Dairy cows: contagious mastitis bacteria

Find out about the types of bacteria responsible for contagious mastitis in dairy herds. 

Back to: Controlling contagious mastitis in dairy cows

Understanding the bacteria that cause mastitis 

Mastitis can be caused by over 100 different bacteria. Most of these reach the udder from the cow’s surroundings. 

Contagious mastitis is spread from cow to cow, and the main features of the bacteria that cause it are: 

  • Persistence of infection within the udder of an infected cow 
  • And/or the ability to be shed in very large numbers from an infected quarter 

Below, we look at the main types of bacteria responsible for contagious mastitis and where they’re typically found.

Staphylococcus aureus 

In the majority of UK dairy herds, contagious mastitis is most commonly caused by Staphylococcus aureus (Staph. aureus), which often leads to persistent subclinical infections. 

Staph. aureus can survive on various parts of the cow and in the environment. In most herds, infection centres on infected quarters and bacteria living on the teat skin. 

This bacterium is still found in only a relatively small percentage of mastitis cases. In a survey of over 6,000 samples from over 500 farms, it was found in only 6% of clinical samples and 9% of subclinical mastitis cases.* 

*B. Payne, J. A. Bradley, E. Coombes, E. Lusby, K. Mining, C. Hunt and A. J. Bradley (2013). The aetiology of bovine mastitis in UK dairy herds. Proceedings of the British Mastitis Conference (2013) Sixways, Worcester, p 59–60. www.britishmastitisconference.org.uk/Previous%20Proceedings.html 

Streptococcus agalactiae 

Streptococcus agalactiae (Strep. agalactiae) is now rare in the UK. It often causes large spikes in Bactoscan and bulk milk SCC.  

It’s occasionally found in herds that lack the basic elements of contagious mastitis control – for example, farms with no history of using antibiotic dry cow therapy or ‘flying’ herds that buy-in lactating cows with no individual cow SCC history.  

Strep. agalactiae was found in less than 0.5% of samples tested in the survey above. 

Streptococcus dysgalactiae 

Streptococcus dysgalactiae (Strep. dysgalactiae) is often found in herds with high levels of teat-end lesions and where teat skin condition is poor. It can live on teats and teat skin, and it spreads between cows by indirect contact, such as on hands or milking cloths. 

A high prevalence of this bacteria is often associated with: 

  • Herds with poor post-milking teat disinfection routines (particularly in bad weather, such as very cold conditions in which teats become chapped and sore) 
  • Herds that use poor-quality post-milking teat dips with little or no emollient 

Strep. dysgalactiae was present in 6% of samples tested in the survey. 


Mycoplasma is a bacteria group rarely identified as a cause of mastitis in the UK, though this may be underestimated because diagnosis is difficult. Even when it is present, it’s rarely the only cause of mastitis on a farm, and other contagious and environmental organisms are likely to be the major issue. 

Mycoplasma bovis is thought to be the most common of several types of Mycoplasma that cause mastitis. Most cases are subclinical, and this causes the greatest loss. It can spread from cow to cow at milking time, as well as by shedding from the mouth, nose and other external mucosal surfaces of an infected animal. 

Cows with Mycoplasma mastitis often respond poorly to treatment, and cure rates using antibiotics are very low. Some farms have used a test-and-cull policy, but this isn’t always needed. Success in controlling the disease is likely to depend on the effectiveness of isolation, which reduces spread while infected cows cure themselves. 

Useful links 

Controlling heifer mastits

Control of environmental mastitis in lactation