""Seems strange since it's used for humans as well ""
Now I don't see how a few drops applied to a hen can possibly cause any effect on a human by the time it's gone though the hen's body. This dosage is negligible compared to the safe human dosage! And I certainly don't see how these few drops make said hen unfit for human consumption for the rest of its life.
I believe people make these sweeping statements about withdrawal periods because there has been insufficient research done on Ivermectin in poultry and the relevant information on withdrawal periods (if any) isn't available.
After all, I wouldn't be worried about a few drops landing on my skin, so why should I worry about any microscopic amount getting into the eggs?
Ivermectin is used in humans, but I wouldn't say to a huge extent. In the UK there is only a cream licensed (used for rosacea). Occasionally unlicensed oral forms are imported to treat resistant cases of scabies.
I agree that the dose for humans (e.g. 200mg per kg bodyweight for scabies) is likely to be well above the dose that you would likely be exposed to by eating eggs/meat from a bird that had been treated once. However that does not necessarily imply safety and there are lots of other considerations, such as:
- Whether multiple doses accumulate in the meat over time and potentially exceed these levels.
- If chickens do not metabolise the drug but store it in certain tissues (not necessarily evenly over all tissues) that could be why the bird is then classed as unfit for consumption forever - we can't assume just because some mammals get rid of the drug in a certain period that birds do the same, or even at all.
- Whether chickens metabolise ivermectin in a different way to mammals, that could result in a breakdown product that is potentially harmful. Direct exposure is not the same thing as indirect exposure.
- Ivermectin is known to transmit to milk - in cows and humans. So eating the eggs/meat of treated birds could then pass on the drug to nursing infants, unknown to the mother.
- Ivermectin is not acceptably safe for all humans and some may be more susceptible to harm from ingestion of even low doses. Does each person possess the relevant information to know whether it would be ok for then to be exposed?
- Even though ivermectin can be used in humans, it is not without risk (nothing is!). The datasheet for the cream states the following: "In accidental or significant exposure to unknown quantities of veterinary formulations of ivermectin in humans, either by ingestion, inhalation, injection, or exposure to body surfaces, the following adverse effects have been reported most frequently: rash, oedema, headache, dizziness, asthenia, nausea, vomiting, and diarrhoea. Other adverse effects that have been reported include: seizure, ataxia, dyspnea, abdominal pain, paresthesia, urticaria, and contact dermatitis."
If you choose to expose yourself that is your choice - and it may not be an unreasonable decision. But the logic that "it will all be ok because this drug gets used in humans" is falsely reassuring and misses a number of potential ways in which harm could result. That is why, in the absence of the research / data, others say not to consume - you may view that stance as an overcautious sweeping statement. I guess it comes down to what you are happy with.