Welcome to The Dairy Mail

by Dr Inge-Marié Petzer

With a proactive approach towards udder health in dairy cows,prevention of mastitis is the main objective rather than trying to cure it. This approach increases the need for early detection of udder infection.

Even in the midst of the technology explosion we live in, we still do not have very effective ways to determine udder infections in the parlour. Only a limited number of diagnostic tests are available to identify sub-clinical mastitis (SCM), and they differ in their sensitivity (ability to identify mastitis) and specificity (ability to identify healthy quarters). Early detection of mastitis in cows reduces production losses, enhances prospects of recovery and a longer productive life for the animal. Once we detect clinical mastitis by means of a strip cup, we have to deal with the disease and an already damaged udder.

More than 2 200 quarter milk samples were tested during herd visits by the Faculty of Veterinary Science at Onderstepoort determining on each the CMCT, conductivity, SCC and microbiology. The reason for this combination of tests was to determine whether using these two tests in parallel, will increase the chances to diagnose sub-clinical mastitis (SCM) or udder infection. The outcome will be discussed later in this series.

In the present series we have been looking at ways to prevent/limit udder infections by early detection of the source of infection on a herd basis – be the source the udder of cows, the environment (bedding, water, milking machine or more) or possibly human carriers. Once the source has been identified, good management and hygiene practices, directed at that particular problem, are put into place to get and keep the problem under control.

Somatic cell count (SCC) has been the golden standard to determine udder health status along with bacteriology for many years. All milk producers these days are familiar with SCC owing to milk buyers using it as a tool for quality assessment and payment. SCC is relatively easy and cost effective to determine in a laboratory, but care needs to be taken that apparatus is of high quality, correctly calibrated and works effectively.

The SCC is an indication of dead body cells (damaged during mastitis) and white blood cells (those that have given their lives fighting the bacteria) that are excreted in the milk. SCC mainly increases when the udder becomes infected or traumatised, but can also increase when the cows experience systemic disease or stress.

Antioxidants and other nutrients effecting white blood cell activity (defence of the udder) include Vitamin E, A and C as well as copper, zinc and selenium. Deficiency of these trace elements and vitamins may predispose cows to more and more severe mastitis. However, in cows receiving adequate trace elements additional supplements thereof has no proven beneficial effects.

The CMCT is a chemical test that determines SCC semi-quantitatively in milk. It is also called the California mastitis test (a misleading name), the mastitis test or the Schalm test. The test reagent reacts with the DNA in the nuclei of cells present in milk and a visible change in the viscosity is an indication of a raised SCC – the more cells present the more distinct the viscosity.

CMCT does not diagnose mastitis – it is only an indication of the level of SCC in the milk.

At the 5th International Dairy Federation (IDF) Mastitis Conference held during March 2010 in Christchurch, New Zealand, an inline CMCT was exhibited that was developed in Australia. This system is connected to the milk line that comes from the milking unit. An aliquot of milk is mixed with the CMT reagent, while the cow is being milked. Viscosity is then measured within seconds after milking has commenced.

The CMCT only provides an indication of increased SCC, despite it also being called the California mastitis test it is not a diagnosis of mastitis. Before performing the test the first time, the spatula needs to be calibrated.

The reason being that the volume of reagent delivered must be equal to that of milk to prevent false positive or negative results. A trial performed at OVI showed that as much as a 0,2 ml difference in volume between the reagent and milk can lead to the over or under diagnosis of high SCC.
Equal volumes of test reagent and milk should be used when performing CMCT.

How to calibrate the spatula
You will need:
• a 10 or 20 ml syringe (not sterile but clean)
• a pocket knife
• a permanent black koki pen
• the CMCT spatulas (all of them)
• Determine the volume delivered by the plunger on your CMCT container. First, ensure that there is no air in the plunger for this will give you an incorrect volume. Press the plunger a few times until the reagent that comes out forms a steady stream.
• Measure the volume that the plunger delivers by pressing the plunger for its full length three or four times into a syringe. Read the volume on the syringe and divide that by the number of times that you pressed on the plunger to determine the average volume.
• Now take the spatula and squirt the exact calculated volume of the reagent into its far right cup.
• Tilt the spatula until just before reagent overflows. Use the pocket knife to mark that fluid line on the surface of the spatula.
• Pour out the reagent, rinse and dry the spatula, and mark the line now clearly with a permanent koki. The spatula is now calibrated for your reagent’s plunger.

CMCT procedure
• This test does not need to be performed in a sterile manner but try not to get dirt in the spatula.
• Use foremilk and milk of each quarter into a cup of the spatula, ensuring that more than the needed volume milk is in the cup.
• Carefully examine the milk by tilting the spatula back and forth and record any flakes or discolouration (such as blood) in the milk.
• Discard excess milk carefully by tilting the spatula until you can just see your black calibration line. The volume of milk left in each cup will now be equal to the volume of reagent to be squirted into them. Also, do not allow the milk from one cup of the spatula to run into other cups.
• Add the reagent to each of the spatula cups with milk by pressing the plunger gently, not to make foam, which will hamper the reading of the results, but completely to add the correct volume of reagent.
• Mix the milk and reagent by rotating the spatula (move in a circular horizontal way) for a few seconds while carefully studying the mixture.