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The means by which bacteria get into the bladder and cause infection is generally from what is known as “ascending infection”. MRSA/MRSI/MRSP may all live normally around the perineum, the skin between the anus and genitals. Bacteria that are in this area may “climb” up the urethra and into the bladder; this is true of many bacteria that cause cystitis, not just MRSA/MRSI/MRSP. Often the bacterias’ “climb” to the bladder is partially aided by the urethral defence mechanisms not working as well as they do normally (for example in older patients, the immune system may not work as well as normal).

In some cases, when treatment is started, we successfully reduce the number of bacteria infecting the bladder can be knocked down far enough for the patient to start behaving apparently normally. However, if even a tiny number of bacteria remain when treatment is stopped, they may cause a repeat infection some time after treatment stops. For this reason, lengthy antibiotic courses may be needed.

Also, as the MRSA/MRSI/MRSP may live normally on the dog’s skin in the perineal area, this means that even if the MRSA/MRSI/MRSP is cleared from the bladder, it may persist on the skin longer and even return to that area after we thought we had cleared it (much like if we take antibiotics, they may kill “good” bacteria in our intestine, but those bacteria will eventually find their way back). If the bacteria can persist or return to the skin, they may re-invade the bladder. This is similar to humans who may be “prone” to bacterial cystitis and need treatment every so often; it is just unfortunate that the bacteria present are MRSA.

It should be noted that the route of traffic from the bladder is generally one-way; huge amounts of fluid are coming to it through the ureters constantly, and exit through the urethra sporadically. This means that it is effectively being flushed in a one-way direction regularly, and makes it difficult for bacteria to get deeper into the system from the bladder as they would have to either travel through the thick bladder wall or up the ureters against the constant flow of urine. As such, these patients are unlikely to develop lots of other signs besides their cystitis, because the infection doesn’t really get much further. This is again true of most bacterial cystitises, not just MRSA/MRSI/MRSP.

The other means by which bacteria may get into the bladder is via the blood stream, most commonly through the kidneys; however in these cases there are normally signs elsewhere in the animal – they are normally much less well than typical cystitis cases.

In the case of the dog in the nursing home, I suspect that its perineum is colonised; however, as discussed by Annette, decolonisation is an inexact science – we don’t even know if it is possible to do it long term. I suspect it may be just as difficult as trying to rid someone of the bacteria that normally live in their guts.

Should the owners be swabbed? No. The reasons are that:

1) If they are MRSA/MRSI/MRSP carriers, this can be considered entirely normal, in the same way as we all have various other Staphylococcus species all over our bodies – if they aren’t causing a problem, there is no point trying to fix them. They can be considered normal, as opposed to being thought of as an infection.

2) If the bacteria are there normally, we probably couldn’t permanently decolonise either the dog or the owners anyway – at present, chances are that we would do nothing if we found MRSA (as it isn’t causing a problem to the owners) and even if we tried to treat it, we would be trying to decolonise them the same way we would try to remove a normal bug from the human gut – as said before, it’ll just come back most likely, and it wasn’t causing a problem anyway – so what’s the point testing for it if we aren’t doing anything about it? And why drive a potential wedge between a dog and its owners when the owners are actually absolutely fine? It would be horrible for all concerned, not succeed, and also be an attempt to treat a non-existent problem.

3) Finding MRSA on the owners would actually not be sufficient to tell us whether they are actually colonized, or just happen to have had some MRSA splatter onto them from their carrier dog (incidental contamination); therefore, again, in the absence of any clinical signs on them, it would take a lot of repeated samples over a period of time to determine whether the bugs were actually colonising the owners. The information gained by doing so would not change how we treated the owners (for the reasons listed above). Therefore, it would be completely pointless to go through the stress and not-insignificant costs of doing this.

The one precaution I would take is that if they were going to have surgery, or be put on medications that may suppress the immune system, it may be worth checking their MRSA status. This would be done by the doctor if necessary.


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