Chicken shaking head? Mystery symptoms?

[Mint413]

About a month ago, we lost a chicken to a mystery illness - scans revealed an enormous, possibly malignant tumor on which the vet could not operate, so we had to put her down. We now have another chicken with the same exact symptoms - she's lethargic, she's lost interest in things she loves, she constantly hunches as though preparing to go to bed for the night, her feathers are mangy and look moth-eaten, and we haven't seen her eating or drinking. We've been noticing these symptoms for about a week, which is how long the previous chicken survived after we first noticed her symptoms.

I doubt we managed to give two of our chickens cancer (though I wouldn't put it past us), and I would be surprised to find that coincidence produced identical cases in the same flock within a month of each other, which means something else is going on. We'd prefer not to go back to the vet, since the vet told us it was cancer, and we're no longer sure we agree - but we would really like to keep her alive. (Of course, the two chickens who have exhibited these symptoms have been our favorites.)

The flock does have coccidiosis - we've never permanently eradicated that infection since it cropped up last summer, but have been keeping it at bay with Corid in the water. Since the chicken now exhibiting serious symptoms hasn't drunk a lot recently, we're planning to squirt Corid down her throat later today. If the problem isn't a severe coccidiosis infection, however, all the Corid in the world won't help. I have no idea what's causing the problem, and I want to identify it so we can treat it instead of just standing here helplessly and losing them. Has anyone else seen similar symptoms? Do you have any advice?

 

[Mint413]

Update: She is alert and drinking (including medicated water), but not eating, and prefers not to socialize or run around like usual. We squirted two types of anti-coccidia medicine down her throat yesterday, along with Nutri-Drench; she struggled more the second time, so we hoped she was getting stronger, but today I see no change. She's also been twitching/shaking her head, as if to dislodge an invisible fly, every few seconds.

 

[Mint413]

Update: She ate a few bugs yesterday, groomed herself, and scratched around in the dirt (in slow motion, as if remembering how to be a chicken), so I was really hoping she was feeling better. But today she is back to sitting in the shade shaking her head. I don't think she can make it much longer without food. We can try giving her more Nutri-Drench, but that alone will only prolong her suffering.