Staph. aureus One Tough Bug

May 10, 2011 09:17 PM

If you have Staph. aureus infections on your dairy, you are faced with limited treatment options.

Culling chronically infected cows is often the best option, since cure rates are low and the infected cows pose a threat of passing the disease to herdmates.

Treatment of clinical cases of Staph. aureus in chronically infected cows is not cost effective because cure rates are less than 35%, says Pam Ruegg, a veterinarian and professor of milk quality at the University of Wisconsin. "In most instances, when the clinical symptoms disappear, the infection simply returns to a subclinical state," she says.

If the clinical case is mild, Ruegg recommends isolating the cow, discarding the milk until it returns to normal and then making a decision about culling the animal. Cure rates for subclinical mastitis caused by Staph. aureus decline with age (81% for cows less than four years of age to 55% for cows eight years or older), number of quarters infected and increasing somatic cell counts.

"Extended duration therapy has been shown to increase cure of clinical mastitis caused by Staph. aureus, and at least five days of therapy is recommended," Ruegg says. "Extended-duration intramammary treatment of subclinical cases may be successful for young cows, in early lactation with recent single-quarter infections. But it should not be attempted for chronically infected cows."

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