In August 2003, I was contacted by Julie with a story about her mouse Archie and a parasitic insect known as the Cuterebra fly. At the time I'd never heard of this parasite. Recently I've received information about another mouse that was infected and thought it would be a good idea to provide some information about this condition.
The Cuterebra fly, also known as the Rodent Bot fly, lays it's eggs in an area frequented by the host animal. When the host passes over the eggs, the eggs attach to the host's coat. When the eggs hatch, the larvae burrow into the skin of the host and grow until just before entering the pupal stage at which time they are ready to leave the host. While in the host the larvae develop in a warble (lump) that grows in size. The larvae breath through a hole in the skin. Generally the larvae exit the host without causing long term problems. There is a risk that they can interfere with bodily functions by entering the trachea and blocking air supply or enter the brain. Clearly this is more likely in a small animal like a mouse. Do not try to remove the larvae yourself. If the larvae is killed inside the host, it will infect the host and possibly kill it. Either allow the larvae to mature and leave the host or, if it appears to be causing problems, have a vet remove it.
For more information on the Cuterebra fly see these sites: