If you are seeing more than 2% cases it is worthwhile trying to work out why. These might be chronic cases of mastitis that are flaring up and-or cows that are acquiring new infections.
One way to differentiate between chronic or new cases is to look at the herd test data for individual cows. By coding each cow at each herd test as either being likely uninfected (<150,000 cells/ml) or likely infected ≥150,000 cells/ml) we can then calculate what proportion of uninfected cows in one herd test become infected by the next (go from <150,000 to ≥150,000 cells/mL between herd tests).
Because herd tests are generally about two months apart, this can be converted to a monthly infection rate by dividing by two. In a well-managed herd, we would expect <3% of heifers and <4% of cows to acquire a new infection each month. You could also look at the cell count history of the individual cows that get clinical mastitis. If there have been one or more herd tests >150,000 cells/mL in the previous or current lactation, it is likely they are chronic infections flaring up, while if the SCC has been consistently <150,000 cells/mL then it is likely they are new infections. Working out whether they are new or chronic infections helps decide on the best preventative regime.
As lactation progresses, the type of bacteria associated with clinical mastitis changes on many dairy farms. Commonly in the first month or so after calving, Streptococcus uberis is the most common bacteria isolated from clinical cases. However, later in lactation, Staphylococcus aureus becomes the most common bacteria in many herds.
S. aureus can occur as either subclinical (ie: high SCC and-or RMT score cases without clinical signs) or clinical mastitis. It is more common in older cows, and in those with chronic infections (ie. those that have had a high SCC at multiple herd tests).
It is a “cow parasite” bacteria that is spread during the milking process by the liners and hands. The most important control measures are to minimise cow-to-cow spread by ensuring infected cows are identified and milked last, and by ensuring teat spraying happens at every milking, using an effective teat spray applied properly.
S. aureus generally has a low cure rate with antibiotic treatment. Research studies in New Zealand and overseas have found the cure rate for S. aureus cases was somewhere between 20 and 30% where conventional duration of therapy was used. Cure rates are lower in older cows, chronically infected animals, cows with multiple glands infected with S. aureus, where teat-end damage is present, and where the isolate is resistant to penicillin. Studies in NZ have found about a third of the S. aureus isolates are penicillin-resistant and require different antibiotics. The only way resistance can be determined is by laboratory testing. However, a clue that resistance might exist is that a higher proportion than normal of treated clinical cases become clinical again within a month of initial treatment. If more than 15-20% of cows recur this should be checked.
Increased bacteriological cure can be achieved by using a longer duration of therapy with the appropriate antibiotic. However, the cost-benefit might become marginal because of milk discard costs.
In some cases creating a three-titter cow and-or culling the animal is the optimal decision. Particularly late in lactation, early dry-off and use of dry cow antimicrobials could be the best option, because the extended period the antimicrobial is above the minimum inhibitory concentration ensures bacteriological cure rates are much higher over the dry period than during lactation.
Clinical mastitis cases occurring in mid to late lactation can be frustrating because of the time taken to find and treat them and also because they are often relatively difficult to cure. Understanding why cases are happening, identifying the best preventative options and choosing the best therapy are all important to minimise the risks and consequences of mastitis at this stage of lactation. Good use of clinical mastitis and herd test records, and of microbiology culture, will help sort these problems out.
• Check out the mastitis module on AgriHQ Academy at academy.agrihq.co.nz to learn more about mastitis.
