Carolina Rosadas ^1* Jerico Baylon ² Claire Greiller ¹ Adine Adonis ² Divya Dhasmana ² Nicholas W S Davies ² Graham Philip Taylor ^1,2

• ¹ Imperial College London, London, United Kingdom
• ² Imperial College Healthcare NHS Trust, Faculty of Medicine, Imperial College London, London, England, United Kingdom

Introduction: Human T cell lymphotropic virus type 1 (HTLV-1) may cause inflammation of the spinal cord, leading to HTLV-1-associated myelopathy (HAM). HAM is a chronic and progressive neurological disorder associated with increased mortality and impaired quality of life. There are limited data on the incidence of HAM, with higher rates seen in Latin America and the Caribbean, compared to Japan. We aimed to determine the incidence of HAM in a cohort of initially asymptomatic people living with HTLV-1 in the United Kingdom (UK). Methods: Longitudinal retrospective observational study of people with confirmed HTLV-1 infection, first attending the National Centre for Human Retrovirology in London, UK, between 1st March 1991 and 31st March 2024. Clinical records were analysed to calculate incidence rate and cumulative incidence of HAM. Secondary analysis was performed to assess HAM incidence in people living with HTLV-1-HIV co-infection. HTLV-1 proviral load at first visit was compared between incident cases and those who remained asymptomatic during follow-up. Results: In a cohort with up to 33 years of follow-up of people living with HTLV-1 in the UK the cumulative incidence of HTLV-1-associated myelopathy (HAM) was 1.35% (4/297), with an incidence rate of 1.98 per 1,000 person-years. All people who developed HAM had high proviral load at first clinic visit. No case of incident HAM was observed in the group of people living with HIV-HTLV-1 co-infection during the study period. Discussion: HAM incidence in the UK cohort is comparable to rates reported in Latin America and the Caribbean and higher than reported in other high-income countries. High HTLV-1 proviral load predates the development of HAM.