Botfly
Botfly in Rabbits:
Botflies are a parasitic fly that burrow under the skin of animals (humans too, but a different spieces found in Mexico and South America), Cuterebra Cuniculi is the species that I found on my rabbits in Michigan. Here's a picture of a botfly larvae under the skin of a rabbit. The pink area in the center of the belly is the botfly larvae.
Botlfly larvae incubates from 16-60 days but may overwinter as long as 7-11 months. The only prevention for botfly is to control the fly population around your rabbits.Here is a picture of the adult fly that lays the larvae, the flies do not eat and live only about 2 weeks.
Removing the larvae:
You must be careful not to kill the larvae while it is still in the rabbit, the injured larvae release a toxin that may kill the rabbit.
You will see a breathing hole at one end of the lump, enlarge the breathing hole with tweezers, and flush the site with peroxide or sterile saline, I have also read that you can cover the breathing hole with vaseline. The larvae will poke it's head out to see what is going on and that's when you use tweezers or hemostats and pull the larvae out, it comes out easily. And then you should kill the larvae, we burned ours. Here's a picture of what we pulled out.
The rabbit should recover completely, but watch for signs that the botfly exit site is getting infected.