Strep Ag. Mastitis

Where Do They Live?

Only in the udders of infected cows or heifers.

How Is Infection Spread?

During milking via hands, equipment and towels used for milking preparation.


Subclinical (no changes in the milk, or signs in cow).


Culture of milk samples. California mastitis test (CMT). Somatic cell counts (SCC) on individual cows.


Reduced production.


Always sensitive to penicillin.

Estimated Cure Rates: Good

Antibiotic treatment during lactation - 80-100% Antibiotic treatment at dry-off time - 80-100%

Preventative Measures

  • Use separate paper towels for prepping each cow.
  • Dip the teats of all cows post-milking with a licensed teat dip
  • Milk infected cows last or with a separate unit.
  • Dry treat all cows.
  • Purchase uninfected cows or heifers.

For more information:
Toll Free: 1-877-424-1300
Author: Ann Godkin - Veterinary Scientist, Cattle/OMAFRA; Doug Dulmage - Dairy Farmers of Ontario; Mark McDougall - Dairy Farmers of Ontario
Creation Date: 17 March 1998
Last Reviewed: 17 March 1998