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Streptococcus agalactiae 
Mastitis in Dairy herds


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Identification, Control and Eradication of Streptococcus agalactiae Mastitis in Dairy Herds

Background:  Streptococcus agalactiae is a gram positive obligate pathogen that affects pre-milking heifers, as well as older cows in dairy herds.  It is considered one of the major causes of economic losses to dairy producers without a control program.

  Although Streptococcus agalactiae can live outside the udder for short periods of time in the right conditions, it is considered to be an obligate pathogen of the udder.  A high percentage of cows may be affected in herds where control procedures are not implemented.  Fomites such as strip cups, towels, milkers’ hands, cross suckling calves, milking machines and other milking equipment and unsanitary conditions are all potential sources of infection in cows.  Even multi-use and hand- mixed antimicrobial mastitis preparations can be a potential source of infection for the udder.

  Streptococcus agalactiae may be transmitted from udder to udder in many ways.  Streptococcus agalactiae breaks the natural barriers of the udder, enters the teat canal, and ascends in the milk through the quarter.  The bacteria penetrates the acinar epithelium, causing edema and extravasation of neutrophils into the lumen, resulting in subclinical or clinical mastitis as well as possible systemic infection.  In later stages, the acini become filled with scar tissue which plugs the glandular-ductal system resulting in a chronic, smoldering infection which decreases milk production and increases the somatic cell count (SCC) of the quarter. Poor udder health due to Streptococcus agalactiae is slowly progressive over time, causing fibrosis and atrophy of the affected quarter.  As a matter of fact, an individual cow with a high SCC typically has lower production that correlates with increased SCC of the herd.

Identification of Streptococcus agalactiae

  As with other causes of mastitis, Streptococcus agalactiaemay cause heat, pain and swelling of the udder as well as abnormal milk consisting of white to yellow clots and flakes.  On closer examination, damage to the teat end may be apparent and can be the result of trauma, improper milking procedures and equipment or freezing.  Poor udder health may be indicators for cows with an infection of the mammary tissue.

  Subclinical mastitis is difficult to detect on visual examination of the milk.  The California Mastitis Test (CMT) is a quick and easy way to identify chronic subclincal cases of mastitis, but is not specific for only this pathogen.  As for clinical cases, the Hymast kit can give results of gram positive (most likely contagious; Staphylococcus, Streptococcus, etc) or gram negative (most likely environmental; E. coli, Klebsiellaspp., etc.) organisms in 12 hours and is used frequently to aid in the diagnosis and treatment protocol of the mastitis.  The most confirmative diagnosis for contagious mastitis is by the isolation and identification of the causative organism.  Culture for Streptococcus agalactiae from the bulk tank with high CMT, Hymast and individual Dairy Herd Improvement Association (DHIA) SCC or linear scores, can identify those cows in the herd that may be infected and/or chronic shedders of Streptococcus agalactiae.  The CAMP test, using a nurse streak of Staphylococcus aureus used to be the gold standard to identify Streptococcus agalactiae; however, it has now been replaced by the Latex agglutination test with high specificity and sensitivity.  Most of bacterial culturing and testing is still done in diagnostic laboratories; however, the practitioner in the field can easily use DMT, Hymast and DHIA records in conjunction with laboratory results during mastitis evaluation.

Control and eradication of Streptococcus agalactiae

  Streptococcus agalactiae mastitis can be eradicated effectively from dairy herds.  The loss of production due to poor udder health associated with Streptococcus agalactiae greatly exceeds the cost of implementation of some simple control measures.

  All cows identified as infected with contagious organisms should be grouped together and milked last in the milking facility.  These animals potentially are a source of infection to non-infected cattle.  Also, newly acquired animals should be housed and milked separately to prevent spread of the organism.  Animals with Streptococcus agalactiae mastitis usually have an excellent response to the use of intra-mammary antibiotics, penicillin in particular.  The cost of the antibiotic and milk withheld from the bulk tank are far less than the long-term expenses associated with herd infection with Streptococcus agalactiae.  However, if the cow is late in her lactation, it may be efficacious to wait until the dry period and use a commercial dry cow therapy (slow-release, wide spectrum antimicrobials).  Often, dry cows are neglected and are rarely evaluated during the dry period predisposing them to udder infections.  Intra-mammary infusion of antimicrobials following the last milking of the lactation (dry cow therapy) can greatly reduce the cases of Streptococcus agalactiae mastitis by eliminating existing infections and controlling new infections early in the dry period.

  Pre-dipping and post-dipping the teat in conjunction with single towel use per animal is effective in the control of all contagious mastitis including Streptococcus agalactiae

infections.  Pre- and post-dippings will help kill any of the organisms that would normally be transmitted into the udder during milking.  Also, regular and thorough cleaning of the facilities and milking equipment with disinfectants will aid in eradication of the organism.  Good hygiene always promotes milk production.

  Heifers should also be considered in the eradication of Streptococcus agalactiae.  Heifers that consume infected milk, whether from the teat or bucket feed unpasteurized milk, are a potential source of infection to their penmates or other cows through transmission of the organisms by mouth.  This may explain why some heifers become infected prior to ever being milked.  Housing these heifer calves separately will easily correct this potential problem.

  Implementing these and other sanitation procedures described above can greatly minimize the economic impact due to Streptococcus agalactiae infections.  These procedures are easily implemented and very cost effective.  Launching this type of program will be a benefit to the producer economically, as well as to the consumer who will be receiving a more sanitary product.

-by Mark James, Class of 2002

-edited by Dr. ChingChing Wu, Head of  Bacteriology, ADDL


1.  Carlton and McGavin: 1995.  Thompson’s Special Veterinary Pathology, 2nd ed. 537-541.

2.  Howard and Smith: 1999. Current Veterinary Therapy, Food Animal Practice, 4th ed.,390-391.

3.  Jones, Hunt and King: 1997.  Vet Path, 6th ed., 1190-1191

4. Morin, Dawn: 1999. Treatment of Clinical Mastitis: Historic and New Perspectives. Proceedings of the 32nd annual Convention of the American Association of Bovine Practitioners. 33-38.

5.  Pasquini, Chris: 1996.  Guide to Bovine Clinics. 192-193.

6. Radostits, Otto,et al: 2001  Herd Health: Food Animal Production Medicine, 3rd ed. 397-429.

7.  Radostits, Otto M., et al: 2000.  Vet Med: A Textbook of the Disease of Cattle, Sheep, Pigs, Goats and Horses, 9thed.  603-686.

8,   Sears, Philip M: 1999.  Management Alternatives in Mastitis Control and Eradication Programs.  Proceedings of the 32nd annual convention of the AABP.  29-32.

9.  Smith, Bradford: 2002.  Large Animal Internal Medicine.  1020-1026.

10.  The Merck Veterinary Manual, 8th ed. 110-115.


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