Mystery disease??

Recently I lost an 5 month old Americana to a mystery disease that I, at first, thought was a parasite or worm. She died within two weeks of showing symptoms. It started off with lathargy, and she would only eat if I was feeding her. She seemed to improve on the corid, however her symptoms were very on and off. She had diarrhea that at first was a mucous-y yellow but turned to a watery green. About two days after she started getting worse again and losing weight, I came into the coop to find her laying on her side. At first I thought she was dead, but when I went to pick her up she opened her eyes, her breathing was really shallow. She lasted about two more hours after that. She was slightly responsive but kept her eyes closed. She couldn't stand or walk properly because her feet would not uncurl. Her breathing became very rapid and she was very shaky. She started convulsing and panting before she died.
We scrubbed and cleaned the coop, and none of our other girls were affected at all. We recently got two red rock chicks and two white rock chicks, all about a month old now. They were completely healthy until I started letting them in a closed off area in the coop during the day, then one of my white rocks started acting funny. Slightly lethargic with little appetite, and she started losing weight. She has diarrhea and today her feet are curled, she cannot walk, and she is starting to have small convulsions and is shaking uncontrollably along with a rapid heartbeat.

We've recently been dealing with a mouse problem in the coop, if that could have anything to do with it.

Please help!

I'm so sorry to hear that. It sounds like a viral or bacterial infection more then a parasite. Cleaning the coop is a great idea, separate the sick hen and try your best to get a blood sample or her droppings to a vet to be tested for what the cause is. I'm not an expert on chicken diseases, so I hope someone else can help.

I hope the rest of your flock remains healthy.

It sounds somewhat like Mareks disease. Were any of the affected chickens vaccinated? If you lose another, contact your state vet for a necropsy. I would also start some poultry vitamins in the water, and check to make sure that your feed is fresh dated or not more than a month old. Smell it and look for off odors or mold. Sorry for your loss, and I hope that you get some answers.