Although about 90% of people with HTLV-1 infection do not report specific symptoms, they have an increased life-time risk of HTLV-1 disease development which, amongst other risk factors, is very much dependent on HTLV-1 pro-viral DNA load. The lifetime probability of developing Adult T-Cell Leukemia/Lymphoma (ATL) is 4-5% for people infected with HTLV-1 (41, 42), but ATL predominantly occurs as a consequence of mother-to-child transmission (MTCT), which contributes 20-24% of all HTLV-1 infections. Therefore, the lifetime probability of developing ATL is about 25% in HTLV-1-infected infants (43). The clinical outcome of ATL depends on the clinical subtype, the timing of diagnosis, and access to treatment. People with ATL associated with HLTV-1 infection have a median survival of 8 to 10 months despite all the advances in chemotherapy and supportive therapy (44, 45).
Japanese colleagues reported that in Japan alone an estimated 1,000 people succumb to ATL every year (HTLV-1 Symposium Tokyo, Japan, July 2018).
In addition, HTLV-1 infection causes chronic, progressive, disabling and painful conditions such as myelopathy (46), polymyositis (47, 48), chronic inflammatory pulmonary disease (49, 50), uveitis (51-53) and dermatitis (54-59). The lifetime risk of HTLV-1 Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP) approaches 4% among people with HTLV-1 infection (60-65), with an average of eight years delay in diagnosis and treatment due to lack of awareness and testing (66). Patients with HAM/TSP may suffer decades of progressive walking disability, chronic severe back and leg pain, incontinence and urinary retention, severe constipation and sexual dysfunction, all of which lead to social isolation (67). HAM/TSP affects both adults and children but mostly middle aged and older women (66).
All the aforementioned HTLV-1 diseases have been observed in people with HTLV-1 infection in Australia (40, 49, 58, 59, 68-71). Recent data collected in Central Australia, in a catchment area of 1 million km2 and in five remote communities (n= 52,121), showed that 30-50% of people were infected with HTLV-1. Infection prevalence was associated with increasing age. Five per cent of the cohort suffered from ATL, and additionally, patients suffered from HAM/TSP (<4%), uveitis, Strongyloides stercoralis co-infection, infective dermatitis and crusted scabies. A total of 9.6% of people with HTLV-1 infection suffered from bronchiectasis, which was more common in male patients. People with positive HTLV-1 serology and high HTLV-1 pro-viral DNA load were at 2.5- and 12.5-fold increased risk of suffering from bronchiectasis. A community-based spirometry study reported that the severity of airway obstruction was strongly associated with HTLV-1 seroprevalence. In Central Australia, higher HTLV-1 pro-viral DNA load was associated with a fourfold increased risk of premature death, especially in people who suffered from bronchiectasis (72).