Persistence pays

There are only three ways to eliminate mastitis from a herd. Infected cows can experience spontaneous recovery. They can be culled. Or they can be treated with an appropriate drug.

Chronic gram-positive mastitis cases, such as those caused by Staph. aureus, are the toughest to crack. It has been estimated that more than 75% of the dairy farms in the U.S. experience some financial loss from this bug.

Untreated gram-positive infections allow the bacteria to penetrate mammary gland tissues. The cow's mammary cells try to close off the infected area by forming an abscess, which later turns into scar tissue.

Drugs have a hard time penetrating this scar tissue. And white blood cells don't have much luck in killing off the bacteria, either. The mastitis bugs just lie dormant until the white blood cells die in three to seven days. And then the released bacteria resume cell division and the infectious process.

"The bacteria's goal is to stay alive no matter what we do," says Austin Belschner, Dairy Program Manager for Pharmacia and Upjohn Animal Health, Kalamazoo, Mich.

Chronic bugs have an extra edge when accosted by antibiotics. Most mastitis drugs were approved by the Food and Drug Administration for treating subclinical mastitis with the least amount of drug possible. So you're not going to see even approved drugs take care of chronic mastitis bacteria in two to three tubes.

If short-duration therapy isn't effective against clinical chronic gram-positives, what about treating cows with more than one shot and for longer periods of time? Researchers looked at extended treatments (ET) in the early 1980s and saw modest improvements. But not enough to justify the time and cost of doing it.

A decade later, when researchers were experimenting with Upjohn's pirlimycin on Staph. aureus, they noticed improved cure rates when the antibiotic was given twice 24 hours apart.

"Cow bacteria was almost gone at 36 hours," Belschner says. "That got us thinking. Should we treat longer?"

Belschner and mastitis researcher Steve Nickerson at Louisiana State University Hill Farm decided to test the ET theory. They worked with the following protocol: Inject chronically infected cows twice with 50 mg pirlimycin in all four quarters 24 hours apart. After the label 36-hour milk discard period, they'd treat all four quarters again twice at 24 hours. After a second 36-hour milk discard period, they'd reinfuse all four quarters a third time. They then followed this up with dry-cow therapy.

"We weren't worried about cost at this point," Belschner says. "We were only looking at cure rates." By treating cows right before dryoff, they had less milk to discard, better efficacy and less risk of milk adulteration.

"But this first trial confounded results," he says. "We didn't know if we were getting a cure from the three-time injections or dry treatments." They are still working with treatment prior to dry-off. No additional data are yet available. Other opportunities soon surfaced for Nickerson to try ET in commercial herds with somatic cell counts (SCC) above 750,000 cells/ml. Using the same treatment protocol, researchers saw 62% of the quarters cured in their first four herds and rapidly decreasing SCC.

Thus far, Belschner says they have more than 500 cows in their data set in multiple herds showing a 40% to 50% cure rate. "But some herds have a cure rate of 10% or less," he adds. In herds with less than a 15% cure rate, researchers believe those cows were infected with a different Staph. strain.

Taking treatment a step further, researchers also are looking at mastitis vaccines in combination with ET. Herds in Michigan and Utah have benefited from this one-two punch. Cure rates were boosted by another 15% to 30%.

"Vaccinating is still experimental," Belschner says. "We're seeing cure rates of 10% to 20% on two tubes, 40% to 50% with ET, and 60% to 65% cures with ET and vaccinating."

Curing cows of mastitis is a noble goal, but at what cost? In Colorado, treatment costs were figured on a 40% cure rate. Per cow, 24 tubes of mastitis treatment cost about $75 and the discarded milk, another $75. Total cure cost: $375 ([$75 + $75] ÷ 40% = $375).

"Staph. aureus cows are usually good milkers," Belschner says. "You'd get $350 to $400 for her as a cull. But you've got at least a $500 to $800 difference in her value as a milk cow vs. a McDonald's cow." Within this difference, he sees the opportunity cost for treatment.

Belschner says FDA considers ET an "on-label" use of pirlimycin, so no extended discard is needed beyond the last dose's requirement. Since it's a prescription drug, it must be bought from a vet.

He adds: "Chronic infections are difficult to cure. You need to balance treatment costs and risks with expected outcomes."

Sidebar:
The basics still don't come in a bottle

Good mastitis management control doesn't start or end with a tube, says Austin Belschner at Pharmacia and Upjohn. It begins with following tried-and-true control measures:

• Clean and dry teats thoroughly—teat ends, too
• Use properly functioning milking equipment. Avoid unnecessary air admission into the teat cups during attachment and take-off.
• Disinfect teats before and after milking. Postdipping is the single most effective practice to reduce new contagious infections
• Milk Staph. aureus cows last or milk them with a separate unit.
• Use dry-cow mastitis treatments
• Maintain a closed herd. Culture milk from purchased cows before adding them to the herd.
• Cull chronic cows.
• Work with your vet on milk quality goals and to assess treatment options.