Mastitis: Streptococcus agalactiae in Cows (Bovis) | Vetlexicon
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Mastitis: Streptococcus agalactiae

ISSN 2398-2993

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Synonym(s): somatic cell count, SCC, Strep


  • Cause: Streptococcus agalactiae.
  • Signs: mild to severe mastitis (subclinical disease common).
  • Diagnosis: bacteriology of milk samples.
  • Treatment: antibiotics.
  • Prognosis: good.

Presenting signs

  • Most commonly a subclinical infection resulting in a dramatically increased somatic cell count.

Acute presentation

  • May cause transient clinical signs which are often short-lived.

Geographic incidence

  • Worldwide although it is rarely diagnosed in UK dairy herds.

Age predisposition

  • All milking cows.

Public health considerations

  • Streptococcus agalactiae Streptococcus spp also causes neonatal septicaemia in humans.
    • Human infection is generally acquired from other human sources, although there may be some risk associated with direct exposure to infected animals or their products.
  • Although some surface antigens seem to be specific to the cow, Streptococcus agalactiae is now recognized to be part of the normal bacterial flora in the human throat, genitourinary tract, and rectum.
  • Mastitis is often the main reason for antibiotic use on dairy farms and better control of mastitis in general is, therefore, crucial if we are to reduce the requirement for antibiotics and reduce the risk of antimicrobial resistance.

Cost considerations

  • Mastitis is the most costly infectious disease affecting dairy cattle, accounting for 38% of the total direct costs of the common production diseases.


Predisposing factors


  • Purchasing infected cows.
  • Partial/no use of antibiotic dry cow therapy Dry cow therapy: overview.
  • Partial/no use of post-milking teat disinfection.
  • Poorly maintained milking machine.
  • Unhygienic parlor practices.


  • Streptococcus agalactiae Streptococcus spp is a highly contagious obligate pathogen of the bovine mammary gland.
  • Infected cattle function as reservoirs of infection.
  • The udder is recognized as the only likely source of the organism in the milk. Consequently, all isolates in the bulk tank are usually assumed to have come from the udder.


  • Streptococcus agalactiae has the ability to adhere to the mammary tissue of cows and the specific microenvironment of the bovine udder is necessary for the growth of the bacterium.
  • The virulence of various strains is related to differences in their ability to adhere to the mammary epithelium.


  • S. agalactiae is a highly contagious obligate parasite of the bovine mammary gland.
  • It generally does not survive for long periods of time outside of the mammary gland.
  • Teat skin and any fomite such as milkers’ hands and milking equipment can be heavily contaminated.
  • The teat canal is the only route of entry into the gland for these bacterial pathogens. As a result, the primary method of transmission is via contact between susceptible, uninfected quarters with fomites contaminated with infective milk.
  • Consequently, the milking time is the primary risk period.


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Further Reading


Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Keefe G (2012) Update on control of Staphylococcus aureus and Streptococcus agalactiae for management of mastitis. Vet Clin North Am Food Anim Pract 28, 203–16 PubMed.
  • Huijps K, Lam T J & Hogeveen H (2008) Costs of mastitis: facts and perception. J Dairy Res 75, 113–20 PubMed.
  • Bradley, Andrew J, Leach K A, Breen J E, Green Laura E & Green M J (2007) Survey of the incidence and aetiology of mastitis on dairy farms in England and Wales. Vet Rec 160 (8), 253-258 PubMed.
  • Kossaibati M A & Esslemont R J (1997) The costs of production diseases in dairy herds in England. Vet J 154, 41–51 PubMed.
  • Keefe G P (1997) Streptococcus agalactiae mastitis: a review. Can Vet J 38, 429–437.
  • Bartlett P C, Miller G Y, Lance S E & Heider L E (1991) Use of bulk tank and milk filter cultures in screening for Streptococcus agalactiae and coagulase-positive Staphylococci. J Food Protect 54, 848-851.
  • Erskine R J, Eberhart R J, Hutchinson L J et al (1987) Herd management and prevalence of mastitis in dairy herds with high and low somatic cell counts. JAVMA 190, 1411–6 PubMed.
  • Farnsworth R J (1987) Indications of contagious and environmental mastitis pathogens in a dairy herd. Proceedings 26th Annual Meeting of the National Mastitis Council 26, 151–5.
  • Wanger A R & Dunny G M (1987) Identification of a Streptococcus agalactiae protein antigen associated with bovine mastitis isolates. Infect Immun 55, 1170-1175 PubMed.
  • Gonzalez R N, Jasper D E, Bushnell R B & Farver T B (1986) Relationship between mastitis pathogen numbers in bulk tank milk and bovine udder infections in California dairy herds. JAVMA 189, 442-445 PubMed.
  • Wanger A R & Dunny G M (1984) Specific agglutination of Streptococcus agalactiae from bovine mastitis by casein components of bovine milk. J Dairy Sci, 67, 2441-2445 PubMed.
  • Guterbock W M & Blackmer P E (1984) Veterinary interpretation of bulktank milk. Vet Clin North Am Large Anim Pract 6, 257–68 PubMed.
  • Jain N C (1979) Common mammary pathogens and factors in infection and mastitis. J Dairy Sci 62, 128-134 PubMed.
  • McDonald J S (1977) Streptococcal and Staphylococcal mastitis. JAVMA 170, 1157-1159.
  • Mhalu F S (1976) Infection with Streptococcus agalactiae in a London hospital. J Clin Pathol 29, 309-312 PubMed.