As can be seen, total reliance on the test results appearing in this Table could lead to a situation in which some of the infected cows would not be identified and would therefore not be treated as required with antibiotics and/or a teat sealant as appropriate.

In some of the dairy farms in Israel, use is made of only one measure – the SCC score at the most recent DHI  – as grounds for making a decision whether to treat with antibiotics. Cows that had an SCC score above 200,000 cells/mL of milk were treated with antibiotics in all four quarters, whereas cows below this measure were not treated at all (Figure 1). In an analysis of the results of 206 cows on one of the dairy farms, the SCC scores were measured and compared after calving with those before calving and the correctness of the decision to treat with antibiotics or not to treat at all was examined. The status of the cows was defined according to the starting SCC score (“healthy”/”infected”) and the changes that occurred in these definitions during the dry-off and were seen during the first DHI test post calving.

Figure 1: The change in SCC before and after the dry period as an indicator of the success or failure of the decision-making on whether to treat/not to treat with antibiotics cows with an SCC score above or below 200,000 cells/mL.

It was found that in “healthy” cows with an SCC below 200,000 cells/mL, the decision not to treat with antibiotics at the cow level was correct in 82% of the cows. At the same time, 18% of the cows were diagnosed as having new infections due to an increase in SCC scores compared to initial SCC score prior to drying off.

In cows that were defined from the start as “infected” (above 200,000), and received antibiotic treatment in all four quarters, the treatment was found to be effective and reduced the SCC in 73% of the cows. At the same time, the SCC score remained high in 27% of the cows even after treatment and they were therefore defined as remains "infected".

A summary of these results shows that in this model and in this work methodology, we will fail to identify 18% of infected cows, and we will treat 27% of cows that are apparently chronic, and the antibiotic treatment would not change anything.

In an advanced analysis of these results, an additional indicator was added to the model to quantify the percentage change in the SCC as “success” / “failure” / “no change”. It was determined that “failure” in treatment would be defined as an increase of more than 10% in the prevailing SCC prior to dry-off, “success” was considered to be a reduction of more than 10% in the SCC at the start of drying-off, and when the change in SCC was less than 10% of that prior to and following dry-off, this scenario would be defined as “no change” (Table No. 2).

Table No. 2. SCC changes in cows of above or below 10% prior to dry-off and after calving as an indicator of the success or failure of decision-making on whether or not to treat with antibiotics based on the last milk inspection prior to dry-off

It was found that in 87.5% of the cows with an SCC above 200,000, antibiotic treatment was effective, and definition of the threshold was probably very close to that of the presence of a pathogenic factor in the udder as expressed in the level of the SCC. In cows defined as “healthy” and not treated, an improvement was found in only 46% of them, 9.8% were found to show no change, while a deterioration was found in 44%. In other words, these cows were not identified as “infected” at the commencement of dry-off (despite having an SCC lower than 200,000). The explanation for this lies in the fact that the SCC results in milk inspections are results of mixed milk from the four quarters. It could be that although the general result was indeed lower than 200,000 at least one quarter was infected with a pathogenic factor that was not treated at the time of dry-off and led to a worsening in the SCC level after calving. Another possible explanation is that this is a new infection due to bacterial infiltration that occurred during the dry period as a result of a lack of an antibiotic sealant on the teat/injury to the teat/a surplus of milk in the udder/ and a milk yield that caused a new infection and an increase in SCC after calving.

What is the level of mammary infection in Israel according to DHI milk inspections in 2017?

In order to obtain a national picture of the annual distribution of dairy farms according to SCC levels in 2017, an analysis of the following results was conducted. Data from 558 farms that performed milk inspections (at least 10 inspections annually) in 2017. More than one million one hundred and eighty-four (1,184,154) inspections were totaled and analyzed, giving SCC level breakdowns of the population of cows examined throughout the year (Table No. 3).

Table No. 3 – Distribution of cows according to DHI SCC levels in 2017 in the Israeli herd on which regular milk inspections are performed

Later on, the farms were divided according to their annual SCC levels and in parallel examinations were also conducted on a percentage of the “healthy” cows – those with an SCC below 200,000 cells/mL of milk at each level (Table No. 4).

Table No. 4 – Milk DHI SCC levels – estimate of the level of intramammary infections, the number of farms (n = 558) at each level of infection and a percentage of the “healthy” cows at each SCC level (Dairy herd book data 2017).

The intra-mammary infection rates increase as the general level of SCC rises in the herd. Since it is accepted that the SCC threshold for infection is above 200,000 cells/mL, then according to these results only 97 farms (17%) are farms with low intramammary infection, while 80% of their cows were “healthy in the udder” on the day the milk inspection was performed (SCC below 200,000). 282 farms (50.5%) had a medium level of SCC and intra-mammary infection, while 74% of the cows in this group were found to be healthy in the udder.

