Treat the pet, not the lab result
The title of this post describes a very basic concept, but one that is sometimes forgotten or hard to follow. The key point is that the goal of treating a sick pet is to make the pet well. Getting well and getting "normal" laboratory data aren't necessarily the same thing.
A question that comes up a lot with MRSA, MRSP and other bacterial infections is "should my pet be tested after treatment to see if the bug is gone?" The answer is usually "no."
One reason to not re-test is just what I said above. The goal of treatment is to make the patient healthy. That doesn't necessarily mean that MRSA, MRSP or whatever bug is causing the problem needs to disappear. In fact, the bacterium that causes a given infection often remains (in smaller numbers) in or on the body somewhere. If it's a skin infection, the bacterium may still be present on the skin where the infection was. However, if the body is handling it well, then it's not necessarily a problem. We have to remember that every animal (and person) has multiple types of bacteria in or on it that can cause disease given the right circumstances. We're never going to eliminate them all. In fact, trying to get rid of all potentially bad bacteria would probably result in bigger problems.
Another concept that I emphasize a lot is only do a test if you have a plan for using the result. If you don't have a clear reason to do the test, and if the result won't impact your decisions or provide information you need in the future, then why do it? While a negative culture might be nice to see, it's not necessarily a guarantee that a particular bug is gone. Furthermore, a positive culture doesn't lead to actions that are any different from those that would be taken if the culture is negative in most situations because, as mentioned above, we're looking for clinical cure (getting better) rather than microbiological cure (getting rid of the bug). Rarely would we extend treatment or do anything different in response to a positive culture in a healthy animal.
The bottom line is post-treatment cultures are rarely needed. There may be some circumstances where testing after treatment is useful and where the results would lead to a defined plan of action, but these are few and far between.
Image: A Mueller-Hinton agar culture plate being used to test the antibiotic susceptibility of a bacterial isolate according to the Kirby-Bauer method. (source: CDC Public Health Image Library #10785)
Skin infections in dogs: Stopping the downward spiral
A large percentage of advice calls that I get about methicillin-resistant staph infections in dogs are regarding skin infections. Skin infections (pyoderma) are a common problem, a leading cause of antibiotic use in dogs, and an often frustrating problem for vets and pet owners alike. One problem is that, unlike many other types of infections, skin infections are often recurrent. This frequently leads to an ongoing cycle of infection-treatment-resolution-infection-treatment-resolution... The net result is some dogs get treated very regularly and for long periods of time with antibiotics, and it's not particularly surprising that highly drug-resistant bacteria like MRSA or MRSP eventually become involved.
Normal, healthy, intact skin is an excellent barrier to bacterial infection. Various bacteria normally live on the skin but do not usually cause infection. Skin infections typically (if not always) develop in response to some underlying skin disease, such as flea allergy dermatitis, food allergy, atopy, Cushing's disease or hypothyroidism. Identifying and treating a skin infection is one thing. Identifying and treating the reason for the infection is another, and that is arguably the most critical component. Ignoring the underlying cause may not be the end of the world for a single infection, because proper treatment and a susceptible bacterium can result in a successful outcome, but ultimately ignoring the real problem can lead to a difficult-to-treat, resistant infections.
Any diagnosis of pyoderma should be accompanied by consideration of the underlying cause. If a cause is apparent, this should be treated (if possible). If a cause is not readily apparent, it should be investigated. By investigated, I mean a real search for the problem, not a cursory examination, half-hearted feeding trial and little more. There is almost certainly an underlying cause and, at the end of the day, time, effort and money are better spent on trying to identify the root issue rather than just throwing round after round of antibiotics at the dog. In some cases, the cause (while it's probably there) can't be identified, but it's definitely worth trying anyway.
If your dog has been diagnosed with a skin infection, ask why it happened. If there is not a clear answer, talk to your veterinarian about the best plan to identify the cause. If at all possible, follow through with the plan. It may include certain diagnostic tests (which cost money) or dietary restriction (which takes effort), but it should be thought of as an investment in your pet's health, as well as a potential way to keep multidrug-resistant bacteria (some of which can infect people) out of the house, to save future treatment costs, and to keep your pet much more comfortable.
