Most mastitis cases are caused by environmental bacteria such as E. coli and Strep uberis. E coli multiply and eventually die, releasing a toxin, and it is this toxin that causes the mastitis signs. In many E. coli cases, the bacteria are long gone by the time clinical mastitis is seen. Research shows that there is no benefit from using antibiotics in mild E. coli cases. It is important to use antibiotics with Strep uberis cases.
Some herds culture clinical cases to decide whether antibiotic treatment is appropriate, and if so, which treatment regime should be followed. In the future, with improved diagnostics, we might have a simple cowside test that will identify mastitis pathogens quickly and accurately, which would be extremely useful. Why use antibiotics if they are not necessary?
Two types of cure
A bacteriological cure is when the bacteria have been eliminated. A clinical cure is when the milk returns to its normal appearance and any clots or discoloration disappear but not all the bacteria may have been eliminated. The aim with successful mastitis treatment is to get a bacteriological cure, but which do we get first?
Think about someone who has pneumonia and is put on a course of antibiotics. At the end of this treatment, the infectious agents will be killed, but the person can continue to have a cough for many months. In this case, there was a bacteriological cure first followed by a clinical cure later.
The same happens with mastitis. There can be significant damage to udder tissue, which manifests as clots that continue to be seen after a bacteriological cure has occurred.
Many cows are treated for three days, with some treatments being extended beyond the data sheet recommendations, which should only happen under veterinary guidance. Intramammary preparations are all licensed. All treatments must prove that they are effective and kill the bacteria for which they are indicated using the treatment regime on their data sheet. If intramammary “Tube A” states that you need three tubes 12 hours apart, then that is all that should be needed to have a successful outcome.
Some farmers continue to treat cows until all clots have disappeared and the milk has returned to normal, but this is often overtreatment. Years ago, I was involved in a medicine trial for mild cases of mastitis. We asked farmers to stop treating cows at the end of the recommended treatment . . . in this case, a 36-hour treatment . . . and wait and let the milk return to normal. This occurred within 24 to 48 hours in all cases.
Remember, the antibiotic has not disappeared completely once you stop treatment. We know this because there is a milk withdrawal period to allow the antibiotic to be cleared from the udder.
It is important to know the bacteria causing mastitis on your farm. You do this by collecting sterile pretreatment milk samples for bacteriology. This should be done on a regular basis for two reasons: First, to fine-tune control measures, and second, to reduce disease and to ensure that the treatments used suit your bugs.
Every farm is different. You may well have two or three bacteria responsible for mastitis. Knowing your mastitis bugs will allow your vet to advise on the best treatments and indicate what sort of cure rates you can expect.
How does one assess mastitis treatment success? There are five ways.1. Recurrence. There is a problem if you have a high incidence of recurrent cases. Recurrence is where you get mastitis in the same quarter in the same lactation. For example, this could be a cow with mastitis in the back left quarter on January 1 and again on March 1. You should have less than 10% recurrent cases.
Recurrence often occurs with Strep uberis and Staph aureus infections. Clinical cases can become subclinical with a raised somatic cell count (SCC). Recurrence can occur if you are using the incorrect treatment. There can also be problems if there is a delay between when mastitis occurs and when treatment starts. This is typically in herds with poor mastitis detection.
2. High SCC after treatment. If a cow had a low SCC before treatment, ideally, she should not have a high count after treatment. If she does, it suggests that she has developed subclinical mastitis. If this is a common problem, it should be investigated further. Be aware that it can take as long as six weeks for the cell count of an affected quarter to drop down below 400,000 cells per milliliter (mL) after a case of Strep uberis or Staph aureus clinical mastitis.
3. Bacteriology after treatment. In an ideal world, you would carry out bacteriology testing before and after every treatment to show that bacteria have been eliminated. The best advice is to wait about a week after the end of the milk withdrawal period, as if there are any traces of antibiotic present, they will stop bacteria growing on agar plates. Polymerase chain reaction (PCR) testing can be carried out at any time as this is identifying deoxyribonucleic acid (DNA) from bacteria.
4. Long duration of treatment. If you need long courses of treatment, then this would suggest that there are problems that should be investigated. There may be times when your veterinarian might recommend extended treatments, but this would be only under certain circumstances and not for every case. You can carry out PCR testing on non responsive cases during treatment to check out the bacteria, as the presence of antibiotics does not interfere with this test.
5. Lost quarters. Some bacteria, in particular E. coli, can result in a quarter drying up because of udder tissue damage. These quarters often return to production in the next lactation.
Track the cows
Mastitis records are a useful way of assessing treatment success. If you have DHIA (Dairy Herd Information Association) and individual cell count records, this is even better. Using these two sets of data together along with bacteriology results allows a good assessment of treatment success.
Now is a good time to assess your mastitis treatments and success rates with your veterinarian and see how this can be improved. How effective is your treatment? Is it as good as you would expect for the bacteria in your herd?
