Mastitis Basics

Categories of Mastitis

Mastitis can be divided into:

  • Clinical and subclinical
  • Contagious and environmental
  • Lactation and dry period infections


Clinical mastitis is when there are obvious changes to the udder tissue and / or the milk.  The milk may appear watery or full of clots, but will have an abnormal secretion.  The udder may appear normal or may be hot and swollen.  Depending on the severity of the infection, the cow may appear normal and behave accordingly or toxic mastitis can occur.

Subclinical mastitis is when there are no visible changes to the milk.  A milker will be unable to identify subclinical mastitis unless he carries out tests, such as individual cow cell counts, or the California mastitis test.  The cell count of quarters with subclinical mastitis will always be elevated above 200,000.

Contagious mastitis: the udder and teats are the reservoir of infection.  Transmission occurs during the milking process from udder preparation by contaminated hands, udder cloths and liners.  Infection establishes on the teat and teat canal.  Bacteria may then penetrate the mammary gland.  Most infections are subclinical and result in raised cell counts.  Control measures include post milking teat disinfection, dry cow therapy and culling.  Contagious bacteria include Staphylococcus aureus, Streptococcus agalactiae and dysgalactiae.  If your herd cell count is over 200,000 then this indicates that there is a problem with contagious mastitis.

Environmental mastitis: the environment is a reservoir of infection.  Infection is transmitted on to the teats between milkings or during udder preparation.  Organisms are forced up through the teat canal during the milking process or after milking if cows are allowed to lie down immediately following milking.  Most infections cause clinical mastitis.  Subclinical infections are less common.  Environmental mastitis is controlled by provision of a clean environment, adequate accommodation for cows, milking through a correctly functioning machine, good udder preparation and pre- milking teat disinfection.  Environmental organisms include E coli, Streptococcus uberis (straw bedding), Klebsiella (sawdust and shavings) and Bacillus.

Lactation period infection is when the bacteria enter the udder DURING lactation. For example this might be from spread of Staph aureus via the liner or an environmental infection of cows are lying in dirty beds.

Dry period infections are picked up when the cow is not lactating. These will almost always be environmental, E. coli and Strep uberis, as the risk of spread of contagious mastitis will be miniscule as the cow is not being milked.

BacteriaStaph aureus
Strep uberis
Strep agalactiae
Strep dysgalactiae
Strep uberis
Source of infectionUdderEnvironment
Type of mastitis seenSub clinical all
Clinical [S. uberis & S. aureus]
Clinical & Sub clinical for Strep uberis only
Time of transmissionMilkingDry period
During milking
Between milking
Infection timeLactationLactation & dry period
Effect on herd cell countRaisedNone unless Strep uberis problems
Common control measuresDry cow therapy
Post milking teat dipping Culling
Good hygiene during milking
Regular liner changes
Milking machine function
Clean environment
Internal teat seal
Clean udders and teats
Pre dipping
Milking machine function
Encouraging cows to stand after milking
Milking machine function