Making decisions about cows with clinical mastitis is one of the most challenging things that dairy producers have to deal with. What treatment should be used? How long should treatment be applied? Should all cows be treated the same way?
In fact, we know that not all cases of clinical mastitis are the same, and thus, shouldn't all be treated the same way. Some cases are short-lived and seem to clear up on their own, while others persist for weeks or may recur several times throughout the lactation. In reality, most cases will appear to clear up within 4 to 6 days no matter what you do. But without intervention, a significant percentage of those cases will become subclinical, meaning that the udder is still infected but no longer showing any symptoms of infection. These subclinical cases contribute high SCC milk to the bulk tank, impair the productive capacity of the udder, pose a risk of transmitting the infection to other cows, and are likely to flare up into clinical mastitis again in the future.
One of the factors that affects whether a case of mastitis is likely to cure (either spontaneously or with antibiotic treatment) or become a chronic infection is the type of bacteria causing the infection. Certain mastitis pathogens are more difficult to cure than others. For example, cows infected with Staph aureus typically have very poor cure rates (20 to 30%) following antibiotic treatment; whereas, a cow infected with a mild case of E. coli mastitis has a 70% or greater chance of cure even without antibiotic therapy.
When milk from a cow with clinical mastitis is cultured to identify the bacteria causing the infection, no bacterial growth is found in 20 to 40% of cases. A small percentage of these cases may be caused by bacteria that do not grow in routine culture conditions (such as Mycoplasma). However, in the majority of these "No Growth" cases, the cow's immune system was able to cure the infection by the time the milk sample was collected. Clearly, there is no need to use antibiotics when no bacteria are present, and this could be 20 to 40% of cases in your herd!
The goal of mastitis treatment should be to reserve antibiotics for infections that are most likely to benefit from treatment, and to employ other management strategies for infections that are not likely to benefit from antibiotic treatment. In particular, knowing what pathogen you are dealing with can help you make the best treatment or management decision for each case of mastitis. On-farm culture is a quick, inexpensive way to get this information. Here's how on-farm culture works:
- A milker identifies a cow with clinical mastitis.
- A sterile milk sample is carefully collected from the affected quarter.
- The milk sample is swabbed onto a culture plate.
- The culture plate is incubated for 18 to 24 hours at 98°F.
- The culture plate is examined and the results are recorded.
- A manager or other worker decides how to treat the cow based on the culture results.