From the NMC Newsletter "Udder Topics", December 2000

Source
The sources of Corynebacterium bovis are infected udders and teat canal.

Means of spread
Spread of C. bovis is from cow to cow at milking.

Basic prevention and control measures
Proper postmilking teat disinfection of cows with an efficacious germicidal teat dip will control the spread from cow to cow. The exceptions to this are teat dips with the active ingredient linear dodecylbenzene sulfonic acid. These products do not control C. bovis or coagulase-negative staphylococci.

Miscellaneous information
Corynebacterium bovis primarily colonize the teat canal and are generally considered mildly pathogenic organisms. Corynebacterium bovis are capable of causing occasional udder infections with a mild increase in SCC and slight reduction in milk production. Although rarely the cause of clinical mastitis, C. bovis are sometimes found as the only organism present in milk samples from cows with clinical mastitis. Antibiotic treatment is not indicated. Dry cow therapy is very effective to eliminate C. bovis intramammary infection.

Pure cultures of C. bovis can be obtained from milk samples from cows with subclinical or clinical mastitis. Whether this organism is present after the primary pathogen has disappeared, or is the actual cause of the infection is debatable. Prevalence of C. bovis is very low in herds that practice postmilking teat disinfection with efficacious products. When diagnosing intramammary infections from blood agar, the tendency is to call all corynebacteria C. bovis, except for Arcanobacterium pyogenes, which is extremely hemolytic.

Culturing comments
Corynebacterium bovis appear as creamy, grey or white, opaque, dry powdery, and small (about 1 mm in diameter) nonhemolytic colonies on blood agar. Scant to no growth is visible after 24 hours incubation at 37 C, but colonies are apparent after 48 hours incubation. When a loop is used to spread the milk sample, colonies are clustered in the primary portion of the streak where milk fat is concentrated. When a cotton swab applicator is used instead of a loop, colonies are widespread over the plate. Corynebacterium bovis are catalase positive. Microscopic examination of culture growth show short, pleomorphic, Gram-positive, nonsporeforming rods, although the shape may range from spherical to oval. Palisade or “picket fence” alignment of cells is common.

Source: NMC Publication "Laboratory and Field Handbook on Bovine Mastitis" (1999) pg. 135.