The importance of sensitivity testing
Successfully treating mastitis starts with an appropriate drug choice. Microbiological culturing and antibiotic susceptibility testing are still the most feasible options worldwide to determine which antimicrobial has the best chance for success. Even when the cow is treated prior to obtaining the results, the information is still valuable to assess whether the antimicrobial used was appropriate. If the cow is suffering from a recurrence of the problem, the correct treatment can be initiated more quickly, especially when the bacteria appeared to be resistant to the product that was initially used.
Moreover, a summary of records of susceptibility results can be used to establish a pattern of which antimicrobials work and which do not for different types of infections on a specific farm over time.
However, using antimicrobials based on in vitro susceptibility is not a guarantee for success. The test measures the growth response of an isolated organism in the presence of a particular drug under standardised conditions. One should take into account that the in vivo situation is far more complex. Staphylococcus aureus bacteria, for instance, are able to hide inside immune cells or to form a biofilm. By doing so, they protect themselves from being killed or inhibited by the given antimicrobials. The success of mastitis therapy also depends on factors such as the appropriateness of the route of administration, the stage at which treatment is initiated, the severity of the inflammatory reaction and the cow’s immunity. Those factors might distort the relationship between the susceptibility results and the clinical outcome in one or the other way.
Those downsides do not outweigh the benefits of susceptibility testing. Combining the results of microbiological cultures and susceptibility testing with clinical information and experience is still very useful in setting up a herd-specific, standard mastitis treatment protocol. Such a herd treatment protocol is yet the only option available to limit the number of treatments, track their success and, above all, to keep frustrations of both farmers and vets due to treatment failure to a minimum.
Author: Dr. Sofie Piepers - M-team University Ghent