How do I link all those? It’s not as big of a stretch as you might think, but it’s definitely getting into some theoretical components.

Dogs are unique from a Lyme disease perspective in that healthy individuals are very commonly tested.

Hundreds of thousands of dogs get tested every year for heartworm, and common heartworm tests also test for antibodies against Borrelia burgdorferi, the bacterium that causes Lyme disease. Seropositivity rates vary a lot by geography and lifestyle, but rates of up to 10% are not uncommon in some areas. The vast majority of those dogs don’t have Lyme disease. They were exposed to the bacterium at some point from an infected tick, mounted an immune response that produced detectable antibodies, and nothing untoward happened to the dog. A minority of exposed dogs ever go on to developing Lyme disease.

The upside: We get lots of surveillance data with all this routine testing.

The downside: There’s often a pressure to “do something” when there’s a positive result, even though doing nothing is usually the best approach.

I think things are improving, but a large number of healthy dogs are still treated unnecessarily with antibiotics (mainly doxycycline) because of positive routine antibody tests.

There are several concerns with that, but the potential for emergence of antibiotic resistance is one of the main ones.  Sometimes, people think that means a higher risk of resistance of Borrelia burgdorferi to antibiotics.  However, I’m concerned about emergence of resistance in the myriad other bacteria that are present in and on the dog, such as staphylococci and E. coli.

The risk of emergence of doxycycline resistance in Borrelia from unnecessary treatment of dogs is basically zero. It’s not because Borrelia can’t become resistant. Presumably it can, but it comes down to some basic ecology. There are a few basic things that need to occur for resistance to be an issue:

  • The bacterium has to be present.
  • A resistant strain has to emerge.
  • That resistant strain has to be spread to other individuals (directly or indirectly).

In dogs that are not actively infected (but still antibody-positive), resistance in Borrelia can’t emerge because the bacterium is not there.

In dogs that are actually actively infected, it’s theoretically possible that resistance could emerge during treatment. However, that would be the end of it because dogs are “dead end hosts” that don’t spread the bacterium any further. Emergence of resistance in reservoirs of the bacteria (in this case rodents, particularly white-footed mice) would be a concern, because then ticks could spread the resistant strain to many dogs (or people).

The main risk of emergence of doxycycline resistance in Borrelia burgdorferi is more likely heavy use of tetracycline in livestock, especially pigs.  Tetracycline is closely related to doxycycline, and the bacterial resistance mechanisms to the two drugs overlap. So, tetracycline exposure is a risk for emergence of doxycycline resistance, in general.

Livestock aren’t reservoirs for Borrelia, but heavy use of tetracycline on farms, especially in feed, could lead to exposure of mice to the antibiotic (when they get into stored or spilled feed, for example), and then theoretically there would be some resistance selection pressure if those mice are carrying Borrelia.

Does this happen?

I don’t know. I’m not sure it’s been investigated. Resistance doesn’t currently seem to be a big deal in Borrelia . However, it makes sense biologically.  While it’s not likely to cause a rapid increase in resistant Lyme disease, we worry about the longterm, cumulative impact of any antibiotic use, in any species (humans and animals). So, I wouldn’t discount it, and it’s yet another reason for us to try to minimize and optimize antimicrobial use in livestock and reduce environmental contamination with antimicrobials (from livestock and other sources).

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