Mastitis Prevention in Dairy Cows
The management of contagious mastitis presents many challenges to the dairy farm veterinarian, including incorporating antimicrobial stewardship into the treatment plan. This module outlines the pathogens, diagnosis, and treatment of contagious mastitis and details the judicious selection of antibiotics for this disease. Alternative control measures and preventive practices that reduce the need for antibiotics are also addressed.
This submodule aims to describe a treatment plan for contagious mastitis that incorporates antibiotic stewardship. By the end of the module, you will be able to:
- discuss proper indications for the use of antimicrobials for contagious mastitis in dairy cattle.
- demonstrate understanding of the need to follow proper treatment protocols when using an antibiotic to treat individual cows for contagious udder pathogens.
- explain how somatic cell counts and culture results are used to formulate a herd antibiotic treatment plan for contagious udder pathogens.
- describe how antimicrobial treatment of contagious udder pathogens can be greatly reduced in herds by using effective mastitis prevention and control programs.
- recognize the futility of using antibiotics to treat cases of contagious mastitis if the cow is to be returned to a herd where she will quickly become reinfected.
Dr. Susan Keller is having a typical day in her active dairy veterinary practice. Her cell phone rings and she speaks with Mr. Oliver McCormick, who owns and operates a dairy farm of about 150 milking cows.
“I’m having some mastitis problems," Oliver says. "You know how we’ve talked before about trying to lower my herd’s somatic cell count (SCC) to get a bonus from the creamery? Well, my last two counts have been over 300,000 cells per milliliter! It’s getting worse instead of better! I think I’d like to treat all the infected cows with antibiotics so that my SCC comes back down."
“It sounds like you may have a mastitis problem going on there. I'll be coming to your place for your routine herd check next Tuesday. Let’s plan on spending some time developing a plan to lower your herd’s SCC. ”
The following Tuesday at Oliver’s dairy farm
After Dr. Keller finishes the routine pregnancy checks, she talks to Oliver about the possible mastitis problem in the herd. She checks both the latest farm Dairy Herd Information Association (DHIA) report and the bulk milk tank SCC (bSCC) report from the milk processor.
Looking at the DHIA report, Dr. Keller reminds Oliver that the SCC for each individual cow is a measure of the degree of subclinical mastitis in that cow. It is estimated that each doubling of the somatic cell count by an individual cow results in the loss of about 1.5 pounds of milk production each day by that cow (6). A farm DHIA report includes reports for each cow and a summary report for the entire herd. Also included are statistics relating to milk production and SCC and a linear SCC score that is a logarithmic transformation of the SCC score. The SCC linear score is related to lost milk production, useful in making management decisions since it reflects how much milk is being lost due to udder infection.
Somatic cell count linear score
A somatic cell count linear score is a log2 transformation of the SCC score. Each one-unit increase in the linear score is associated with a doubling of the somatic cell count and an estimated loss of 1.5 pounds of milk production per day (6).
Purposes of somatic cell counts
- Identify individual cows affected with subclinical mastitis
- Monitor herd improvement after management changes are instituted
- Identify herd mastitis problems as soon as possible
Interpretation of somatic cell count linear score for an individual cow
- 0 and 1: no evidence of subclinical mastitis
- 2 : low suspicion of subclinical mastitis
- 3 and 4: suspect for subclinical mastitis
- ≥ 5: subclinical mastitis
- 7−9: indicates that the cow is shedding extremely high numbers of somatic cells and is on the verge of showing signs of clinical mastitis.
A herd goal should be to have >80 percent of the animals with SCC linear scores ≤3.
As another way to look for evidence of subclinical mastitis in the herd, Dr. Keller checks the bulk milk tank SCC report. A reasonable goal for bSCC is < 200,000 cells per ml. Federal and state regulations forbid consumers from purchasing Grade A milk from dairies that have a bSCC > 750,000 cells per ml. Dr. Keller confirms that the bSCC has been slowly increasing and is now > 300,000 cells/ml.
Common mastitis pathogens (2):
Less common mastitis pathogens:
Contagious Udder Pathogens (2):
Environmental Udder Pathogens (3):
Environmental udder pathogens:
Examples of mastitis pathogens with identified environmental risk factors (3)
Dr. Keller then asks to see the herd treatment records that Oliver maintains. The records indicate that at least 25 percent of the herd was treated for clinical mastitis at least once during each lactation period. Dr. Keller explains that a 25 percent rate of clinical mastitis is typical for most dairy farms and indicates that this herd’s problem with high SCC is likely due to subclinical mastitis and not excessive clinical mastitis.
