We have not dewormed them, but we are looking into it now. My husband was asking around and apparently gapeworm is in the soil in our area. A few of our birds are making a barking-like noise, and one of our young lavenders just died. Like 15 minutes ago. I really am not good at this. Between possible mareks, bug infestation, now maybe gapeworm, paralyzed birds, and death....heartache.
Ah, I totally understand how you feel.
No one starts anything as a master. We all have to learn. And the chicken keeping learning curve is a steep one at the beginning. I didn't start with nearly as many birds as you have, but the ones I had came sick with bronchitis and bumblefoot and I just had no clue what to do. There was so much frustration and heartache. Thanks to the kind and experienced people on this forum who helped me, my skills have come a long way. And yours will too. Hang in there. You just need to take it one step and one lesson at a time.
The internet seems to want new chicken keepers to think all of their chickens are dying of more "exotic" sounding diseases like Newcastle, Avian Flu, Marek's, etc. While these diseases do take a toll, in reality, I'd venture that most chickens backyard flocks perish from much more mundane things like worms, coccidiosis (a protozoa parasite in soil), mites, lice, fleas, and indigestion that turns into impacted or sour crop. These are all treatable if caught in time. That doesn't mean every bird will survive. But it means you can be proactive and empower yourself to ensure good healthy lives for many of them.
I hope Nova's illness is one of these treatable and mundane ones. But if not, I'd like to offer a few lessons I've learned to help overcome that sense of frustration and heartache when nothing seems to be going well...
First thing I did -- and advise you to do -- is make a doable schedule -- one you can actually execute -- for maintaining a healthier flock. People with smaller flocks (like 5 to 15 birds) can get away with treating symptoms as they appear (but even then, it's often too late). As my own flock has grown, I've adopted a more scheduled regime myself.
With a lot of birds like you have, it will be easier on you to do things like parasite inspections and deworming on schedule. And catch problems before they become fatal.
Right now, you are playing whack-a- mole with all of these problems. Which is why it's so hard. For now, you can continue to try to help ailing birds one by one, but one you get on a maintenance schedule, you will start to see far fewer problems!
So now you know you have external parasites like mites and lice around. Any backyard chicken keeper who claims they've never seen bugs on their birds is either delusional or not looking! Treat all birds showing symptoms.
Permethrin is also available in a liquid 10% solution (which must be highly diluted!!!) which you may find easier to apply to individual birds. I apply the liquid to the chickens with a paintbrush, getting it down to the feather bases.
After you treat any individuals with symptoms AND clean and treat the coops, repeat on both birds and coops in 10-14 days. This is VERY important. Insecticides often don't kill the eggs of the parasites. You need to kill any bugs that hatch after the first treatment.
For the coop:
Then, make a schedule to clean, replace bedding and treat your coops every 2 months, or more often as needed. The more you do this routinely, the less of a big project it is when you do.
For the chickens:
Make weekly inspections of birds for mites, lice, any bugs. Check their feet for bumblefoot while you're at it . If you don't know what bumblefoot is yet, learn about it. Check them at night on their roosts with a headlamp or flashlight. Treat individual birds as needed. Remember to follow up after 10-14 days to kill any newly hatched bugs.
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That routine will help you get the external parasites under control. Now, what about the internal ones like worms.
Of all the internal worms that affect chickens, gapeworm is actually the least common. But if you have it in your soil, you likely have common ones like roundworm and tapeworm too. But don't worry! Like treatments for external parasites, a good dewormer will work on all of the common culprits.
Since you haven't dewormed your flock and you have sick birds, I'd recommend deworming all of the birds older than twelve weeks. Use a product with Fenbendazole or Albendazol as the active ingredient. I use a powder mixed in their feed because the powder tends to settle in water. But if you can get a premixed liquid, great. Both dog and goat dewormer work on chickens, but measure dosage carefully. There are many threads here on BYC with proper dosage information. Younger/smaller birds get a smaller dose than heavier/adult birds (dewormer is dosed by weight).
For any sick birds you have in isolation, you can deworm them individually with dog dewormer (that comes in a little plastic syringe). Just pop a pea sized dollop inside their beaks. Repeat in 10 days. Worms also lay eggs inside the chicken and the larvae need to be killed in the follow up treatment.
You can choose to deworm every 3 months, or even every six months as a precautionary measure, or do it when you see symptoms like lethargy, pale combs, and watery or foamy yellow diarrhea.
Another treatable thing to learn about and be prepared for is coccidiosis. If you don't know what it is, learn about that too. It mostly affects young chicks, but can strike adult birds too. They symptoms are similar to worms, but there's a separate treatment for it called Corrid. Always keep some on hand.
There's many other afflictions: common respiratory diseases, aspergillus, injuries, bumblefoot...organ failures like
@Eggcessive points out. Be on the lookout for sneezing, raspy breathing, limping. I know it sounds overwhelming, but the longer you have chickens, the more your brain just registers little details that seems "off" while you are feeding them or around them. It becomes automatic after a while.
Older hens often develop reproductive problems like ovarian tumors or egg yolk peritonitis after their second or third laying season, especially if they have laid a lot of eggs. Sometimes you can help them, sometimes you can't. It's very sad, because those are the ones we tend to bond with as they age. It's just something to be aware of.
Keeping chickens is a labor of love. They are intelligent and complex living creatures, not egg machines It certainly sounds like you sincerely care about their welfare and want to do well by them. You will learn as you go, one day, one chicken at a time.
We started her on Pultry Cell and Vitamin B Complex in case the Corid depleted her of the Vitamin B. (This was explained to me by 
