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.
Staph aureus can occur as either subclinical (i.e. high somatic cell count and/or rapid mastitis test score cases without clinical signs) or as clinical mastitis. It is more common in older cows, and in those with chronic infections (i.e. that have had a high somatic cell count at multiple herd tests).
Staphyaureus is a “cow parasite” bacteria that is spread during the milking process via liners and hands. Hence, the most important control measures are to minimise cow to cow spread by ensuring that infected cows are identified and milked last and by ensuring that teat spraying occurs at every milking using an effective teat spray applied properly.
Staph aureus is difficult to cure. Research studies in New Zealand and overseas find that the cure rate for staph cases is somewhere between 10% and 30% where “conventional” duration of therapy is used. Cure rates are lower in older cows, chronically infected animals, cows with multiple glands infected with Staph aureus, where teat end damage is present, and where the isolate is resistant to penicillin. Studies here in New Zealand have found that about a third of the Staph aureus isolates are resistant to penicillin and require the use of different antibiotics. The only way that resistance can be determined is by laboratory testing. However, a clue that resistance may be present is that a higher proportion than normal of clinical cases that are treated become clinical again within a month of initial treatment (i.e. if more than 15% to 20% of cows recur this should be checked).
Increased bacteriological cure can be achieved by using longer duration of therapy with the appropriate antibiotic. However, the cost-benefit of doing so may become marginal due to milk discard costs.
In some cases creating three-titter cows and/or culling the animal is the optimal decision. Particularly late in lactation, early dry off and the use of dry cow antimicrobials may be the best option. The longer the antimicrobial is above the minimum inhibitory concentration during the dry period (relative to during lactation), the higher the bacteriological cure rates .
Cognosco is shortly commencing a study to assess three different treatments for clinical Staph aureus cases. This study will involve collection of milk samples before treatment for bacteriological culture and then re-sampling of those glands from which Staph aureus was grown prior to treatment.