Treatment of gram-negative clinical mastitis

Many dairy farms practice culture-based treatment of clinical mastitis. This is great, because judicious use of antibiotics requires us to use antibiotics only on animals that are likely to benefit from anti-biotic treatment, and on most farms, less than 50% of cows with mild or moderate clinical mastitis meet this criterion. 

It is also great because it saves a whole bunch of money. Most of the available veterinary literature suggests that treatment of grade one and grade two clinical mastitis caused by the most gram-negative organisms is of little value. This is because the spontaneous cure rate is about the same as the cure rate following treatment, and other outcomes, culling, for example, are similar for treatment or no treatment as well. Most farms that practice culture-based treatment do not treat grade one and grade two clinical mastitis caused by gram-negative organisms.

A recent paper in the Journal of Dairy Science (2023 Bruno, et. al.) might make us think a little differently.

For this study, causative organisms were first identified as gram-positive, gram-negative or no growth by on-farm culture, and then confirmed by MALDI-TOF. MALDI-TOF is a new system that offers increased capabilities to correctly identify organisms. Cows with any gram-negative organism confirmed by MALDI-TOF were then allocated to one of three groups: no treatment, treatment with Spectramast LC for two days and treatment with Spectramast LC for five days.

Mastitis cases were scored as grade one: mild, with only abnormal milk, grade two: abnormal milk plus changes in the udder, and grade three: severe. Only grade one and grade two cases were used in the study. A total of 423 quarters were enrolled in the final analysis. Of these, 135 were not treated (CON), 133 were treated for two days (SP2) and 155 were treated for five days (SP5). E. coli was isolated in 87% of the gram-negative cases, Klebsiella was isolated in 7%, and the remainder classified as other. Out-comes measured were bacteriologic cure on days 14, 21 and 28 post treatment by assigning a clinical score based on clinical signs at multiple days post treatment, mastitis recurrence and by looking at culling and death rates.

Bacterial cure rate was higher for treated groups, both for two and five days than for controls. However, if results were sorted by clinical score at enrollment, almost all of the improvement in treated groups was due to increased cure rates in cases of moderate mastitis. The differences in cure rates for grade one cases between controls and treated cows were not significant. Clinical scores were better for cows in both treated groups compared to control cows. The return of milk to normal appearance was no different between treated groups and the control group. Since treated and non-treated both resolved by day five, there was no difference in the number of days of discarded milk for the control group versus the SP2 group, though milk was discarded for eight days in the SP5 group.

The proportion of quarters with recurrence of clinical mastitis was greater in the control group as compared to the SP2 and SP5 groups, but there was no difference in recurrence between SP2 and SP5 groups. Treatment also reduced culling. Culling within 14 days after diagnosis and through 30-, 60- and 90-days post diagnosis was less in both the SP2 and SP5 groups compared to the control group. Thus overall, treatment with Spectramast LC for either two or five days had better outcomes than not treating. It would have been valuable to be able to compare outcomes for cases caused by E. coli versus Klebsiella, but the small number of Klebsiella cases did not allow for that analysis.

There has only been one other peer-reviewed study that showed benefit from treating non-severe gram-negative mastitis. In that study, the benefits were mostly reduced culling. Thus, this is really the first study suggesting that treating non-severe E. coli mastitis infections makes economic sense on a dairy. It is particularly interesting that there was no difference in the required milk discard between the control group and the SP2 group. There were few benefits for extending the treatment to five days, and the increase milk discard with extended treatment would be expected to easily outweigh the benefits of five-day treatment. While bacteriologic cure rates were improved by treating moderate cases and not mild cases, clinical scores were better for treated mild cases versus the controls. In addition, the authors point out that in a non-research setting, farms may have difficulty correctly characterizing cases as mild or moderate and suggest that it would be advantageous to treat both mild and moderate cases for two days.

Science is always changing, but on the other hand, one study does not necessarily mean one needs to change any particular practice. If you think you may benefit from changing your current treatment protocols, you should discuss this with your herd veterinarian.

Jim Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minnesota. He consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at [email protected].