“Use it and lose it” is often said when it comes to antibiotic resistance concerns. Every time we use an antibiotic (in a person or animal), there’s some potential for resistance to emerge. The more we use antibiotics, and the worse we use them, the greater the risk, generally speaking.
Questions about the (rampant) use of doxycycline for treatment of Lyme disease and the potential for development of resistance come up periodically. For me, it’s mainly in the context of (over)treatment of dogs that come up positive on screening tests but are clinically normal. However, the same concept applies to people.
So, what’s the risk?
From a Lyme disease standpoint, it’s not much of an issue. That may not make sense at first glance, but when you think about it, it actually does.
Let’s look at some scenarios:
- If a dog is not actually actively infected with Borrelia burgdorferi, the bacterium that causes Lyme disease, antibiotic use can’t result in resistance in that particular bug since it’s not there (but there are still all the other bacteria in and on the dog’s body to consider too… see below).
- If a dog is infected with the bacterium and it is treated with doxy, odds are pretty good the bacterium will die, so still no resistance.
- For the B. burgdorferi in an infected dog to become resistant, it has to do it fairly quickly in the face of treatment. It’s most likely to occur from acquisition of a resistance gene from some other bacterium, given what we know about the mechanisms of doxycycline resistance in other bugs. So the B. burgdorferi needs to encounter another bacterium harbouring a doxy resistance gene. If B. burgdorferi caused infections in the gut, respiratory tract or on the skin, that would be more likely. However, in blood, within tissue and in joints, there’s much less of a chance of encountering another bacterium, let alone one that’s doxycycline resistant.
Okay, so in the very unlikely scenario of a resistant bug meeting B. burgdorferi, and a resistance gene being swapped, what now?
- Well, not much from a population standpoint. It could make Lyme disease harder to treat in that dog, but since dogs are dead end hosts (they aren’t a relevant source of infection of ticks), resistant B. burgdorferi would stay in that dog.
None of this is meant to say “sure, doxy for everyone!” Unnecessary doxycycline use is still a concern, but it’s a concern because of development of resistance in the myriad other bacteria that are present in the body, not B. burgdorferi. Creation of a wide range of other doxycycline resistant bacteria is the concern (and it’s almost certainly happening).
Resistant Lyme disease shouldn’t be an issue. If it occurs, it’s going to happen by the bacterium picking up tetracycline resistance genes in the wild, and there’s not much reason for a bacterium to go to the effort of picking up and maintaining an extra gene if there’s no benefit (because there should be little chance of being exposed to those antibiotics in the wild). It’s possible it could happen, since there are lots of resistance genes out in circulation, and there may be artificial/human influences (e.g. antibiotic use in livestock, resulting in exposure of wildlife to resistant bacteria), and because bacteria naturally develop some resistance genes to fight amongst themselves. However, odds of this creating a sustained, significant population of resistant B. burgdorferi have to be low.