Welcome to The Dairy Mail

by Dr Inge-Marié Petzer

In the first part of our two-part series in udder health, we focused on the importance of early detection of mastitis in cows and how to perform the CMCT procedure. In the second part, we take a look at CMCT readings, monitoring udder health and measuring success with CMCT during lactation.

Practical uses of the CMCT
On herd basis
Ideally 85% or more of all lactating cows should be CMCT negative. It can be used as a screening test of the herd, not as a diagnostic test.

It can also be used as a follow-up on all cows with high SCC on cow milk samples (E-report) looking at the inter udder relation (compare the four quarters of the udder with each other).

Monitoring of udder health
Individual cows
The purchasing of lactating cows should always be followed with aseptic quarter milk samples to obtain a bacterial profile. It may sound strange but you are buying the udder more than the cow – that is mainly where the money is going to come from. If no quarter milk samples are evaluated it would be the same as buying a bull without testing its fertility.

However, the CMCT can be done on cows preferably not later than six hours after the last milking. Any quarter that shows a distinct gel formation should be considered infected. Purchased lactating cows that are not cultured must be segregated at milking and milked last until a milk culture can be examined.

Examine and monitor fresh, post-calving cows to determine whether the udder is normal after the dry period.

A CMCT can be done on fresh cows and heifers by their third or fourth milking. It is difficult to get accurate CMCT readings on colostrum. When one of the quarters is significantly more positive than the other, samples should be taken from all quarters for culturing.

She should be separated from the group or milked just prior the mastitis group until results are available. If this is not practical, the milking unit used to milk her, should be back-flushed with great care with a fast-acting disinfectant prior to milking the next cow.

Fresh cows do not have CMCT positive quarters unless an infection or serious injury has occurred. Use CMCT weekly on fresh cows that tested positive in one or more quarters – this will provide you with more detail on the progress of the infection – is it getting better, worse or does it remain the same?

CMCT positive
The procedure followed depends on which bacteria have been isolated from the cow. She does not need to be milked separately, as it is a non-contagious bacterium. When it is contagious Staphylococcus aureus (STA) and Streptococcus agalactiae (SAG) (see earlier in this series), she needs to be segregated at milking. Streptococcus agalactiae cows need to be treated. Approximately 50% of environmental infections in fresh cows will spontaneously cure and disappear within 15 to 30 days in milk. If the CMCT score ­increases at 14 to 21 days in milk compared to day 2, the cow (quarter) will require more intense attention at milking time. Antibiotic therapy will be necessary when the cow’s quarter or life is being threatened.

All quarters show gel formation
When a fresh cow has all four quarters showing some gel formation, but the CMCT reading is the same for all quarters, the cultures are negative on the four quarters. The increased CMCT readings are because of udder conformation, age, injury or other trauma such as milking. As days in milk increase in this cow the CMT readings may decrease or stay the same with no infection present.
Should fresh cows be treated based on CMCT results? The answer is a definite no. The CMCT identifies quarters with high SCC, indicating a chance of being infected but colostrum and stress may play a big role early in lactation.

CMCT measures success
There are two uses for the CMCT post treatment namely to indicate when the SCC is low enough to add the milk to the tank (check when antibiotic withdrawal time is done) and to monitor if the cow was possibly cured from the mastitis.

After intramammary treatment, the SCC in the treated quarter will remain increased for a period owing to irritation caused by the remedy on top of the inflammation. This may affect the CMCT reading for up to three weeks after treatment. A positive CMCT on a quarter at three weeks post-treatment would indicate a failure of treatment.

One negative CMCT on a treated quarter at three weeks post-treatment (clinical cure) would not guarantee that the quarter is free of all organisms (bacteriological cure). However, if subsequent CMCT testing at two-week intervals remain negative, chances are better that she is cured.

When you use selective therapy, use the CMCT to help determine which cow to treat with dry cow therapy or which cows need special care or treatment during the dry period when blanket dry cow therapy is used. Lactating cows can be tested at the last milking prior to drying off.

When this data is compared to the CMCT readings at drying off to that of the animal at the fourth to sixth milking after calving, two results are possible:
• The quarter was CMCT positive at dry off and is negative at calving = cure, owing to dry treatment or spontaneous cure.
• The quarter was CMCT positive at dry off and is still positive at calving = there are two conclusions to this finding:
• The quarter is still infected with the same infection that was present at dry off and the treatment failed. This is common with STA infected cows.
• The quarter was cured of the infection that was present at dry off but it became reinfected prior to calving with a different organism.

Culturing CMCT positive quarters at dry off and again at calving would be the only way to distinguish between these two findings. However, if the majority of CMCT positive quarters at dry off are also CMCT positive at calving, one should suspect that those quarters are chronically infected and did not respond to dry treatment.

Cows that maintain a chronic infection through the dry period should be separated at milking time. The causative organism is likely to be STA that did not respond to dry cow therapy and will not likely respond to any other therapy.

A high percentage of non-cures with dry cow therapy may warrant a change in dry cow antibiotic. Choose a new dry cow antibiotic and continue to follow CMCT results at dry off compared to calving. At least 50 infected cows will be needed to form a conclusion on the efficacy of the new dry cow antibiotics.

A quarter is CMCT negative at dry off and positive at calving and was infected during the dry period or at calving. A high percentage of new infections during the dry period would implicate environmental organisms. Improve the sanitation of the housing for dry cows, pre-fresh cows and maternity cows.

A quarter is negative at dry off and is still negative at calving – that is what we need. In the next issue, we will be looking at electrical conductivity as a cow side-test followed by the information we obtained by using the CMCT and conductivity test in parallel.