Don't be so sure of that. Marek's lives in the soil. Many people recommend burning everything down... before starting over.A final update that I took this little hen, named Chai, to the vet last week to have her euthanized and to get help sending her off to the lab (a 7 hour drive away) for testing. She was a champ til her final day, when fairly advanced ascites was making it hard for her to walk.
Here is the main finding of her necropsy:
"There is severe metastatic adenocarcinoma in this bird, most likely of ovarian origin. There is metastasis to the brain that could explain the neurologic signs. The sciatic nerves were unremarkable. This is a spontaneous disease, unlike Marek's associated lymphoma. There were no histologic lesions in the crop to explain crop stasis. There was no metastasis to this tissue, or evidence of bacterial or fungal ingluvitis."
I am relieved that I'm not dealing with Mareks. This result still doesn't explain why three other young hens of mine fell ill and died around the same time this hen first got sick in 2023. I guess I'll never know - but am very glad to now know I can consider getting more or hatching chicks without having to worry about Mareks.