Clinical mastitis cases reduce production and can cost lost dollars due to discarded milk, cost of treatment products, lost bonuses and extra labor.
Clinical mastitis is caused by bacterial infections of the udder.
Prediction of clinical mastitis is difficult.
Several tests exist for determining specific mastitis pathogens.
Proper handling and use of the results of any test available so that the success rate and the cost of the test are reasonable can be a management dilemma
The impact of mastitis on milk production, milk quality and the economic consequences are well known to dairymen. Reduced production, lost bonuses, discarded milk, treatment products, extra labor, vet calls and a certain small percentage of deaths from clinical cases cause substantial annual losses depending on the assumptions made. It has been estimated each clinical case causes roughly a $100 loss, depending on what is included.
The ideal mastitis diagnostic tool would be one that could accurately predict the onset of clinical mastitis well before it became a clinical case so that preventive action could be taken.
Since mastitis is caused by bacterial infection of the udder, bacteria are always the concern. Clinical mastitis onset differs somewhat depending on bacterial cause. Coliform bacteria may produce sudden and severe clinical cases with little advance warning while Staph aureus tends to linger as sub-clinical and occasionally causes clinical flare-ups. Most bacterial causes of mastitis produce sub-clinical infections of varying duration. This period and the symptoms are important from a diagnostic standpoint.
Predicting clinical mastitis has been difficult for several reasons. First, we don’t test milk from every cow each milking. Monthly samples taken to determine production is inadequate for this purpose. Secondly, a composite sample of all four quarters tends to be less sensitive than individual quarter samples. A component detectable in quarter samples may be diluted in the blended sample to a level below the detection threshold of tests.
What indicators may predict problems? Bacteria or bacteria constituents, changes in milk constituents, bacterial antibodies in milk, damaged tissue components, somatic cells or cell components or a combination of several things could prove helpful.
The gold standard for determining udder infection status is milk culturing. Finding mastitis pathogens in milk is a clear indication of potential problems especially with certain bacterial species. However isolating no pathogens from clinical samples is common and rather confusing.
The Hy-Mast test, from Pharmacia, is used on the farm to test milk samples for presence of mastitis pathogens. It takes approximately 8-12 hours and distinguishes between gram-negative and gram-positive bacteria. These two bacterial groups respond differently to antibiotic therapies so knowing which is present is important.
More traditional milk sampling and culture programs, while not rapid cow-side tests, are still excellent for determining specific mastitis pathogens. Milk culturing using a quality lab is still the best and most reliable mastitis diagnostic tool. A key for success is a clean milk sample that is properly stored and handled from the farm to the laboratory. Failure in any of these areas can lead to meaningless information.
Cows infected with Staph aureus produce specific antibodies against the organism that can be detected with an appropriate test. Originally such a test, Pro-Staph, was marketed in areas of the US for this purpose. Now the SAATK test, which does the same thing basically, is available in some locations. The test detects the presence in milk of antibodies against Staph aureus. Their presence suggests the cow has or has had a Staph aureus infection. It represents a tool to screen cattle for Staph infections and cows testing positive should then be sampled and cultured to verify infection status.
Somatic cell movement into the udder in large numbers occurs quite early in the development of mastitis so the quantity in milk is a useful measure of infection status. Counts less than 200,000/ml generally indicate good udder health. Counts in the 300,000-500,000/ml suggest a challenge exists. The action of somatic cells may control and eliminate the problem. On the other hand a cell count may be on the way to a much higher level associated with onset of a clinical case. Monitoring cell count concentrations over time helps determine infection status changes.
Measuring somatic cell concentrations in milk has been the best-developed and most widely used practice for estimating udder health. There are quick easy to use cow-side tests as well as central lab tests that provide information in a few days.
Estimating somatic cell concentration using the CMT test has been a valuable tool for cow-side evaluation. The CMT is simple, economical, fast and easy to use as a cow-side test. While the cell count range of each category is quite large it still is a good tool. It provides no indication of bacteria type but the score can be used to determine infection status of individual quarters.
Infection of the gland by bacteria eventually produces tissue damage. This has been monitored in milk by measuring concentrations of the enzyme, NAGase. The higher the NAGase concentration the more likely the presence of pathogens and clinical infections. The testing of milk for NAGase has generally been based on laboratory testing of samples but at one time there was a NAGase test kit for farm use sold in the US. Whether or not it is still available is unknown. This test is used in some countries but is not commercially available in the US to my knowledge.
Other milk component tests have also been evaluated over the years but none have emerged as commercially viable in the US.
Mastitis infections produce inflammation and tissue damage. When this occurs, blood components infiltrate into milk changing the proportions vs. regular milk from infection free udders. Sodium and chloride ions are elevated in mastitis milk making it a better conductor of electrical current. This concept has been studied extensively to determine its use in detecting infections in milk from quarters or the whole udder.
Milk conductivity research has verified the salt content in milk changes due to mastitis. Second, changes in milk conductivity may precede the onset of clinical mastitis. Third, each cow is unique so cow to cow comparisons of milk conductivity are not acceptable. Within cow quarter comparisons are best but it takes sufficient computing power to analyze the data and make such comparisons.
Milk conductivity may have merit for mastitis detection but success depends on the reliability of equipment, standardization of the sampling procedure and proper analysis of the data.
Hand held conductivity testers are available for checking suspect cows and there are commercial systems available that incorporate conductivity sensing directly into the milking cluster.
Mastitis monitoring tools have to be sensitive enough to pick up a high percentage of actual infections but not accumulate a lot of false positive cows. Tests failing on either count are unacceptable.
The key question with all tests is how the results are handled and used so that the success rate is high and the costs are reasonable? That is the management dilemma. Software routines that monitor incoming data and compare it to historical benchmarks can be helpful in determining when significant changes are occurring and further evaluation is necessary.