Canadian Quality Milk On-Farm Food Safety Program - Centre ...

Canadian Quality Milk
Clinical mastitis refers to mastitis that leads to obvious changes in the milk, the
quarter, or the animal. Subclinical mastitis usually does not cause obvious changes in
the animal or milk and can only be identified using a test such as the SCC or CMT.
With only a few exceptions, microbes that cause mastitis enter the
udder through the teat. As a result, only one or two quarters are
infected at one time. Less commonly, infections can spread through
the blood. When this happens, all four quarters usually become
infected; therefore, it is wise to sample all quarters for bacterial culture.
Mastitis can be separated into two types, according to the source of
the microbes causing the mastitis:
• Contagious mastitis can be either clinical or subclinical, but
most often is subclinical.
• Environmental mastitis can also be clinical or subclinical, but is often clinical.
Although we differentiate contagious and environmental to help understand how to
control and prevent mastitis, an animal may have both types of mastitis at the same
time.
3.2.4.1 Contagious Mastitis
Contagious mastitis is an infection of the udder with microbes that originated in the
udder of another infected animal. With only a few exceptions, the microbes that cause
contagious mastitis enter the uninfected quarter through the teat canal. The infection is
usually spread during milking.
The microbes that cause contagious mastitis are adapted to live in the udder (for
common names see Table 10). As a result, they can survive for long periods of time in
an infected quarter. In some cases, the infection may last for the entire life of the cow.
In most cases, contagious mastitis causes neither clinical mastitis nor obvious changes
in the milk. Subclinical infections caused by contagious microbes are often best
detected using individual cow/heifer SCC information or the CMT. These tests can be
used to detect which animals or quarters are infected, but they cannot tell which
microbe is actually causing the mastitis. A milk culture is needed to determine which
microbe is causing the mastitis.
Like any contagious disease, contagious mastitis can be carried into a previously
uninfected herd by the introduction of an infected cow or heifer.
The symptoms of
clinical mastitis and
the tests used to
identify subclinical
mastitis identify
which animals have
udder infections.
However, they do
not tell what is
causing the mastitis.
June 2010 3—13