Microchipping rats and mice

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The text on this page is taken from an informal compilation of opinions of contributors to the online VOLE List. As such, they are not peer reviewed and may contain differences of opinion. Those wishing to contact the list may contact Adrian Smith.

I would be interested to hear your collective views on microchipping rats and mice. There seems to be a considerable variation in opinion as to whether the “newer style” chips, many of which are delivered via a 21g needle, should be implanted under anaesthesia, or physical restraint. I note that some of the review articles on identification of methods seem to avoid mentioning the issue, and others say “its a matter of preference”. There seems to be a lack of published data on the immediate adverse effects (if any) of implantation, the few papers out there seem focussed on tissue reaction long term etc. So if you microchip your rats and mice do you have a policy, is it based simply on consensus, or do you advise one approach or the other and leave it up to investigators to decide?

I have been doing quite a lot of digging into the literature, and can get some evidence base that:

  • In people, decreasing the needle size does reduce (slightly) pain on placing a needle subcutaneously
  • Somatosensory (and nociceptor) innervation of skin is very similar in rodents and humans but central processing differs (so nociceptor stimulation in skin should produce pain, and less stimulation should cause less pain).
  • One interesting paper showing bevel up injection causes less pain than bevel down (remember having that debate regarding ease of getting a needle into a vein…).
  • One non-peer reviewed study showing implants of the smallest chips didn’t change growth rate in juvenile mice.
  • Variations in SOPs on lab animal unit web sites from “no anaesthesia” to “anaesthesia if preferred” to “give an anaesthetic”.
  • Recommendations from assorted non-lab organisations of anaesthesia “to prevent risk of injury” to small mammals and birds - but not to avoid pain.
  • Most manufacturer’s of microchips seem to assume no anaesthesia needed.

We do chip mice in some of our facilities. Considering the risk benefit/aversiveness of anaesthesia (especially multiple anaesthesia during the experimental life of the animal), we aim to anaesthetise and pair the chipping with another invasive procedure such as implant of tumoral cells for instance. While we did check that anaesthesia doesn’t affect the behaviour of the implanted cells, we didn’t look into it from the animal’s perspective.

I should add that quite a few of our scientists would rather anaesthetise to chip for their own comfort anyway…. None of this is properly evidence based, but there you go.

Microchipping, like other procedures, were regulated by the SOP and never left up to discretion of the investigator.

21G needles are at a diameter that I would be considering not using anaesthesia, especially if into the scruff (I would want to see base of the tail first hand but doesn't look too bad in the video I saw).

The reason being even brief volatile anaesthesia can have a level of unpleasant recovery in humans e.g. "Emergence delirium in children undergoing dental surgery under general anesthesia", so until there is good evidence otherwise, I would think the brief pain of insertion would be less "harm" than the anaesthesia.

In the case of the larger chips (which many still use) a very large needle is being inserted into a very small animal. Needle stick accident risks to both animal and human are reduced if the animal is immobilised, so the procedure becomes less error prone, which when many hundreds are being done is important. I also know of historical cases where chips would re-emerge or at least hamper wound healing if left too close to the wound. An immobilised animal allows for creating a s/c tunnel of decent length to deposit the chip away from skin wound and massage all into place. For these reasons I always advocated brief anaesthesia.

I collected quite a few opinions, totally opposed: Strong views about absolutely you need to do it under GA, "it is painful, period" and strong views about the opposite as concerns about the gas aversion and the stress caused by being exposed and knocked down by a gas to then awake feeling unsteady and dizzy.

I reviewed some papers and I also had a conversation with a researcher from Switzerland regarding their experience microchipping wild mice without anaesthesia. Then I observed the procedure both ways with and without it. My conclusion was, and this is my personal view, aligned with the opinion of the Switzerland researcher, that the handling was the stressful part for the mice, not the injection. The anaesthesia was adding extra stress, as taking longer for the whole experience and the animal needed to be handled anyway. I also believe that the smallest microchips that we have now available, together with the long bevel designed to be "painless" has really made it less painful and I don’t see the justification for the anaesthesia. Adding, as long as the restrainment technique is good.

Just a thought, without ever having observed or compared the two methodologies (although the thinking below could apply to many situations)... If one considers this from the point of view of a prey species (and with apologies for any excessive anthropomorphism), then possibly gaseous anaesthesia having been gently transferred to a knockdown box would be puzzling and confusing whereas, no matter how good the technique, being restrained by scruffing would feel like a predator attack and a prelude to being dispatched. If I was a mouse, I'm pretty sure I'd 'prefer' the former - and forget it quicker.