You're speaking with Dr. Michael Salkin. Welcome to JustAnswer. I'm currently typing up my reply.
Janet, I'm sorry to hear of this with your chicken. Can you elaborate on her "struggling to walk", please? Is she having a problem rising from a sitting position? Is she lame on one or both legs? Is she ataxic ("drunken sailor")? Is she walking like a penguin? Is she squatting often as if to lay? Has she been laying? If so, has laying abruptly stopped?
Thank you for the additional information. Have you been able to palpate (feel) her to see if she has lost considerable weight? Has her comb and wattles become pale or discolored? Are you seeing any other symptoms of illness - a change in her eyes, swelling in her face, nasal discharge, gaping, increased respiratory rate, sneezing, coughing, regurgitation/vomiting, distension of her coelom (abdomen)?
If she has gone light her struggles are likely to result from a chronic disease process/anemia - chronic infection, metabolic disorders, or perhaps neoplasia (lymphoid leukosis, Marek's disease, e.g.). Unfortunately, her symptoms can indicate any number of illnesses or health issues in chickens. In avian medicine, there's rarely one cause of such a presentation, so we usually begin with a list of differential diagnoses and use lab tests, X-rays, and physical exams to differentiate one from another. Necropsy of a newly dead or a sacrificed severely ill bird then refrigerated (not frozen) can be an important diagnostic particularly in large flocks. With this in mind, your best course of action is to reach out to your county-extension poultry personnel or avian-oriented veterinarian for help.
It's best to approach the diagnostic process with a clear sense of her financial value to your operation. Although some services such as your county animal disease diagnostic laboratory might be available free of charge through a county agency or land-grant extension office, the expense of some diagnostic tests and treatments can add up quickly. While it’s always worth your time and money to identify a bacterial or viral infection that could potentially impact more than one member of the flock, this might not be the case with a condition that only affects one hen. It frustrates me that I can't be more specific for you but such is the dilemma of poultry owners and vets alike. In the UK, you're now prohibited to even purchase drugs at your agricultural merchant and so my suggesting your presumptively treating her with this or that isn't helpful. You have to go through an avian vet (please see here: bvpa.org.uk).
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