Botfly Larvae/cuterebra or Warble
A bot fly larva is the developing form of an insect. It is the stage after it hatches from the egg and before it becomes an adult fly. The lump in the animal's skin caused by the growing bot is called a warble, and the opening in the animal's skin is referred to as the warble pore. There may be just one larva or there may be more in different parts of the host animal's body...the larva feeds upon the animal's tissue exudate. In the early stages of development the larvae may be light colored and generally darken as they mature. The area can devleop an abcess from the chronic irritation. The Bot fly warble is often misdiagnosed as a simple abcess....
If the bot fly larva is not in an area that restricts movement or other functions, it can be left intact and will drop off at maturity. The sight on your animal is gruesome, but in many cases they only cause mild irritation for the animal. Once the warble drops out, clean the wound with an antiseptic and apply topical ointment if needed. On very small mammals such as mice, the warble can be life threatening -- it may need to be surgically removed.
If the bot fly is killed while it is living under the skin of the mammal, it can release a toxin whcih can cause anaphylactic shock (this is one reason why we don't try to kill it while it is still there). Oral antibiotic treatment may be indicated if a secondary infection develops in the warble.
A mature bot fly larva is large enough to be carefully removed using tweezers or forceps. Sometimes slow, steady pressure AT THE BASE of the warble can push the larva out thru the opening. Be sure not to kill the larva in the process. The warble pore can then be flushed with chlorhexiderm@ or saline solution and a topical antibiotic applied if needed.