Dr. Keller: “I see that 30 percent of your cows have an SCC linear score of ≥5. This means that your bSCC isn't coming from just a few cows, rather, many cows are contributing to your high SCC problem. Furthermore, I see that only 54 percent of your cows have a somatic cell count linear score of ≤3. A reasonable goal for you would be a linear score of ≤ 3 for >80 percent of the cows in your herd. The DHIA report also compared each cow’s SCC linear scores from this month with those on last month’s report, and I see that a high proportion of the cows with high SCC linear scores this month did not have high scores last month. This means that they were recently infected and your problem is expanding."
Somatic cell count linear score that shows an increase in current SCC linear score as there is an increase in the previous month
Dr. Keller shows Oliver the chart, pointing out the cow identification numbers in the upper left quadrant that have new infections. These are the cows that had low SCC linear scores last month and high ones this month.
Oliver wants to know: “Are these the cows we're going to treat with antibiotics?”
Dr. Keller says, “You have the right idea, but we only want to treat the cows that have a high likelihood of treatment success. Also, limiting antibiotic use will help prevent antibiotic resistance and thereby preserve the usefulness of antibiotics for the future.
“For one thing, there are some cows in this group that are poor candidates for treatment. For another, we need to know what types of bacteria are causing these infections before we select an antibiotic. For infections caused by the bacteria that we call ‘contagious udder pathogens,’ cows that are cured with an antibiotic treatment will just get infected again unless we make some management changes to prevent cow-to-cow transmission.”
Staphylococcus aureus (4)
- S. aureus can survive inside white blood cells (intracellular).
- It is intermittently shed from the mammary gland, so milk must be cultured repeatedly to be sure a cow is truly negative.
- There is a 75 percent chance of detection with one culture from an infected cow.
- There is a 95 percent chance of detection with three cultures collected on consecutive days from an infected cow.
- In chronic cases, fibrosis and microabscess formation in the udder decrease the probability of adequate drug distribution to the site of infection.
Streptococcus agalactiae (4)
- S. agalactiae colonizes the epithelial surface of teat ducts.
- The udder is the reservoir of infection.
- Toxin elaboration results in inflammation.
- Organisms are shed in the milk in high numbers.
- Antibiotic therapy for lactating cows is highly successful provided the bacteria are not resistant to the antibiotic used.
Dr. Keller continues: “Not only will the culture results give us guidance regarding which antibiotic to use, the culture results can also help identify preventive procedures for your mastitis problem. Our first step will be to culture the cows with the highest SCC linear scores so we can tell which bacterial species are causing your SCC problem.”
Dr. Keller obtains milk samples from all cows with a SCC linear score of over 5 and sends them to the state veterinary diagnostic lab for culture.
The following week, Dr. Keller reports the culture results to Oliver McCormick.
Culture Results of All Cows with a SCC Linear Score >5
|Cow ID||Culture Result|
Dr. Keller explains: “You have several different kinds of bacteria that are responsible for your subclinical mastitis problem. About 55 percent of the cows with high SCC linear scores are infected with Staphylococcus aureus and 30 percent have Streptococcus agalactiae.
“Staphylococcus aureus and Streptococcus agalactiae infections are considered to be ’contagious udder pathogens.’ Infection with these pathogens is usually strongly associated with milking technique, and they are spread from cow to cow. Even if you cure a cow, she will likely be immediately reinfected from one of her herd mates.”
Dry Cow Therapy (4)
- “Dry Cow Therapy” (DCT) refers to a long-acting antibiotic intramammary infusion administered after the last milking of a lactation, and therefore immediately before her “dry period.” The dry period is usually about two months long, and therefore DCT starts when the cow is approximately seven months pregnant.
- Dairies that have successfully controlled mastitis by proper management might only use antibiotic dry cow treatments for those select cows that appear to be subclinically infected with mastitis pathogens, or they may use selective therapy in combination with internal teat sealants.
- Dairy farms with high rates of subclinical and clinical mastitis in early lactation should routinely administer an antibiotic dry cow treatment to all their cattle at drying off.
