There are a variety of options for cows with chronic high somatic cell counts. You can sell them. You can ignore them and hope they get better. You can culture milk and treat appropriately. You can treat all of them. You can do a combination of things. But what should you do? Do you have a plan? Some farmers do not have any sort of plan and only address cows like this when the herd SCC gets too high. We can do better.
    First, why the heck do cows become chronically high, why do some never show clinical mastitis and why do many show no bacterial growth on milk culture? Dr. Pam Ruegg from the University of Wisconsin-Madison believes these chronic cows are animals with failed inflammatory responses that did not fully clear the organisms from the udder. Cows that are culture negative likely have low levels of organisms, below the detection limit. Clinical mastitis happens when the inflammatory response causes changes in the appearance of the milk, swelling of the udder, or in the case of toxic mastitis, signs of systemic illness.
    Back to the plan: what to do with them? One method is to develop a subclinical mastitis plan for your dairy. It might look something like this: Following test day, subclinical cows are sorted into lists. The first list is new infections. These are cows that were low on previous test, or in the case of fresh cows, low at the last test in the previous lactation, but are high on the current test. The second list is chronic cows. These are cows that have been high for two consecutive tests, or if fresh, high at dry up and after calving.
    Examine the list of new infections. Remove any cows that have been treated for clinical mastitis since the last test since they will be managed by your clinical mastitis plan. You might consider removing cows that are more than about 200 days in milk or more than 150 days pregnant since these animals will be dried up in the near future. There are not many lactation days left to pay for possible improvements in milk production due to treatment, and they will be treated at dry up soon anyway. The remaining animals on this list would be good candidates to examine for signs of clinical mastitis or to take samples for milk culture.
    Culturing the milk only makes sense if you plan to do something with the results. That something might be antibiotic treatment, or it might be culling if your reason for culture was to look for Prototheca, Staph aureus or other contagious organisms. Treating cows with subclinical mastitis is not usually recommended because of poor economics. The biggest reason is because you do not need to discard milk from untreated, subclinical cows since the milk looks normal, and thus, the cost of treatment is greater when the value of discarded milk is considered. However, if your farm typically has excellent milk quality and you need milk to feed calves, there may be no additional cost from discarding milk. A specific treatment plan for subclinical mastitis needs to be developed for your farm by your veterinarian, but consider that some cows infected with some organisms are likely to self-cure, while cows infected with other organisms are more likely to become chronic or show up with clinical mastitis in the future. For example, according to Ruegg, infections caused by E.coli spontaneously cure 80-95% of the time, but infections caused by environmental Streps only cure spontaneously 28-30% of the time. Facts like this are important when devising a treatment plan.
    Now look at the list of chronic infections. First, identify fresh cows that had a high first test and high last test in the previous lactation. Look at their records from the last lactation. Cows that had more three or four consecutive high tests last lactation should be considered as do-not-treat cows. This means that should they get clinical mastitis this lactation, they will not be treated, but milk will just be discarded until the appearance returns to normal. Understand that the appearance of the milk has little to do with infection status, and most cows’ milk will return to normal four to six days after abnormal milk first appeared, treated or not. Cows that have persistent high somatic cell counts are not likely to respond to treatment, so marking them as “do not treat” saves money and hassle. Next, examine cows that have had at least two consecutive high tests. Cows that have four or more high consecutive tests (on herds that test monthly) are not likely to respond to treatment for clinical mastitis, so they should be marked as do-not-treat cows as well. Cows with two or three high tests should be put on a watch list. Once they have three or four consecutive high tests they should be marked as do not treat. Many dairies treat cows with clinical mastitis without looking to see if they have chronically high somatic cell counts and thus treat cows that have little change of responding to treatment. To complicate matters, since milk returns to normal whether cured or not, owners and managers often think treated cows responded when in fact, they did not.
    Part of the subclinical mastitis plan should be monthly monitoring of the big three SCC indicators: New infection risk, percent chronic infections and percent high on first test. They tell the story of udder infections on your dairy, and when combined with monthly rate of new clinical infections and herd SCC, indicate the overall udder health status on your dairy. Subclinical infections are a much larger part of that story than clinical mastitis, so having a subclinical mastitis plan will have an outsized impact on udder health.
    Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minn. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at bennettnvac@gmail.com with comments or questions.