Mastitis is still one of the most common and expensive problems on farms, even after improvements in reducing subclinical cases over the past 20 years. Because several different forms of bacteria can cause it, the right diagnostic test can help producers catch infections earlier and make better calls on treatment, prevention and culling.
“Finding mastitis before it turns into a bigger problem is the best way to limit the damage,” says Pamela Ruegg, the David J. Ellis Chair in antimicrobial resistance at Michigan State University.
Ruegg says no single test answers every question. Producers need to know what each tool measures and where it fits in a whole-herd milk quality program.
“We have a bunch of tools in our diagnostic toolkit, and there are different ways to test for mastitis,” Ruegg says. “We have to know when to use each of those tools.”
Three Categories of Mastitis Tests
Most milk quality programs use a mix of older, familiar tests and newer diagnostic options. Ruegg sorts them into three groups: inflammation, culture and molecular.
- Inflammation tests measure inflammation, including somatic cell counts and automated sensors for milk conductivity or lactate dehydrogenase (LDH). These tests show whether a cow is responding to an infection, but they do not name the organism causing the trouble.
- Culture tests identify the pathogen through bacterial culture. Lab and on-farm cultures show which bacteria are present and support culture-based selective treatment protocols. Culture results also help shape prevention plans by showing whether contagious or environmental pathogens are involved in the herd.
- Molecular tests include polymerase chain reaction (PCR) testing.
“PCR testing detects DNA fragments from both live and dead bacteria,” Ruegg says. “It’s a really sensitive test for determining the type of bacteria in the milk sample, but it can’t tell us if the infection is active or if it’s already been cleared.”
Ruegg also points to MALDI-TOF, a lab method used after bacteria grow in culture. The technology helps labs identify bacterial species more precisely, especially organisms once lumped together, such as environmental streptococci.
Detection Isn’t Always Diagnosis
Ruegg says producers should know the difference between finding inflammation and diagnosing an active infection.
Subclinical mastitis is generally defined as a somatic cell count above 200,000 cells per milliliter. But inflamed quarters do not always turn up a positive culture result.
“In many inflamed quarters, when we sample them, they may be culture negative,” Ruegg says. “That may indicate freedom from infection or simply a false negative.”
She stresses that every diagnostic test has limits. Highly sensitive tests, including PCR, find more infected cows but can also create more false-positive results. More specific tests, such as bacterial culture, cut down on false positives but may miss infected cows because of false negatives.
Ruegg recommends using several pieces of information instead of leaning on one test alone.
“The best way to use tests is to put them all together,” she says.
For example, a cow with somatic cell counts staying high but repeated negative cultures may still have subclinical mastitis, so more testing may be worth the time.
Herd Prevalence Changes How Results Should be Read
The number of infected cows in a herd also affects how producers should read test results.
“If Mycoplasma bovis is rare in your herd, a single positive test has a low predictive value,” Ruegg says. “You should confirm it with another test before culling because it could be a false positive.”
Herds with an established Staphylococcus aureus problem need a different approach.
“In a herd where you know you have a problem with Staph. aureus, a negative culture from a chronic cow is likely a false negative due to intermittent shedding,” Ruegg says. “You should consider the cow to be infected until proven otherwise.”
Start with a Management Plan
Ruegg encourages producers to decide how they will use results before they collect samples.
“Never perform a diagnostic test unless you have a predefined management plan for the result,” she says.
She recommends using inflammation-based tests, such as somatic cell counts, to monitor herd performance, then using culture or PCR to support cow-level treatment and management decisions.
Ruegg also encourages producers to work with their herd veterinarian when reading results, especially with contagious pathogens where herd prevalence affects test accuracy.
“Test results are only useful when they’re used to make a management decision,” she says. “The selection of which test to use depends on the action you’re going to take.”
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