Dr. Keller says, “The next step is to determine what antibiotic will likely be effective against the strains of Staphylococcus aureus and Streptococcus agalactiae that you have circulating in your herd. We call this profile of antibiotic resistance an ’antibiogram,’ and it can be used for selecting the type of antibiotics that are most likely to be effective. We want to give strong preference to the older antibiotic products that are approved for use in lactating dairy cattle, that do not interfere with antibiotics commonly used for human medicine, and yet should be effective in killing the bacterial strains prevalent in your herd. This will help preserve the efficacy of antibiotics for the future.
“The good news is that you can eliminate these two pathogens from your herd, and with proper management and biosecurity programs, you can keep them out.
“The most important factors for preventing these infections relate to the milking procedure. To make sure your milking equipment is operating properly, I’d like to have your milking equipment checked out by your dealer. Then I'll return when the cows are being milked to evaluate the techniques used in your milking parlor.”
Milking Equipment Performance and Use (5)
- Milking equipment function must be optimal to avoid transferring pathogens from one cow to another.
- Improperly functioning milking equipment can lead to teat end impact, which occurs during milking when milk jets back up the teat canal, potentially carrying pathogens that can establish an udder infection.
- Poorly operating milking equipment can also be traumatic to the teat skin and streak canal, thereby predisposing to infection.
A couple of days later, Dr. Keller returned to the farm during the afternoon milking. The equipment dealer had left a report stating that the milking system was operating at an optimal level.
The following assessment was made of their milking procedure:
1. Premilking stimulation (1=poor, 5= excellent): 4−5
2. Individual towels for each cow? (No, Yes): Yes
3. Teats and lower udder adequately cleaned? (No, Yes): Yes
4. Adequate drying? (No, Yes): Yes
5. Overall evaluation of premilking technique? (1=poor, 5= excellent): 5
6. Post-milking teat sanitation technique (Dip, Spray): Spray
7. Type of product used for post-dip? (None, Bleach, Iodophor): Bleach
8. Overall evaluation of post-milking process? (1=poor, 5= excellent): 3−5
Dr. Keller asks Oliver to use 1 percent Iodophor teat dip rather than a teat spray that often does not cover the backside of the teat.
Once again, Oliver asks the inevitable question: “Now that we know what bacteria are causing the problem, which cows do we treat with antibiotics? Since the cows that are infected with Streptococcus agalactiae are more likely to respond to antibiotic treatment, can we start injecting those first?”
“Yes, we should treat the cows infected with Streptococcus agalactaie, but we should not use an injectable antibiotic,” says Dr. Keller. “The abscess and scarring in the mammary gland reduce the ability of injected drugs from reaching the site of infection. Whenever possible, we should use antibiotics according to their approved label directions, and there are no antimicrobial agents approved for subcutaneous, intramuscular, or intravenous administration for the treatment of mastitis in dairy cattle. Instead, we will use an intramammary antibiotic. With Streptococcus agalactiae infections, we can expect a high cure rate (~90%) using an intramammary antibiotic. There is no need to give injectable antibiotics that may be less effective.”
Possible antibiotic treatments for Streptococcus agalactiae
- Intramammary penicillin
- Intramammary amoxicillin
- Intramammary cephapirin
- Intramammary pirlimycin
- Intramammary ceftiofur
Poor antibiotic choices for Streptococcus agalactiae
- Intravenous oxytetracycline
- Intramuscular oxytetracycline
- Intramammary gentamicin
- Subcutaneous florfenicol
- Intramuscular ceftiofur
Dr. Keller: “The more difficult problem is Staphylococcus aureus. To control S. aureus, we need to identify the cows that have only recently been infected, because these are the best therapeutic candidates. Chronic infections—which we identified earlier based on SCC linear scores—have a much lower likelihood of responding to treatment. Cows that are chronically infected with S. aureus are very difficult to cure by antibiotic treatment because of the microabscesses and scar tissue that this particular organism causes in the mammary tissue. We only want to use antibiotics for those animals that are most likely to respond.
