Hi i would also like to know how to add garlic to diet. Thanks in advance.
There's a few ways.
For best results, you'd go the extra mile (extra bit of effort daily) and crush or mince unpeeled or peeled raw garlic, average of a clove per bird per day, and just throw it into their scratch. If they're not used to it they may well be wary of it to start with. Mixing it with their feed gives them a taste of it, even if they avoid it to begin with, and soon enough they will medicate as necessary. Not all garlics are created equal, lol, so if one strain or source doesn't deliver try another. I also regularly give them things like raw onion, cayenne pepper, black pepper, etc to help control any adult worms they bring into the flock, and these spices are also great for general health, being anti-inflammatory etc. I don't scrimp much on them because our health depends in large part on the health of the animals and plants we eat.
I give them a handmade mix including coarse grains and soak it overnight for greater nutrient absorption and less food waste, as it results in better feed conversion, and the grains etc being wet allows the other additives like garlic, kelp, herbs, spices etc to bind to the grains and not be wasted. Fresh raw garlic gives better disease control and prevention than any other form of it.
For good but not the best results you could use pre-minced and dried garlic which you can buy by the kilo at produce stores, where it's used for horses etc. You can also buy it minced and undried in tubs of a kilo or more. When it's not eaten within a day of being minced the Allicin dissipates and that's one of the most powerful natural antibiotics in garlic. Still, there are many in there which survive drying etc, and the sulfur buildup over time is what tackles the parasite issue. The main thing about using raw garlic is that when fresh it's more antibiotic, antibacterial, antiviral, antimicrobial, etc.
I'd recommend also using pet/livestock grade Diatomaceous Earth or ash or hydrated agricultural lime in their cage floors and any areas they frequent for dustbathing, to kill adult lice and oocysts and hasten the control over parasite reproduction. If they are heavily infested, best to consider using something topical rather than waiting for the sulfur to build up in tissues (you can use it topically too) just in case any become overloaded in the meanwhile by the parasites.
Best wishes.