Hi, Steve --- You really need to REMOVE the maggots. Spraying them with an insecticide is not enough. It will kill some but just bother the others enough to move to a new spot or burrow deeper.
How do I say this? They are not "after" decayed flesh. Flies are attracted to the smell of blood, lay eggs that hatch in 24 hours into the larval form (maggot) which needs to feed for about 5 days (depending upon the species of fly) and then the larvae develops a brown pupae where it metamorphoses into a fly.
During the larval stage, the maggot makes wounds much, much worse..... attracting more flies who lay more eggs bringing on a miserable death to the poor animal.
You need to put this duck humanely out of her misery OR you need to bring her inside and treat her wounds. A cardboard box or a dog crate or a large rabbit cage with straw or old towels in the bottom --- anything that you can keep clean. Plus food & water. She will need to be in till the wounds heal.
HOW TO GET the MAGGOTS OFF: this is very simple, have someone hold the duck over a bucket with some salt or ashes in the bottom while you use your thumb to brush the maggots off that you can into the bucket. Then use tweezers to pick off EVERY LAST maggot. (I have done this; you can do this. Last time, it took about 45 minutes. I have spent 3 hours at this so you may want to sit down.) Have a small jar 1/2 full of hydrogen peroxide handy. Use the tweezers to pick off a maggot and dunk it into the hydrogen peroxide then wipe the tweezers on a paper towel then get another maggot...... You wipe the tweezers on the paper towel because if not you introduce too much hydrogen peroxide into the wound where it will foam making it hard to see. Also, the hydrogen peroxide will not kill the maggots but they do not like it so they will begin moving to a new spot making it harder to track them all down. Once they are in the jar, 90% of them stay. Keep an eye out for escapees which won't matter if you are outside
I have tried various methods over the years; this one is the quickest. The point is to use tweezers to pick the maggots off --- what you do with them is up to you.
Also, maggots that are, say, 5 days old are much larger than, say, 1 day old so they may not all look the same. Don't miss the little ones.
Once you have all the maggots off, look to see if there is a 2nd or 3rd site.... clear them as well. Then check the vent as well.
Any wounds need flushed with an antiseptic. Hydrogen peroxide is actually good for this as the foaming action will debride the wound. Now you need to apply a thick coat of a topical antibiotic like Neosporin. I have also used Bag Balm made for cow's udders. Do not bandage.
If the wounds are more than superficial, I would add an oral or injectable antibiotic appropriate for the species.
You will need to check periodically to see if you missed any maggots. There are always a few. They will migrate to a new spot.
Remember, it is the smell of blood from a wound no matter how small that attracted the flies in the first place. NOTHING is better at cleaning blood out of feathers, fur or clothing than hydrogen peroxide.
Keep the duck inside till the wounds heal over --- at least 5 days.
If you have any questions or problems, I will check back. Please trust me on this.
EDIT: Regarding the antibiotic we need a clarification. In the Clinic, I would give an injectable and send home an oral for follow up. But I wrote this so it can be done completely at home without vet care. For poultry including waterfowl, you can go to the feed store or farm supply and look for injectable Penicillin. With minor wounds, the topical antibiotic (Neosporin) will suffice. What is the difference between minor and major? Well, that is subjective --- go with your gut. If you suddenly found yourself with a wound like that, what would you call it?
All I can say, the last maggot case I treated just last week had a wound about the size of a 50 cent piece that was fairly shallow. It was NOT bleeding; just oozing mostly clear fluid. This was on a small Indian Runner duck. I picked the maggots off w tweezers, flushed the wound w hydrogen peroxide, patted it dry and applied Neosporin. She came inside to live in a dog crate lined w towels that I changed 2x per day. Good food, water & vitamins plus checking the wound every day and reapplying Neosporin. Keep an eagle eye out for missed maggots. Duckie went home to owner to live outside after 7 days.
