More reading:
@casportpony
Yes, that's what I read too...Fenbendazole has shown some effectiveness against the most common tape, but there is some inconclusiveness. Vets tend to still prescribe it for that purpose as a matter of course, even though it is not conclusive.
I've read and heard good things and bad things about internal worms and Ivermectin, and I honestly think it depends upon the population you've got in your area. Literature clearly shows worms build resistance to drugs, and worms have shown ability to build resistance to Ivermectin. I think some areas which have had over treatment of one type of med, Ivermectin in particular, become resistant.
Therefore, I personally prefer rotating control meds because of the tendency of worms to develop resistance. What works today won't tomorrow...so rotating helps to reduce resistance build up. It then becomes a case of trial and error for the poultry owner, with the wisest course to administer then take fecal samples, but often that is time consuming and not often done....so what tends to happen is administration and visual observation with colloquially reported results. Rotation, and literature, can help when fecal samples are not desired.
LofMc
A recent study showing effectiveness of Ivermectin, if given in repeated dosages over 3 to 4 weeks (EDITED: oops...this particular study shows fecal samples were taken over a course of month after one treatment...I have read a different study that I've misplaced that showed efficacy improved with repeated doses...will look for that):
http://www.medwelljournals.com/fulltext/?doi=rjpscience.2011.41.44
http://www.aensiweb.com/old/aeb/2011/2002-2005.pdf (same article in pdf form)
Older study referenced showing Ivermectin effectiveness on several worm types in poultry:
https://books.google.com/books?id=8...of ivermectin on helminths in poultry&f=false
EDITED TO ADD: this study found again while looking for the study that I know showed 2 control groups and that efficacy of Ivermectin improved over repeated dosages...will continue to look for that one...possible I've confused it with the standard usage of 3 treatments for mites/lice with Ivermectin:
http://www.ncbi.nlm.nih.gov/pubmed/2267731
A study done on pigeons showing efficacy of Ivermectin on nematodes
http://thejaps.org.pk/docs/16_1-2_2006/Basit.pdf
A study in Bangladesh showing efficacy of Ivermectin on nematodes for ducks:
http://eurasianjvetsci.org/pdf/pdf_EJVS_976.pdf
The Literature:
Merck's Veterinary Manual lists Ivermectin as effective (though not currently approved "on label")
http://www.merckvetmanual.com/mvm/poultry/helminthiasis/overview_of_helminthiasis_in_poultry.html
A typical study which shows the non-satisfactory results of Ivermectin (note it was only one treatment...the positive study showed that 3 to 4 treatments were necessary for eradication)
http://onlinelibrary.wiley.com/doi/10.1111/j.1439-0450.1989.tb00635.x/abstract
Another synopsis of studies showing the non-efficacy of ivermectin, fenbendazole and oxfendazole in poultry for helminths
http://www.reeis.usda.gov/web/crisp...sms-established-in-arkansas-farm-animals.html
Since Ivermectin is cheap and easy to apply, it can be a good choice if your area (and more importantly flock) has not shown resistance.