Pathogens - The cause of mastitis

Origins of infection: contagious and environmental

Disease-causing bacteria are called pathogens. The most common mastitis pathogens are found in the udder tissues, spreading from cow-to-cow (contagious pathogens) or in the herd's surroundings (environmental pathogens), such as bedding materials, manure and soil. This distinction may be important when assessing the challenges present in a herd and the measures which may be taken to reduce or treat mastitis:

  • Contagious pathogens that cause mastitis tend to live on the cow's udder and teat skin and transfer from affected cow (or quarter) to unaffected cow (or quarter) during milking. They adhere easily to the skin, colonising the teat end and then 'grow' into the teat canal, where infection occurs; because of this, post-milking teat disinfection and dry cow therapy play an important role in controlling contagious mastitis. Farms with a high level of contagious mastitis often have high Somatic Cell Counts (SCCs) with relatively normal Bactoscan results.
  • Environmental mastitis pathogens - present in the housing and bedding - can transfer during milking or between milkings, when the cow is loafing, eating or lying down. The pathogen can enter the teat canal by force during milking, for example, when liner slippage occurs. These environmental pathogens do not generally possess the same ability as contagious pathogens to adhere to and colonise the teat; dry cow therapy has little value in their control as these kinds of infections do not carry from one lactation to the next. High levels of environmental pathogens in a herd may cause normal SCCs but higher than average Bactoscan results.

Mastitis pathogens can infect cows both during the dry period and when cows are lactating, and it is important to identify and recognise the source of these infections, as approaches to control, prevention and treatment of the pathogen's effects can differ according to whether the infection occurs when the cow is dry, or in lactation.

Major and Minor Pathogens

The main mastitis-causing pathogens are Escherichia coli (E. coli), Streptococcus uberis and Staphylococcus aureus, and a wide variety of other organisms have been identified as potential mastitis pathogens. These organisms are termed major pathogens and are generally regarded as those commonly associated with clinical mastitis in dairy cattle.

It is not always possible to identify the causative pathogen of the case of clinical mastitis from the symptoms presented without laboratory testing of milk. Other bacteria that may be present in the udder and often have an overall beneficial effect on protection from infection caused by major pathogens, due to the production of natural anti-bacterial substances or competition with other bacteria, are termed minor pathogens. Due to their complex interaction with the udder they can be implicated in instances of increased SCCs and thus the incidence of sub-clinical mastitis but they do not usually cause clinical forms of the disease.