Mastitis treatments depend on the cow and the bug
How do you know your mastitis treatment protocol is actually working? And would the infection spontaneously cure had you done nothing?
Those are the big questions faced by dairy producers and their veterinarians, says Pam Ruegg, a veterinarian and professor of milk quality at the University of Wisconsin. That’s especially true on farms where contagious forms of mastitis—Streptococcus agalactiae (Strep. ag.) and Staph. aureus—have been largely solved.
"Most cases of mastitis caused by environmental pathogens present with mild or moderate clinical signs," Ruegg says. In fact, a summary of four studies involving 1,300 cows shows that 90% of the cows had mild or moderate cases of mastitis. Even in a study of coliform cases only, just 22% of the 144 cows in the trial were severe cases.
"Clinical signs will normally abate for the majority of cows within four to six days with or without treatment, but disappearance of clinical signs does not always indicate that the quarter has been successfully cured," Ruegg says. "While the milk appears normal, many of these cases may have simply regressed to a subclinical state."
The cow herself is a big determinant of cure rates. "Older cows have both a greater risk of mastitis and less chance of [treatment] success," Ruegg says. Overall cure rates (both bacteriological and clinical) are typically greater than 50% for first-lactation cows but fall to 25% to 30% for fourth-lactation and older cows.
Cows with previous cases of clinical mastitis have a reduced probability of success. Overall, when a cow has clinical mastitis, there is an 18% chance of her having a second case within 60 days. Early-lactation cows have a higher probability of recurrence than late-lactation cows.
Cows that had a clinical case of mastitis in the previous lactation are four times more likely to have clinical mastitis in the next lactation (6% versus 23%). Cows with a high somatic cell count (SCC) also show slower response to treatment.
"Before treatment, check the cow’s history," Ruegg says. If the cow has a history of high SCCs, extended-duration therapy might be warranted. But if she has had no prior clinical cases and a low SCC, and if no bacteria grow from culture, no treatment might be the better option since she’ll likely return to normal milk in four to six days.
Know your bug, Ruegg advises. "Therapeutic cure rates for mastitis caused by yeasts, pseudomonas, myco-plasma, serratia, nocardia and prototheca are essentially zero, regardless of treatment."
In contrast, environmental streptococci typically respond well to antibiotics, with a low spontaneous cure rate and high rates of recurrence when antibiotics are not given.
"We have to culture cows to identify pathogens. Even if a diagnosis is not immediately available, farmers can readjust therapy when the pathogen is diagnosed 24 to 48 hours after beginning treatment," Ruegg says.
On farms that have controlled contagious mastitis, 25% to 40% of cases have no bacteria recovered because the cow’s immune system kills it prior to sampling. "If you treat all cases, it masks the spontaneous cures of the no-growth samples that have already occurred," Ruegg says. Plus, it overestimates the effectiveness of your treatment protocols and wastes antibiotics.
|Older cows with recurring mastitis infections are often more difficult to cure than first-calf heifers suffering their first case. Photo: Rick Mooney
Extended-duration therapy has its benefits, but not in all cases. "There is considerable evidence that it increases cure rates for pathogens that have the ability to invade secretory tissue, such as Staph. aureus and some environmental streps," Ruegg says.
However, research to support its use to treat pathogens that infect superficial tissue (coagulase-negative staphylococci and most E. coli) has not been published, she says.
"When extended-duration therapy is routinely used for all cases without regard to causative pathogen, considerable milk is discarded unnecessarily. This is especially true for the 20% to 30% of clinical cases on most farms that are microbiologically negative."
Systemic antibiotic treatment of acute and severe coliform mastitis is generally recommended. A Michigan study shows that cows with severe coliform mastitis are more likely to die or be culled (37%) than cows that receive systemic therapy (14%). But a study of cows with mild or moderate coliform mastitis showed no differences in culling, loss of quarter, cures or recurrence. In a study of acute induced coliform mastitis, systemic therapy improved cure rates, reduced body temps and improved appetites.
The take-home message: Systemic antibiotics improve survival and clinical outcomes of cows with severe coliform mastitis. But routine use of systemic therapy is not recommended for mild or moderate cases, Ruegg says.