The purpose of this study is to see if taking mepolizumab is safe and effective in treating Hypereosinophilic syndrome (HES) in children and adolescent patients ages 6 to 17 years who are receiving standard of care (SoC) therapy. Mepolizumab is administered through a subcutaneous injection (shot). Participation in the study will last for about 64 weeks approximately (1 year and 3 months).

Candidates for this study may or may not report disturbances in odor perception as their primary reason for seeking treatment at MUSC. This study is designed to collect long term, observational data from patients who are being treated with routine clinical care in health clinics at MUSC. Data from clinical questionnaires will be de-identified and stored in a database.