In Gail Damerow's Chicken Health Handbook, she recomends ivermectin injectable liquid, 1%, to use as both a wormer and to kill external parasites. I've given it to mine, several times. I put 2 cc's per gallon in the drinking water, for two days. It will get rid of both internal and external parasites. It takes a few days to get all through the bird's system and into the skin to kill the bugs, but it works. It's an off-label use, it sold for hogs and cattle. I use it on poultry and dogs, too. It seems a little pricey, but it lasts a long time. One $35 bottle has lasted over a year. That's using it for a good sized flock, (always at least 25 birds, sometimes as many as 60 or more, like now) 4 dogs, and trough this summer, 4 hogs. I still have enough left to treat the chickens again, and maybe the hogs, too.
I follow the withdrawal times for cattle, since it isn't labeled for poultry. I normally treat the poultry during the fall molt, when they aren't laying much anyway, so residue in eggs isn't really an issue. Withdrawal time before slaughter in cattle is 35 days, but I don't sell eggs again for a minimum of 6 weeks, longer if they still aren't laying much by then. I use the eggs myself after 10 days. The amount of ivermectin the birds ingest is really tiny. By the time it's worked down to what's left in an egg, I don't think it's a problem for humans. But I wait to sell eggs, to be on the safe side.
Ivermectin is used to treat humans for certain parasites as well, so I don't believe consumption of meat or eggs from animals treated with it poses any significant risk. I tell my egg customers what my birds have been treated with, so they can decide for themselves if they want to use the eggs. So far, nobody has objected.
At the same time that I treat with ivermectin, I clean and spray the coop. I've used kennel dip in the past to spray the coop, but this year I'm trying a neem based fruit tree spray. Neem kills all kinds of insects, and it's safe for poultry and mammals.
Here's a link to the wiki site about ivermectin.
http://en.wikipedia.org/wiki/Ivermectin