What is CO and what do you dust them with? I have heard of dusting with DE and I have that but have never actually dusted a chicken, I just mixed it into their dust baths.
How do you build a strong immune system in your flock?
CO is Castor Oil and I use it on legs and feet to treat or prevent scale mite. I dust with pyrethrin powder, which you can order online....you'll not find it too often in many garden centers, though you can find permethrin powder and many old timers use that as well.
Building strong immune systems starts before two cells divide in that egg. The parent birds need to be hardy, healthy and strong, with the most naturally nutritious feed they can ingest, access to clean soils and adequate forage.
Then that chick needs to be on the ground in the coop immediately after hatch, just as soon as it's dry and can walk adequately, that chick needs to be exposed to the germs of the flock, of the soils and the environment in which they will be living. Any weak chicks need to be killed immediately, not held over to "give them a chance", babied around and given vitamins or meds to "help" them survive, etc. In the wild they would have none of these and they would not survive, thus insuring strong birds in that line. Better even if hatched under a broody mama, where they will be exposed to the proper germs and eliminated if they are weak.
No vaccines, no meds, no overfeeding, plenty of exercise, sunshine, huge ventilation, clean soils, natural diet while growing. Cull all birds that do not thrive on this life, who do not lay well on this life, are too old to lay, have never shown hardy vigor, poor social skills(flock picks on them...there's always a reason, cull the bird), poor temperament, constantly broody, fail to regrow feathers quickly nor look bright eyed and healthy. Acting sick? Give it a day or so to see if it will recover...do not isolate it from the flock and baby it around. If it doesn't recover in a day or so, cull it. Open it up and explore the reason, learn about the birds as you go along.
No excess cleaning and disinfecting of the coop, the feeders, the waterers, etc. Just regular cleaning on an as needed basis...there is no such thing as sterile or germ free when dealing with chickens and it's silly to even think it. They walk in their own feces every day. A swish to remove extra dirt, rinse and clean water to refresh is good enough for chickens.....they love to drink out of the muddiest, nastiest water out there even with pristine water right next to it. Maybe that nasty water helps them form immunity, who knows?
A good, cultured and composting deep litter on the coop floor and in any runs or spare pens. A varied diet of good foods found out on range, any veggies and fruits available from the garden, any meat scraps, snakes, lizards, etc. they can pick up along the way. Fermenting the feed helps but is not a cure all or a complete answer to good health, nor is DE, ACV, high pro feeds or calcium supplements.
Choose breeds that are known for natural hardiness, that do well out on range and will forage well for a natural diet...birds that haunt the feeder all the time can be culled, as they are getting no exercise and are eating bagged feed most of the time. It's the equivalent of a couch potato who eats processed foods all the time...are they going to have a good immune system and be as healthy as the rest? Not likely. If Suzy can make an egg each day on mostly foraged and natural foods and Stella can only make an egg each day if she eats lots of processed feed, guess which one gets to stick around?
No heating the coop in the winter....good, fresh air in the coop to move stale air and germs up and out of the coop. Any bird that needs heating in the winter, you can cull...they aren't hardy enough to breed or keep in the flock. Any bird that does poorly in extreme cold and in extreme heat, you can cull....they aren't strong enough to pass along genetics.
Now...do that season after season and you will have a flock that can withstand anything that comes along. I can't stress enough the power of the yearly or even bi-annual cull to insure you have the strongest birds in the coop. I can't stress enough the importance of low stocking rates on your soil, of good airflow in the coop, of exercise and natural foods and of maintaining only the strongest genetics.
Yeah...it takes work and commitment, but the end product is worry free.