Making a plan for chronically high SCC cows

Number 1399. She is high again on the monthly somatic cell count test. This is the fourth time. She is now what our vet clinic considers a chronic cow. What are you going to do with her? Sell her? Ignore her? Treat or culture her? Let’s make a plan.

Some farmers do not have any sort of plan and only address cows like this when the whole herd SCC gets too high. Subclinical infections are a much larger part of the SCC story than clinical mastitis, so having a subclinical mastitis plan will have a large impact on udder health and farm economics.

First, why do cows become chronically high, never show clinical mastitis and show no bacterial growth on milk culture? Researchers believe chronic cows are the result of animals with failed immune responses that did not fully clear the organisms from the udder. It can happen with any mastitis pathogen, although I find the Streptococcus species can be a common cause of chronic cows. Chronically high SCC cows that culture negative probably have such low levels of organisms that we cannot detect them on culture plates. Perhaps detection would be better with mass spectrometry at a diagnostic lab, but this is less practical and more expensive to implement than on-farm (or vet clinic) culture. Remember, subclinical mastitis is different than clinical mastitis, which is when the inflammatory response causes changes in the milk or udder or shows signs of systemic illness.

Should all subclinical cows be treated? I don’t recommend blanket treatment of subclinical mastitis. Because you do not need to discard milk from untreated subclinical cows since the milk looks normal, the cost of treatment becomes greater than the value of discarded milk.

So, back to the plan: what to do with high SCC cows? Since Dairy Herd Improvement Association test day generates several lists of cows based on SCC, I use the following method. (Note: The following suggestions assume that DHIA testing is performed monthly. I use a similar method for herds with less frequent DHIA testing with some alterations.)

The first list is new infections. These are cows that were low on the 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.

Examine the list of new infections. Remove any cows that have been treated for clinical mastitis since the recent test. Consider removing cows more than about 200 days in milk or more than 150 days pregnant, since these animals will be dried up soon. 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. If you utilize selective dry cow treatment, ensure that cows close to dry off who show up on the new infection list receive intramammary antibiotics and teat sealant. The remaining animals on this list would be good candidates to examine for signs of clinical mastitis or to take samples for milk culture. Use the California Mastitis Test paddle test if you are unsure of the quarter affected.

The second list is chronic cows. These are cows that have been high for four consecutive tests or, if fresh, high at dry up and again for multiple test days after calving.

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 three or four consecutive high tests last lactation should be considered as do-not-treat cows, except if they are toxic. This means they should get clinical mastitis this lactation; they will not be treated, but milk is 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 4-6 days after abnormal milk first appeared, treated or not. Cows that have persistently high SCC are not likely to respond to treatment, so marking them as “do not treat” saves money and hassle. Cows that have four or more consecutive high tests in their current lactation are not likely to respond to treatment for clinical mastitis, so they should be marked as do-not-treat cows as well.

Cows with three high tests should be put on a watch list. Once they have four consecutive high tests, they should be marked as do not treat. Many dairies treat cows with clinical mastitis without looking to see if the cows have chronically high SCC and thus treat cows that have little chance 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.

On some farms, I will note the watch cows with three high tests on their cow cards with “Wait and see.” In some situations, I will also make a list of first-lactation cows with three high test days to CMT and culture in hopes of cleaning these young cows up for subsequent lactations, though I don’t currently track the success of this method.

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.

Megan Weisenbeck is one of six veterinarians at Northern Valley Livestock Services in Plainview, MN. She practices primarily dairy production medicine in Minnesota and Wisconsin. Megan can be reached at [email protected].