Name that bug… What causes mastitis in Canada?


Mastitis Hurts

“Mastitis is like an iceberg – effects float with only a fraction of their bulk above the water”

Sigmund Freud quote, altered to fit mastitis in cattle.

This wouldn’t be a mastitis article if I didn’t mention that mastitis is among the costliest diseases of dairy cows. A recent study found that the cost of mastitis, both clinical and subclinical mastitis, was $660 – on a per cow per year basis [1].  For a little perspective, that means that if you milk 100 cows, mastitis is costing you $66,000 per year! Sure, about half of this cost is things we can see – culled or dead cows, drugs, discarded milk – the top of the iceberg. The other half is strictly lost milk production following the damage of the mammary gland – the unseen but very real effect.

So, what causes mastitis in Canada?

If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. If you know neither the enemy nor yourself, you will succumb in every battle.”

Sun Tzu, The Art of War

There is a huge variety of bacteria, fungi, and algae that can cause mastitis in cattle. Several Canadian researchers have uncovered the bacteria that cause mastitis. So, who makes the top ten?

When looking at clinical mastitis, Figure 1 outlines the top 7 bacterial causes of clinical mastitis across Canada [2].

Figure 1: The Incidence rate of Clinical Mastitis across Canadian Dairy Farms.

There are regional differences in the relative frequency of each bacteria, but the top 4 are consistent: Staphylococcus aureus, E.coli, Streptococcus uberis, and non-aureus Streptococci. What is not shown is the high level of “No bacterial growth”.

What are the most common causes of subclinical mastitis – that form of mastitis that you can’t see with your eyes? Figure 2 outlines results from a recent Canadian study.

Figure 2. Bacteria causing subclinical mastitis infections in Canadian dairy cows

There are those 2 familiar foes – S.aureus and non-aureus Staphylococci!

Common suspects

The Bacteria Contagious or Environmental? Where does it live? Most common type of mastitis Response to antibiotic therapy
Staphylococcus aureus Contagious In the udders of cows Mild clinical and subclinical

Often chronic (long-term)

Very poor
Non-aureus Staphylococcus Both On the skin of cows


Mild clinical and subclinical Good
E.coli Environmental Wherever there is manure, water, or organic matter Severe clinical mastitis (e.g. watery milk, fever, swollen udder) Often spontaneous cures

Systemic antibiotics given to treat blood infections

Streptococcus uberis Both Organic matter in the environment Udder of cows Mild to moderate clinical mastitis

Often chronic

Contagious – bacteria is adapted to live primarily in the udders of cows and is spread from cow-to-cow at milking through the milk.
Environmental – bacteria lives mostly in the environment. Infections occur primarily between milkings from an unhygienic environment.
Both – Bacteria can be passed from cow-to-cow during milking and from the environment to cow.
*Note: these definitions are not set in stone. Contagious bacteria can survive in the environment and infect cows. Likewise, some other environmental bacteria are able to cause long-term mastitis infections and transmit from cow-to-cow.

Knowing the risk on-farm

To effectively control mastitis, an accurate diagnosis of the problem must be made. This means that milk samples must be taken from infected cows for laboratory diagnosis. Each farm can have its own unique bacterial challenges. All cows with clinical mastitis should be tested to determine the best course of treatment. Also, routine testing of subclinical or high SCC cows will give you a herd picture of the type of bacteria causing an issue.

A note on “No Bacterial Growth”

Around 20-40% of milk samples submitted to the lab for culture yield “no bacterial growth”. This can be frustrating for anyone that wants answers for their mastitis problems. What does it mean?

  • First, the cow’s immune system might have already cleared the bacteria from her udder. Her mild may still have signs of mastitis – it can take time for this inflammation to subside after the bacteria is cleared. E.coli bacteria is typically cleared from the udder quickly – the real damage comes from the cow’s immune response.
  • Second, the bacteria causing the infection may be too low to detect. S.aureus is a prime example of a bacteria who can be shed at low levels yet cause significant clinical mastitis.

Prevention is key

There have been volumes devoted to uncovering and implementing mastitis prevention programs on-farm. Recently, Dr. Thomas Hemling devised a 7-point plan for controlling mastitis (Figure 3) [3].

Figure 3. 7-Point Plan for Mastitis Control [3]

This 7-point plan takes into consideration cow immunity and vaccination, environmental challenge, bacterial identification, and management to ensure robust, holistic mastitis control is possible.

Hipra and Mastitis Prevention

Hipra, a leading multinational veterinary pharmaceutical company, strives to be the reference in prevention for animal health. That is why the focus has been on developing industry-leading vaccines to prevent disease in animals. In Canada, Hipra markets Topvac – the only vaccine licensed to reduce the incidence of sub-clinical mastitis and the incidence and the severity of the clinical signs of clinical mastitis caused by the top 3 causes of mastitis in Canada: Staphylococcus aureus, E.coli and other coliforms, and coagulase-negative staphylococci.

What’s more, effective prevention requires knowledge of the cause and impact of disease on the farm. Hipra offers two tools to help you better understand the udder health challenges on farm:


  1. UDDERCHECK, a PCR-based diagnostic tool to identify the bacterial causes of mastitis, and
  2. STARTCost, an online calculator to understand farm-specific economic impacts of mastitis.

To learn more about preventing mastitis, visit


[1] M. Aghamohammadi, D. Haine, D. F. Kelton, H. W. Barkema, H. Hogeveen, G. P. Keefe and S. Dufour, “Herd-level mastitis-associated costs on Canadian dairy farms,” Frontiers in Veterinary Science, vol. 5, no. MAY, pp. 1-12, 2018.
[2] R. Olde Riekerink, H. W. Barkema, D. F. Kelton and D. T. Scholl, “Incidence rate of clinical mastitis on Canadian dairy farms,” J Dairy Sci, vol. 91, no. 4, pp. 1366-1377, 2008.
[3] T. Hemling, “7 Point Plan for Mastitis Control,” M2 Magazine , 2018. [Online]. Available: [Accessed 29 April 2020].