Dear lady; You are right to worry about parasites transmitting diseases and other micro parasites to humans or other pets/animals, and your responders have given you great ways to use the very effective sevin, all that we ever had to use, against lice. There seem to be several sizes and they have different stages of size, different types of evidence of their juvenile and cocoon stages, which those bald spots on your chickens indicate they had something causing loss of plumage. You might see balls or lumps of whitish stuff, like popcorn, from the size of a pinhead to a quail egg size, usually at the base of feathers, and rump area can be the worst, although the area at the breast, where hens usually have lost feathers to nesting, hard breeding and parasites. The liquid sevin was formerly an agriculture/horticulture application, as the powder was, and I agree with your responder who wisely tells you that the powder form sticks well to a bird...my turkeys have been lice free for months from one good application of sevin powder, but it must be when the lice are in early stages. the sevin appears to actually kill the nits, a seed like attachment to feather bases that cocoons the fetal lice; because these structures are dried up and dead a week later with no new hatched lice on the birds. However, should your infestation get as bad as you described; with floppy, underweight birds; you then have secondary medical conditions that we have followed breeders recomendations on: Tetracycline from the animal medicine counters at the agriculture supply stores, the water soluble form has successfully killed whatever has opportunistically attacked the lice-weakened birds, especially coccidiosis if it is present in the environment, which it is rampant in our area and others, tetracycline powder at a 4 tablespoonfulls in a quart mason jar full with clean water, boiled and cooled if possible, and your ill appearing birds of underweight, underactive; give an eyedropper full if they will not suck some up with a beak dipped in it. we have never lost a bird that has been given a dose of 4 mls to 10 mls of this tetracycline to water ratio by their mouth, unless they were too far gone. After a week of daily dosing, you should pursue a wormer ration by mouth, like wazine which is also water soluble. It is difficult to divide down the huge flock dosing that these manufacturers print on labels, but with an hour of brain busting, you can do it, looking up conversions of liters, mls, cc's which are hte same as ml's, ounces, quarts, gallons, etc. Once, I wrote down a chart for each of these medicines, and pasted it to our mason jar shelf, so when we went to get one for making medicine, we could look up how many tablespoons or teaspoons to mix into our quart, that one quarter of a gallon. while you can make a regimen of wazine twice a year in your water containers in pens, so that over the course of a few days, every member of that pen got some medicine, it is necessary to treat sick birds right away, without burning a hole in their gut with caustic meds, like we know that antibiotics are....just try tasting that diluted liquid medicine you are giving to your birds. we like to mix in water soluble probiotics, vitamins, proteins like musclebuilding powders we use on ourselves that mix with milk...speaking of milk, any available goat milk is the best thing for ill creatures. good luck, and always remember that weekly picking up of your birds is the best way to acquaint yourself with their healthy weight, so that when they slip down, you can identify a problem before it detracts too much of their available vigor. birds can go fast, so you have to be faster.