Aloha! You're speaking with Dr. Michael Salkin
I'm sorry that your question wasn't answered in a timely manner. Paleness indicates profound anemia but not the etiology of that anemia. While both external (fleas, mites, lice) and internal (roundworms, coccidia, e.g.) parasites are often incriminated, we also have to consider the chicken anemia virus, other viruses such as Marek's and lymphoid leukosis, and any chronic disease process - nutritional, infectious, or malignant.
In veterinary medicine, there's rarely one cause of a condition, so we usually begin with a list of differential diagnoses and use lab tests and physical exams to differentiate. With this in mind, your best course of action is to reach out to your county-extension poultry personnel or veterinarian for help in differentiating the various causes of anemia. Veterinarians can perform a physical exam and run diagnostic tests, including X-rays, to distinguish between the different etiologies. You can also see here: http://www.bvpa.org.uk/
Approach the diagnostic process with a clear sense of your hen's financial value to your operation. Although some services might be available free of charge through a 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.
Please respond with further questions or concerns if you wish.