The remainder of the farms were found to be at high (above 300,000) and very high infection levels (above 350,000) which reduces their ability to perform selective dry-off because nearly 40% of the cows were defined by the system as being “infected” in the udder. Based on these data, it is also possible to predict the “healthy” groups of cows in the total herd at each SCC level (Figure No. 2) and to suggest the parity in which selective dry-off can be more successfully recommended.

Figure 2. Predicting the results in terms of the percentage of cows in the herd that are “healthy in the udder” (cows with an SCC below 200,000) according to the results of the milk inspection by parity (according to the data in the Herd Book – DHI -Milk Inspections 2017)

Under this model, the dairy farmer receives information every month on the results of the most recent milk inspection – e.g., which lactations are expected to be “healthy”. The data are for the last month and of course, include the cows designated for “dry-off” at that time. One is therefore able to focus on these lactations while fully and completely implementing the selective dry-off program (see below).

For example, where SCC levels in a herd are lower than 200,000, the “healthy” cow ratio is very high, and selective dry-off is therefore recommended for all the cows. With a rise in SCC levels, comes a fall in the ratio of “healthy” cows and the recommended lactations for selective treatment were therefore reduced.

In a herd with an SCC score above 300,000 cells/mL of milk, it is recommended to selectively dry-off the first and second calving cows only. Where an SCC score is above 350,000 cells/mL of milk, drying can only be successfully conducted in 1st lactations cows or not at all.

During working with this model, efforts are concentrated on the “healthy” groups while the other groups remain under the blanket method of drying as it used to be before taking up the current process. When implementing this model, the use of antibiotics is reduced in the “healthy” groups and, concurrently, the health of the udder is not excessively endangered because of non-identification of infected cows. There is no doubt that those wishing to take up “selective drying” in isolation from the monthly SCC results and different lactations can do so, provided they implement the drying program without shortcuts and make the best use of diagnostic and identification tools before making the therapeutic decision, thereby ensuring the success of the move.

For the first time, we present here a plan which includes the timetable and the stages for the various actions that should be carried out, including the gathering of information and the documentation required during the process. In this method of work, the broadest possible database is created, which is used to make the most correct therapeutic decisions at the quarter level, but it does require follow-up throughout the dry period (one must remember that some quarters/udders are not treated at all) and after calving with the objective of detecting a change in intra-mammary infection tendencies as early as possible due to having made incorrect decisions.

Making a therapeutic decision:       Quarter 1 – teat sealant only

                                                            Quarter 2 – NO treatment

                                                            Quarter 3 – antibiotics & teat sealant

                                                            Quarter 4 – No treatment

Savings:               75% in use of antibiotics

Expenses:            50% addition of a teat sealant

During the dry period – especially in the first weeks and after calving – changes in the udder and in the cow.

After calving –Need to sample cows with abnormal SCC or with a clinical teat infection whose condition worsened during the dry period.

1. A selective drying process and a reduction in the use of antibiotics are the obligated reality in the world and in Israel and cannot be ignored. However, in order to carry out these tasks successfully, we must use a broad database at the level of the herd and the level of the individual cow. The rule that guides us is the use in parallel of existing information and various and diverse diagnostic tools in order to identify the infected/healthy cows. After gathering as much information as possible, treatment can be given at the quarter level, which may comprise four different options: A. No treatment; B. Antibiotic treatment; C. Treatment with a teat sealant only; D. Combined treatment of antibiotics and a teat sealant.

2. The correct use of a teat sealant prevents new infection (4.5 times greater) and cure rate is 2.4 times greater than cows that were not treated with it. This treatment saves antibiotic use where it is not needed, and the udder/the quarter has been found to be pathogenic free. On the other hand, it is a feature that prevents new infection because of its physical ability to seal the teat canal against penetration of hostile elements.

3. Carrying out selective treatment on the basis of SCC results only, or performing a CMT test, does not give the proper results and some of the infected cows will not be identified and not be treated in the dry period, which is a critical period for possible recovery.

4. The laboratory microbiological diagnosis at the quarter level, even if performed one week before dry-off, is the closest result to reality, and gives the best assessment of infection/health of the udder/the quarter.

5. The addition of the sensitivity test to the diagnostic result also allows us to know clearly with which antibiotics to treat/not to treat, since we know the sensitivity/resistance of the pathogenic cause to the treatment substance.

6. Despite all of the above, the dairy farmer/the farm’s veterinary must understand that selective dry treatment may include risks of an increase in new infection, compared with not providing treatment to infected cows/quarters that should have been treated during this period. In order to reduce the damage that may be caused by making erroneous decisions, one must take into account the monthly SCC level, which will lead us to the “cleaner” lactation groups and from there follow the action plan as presented.  At high SCC levels (300,000 cells/mL of milk), one should consider whether it is possible in the given situation to conduct the selective drying-off process properly or whether it is first necessary to bring about an improvement in the intra-mammary infection in the entire herd (in the general SCC) and only then to activate the selective drying program.