Wound Myiasis: Maggots By Any Other Name
'Tis the season... fly season. And just like every other living thing on the planet, the flies need to reproduce too, which means sooner or later you're going to find the maggots. Maggots really aren't so bad... until you find them living in the hide of some poor creature, usually in the decaying tissue of a wound that happened a few days earlier, which a few species of flies thought would make a good nursery. At that point I think most people would agree that maggots have a pretty high "ick" factor.
Myiasis is the technical term for infestation of the tissues with fly larvae (aka maggots). There are three kinds of myiasis: obligatory, facultative and accidental. They all occur when adult flies are attracted to moist or oozing tissues (such as an open wound) in which they then lay their eggs. The eggs ultimately hatch and the larvae hang around in the local tissue for a few days until they fall off to pupate on the ground to produce more adult flies. Obligatory myiasis is a reportable disease caused by certain species of Cochliomyia and Chrysomyia, which are commonly known as screwworms. The reason screwworms in particular are such a concern is the larvae of these species will actually invade and digest healthy, living tissue. Thankfully we don't have these species in Canada, but they do occur in Central and South America, Africa and Asia. Facultative myiasis is caused by blowflies and fleshflies of several genera. These flies lay their eggs in decomposing (dead) tissue, typically in wounds or skin mascerated by moisture or chronic fecal soiling, as well as carcasses of dead animals. Although the larvae do not invade living tissue like screwworms, they do secrete enzymes which may cause enlargement of the wound. Accidental myiasis occurs when flies of the Muscidae family lay their eggs in a wound while feeding, even though animal tissues are not normally used by these species for breeding.
Although myiasis may not be common in horses, it certainly does occur. The presence of maggots, particularly in a wound, indicates that there is necrotic (dying) tissue present, which should be removed. Furthermore, the maggots you may find on a horse are by no means "medical grade" - the maggots that you may have heard of being used in human medicine are purpose-bred and kept very sterile. Secondary bacterial infection is common with wound myiasis, and in large or deep wounds this may even result in septicemia. Larvae-infested wounds are typically painful, with a foul smell and a lot of discharge. Treatment involves removal of the maggots, debriding all of the dead or unhealthy tissue, cleaning the entire wound, applying an appropriate dressing (if possible), and use of topical insecticides to prevent future infestation. Antibiotics may be necessary if secondary bacterial infection is present.
There are a few things you can do to help prevent wound myiasis.
- Avoid doing surgical procedures during fly season. If a procedure must be done, monitor the incision carefully until it is completely closed and keep it as clean and dry as possible so flies aren't attracted to it. Use of topical insecticides can be very helpful too, but check with your veterinarian about what kinds of fly spray to use around surgical sites.
- Check your horse carefully on a daily a basis for any flesh wounds, and if you find one make sure it's properly cleaned and treated as soon as possible.
- It's also important to prevent skin and hair from becoming caked with mud or manure.
People are also susceptible to wound myiasis, but the keys to prevention are the same: keep wounds clean and dry, keep wounds covered if possible (and also keep bandages clean and dry), and keep the adult flies away!
Image: Screwworm larvae armed with sharp mouth hooks that they use to tear at flesh. (source: www.vetmed.ucdavis.edu)
