Dairy NZ

Subclinical mastitis shows changes that are NOT VISIBLE on initial inspection.  Tests are required to detect subtle changes in the milk.  The changes in milk and the relevant tests are summarized in the Table.   Note: Subclinical cases of mastitis are not usually treated with lactating products.  It is more cost effective to: Monitor for a few days/weeks and allow the cow to reduce the SCC herself.  This applies to subclinical mastitis found in the colostrum period or following the first 1-2 herd tests Treat such infections with dry cow antibiotics therapy (DCT) when the cow is dried off at the end of her lactation.

Healthy udders should be the main goal for all herds

Tests used to detect the subtle changes in milk, indication subclinical mastitis. What farmers need to know…

The RMT measures the presence of somatic cells in milk. 

Events that increase milk SCC include:

• Clinical mastitis
• Subclinical mastitis
• Colostrum
• Involution (drying off process at the end of lactation)

The RMT is probably the best cow-side tool for screening cows before they go into the bulk tank, to gauge risk of affecting the BMSCC.

It can be used to test cows for subclinical mastitis in the colostrum period.  Use with caution in the first 48 hours after calving, as very high numbers of somatic cells in colostrum will cause mild positive reactions. However, infected quarters will still show stronger reactions compared to uninfected quarters.

Ideally use on the third or fourth day after calving to screen for subclinical mastitis that need monitoring.

The conductivity meter (or resistance meter) measures the presence of salt ions in the milk.

 Events that increase salt ions in milk include:

• Clinical mastitis
•  Subclinical mastitis (some cases)

 A relative threshold change in conductivity (resistance) is used to find an infected quarter. 

Conductivity (resistance) MUST be measured and compared across different quarters within a cow, not across different cows.

Look for large differences between quarters.  Be wary if all four quarters show a high (or low) result. 

Conductivity meters can be used in the colostrum period - use on the third or fourth day to screen for subclinical mastitis that needs monitoring.