What’s the big deal about a couple cases of subclinical mastitis? After all, you can’t see any change in the milk and the cows don’t appear to be sick or uncomfortable. Sure, your bulk tank somatic cell count may be higher than you prefer, but there’s not much you can do about that. Besides, when it comes to mastitis, you focus on what’s important — the clinical cases.

This attitude couldn’t be more wrong. On average, mastitis will cost you about $200 per cow this year. And for every one clinical case, there are 15 to 40 subclinical cases lurking in your herd. (Subclinical mastitis is defined by cows with a somatic cell count of more than 200,000, but no visual signs of mastitis.) According to researchers at Virginia Tech, these subclinical cases may account for 70 percent of total milk loss due to mastitis.

Ultimately, to address subclinical mastitis, you must go back to individual cows and find those that need attention or require action, says Dave Rhoda, veterinarian with the University of Wisconsin’s Milk Money program. “These are obviously not emergency cases, but your goal should be to find the right cows at the right time.”

Here’s how to develop an action plan for individual cows with subclinical mastitis.

1. Target chronic cases

Identify cows that have had a somatic cell count exceeding 200,000 for two consecutive months or more. This threshold may vary by dairy and herd goals, and depending on your risk and risk tolerance, says Rhoda. But, by definition, research has shown that cows with a SCC greater than 200,000 have at least one infected quarter. So, that makes the 200,000-SCC threshold a good place to start.

If you have a large number (10 percent or more) of chronic subclinical cows and you are just beginning to address subclinical mastitis in your herd, narrow your focus to cows that have had SCC scores exceeding 200,000 for five consecutive months. This will give you a more manageable number of animals to address, suggests Ynte Schukken, director of CornellUniversity’s Quality Milk Production Services.

As you improve your control and prevention programs, drop that threshold back to two consecutive months for aggressive subclinical mastitis management.

2. Test further

Use a California Mastitis Test (CMT) or other on-farm test to identify infected quarters in these chronic cows. This is a good time to examine teat ends on these cows, also.

Next, culture milk samples from infected quarters to determine the pathogen present.

Flag these cows so that employees in the parlor can closely monitor them for signs of clinical mastitis that may develop, says Larry Smith, OhioStateUniversity animal science professor, emeritus.

3. Check histories

In order to determine the best course of action, you need to know more about each cow’s history. Check individual records to determine lactation, past infections and duration of infection. Note days in milk when subclinical infections were first recorded. It is also helpful to log whether any management practices changed and, if so, when they occurred. Arrange the data in chronological order to see if cyclic patterns emerge or if infections are the result of a perpetual, long-term problem.

4. Individual action plans

With cow history and culture results in hand, now it’s time to make a decision for each individual cow. Here are your main options:

Ignore. “If  BTSCC (bulk-tank somatic cell count) is pretty good, I’m probably going to ignore a cow with subclinical mastitis unless something breaks,” says Smith. A prime example of when you could ignore a chronic subclinical cow is if her SCC fluctuates between 200,000 and 300,000 from month to month, but she doesn’t break with clinical mastitis. You also may want to ignore new subclinical cases when a cow’s SCC is below 250,000.

•  Segregate. If your facilities allow, segregate these chronic subclinical cows from lower-scoring herdmates. This is especially important if culture results reveal a contagious pathogen. Herds with Staph. aureus or mycoplasma infections are prime candidates for this approach. In larger herds, isolate these cows into a separate mastitis group and milk last or install automatic teatcup sanitizers or backflush units on each milking unit. In smaller herds, simply milk these cows last.

• Early dry off. At dry off, consider extending the dry period for chronic cows by 30 to 60 days and use dry-treatment therapy to eliminate the infection. Or, dry off the infected quarter any time during lactation — if it is persistently, chronically infected. This may be preferable for early-lactation animals if the subclinical infection is chronic.

Cull. Use individual cow histories to help decide whether you want to keep a cow in your breeding pool or remove her from the herd, suggests Rhoda. Look for cows with persistently high SCCs that carry over from one lactation to the next. These cows are prime culling candidates. Also, based on CMT results, look for cows with three or more infected quarters; they probably should be culled. One other culling category is cows whose milk has been withheld from the bulk tank for 28 to 30 days or more, or have been treated three times or more for mastitis.

Treat. Do not automatically treat lactating cows with an antibiotic simply because of elevated SCC. Treatment of subclinical cases generally occurs at dryoff.

“Trying to get these cows uninfected is not very easy,” saysSmith, who is not a fan of antibiotic therapy for subclinical mastitis. The main exception, he notes, is when a dairy is facing action due to multiple bulk-tank somatic cell counts above the legal limit.

“Treatment depends on the pathogen and the cow,” says Schukken. Use pathogen test results and cow histories to determine whether antibiotic therapy will be effective and which tools to use. When analyzing these factors, put a strong emphasis on early-lactation animals and younger cows. “For example, there is no value in treating an eight-year-old cow with three infected quarters,” he asserts.

Use DHI records to monitor effectiveness.

5. Watch for new cases

Chronic cases of subclinical mastitis are not the only category to consider. Also watch for new infections. That is, cows that did not score above your SCC threshold last month, but did this month. Use the same strategy for these animals, as mapped out in steps two through four.

6. Set targets and teams

Once you’ve had a system in place for a few months, take some time to assess if it works or not. For example, chronic subclinically infected cows should comprise no more than 5 percent of the herd, advises Schukken. Results outside of this parameter should cause you to examine your overall mastitis-control program.

To help ensure success, Rhoda suggests that you set up a team or at least empower one person to address subclinical mastitis on your dairy. They should follow the steps outlined above once each month. You’ll be surprised how much this will help improve milk quality and cow health.