Looking for Staph mastitis in the right places.
Dairy Pipeline: March 1999
Gerald M. (Jerry) Jones
Extension Dairy Scientist, Milk Quality and Milking Management
Although much of the recent emphasis on mastitis prevention has been on clinical mastitis caused by enviromental streptococci and coliform, many herds continue to have problems with subclinical and recurrent Staph aureus infections that seldom are clinical. At its 1999 Annual Meeting, the National Mastitis Council revisited Staph aureus mastitis.
Scientists from Italy (Zecconi and Piccinini) described how these bacteria become more and more adapted to the host and that, although numerous strains of Staph aureus were identified in a herd, 1-2 strains may survive. Consequently Staph aureus infections may be found when SCC are less than 200,000, particularly in early lactation and such infections may become chronic but still subclinical.
Dr. Jerry Roberson, from the Virginia-Maryland Regional College of Veterinary Medicine at Virginia Tech, explained that the most important reservoir of Staph aureus is an infected udder and the infection is primarily transmitted from cow to cow during milking. Hands of milkers are an important site and the presence of Staph aureus on workers' hands has been found between milkings. According to Dr. Roberson, milking time hygiene alone was insufficient in controlling the disease.
Staph aureus infections have been isolated from bred heifers and dry cows. Roberson reviewed evidence that this can spread Staph aureus infections. The control program described by the Italian scientists included segregation of infected cows and milking this group last (which many Virginia dairy herds claim they can't do), diagnosis on quarter milk samples, sampling after calving, and choosing dry cow treatment based upon sensitivity tests.
It is my opinion that if your SCC is above 200,000, you should: (1) Use the DHI SCC to identify cows with subclinical mastitis and use the CMT paddle to isolate which quarter(s) to sample; (2) sample all heifers and cows at 7 and 14 days after calving (you can freeze samples for as long as 6 months, and ask the culturing lab to plate 0.1 ml of milk rather than the customary 0.01 ml.); (3) wear nitrile gloves when sampling cows but also when milking; (4) isolate Staph aureus infections at milking time either by segregation and milking last or backflushing milking units; and (5) select dry cow treatment that infections in your herd are susceptible to.