Soussan Moussavi - Student Research Committee, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran
Roya Mallem - Student Research Committee, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran
Zohreh Vahidi - Inflammation and Inflammatory Diseases Research Center, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran
Saeede Anvari Ardakani - Department of Neurology, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran

Human T-lymphotropic virus 1 (HTLV-1) -associated myelopathy/ tropical spastic paraparesis (HAM/TSP) is a progressive disease of the CNS that causes weakness or paralysis of the legs, lower back pain, and urinary symptoms. HAM/TSP imposes a substantial health burden inMashhad, Northeast of Iran, where the HTLV-1 infection is endemic. Since enhancing immunity is one of the treatment strategies, vitamin D (vitD), as an immunomodulator, might have some effects on HAM/ TSP clinical symptoms. Our study aims to investigate the association between HTLV-1 proviral load, serum calcium and vitD levels, and the severity of HAM/ TSP clinical symptoms. Materials and Methods: Inthis cross-sectional study, data were collected from 41 HAM/TSP with positive serum anti-HTLV-1 antibody who referred to the Ghaem Hospital Neurology ward, affiliated to Mashhad University of Medical Sciences, Mashhad, Iran, from 2014 to 2017. We investigated theassociation between laboratory findings and clinical symptoms, including motor disability, the severity of spasticity, and urinary disorders. Results: Subjects without urinary problems had higher mean vitD levels (p = 0.04). Except for urinary incontinence, vitD was notsignificantly associated with the severity of HAM/TSP clinical symptoms (p > 0.05). There was no significant association between proviral load, serum calcium level and HAM/TSP clinical symptoms (p > 0.05). HTLV- 1 proviral load and serum calcium (p = 0.2) and vitDlevels (p = 0.09) were not notably associated, as well. Conclusions: The current study illustrates that there was a significant association between one of the HAM/ TSP clinical symptoms and serum levels of vitD.