The issues why some medications are not permitted is that the companies did not request another authorization after if ran out. It costs money and if they don't make enough money (for them) or there are different medication where it would more profitable they might not renew the permit. For that they need to put lots of money towards that permit. They need to do new studies which involves peoplel working, over head within a building and probably fees.
I think they don’t allow dewormers for cats, dogs and show birds to use for chickens in the chicken industry (consumption), because these residues in the eggs and meat are not healthy for humans.
The mighty chicken industry has enough money for research. And we have a agricultural university that does a lot of research (WUR) Non profit and for payment .
I found very good info on chicken health:
Title: Health organic laying hens (farming)
https://edepot.wur.nl/36929
Below two pieces of info from this report.
Page 46-48. If you are interested you can use google translate (page by page) to read the whole report.
Resistance to worms
Chickens can develop a resistance to worms to some extent, keeping infestations at a low level. This resistance is maintained as long as the animal is infected with some worms. If the animal is free of worms, this resistance disappears. For this reason, it is not recommended to treat hens with minor infestations.
Treatment against worms
Only the active ingredient flubendazole (Flubenol, Solubenol) is permitted in the Netherlands for the regular control of worms. There is no longer a waiting period for this product, so that its use has no consequences for the sale of the eggs.
Two strategies are possible when using a wormer:
- worming if necessary
- regularly according to a fixed schedule (also called strategic by the GD)
called deworming). With a regular schedule, it is recommended to deworm every 6 to 8 weeks or, if there is a high infection pressure, every 5 weeks.