EPIDEMIOLOGY Incidence of Human T Cell Lymphotropic Virus Type 1-Associated Myelopathy/Tropical Spastic Paraparesis in a Long-Term Prospective Cohort Study of Initially Asymptomatic Individuals in Brazil Luiz Cla ´ udio Ferreira Romanelli, 1 Paulo Caramelli, 2 Marina Lobato Martins, 1 Denise Utsch Gonc ¸ alves, 2 Fernando Augusto Proietti, 3,4 Joa ˜ o Gabriel Ramos Ribas, 5 Marcelo Grossi Arau ´ jo, 2 and Anna Ba ´ rbara de Freitas Carneiro-Proietti 1,3 Abstract The incidence of human T cell lymphotropic virus type 1 (HLTV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is not well defined in the literature. Several studies have reported different incidence rates, and recent publications suggest a higher incidence and prevalence of HAM/TSP. The interdisciplinary HTLV Research Group (GIPH) is a prospective open cohort study of individuals infected with HTLV-1/2. This study describes the demographic data and HAM/TSP incidence rate observed in 181 HTLV-1-seropositive individuals and compares the results with previous reports in the literature. HAM/TSP was diagnosed on the basis of the World Health Organization diagnostic criteria and De Castro-Costa et al. [Proposal for diagnostic criteria of tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM). AIDS Res Hum Retro- viruses 2006;22:931–935]. Seven HAM/TSP incident cases were observed during the follow-up. The HAM/TSP incidence density was 5.3 cases per 1,000 HTLV-1-seropositive cases per year (95% confidence interval: 2.6–10.9), with a mean follow-up of 7 – 4 years (range: 1 month to 15 years). HAM/TSP was more frequent in women in their 40s and 50s with probable infection via the sexual route. The HAM/TSP incidence density among HTLV-1- seropositive cases observed in the present study is higher than that in previous studies. HAM/TSP may be underdiagnosed in countries like Brazil where HTLV infection is prevalent. Orientation and prevent transmis- sion of HTLV programs are needed. Currently, preventing HTLV-1 transmission is the most effective way to reduce the impact of HAM/TSP on society. Introduction A pproximately 10 to 20 million individuals are infected with human T cell lymphotropic virus worldwide, mostly with type 1 (HTLV-1). 1–3 The seroprevalence rates differ with respect to geographic area, 4 and reports suggest HLTV-1 seroprevalence increases with age and is higher in women than men. 5–10 In 1986, this virus was the first human retrovirus to be associated with a neurological condition, which was named HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP). 11–14 The majority of people infected with HTLV-1 remain asymptomatic throughout life. 1–3 Therefore, the actual prevalence and incidence of HAM/TSP have not been determined with certainty. The present study aimed to determine the incidence of HAM/TSP on the basis of the 15-year follow-up period of 181 asymp- tomatic HTLV-1-infected individuals. Materials and Methods The Interdisciplinary Research Group on HTLV-1/2 (GIPH) aims to study the natural history, clinical manifesta- tions, epidemiology, and other aspects of HTLV infection. 15 Since 1997, this open cohort study has been following indi- viduals infected with HTLV in Minas Gerais, Brazil. The co- hort comprises over 570 HTLV-infected and 166 noninfected 1 Fundac ¸a ˜ o Hemominas, Belo Horizonte-Minas Gerais, Brazil. 2 Universidade Federal de Minas Gerais (UFMG), Belo Horizonte-Minas Gerais, Brazil. 3 Faculdade da Sau ´ de e Ecologia Humana (FASEH), Vespasiano, Minas Gerais, Brazil. 4 Centro de Pesquisas Rene ´ Rachou, Fiocruz, Belo Horizonte-Minas Gerais, Brazil. 5 Rede Sarah de Hospitais do Aparelho Locomotor, Belo Horizonte-Minas Gerais, Brazil. AIDS RESEARCH AND HUMAN RETROVIRUSES Volume 29, Number 9, 2013 ª Mary Ann Liebert, Inc. DOI: 10.1089/aid.2013.0086 1199