"Most cows that are chronically infected should be culled. If we do not identify the chronically infected cows and just treat all the cows with antibiotics, the chronically infected cows will not be cured and will remain a source of S. aureus for other cows in the herd. Finally, the infected cows that are not culled should be segregated from the rest of the herd and milked last, or with separate equipment, to prevent spread to the rest of the herd. Cows should receive our intramammary treatment. In order to avoid antibiotic residues, we do not want to treat cows that are about to be culled due to low production, old age or other problems. Chronically infected cows are much less likely to respond to treatment and are more likely to become reinfected, so these cows should probably be culled instead of treated. Cows that are late in lactation should be dried off a few weeks earlier than originally planned and given an antibiotic dry treatment, instead of an antibiotic licensed for lactating cows.”
||A picture of a sagittal section of a cow’s udder to demonstrate distribution of a dye administered by intramammary infusion, in the front quarter (nonmastitic, left of red line) or the rear quarter (mastitic, right of red line). The front resulted in widespread diffusion of the dye, while the dye poorly diffused in the rear quarter. The limited diffusion of the dye in the mastitic quarter is due to fibrotic change and abscess formation. This demonstrates why some forms of mastitis are not easily treated with antibiotic intramammary infusions.|
Which cows infected with Staphylococcus aureus (Staph) or Streptococcus agalactiae (Strep ag) are good candidates for antibiotic therapy?
Mastitis Treatment and Prevention
Summary of Mastitis Antibiotic Treatment Guidelines
- Use antibiotics to eliminate an infection, not to just reduce the signs of infection.
- It is important to maintain a sufficiently high antibiotic concentration at the site of infection.
- If not responding well, consider using approved drugs with increased dose frequency and duration of therapy rather than quickly resorting to last-resort, broad-spectrum antibiotics.
- Culture and test the identified organisms for antibiotic resistance by determining the mean inhibitory concentration (MIC), which can also be helpful in determining the correct dosage to administer. If you use systemic therapy, calculate the regimen based on the unique pharmacological parameters of the antibiotic and the MIC of the bacteria you are trying to kill.
Remember that treating cows with antibiotics should be a small part of your mastitis control program. The long-term solution lies in changing your management to prevent transmission. Many herds have completely eliminated both Staphylococcus aureus and Streptococcus agalactiae from their herd by following some simple management procedures.
Prevention and Control of the Contagious Udder Pathogens
1. Proper milking technique in the parlor to prevent teat injury and cow-to-cow transmission of pathogens
- Proper management to prevent cow-to-cow transmission of pathogens by fomites
- Routine use of individual cow, disposable paper towels (versus cloth towels) for cleaning the teat
- Trimming long udder hair
- Automatic backflush system used after each milking
2. Routine use of germicidal teat dip applied after milking
- Iodine—high (1%) and low (0.1–0.25%) concentrations
- Quaternary ammoniums
- Sodium hypochlorites/Clorox—very irritating
- Anionic acids (naturally occurring soaps)
3. Antibiotic dry cow therapy used according to label specifications
4. Treatment of cases of acute clinical mastitis
5. Cull cows with chronic infections
6. Keep infected cows segregated and use separate milking equipment to prevent transmission throughout the herd
Potential causes of teat trauma
- Inappropriate vacuum: A vacuum level of 10.5−12.5 inches at the teat end during peak milk flow offers the best combination of rapid, complete milk removal with a minimal amount of physical harm. (A vacuum gauge should read 15 inches of mercury or 50 kilopascals.)
- Inadequate massage: A cycle refers to the total time in seconds that a pulsator takes to complete one milk phase and one massage phase. The massage phase is the "rest phase" of the milking process, and should not be less than 0.35 sec. The second phase is the "milking phase." Together the two phases are called the pulsator cycle.
- Pulsation ratio: Usually 50:50 or 60:40. The pulsation ratio is the length of time in each cycle that the pulsator is in its milk phase compared to its massage phase.
Key points regarding the use of antibiotics for subclinical mastitis caused by contagious udder pathogens
- Remember that treating cows with antibiotics should be a small part of your mastitis control program. Limiting antibiotic use will help prevent antibiotic resistance and thereby preserve the usefulness of antibiotics for the future.
- Culture milk samples from cows with high somatic cell count linear scores to determine the causative agent and its antibiotic resistance pattern.
- Do not waste antibiotics by treating lactating cows that will soon be culled or dried off.
- Give strong preference to antibiotic products that are approved for use in lactating dairy cattle.
- It is futile to treat infected cows with antibiotics if you do not take management steps to decrease the rate of transmission of contagious mastitis pathogens among the cattle. Otherwise, the cattle you treat today will be reinfected tomorrow.
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