How to detect subclinical mastitis

If you thought "looking for a needle in a haystack" was difficult, try looking for something you can't even see. That's how many producers feel about subclinical mastitis.

Lactating animals with subclinical mastitis show no clinical signs or visible changes in the milk, but they do have somatic cell counts greater than 200,000, and that can be a valuable clue. 

Use these five steps to detect subclinical mastitis in your herd.

1. Get SCCdata for every cow

Individual cow results for somatic cell count are available through Dairy Herd Improvement test-day data. If you are not currently on DHI, you have a couple of options: (1) use a direct cell counter or (2) test every cow using the California Mastitis Test. While the CMT does not give the somatic cell count, it does indicate potential problems by the amount of thickening or gelling that occurs when a re-agent is added to the milk.

Or, you could go on DHI-test for a couple of months, says Andy Johnson, milk-quality veterinarian from Clintonville, Wis. This gives you the data you need to assess the status of individual cows, and will generally cost less than the amount of money the dairy is losing from lost milk production and lost milk-quality premiums. In a large herd, says Johnson, milk production and premium losses can easily total $3,000 to $5,000 per month.

The cost of DHI testing starts at about $1 per cow for owner-sampling, and about $2 per cow for supervised testing, explains Jill Makovec, manager of education and information at AgSource in Verona, Wis.

2. Analyze the data

Once you have SCC data on each cow, start identifying targeted groups of animals. These data can be sorted using Dairy Comp 305 or other dairy-management software, explains Dave Rhoda, veterinarian with the University of Wisconsin"s Milk Money program. Use the one that you and your veterinarian are most comfortable with.

DHIA also offers a report called the "udder health management summary" that details many of the groups discussed here. 

3. Examine first test-day data

Examine SCC data for all newly freshened animals, and find the ones with SCCs greater than 200,000. Johnson recommends that no more than 15 percent of newly fresh animals fall into that category. 

Rhoda takes this one step further by looking at first-calf heifers and cows separately. This, he says, will help you pinpoint where infections are occurring.

4. Look for new infections

"Newly infected animals" are those that had a low SCC last month but are now above 200,000. To find them, compare data from two consecutive months, using herd-management software or the DHI report mentioned above. Johnson recommends that less than 7 percent of your lactating animals fall into this category each month.

5. Find chronically infected cows

Cows that have a SCC above 200,000 for two or more consecutive months, but lack clinical signs, are chronically infected. Makovec recommends the prevalance of cows with elevated SCC be less than 15 percent. She includes chronic and new infections in this goal.

Next steps

Once you have identified animals in the target groups, you can develop an action plan for each cow. To learn how, please see "Reduce subclinical mastitis cow by cow," on page 27.

You also can use the records to find sources of infection and develop prevention plans.