CMT could cut dry-cow antibiotics use by half, study shows

Using the Californian Mastitis Test (CMT) at drying off could help cut antibiotics use by 50% without impacting clinical mastitis, a study has found.

In the first large-scale investigation to identify at-risk quarters in high-cell count cows, veterinarian researchers from Quality Milk Management Services (QMMS) found quarter-level antibiotics administration reduced antimicrobial use/cure.

However, the study concluded it was “unjustified” in animals with low-cell count.

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At the recent British Mastitis Conference, lead researcher Andrew Bradley said the findings suggest selective use of dry-cow therapy at the quarter level in cows with high-cell count – on well-managed farms with low somatic cell counts (SCC) and a low prevalence of major mastitis pathogens – could result in a substantial reduction in antibiotics.

However, he said the addition of antibiotics is probably unjustified in low SCC cows and is unlikely to result in significant gains.

Quarter-level application of antibiotics has previously not been recommended due to the risk of missing a major pathogen infection in another quarter.

However, evidence shows independent quarters are less marked with environmental pathogens than contagious ones. Contagious pathogens are more likely to spread between quarters and become persistently infected.

Concern is growing around prophylactic use of antibiotics at drying off, but researchers say the highest proportion of antibiotics comes from systemic (injectable) use.

About the study

Six commercial dairy herds in Somerset, with bulk milk samples less than 200,000 cells/ml, took part in the study, with data collected from 766 cows (3,064 quarters).

Cows were split into two groups:

1. Infected (381 cows): Those that recorded SCC higher than 200,000 cells/ml on three consecutive tests, with cases of clinical mastitis
2. Uninfected (383 cows): Those that recorded SCC below 200,000 cells/ml on three consecutive tests, with no previous cases of clinical mastitis.

Each group was randomly assigned to one of three treatments.

Bacteriology and SCC data were then collected again from each quarter at calving, at seven and 14 days into lactation and, finally, 100 days post-calving.

Results from the QMMS study

• Major pathogen infection was low and minor pathogens much higher at drying off and calving.
• Gram-positive infections were also very low in both infected and uninfected groups (around 2.5%) at drying off.
• On average, 10 tubes/cure were used for major pathogens in the high-cell count group when a selective quarter approach was not used.
• In the high-cell count group, antibiotics use could be reduced by 50% by targeting individual quarters that had negative CMT results.
• In lower-cell-count cows (uninfected group), the cure rate did not differ across the three treatment protocols.
• Infected cows from the SQLT1 group were at lower risk of being infected at calving with a minor pathogen, compared with the SCLT group.
• No differences were seen in clinical mastitis in the first 100 days of lactation.