Bot Fly Larva

Subject: Bot Fly Larva
Location: North Bay, Ontario, Canada
August 21, 2014 10:25 pm
Hi there,
I am located in North Bay, Ontario, Canada. I have recently found a mouse inside my house walking around pretty slowly. I put gloves on and picked him up to put him outside and when I looked at him I saw a weird brown thing protruding from his side. Upon closer examination I determined it was alive and I recognized it as a bot fly larva that I had read about online a while ago while researching animal parasites. I pulled it out carefully with tweezers, plus about 5 other ones. They were quite large. I have a video of this extraction. I estimate the larger ones were roughly 3cm, maybe slightly larger. Definitely matched the description of rodent bot fly larva. I kept the mouse in a container and fed him until his wounds healed and let him go.
A couple days later (before I let the other mouse go) I was cleaning out and removing a big work tent that was in our backyard that had been used for our house renovations. It was damp, lots of wood scraps etc. I emptied a basket of garbage wood and a mouse emerged from the stuff I was dumping. He was slow and you could actually see two huge bot flies hanging out of him. Very disturbing.
Due to the fact that I have worked extensively in that gross work tent, plus the other mouse was found in our house full of the parasites, some serious questions have come up.
Firstly, how concerned should I be regarding bot fly infections on/in me or my two cats? Is there something I should be looking for on the three of us (obviously a gross black worm thing, but I would prefer to catch it waaaay before that).
Secondly, is this normal??? Are bot flies common this far north? Should I be reporting this, and if so, then to who?
Lastly, how do I avoid coming into contact with the eggs? Are there common types of material they are laid on or environments I could perhaps minimize in order to dissuade them from being laid near my house?
Thank you for your help with this.
Signature: Kate Griese

Bot Fly Larva
Bot Fly Larva

Dear Kate,
Thank you for your thorough and engaging request.  You are correct that this is the larva of a Rodent Bot Fly.  A link from that posting is no longer valid, however we did quote from what might have been the College of Veterinary Medicine at Michigan State University which stated:  “
Cuterebra is a normal bot fly of rodents and rabbits, but can also infect cats, dogs, and man. ”  This online library seems to support that cats can become hosts to Rodent Bot Fly larvae.  Companion Animal Parasite Council indicates:  “Cats and dogs are accidental hosts.”  VCA Animal Hospitals indicates:  “Cats are accidental hosts of Cuterebra larvae. They are most commonly infected when they are hunting rodents or rabbits and encounter the botfly larvae near the entryway to a rodent’s burrow. Most cases of warbles in cats occur around the head and neck.”  BugGuide data on sightings indicates that you are in the normal range for Rodent Bot Flies.  We believe it is highly unlikely that a human will be parasitized by a Rodent Bot Fly.  We will attempt additional research on this when time permits.

1 thought on “Bot Fly Larva”

  1. I live in southern Ontario. I own a Chihuahua, we spend alot of time outside together. Last summer while she was sitting on my lap a weird long slender brown fly landed on its back. I brushed it off but like it was attached to a bungee cord it would fly right back to the spot i just brushed it off of. It seemed to be using its wings to flap and burrow into the dogs fur. It seemed so desperate to do whatever it was doing. I thought it was trying to bite the dog or lay eggs on or in it.
    Now just today I came in from a walk and an identical fly was on the dogs back. I thought it was just a piece of fuzz so i brushed it off with similar results as last summer. Their was no larva last time I hope im wrong or I got to it before it laid any eggs.


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