Untreated mastitis – is that wise?

Some bacteria that cause mastitis respond to common mastitis treatments and should be treated that way. However, evidence shows that many cases of clinical mastitis do not respond to antibiotic treatment and so should not be treated with antibiotics. How can someone know the difference?

The answer is by culturing the milk of clinically infected cows to at least categorize the pathogens as a type that responds or a type that does not. Many will say sending milk away for culture takes days and by then the cow should have been treated. That may be true, but dairy producers culturing their own milk is an alternative with a much shorter time until the results are known (see article “On-YOUR-farm milk culturing?”).

At least three dairy producers in northeast Michigan do their own on-farm milk cultures. All three of these producers consistently produce milk from cows with average bulk tank somatic cell counts (SCC) less than 100,000. Culturing milk is part of their complete program for cow health and quality milk, not only because it provides information for treating the cow with mastitis, but because it helps them monitor the herd udder health and fine-tune their prevention program.

Julie Nelkie and her daughter, Abigail O’Farrell, at Lemajru Dairy Farm, LLC of West Branch have been culturing milk samples for about 7 years. Charina Dellar of Dellar Dairy Farm in Harrisville got started almost two years ago and Keith Kartes of Circle K Farms in West Branch has been culturing milk for about a year now.

Even with a low average SCC, do these farms think it is worth it to culture clinical cows? “Yes” is how they all responded emphatically. In fact, they agree that culturing milk is a key to their maintaining low SCC.

Culturing milk provides greater information with which they can manage both the individual and the herd. In regard to the individual, it means that they make the decision often to not treat cows with clinical mastitis. Does the decision not to treat have risks?

Researchers have examined that question. Studies involving eight dairy herds in two states; Minnesota and Wisconsin, as well as Ontario, Canada, looked at the effect of treating all clinical cows or just treating cows likely to respond to antibiotic based on on-farm culturing. There were a total of 7,360 cows on these farms with 422 cases of mild or moderate mastitis during the study period.

Articles by Dr. Alfonso Lago et. al. detailing the results of this study and an associated one were published in the September 2011 issue of the Journal of Dairy Science (94:4441-4456). A second article details the impact of treatment decision on cow performance measures.

In the first article, the authors reported on infection results. In the eight herds, cows were divided into two treatment groups: the positive-control group where all cows with clinical mastitis were treated with antibiotic and the culture-based treatment group where the herdsman would culture milk from mild and moderate cases of clinical mastitis and treat (same treatment) only those cows that had gram-positive (staphylococci or streptococci) growth.

It is important to note that in the study, treatment was delayed until cultures were complete only for cows with milk to moderate clinical mastitis. Treatment should not be held from cows with severe (fever, off-feed, depressed) mastitis while waiting for culture. Many of these types of mastitis can result from life-threatening coliforms, and while culture is still useful for management decisions, antibiotic and supportive therapy should be given immediately.

Among the eight herds recruited for this study, personnel from four farms already did on-farm milk culturing and four were trained to do it for the course of the study. They used bi-plates that distinguished between gram-positive and gram-negative bacteria.

The researchers report on comparisons of how cows did in response to each of these treatment groups. The first thing they note is that only half the number of cows in the culture group were treated (initially or later) compared the positive control group, in which all cows showing clinical mastitis were treated. Therefore, on-farm culturing cut antibiotic use and treatment costs in half.

But how did those cows do that were not treated? Four different measures were used to answer that question:

• Days to clinical cure – The time (days) until milk appeared normal again; that is, the absence of clinical signs of mastitis. No significant difference.
• Bacteriological cure risk – The frequency that there was no subsequent bacterial growth when checked at 14 days and 21 days after mastitis was initially discovered. No significant difference.
• New intramammary infection (IMI) risk – The presence of a new bacterial species growing at days 14 or 21 that was not originally cultured. No significant difference.
• Treatment failure risk – An overall measure of treatment success that combined measures including bacteria cultured at 14 or 21 days – whether the original or new species, if a second flare of clinical mastitis required treatment after initial, or if the animal was culled or died by 21 days. Any of these was called treatment failure. No significant difference.

In addition, they looked at how long the milk from mastitic cows was kept out of the tank and found that the withholding time for the culture-based group was reduced by one day compared to the other group.

Based on these results, the authors concluded that on-farm cultures on these farms reduced antibiotic use significantly and days of nonsaleable milk with no loss in mastitis control on the farms. On-farm culturing resulted in win-win decisions.

Indeed, that is the same result seen by the producers in northeast Michigan.

Source: Phil Durst, Michigan State University Extension

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