Dealing with Chronic Mastitis - Definitions, Records, and Best Management Options

Have you ever looked at your DHI Somatic cell count (SCC) records and wondered why so many cows continue to have high SCC or repeat cases of mastitis? Why does this happen? What can you do differently to avoid these same cows from being infected again? What can you do differently to prevent the next cow from having mastitis repeatedly over her lactation? Do you have more chronics or repeat mastitis cows than you should?

This article discusses the definition of chronic mastitis, the importance o f records to track when chronic mastitis begins and how to determine the best management options to reduce the impact on your herd.

Definitions

The term chronic mastitis is used to describe mastitis that has a high chance of being resistant to therapy, lasts for a long time, or occurs repeatedly in the same cow or quarter. Often, culling may be the best choice for a cow with chronic mastitis, especially when mastitis is present in more than one quarter, or occurs repeatedly or continually in early lactation. Culling is often the best management choice due to the economic impact -- production loss and treatment costs -- and the risk of spreading the mastitis to additional cows versus the cost of culling and replacing her with a non -infected first lactation heifer. For culling to work economically when dealing with chronic cows, there must be non-infected healthy heifers calving in a barn set up for excellent mastitis prevention. There is nothing more frustrating or costly than culling cows for mastitis and soon after having previously non-infected cows become infected and eventually chronic.

Because labeling a cow as chronic is often dire and leads to terminal management actions, it is best to use economics and multiple records before tagging a cow as chronic. Chronic mastitis is defined several ways. The definition used in the Pa Dairy Tool SCS Analyzer, classifies mature cows with three or more test day SCC over 300,000 and with at least one of those occurring in the most recent 3 tests. We know that cows with an SCC over 200,000 have a high probability of mastitis. However, cows with SCC consistently over 300,000 should be evaluated more closely, hence the reason for increasing the decision-threshold in the report. First calf heifers are classified as chronic if they have 3 or more DHI tests above 200,000 and with at least one of those high tests in the most recent 3 months. We expect heifers to have less mastitis and cure more quickly, so the threshold is lower (stricter) for this group to be tagged or labeled as chronic.

Using Records to Determine Best Management Options for Chronics

Mastitis quarters of chronic cows will likely not cure when treated; making these cows likely candidates for culling. If the chronic mastitis infection is only in one quarter and is caused by a non- cont ag ious organism, there are management strategies to keep the cow in the herd, to reduce the chance of spreading the infection to clean or non-infected cows, and to reduce the impact on your ability to receive higher milk quality premiums. Options might include “killing” the quarter or diverting milk from the single quarter (short-term strategy) away from the bulk tank milk. The more quarters infected in a chronic cow, the greater the likelihood that antibiotics will not work to clear the infection. The more chronic cows you have in your herd, the tougher your choices. More information is needed to make good short and long-term cost-effective decisions.

Clinical mastitis and treatment records, when combined with SCC records, can help you make even better decisions about chronic cows. If a chronic cow -- based on SCC records -- has had several episodes of clinical mastitis despite quick diagnosis and strategic treatment, culling is likely the best management option. Of course, this assumes that treatment was targeted for the specific bacteria – identified by culture, that the best drug used – based on sensitivity, bacteria, cow history, and possibly DNA testing, and that the drug was used for the appropriate time period – according to label directions or extended-use instructions by your herd veterinarian. If a chronic cow has had a new infection in the current lactation, was clean in her previous lactation, and has not been treated with antibiotics, treatment may be the best management option. Early dry-off could also be a best management option if the cow is genetically superior to herdmates, late in lactation, pregnant, and close to dry-off. If the same cow was milking in only 3 quarters, has Staph aureus in a new quarter that doesn?t respond to treatment, then culling for mastitis becomes a better management option. Combining DHI SCC and clinical events and treatment records will help you make more profitable decisions about chronic mastitis cows. Mastitis cost records for your herd and individual cows will lead to even better decisions.

Need for monitoring records to understand where/ how chronics begin

When reviewing producer records, I classify cows as multi -lactation chronics if their SCC was high in the previous lactation – based on their last test SCC before dry-off or the previous lactation SCC was above 300,000 and if the SCC is still above 300,000 at the first test in the new lactation. These multi-lactation chronics should be watched carefully after freshening to determine best management option. Information from observing cow signs of mastitis, using the California Mastitis paddle – to identify subclinical severity and infected quarter, DHI SCC records at first test, and possibly new culture results can all help you to make the best decision. These cows likely did not respond to treatment or developed a new infection – either case classifies these cows as having chronic mastitis. Herd records and protocols should be reviewed to determine if overall dry period treatment management - - which is intended to cure chronics and prevent gram positive new infections -- is still effective.

First calf heifers should enter the milking string without mastitis infections. If they develop or have mastitis at calving or continue to have a high SCC for the first 90 days, they can be another source of chronic mastitis. These chronics could be the result of trauma during the heifer rearing period leading to a persistent infection or could be the result of group bedding management or milking related issues -- milking management or machine related risks. Treatment philosophy may also lead to development of chronic mastitis. Use of dirty tools, poor hygiene, and delayed antibiotic treatment, and/or misuse of drugs may lead to chronic infections. Rare or occasional chronics in these young animals may be related to poor immune system response through genetics or teat end damage. All potential risk factors should be evaluated to enhance best management decisions.

Chronics that develop during lactation may be the result of bedding management choices or may be the result of milking management and/or milking machine malfunction. Treatment management is also a likely contributor to development of chronics in some herds. Poor choice of antibiotics , failure to treat cows early in the infection, poor sanitation during treatment, and poor choices for length of treatment are common contributors to the number of chronics in dairy herds. Risk assessments can be helpful to distinguish between likely reasons for high herd levels of chronic mastitis.

Complete mastitis management risk assessments are a painless and cost-beneficial way to pinpoint causes of new infections and chronic mastitis and to achieve higher milk quality bonuses. Use of milk quality target teams can be a great way to establish protocols and monitor herd progress toward achieving higher milk quality premiums and reducing mastitis costs.

------- Sandy Costello, Ph.D. - Southeast Region Dairy Herd Health Extension Educator, Cumberland County