Clinical, virological and immunological characteristics of people living with HTLV at a Swedish tertiary referral center

Background The prevalence of HTLV in Sweden is presumed to be low, however comprehensive data are lacking. This study describes clinical and laboratory characteristics of patients with HTLV infection followed at the outpatient clinic of Karolinska University Hospital in Stockholm, Sweden. Methods We performed a retrospective analysis of all HTLV positive patients followed at our clinic between January 2013 and May 2024. Demographics, mode of transmission, HTLV type, proviral load (PVL), co-infections, diagnosis of HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP) or adult T-cell leukemia/lymphoma (ATLL), and immunological markers were extracted from medical records. Results We included 102 patients (82% HTLV-1, 18% HTLV-2). The median age was 56 years (IQR 42–62; range 23–81) and 54% were women. Twenty-nine percent of the patients were born in Sweden, while the rest had other countries of origin (33% Middel East; 16% South America; 11% Africa, and 5% the Caribbean). HTLV infection was identified by contact tracing (35%), blood donor or IVF screening (31%), and symptom-based testing (7%). Mother-to-child transmission was considered a possible route of infection in four Swedish born individuals. Ten individuals (12% of people living with HTLV-1) had HAM/TSP, and two were diagnosed with ATLL. Patients with HAM/TSP had higher median PVL than asymptomatic carriers. The proportion of regulatory T cells were higher in symptomatic individuals and was positively correlated with PVL. Conclusions This study provides the first comprehensive characterization of a cohort of people living with HTLV in Sweden. Despite the low national prevalence, we observed a notable proportion of symptomatic cases, including HAM/TSP and ATLL. We also observed four cases of MTCT in individuals born in Sweden.