Brahma hen cannot stand.

If, indeed, this came on suddenly over the last 24 hours, I would suspect neural toxin poisoning. I've had some chickens stricken in various degrees of lameness by picking up grit from soil contaminated by hydraulic fluid. It wasn't until a small chick became suddenly and seriously incapacitated that I finally discovered the source was a leaking log splitter in the middle of their free range area.

Depending on how much of the petroleum distillates that a chicken ingests, they can become slightly lame due to minimal brain damage, or they can die quickly from consuming a lethal amount. I once had a young pullet die overnight after consuming grit where paint thinner had been spilled. The first symptom of this neurotoxin poisoning is poor balance quickly turning into loss of mobility.

Petroleum distillates aren't the only toxin that can cause these symptoms this hen has. Poisonous mushrooms, moldy feed or hay, rotting vegetables that have started to grow mold, some plants and even some insects can cause neurotoxin poisoning. If caught early, you might be able to save the chicken by giving them activated charcoal or a molasses drench that will clean out the intestines. Epsom salt drench is also useful.

However, the toxin usually works so quickly that the damage has been done long before we discover the sick chicken.

Please understand that I can only guess about this hen, not possessing the ability to see her or know all of the environmental factors she's been exposed to. Others may have a completely different and more accurate assessment.

 

It could be a lot of things from internal parasites to external parasites to illness.

Check her for mites as they can cause anemia and weakness.

Check for worms, hopefully with a fecal float, as that can cause anemia and weakness.

Consider coccidiosis if she has had runny, blood tinged poo.

Check for egg binding. That can at times cause nerve impingement that prevents them from standing. Likely that also includes significant internal infection.

Consider wasting Marek's. She would be about the right age. Slow wasting illness strain where they just lose interest and fade away. Not all Marek's strains cause the classic paralysis and quick death.

She could also have internal cancer or liver disease. Check her abdomen for fluid build up which would be a sign of heart or kidney failure.

And...I hope you changed shoes and disinfected yourself before entering your flock area.

My thoughts.
LofMc

 

...and yes, I agree with azygous....consider toxins as well.

LofMc

 

Uh oh...Cy is this one related to the necropsied one from earlier?

 

There is more to the story then?

How many birds have shown symptoms? How close in time?

 

I would look into reproductive disorders which can cause an interruption in laying, decreased appetite, weight loss, runny poops, sometimes an enlarged lower belly from internal masses or fluid, lethargy, slow or poorly functioning crop, and the tail down posture. Most of the time, they die when they stop eating, and we do not know for sure what was wrong until we do a home necropsy and look at the organs. This just seems to happen so often. I hope it is something that may have a possible cure. Here is a good link to read:
http://www.theveterinaryexpert.com/backyard-poultry/egg-yolk-peritonitis/