Teriflunomide in HTLV-1 Associated Myelopathy/Tropical Spastic Paraparesis

* INCLUSION CRITERIA: In order to be eligible to participate in this study, an individual must meet all of the following criteria: * 18 years or older * Diagnosis of HAM/TSP as defined by WHO criteria, including a positive HTLV-1 EIA and confirmatory Western Blot. * Enrolled in 98-N-0047 * Patient must be willing and able to comply with all the aspects of trial design and follow-up. * Negative QuantiFERON-TB gold, or completion of latent tuberculosis infection treatment, per CDC and National TB Controllers Association recommendations in the event of a positive test result --In the event of an indeterminant result, the test will be repeated. Should two consecutive tests yield indeterminant results, a chest x-ray will be performed to rule out radiographic evidence of a latent TB infection. Negative imaging will enable the subject to qualify for participation in the study. * Ability to take oral medication and be willing to adhere to the protocol regimen * Patients must be able to provide informed consent * If able to become pregnant or to father a child, patient must agree to commit to the use of a reliable/accepted method of birth control (i.e. hormonal contraception (birth control pills, injected hormones, vaginal ring), intrauterine device, barrier methods with spermicide (diaphragm with spermicide, condom with spermicide) or surgical sterilization (hysterectomy, tubal ligation, or vasectomy) for the duration of the treatment arm of the study and for two years following cessation of treatment with teriflunomide. EXCLUSION CRITERIA: An individual who meets any of the following criteria will be excluded from participation in this study: * Alternative diagnoses that can explain neurological disability * History or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial or interfere with participation for the full duration of the trial; or not in the best interest of the subject to participate, in the opinion of the treating investigator * Severe immunodeficiency, bone marrow disease, or severe, uncontrolled infections. * Liver dysfunction, as indicated by baseline aspartate aminotransferase (AST) or alanine or alanine aminotransferase (ALT) greater than 2 times the upper limit of normal * Positive serological evidence of HIV, HTLV-II, Hepatitis B or C * Treatment with immunomodulatory/immunosuppressive therapy is exclusionary except in the cases below: * current use of topical steroids * Study participants who have been prescribed \<= 10 mg prednisone (PO) per day will be required to discontinue this medication for a period of at least 3 months (90 days) prior to enrolling in 21-N-0016. Additionally, participants who have been prescribed burst therapy with an oral steroid as an outpatient over the course of \<= 10 days (e.g., Medrol Dosepak \[methylprednisolone\]), can enroll after discontinuing burst dose oral steroids for at least 3 months. * Pregnant or lactating women. * Treatment with other investigational drugs within 6 months before enrollment * Known hypersensitivity to teriflunomide or leflunomide * Concomitant treatment with leflunomide