"Contagious" mastitis pathogens

Causative agents of contagious masitis

The following bacteria are traditionally classified as "contagious" pathogens in mastitis:

  • Staphylococcus aureus: is typically known for causing subclinical and chronic cases of mastitis that are difficult to treat.
  • Streptococcus agalactiae may also cause chronic subclinical mastitis, causing high cell counts and even an increased bacterial count in the bulk tank
  • Mycoplasma: usually responsible for mastitis with a marked decline in milk production.
  • Corynebacterium bovis causes only a slight increase in somatic cell counts. They are indicative of the success of udder disinfection at milking.
  • β-haemolytic streptococci (e.g. Streptococcus dysgalactiae): clinical and subclinical mastitis with deterioration of the milk quality.

Staphylococcus aureus

Gram-positive cocci that colonise the skin and the areas of hyperkeratosis that may be present on the teat. They usually cause subclinical and chronic mastitis, although the variability of the strains also allows other forms of presentation (even peracute gangrenous mastitis in ewes).

Staphylococcus aureus

Recently purchased cows can be a source of transmission. In heifers, the appearance of infections before calving is possible due to intersuckling. Flies play also an important role as vectors in the transmission and spread of the disease.

Identification of cows with mastitis caused by S. aureus can be obtained by microbiological culture. Somatic cell count may be useful to identify those animals from which to take a bacterial sample, yet an increase in the SCC is not observed in all animals.

The treatment of mastitis caused by S.aureus is complicated. Staphylococci protect their colonies with exopolysaccharide capsules. Those hinder both the action of antimicrobial drugs and the immunisation by vaccines. Another survival strategy of S.aureus is its capacity to live inside macrophages and epithelial cells. Finally, some S.aureus strains have developed the capacity to neutralise penicillin by producing a penicillinase (or betalactamase). Strangely, this not only decreases cure rates after treatment with penicillin: for some reason these strains seem to be hard to cure at all, no matter which antibiotic is used and for how long it was used. Penicillinase resistance is therefore a valuable prediction parameter for bacteriological cure. Cows infected with a resistant strain should be considered for culling.

Prevalence of staphylococci in dairy herds

Normal herds

S. aureus



18 %

14 %


15 %

23 %

Herds with a high prevalence of S. aureus

S. aureus



40 %

11 %


28 %

24 %


Streptococcus agalactiae

Streptococcus agalactiae

S.agalactiae are Gram-positive cocci that can only survive inside the udder. Their life expectancy in the exterior does not exceed 10 to 15 minutes, although they can survive longer in dirty milking equipment or in puddles of milk. They cause subclinical and clinical mastitis, in addition to increasing the number of bacteria in the bulk tank. There are several strains that can be differentiated by the different types of haemolysis they cause.

Transmission between animals occurs via the milking unit or the equipment used for during milking (cloths, hands). The introduction of infected animals is the trigger for the transmission of the disease in a herd.

Although treatment with ß-lactam antibiotics is usually successful, there are strains with problematic protective capsules and adhesins.


This contagious pathogen can be found in the mammary gland as well as in the respiratory tract, joints and urogenital area. It has a high prevalence in certain geographical areas such as the USA and China. It usually causes mastitis associated with a drop in milk production and often affects more than one quarter of the udder.

The disease enters the farm when infected animals are purchased. Transmission occurs by intramammary route as well as by airborne and oral route.

Mycoplasma does not grow on normal culture media (blood agar); the only way to obtain isolates is to use PPLO selective media for this type of microorganisms.

There is no existing efficacious treatment for Mycoplasma, so management in problem herds should focus on segregation and culling in order to eradicate the disease.

Corynebacterium bovis

These are Gram-positive bacilli that can be found on the skin and in the teat canal. They have always been considered as a secondary pathogen, as they only cause mild increases in cell counts. They are associated with failures to disinfect the udders properly, especially after milking.

Several species are included in the C. bovis group (C. equi, C. murium, C. ovis, C. suis, C. renale). Some of them have a more important repercussion on the control of cases of mastitis, since they act as true contagious agents, with significant increases in cell counts and a strong persistence of the infection. Their count at bulk tank level is an indicator of the degree of disinfection of the udders.

β-haemolytic streptococci

Streptococcus equi subspecies zooepidemicus, Streptococcus dysgalactiae subspecies equisimilis and Streptococcus canis are included in this group. They cause clinical and subclinical mastitis with deterioration of the milk quality, without affecting the overall health of the affected animals.

They are transmitted from one animal to another exclusively through infected milking equipment, although microorganisms of this type have been isolated in bedding materials.

Treatments are less effective than against S. agalactiae, but acceptable results can be achieved with penethamate and/or penicillin (75 % of success).

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