HTLV-1 and -2 infection have been described in people with HIV-1, Hepatitis B virus (HBV) and Hepatitis C virus (HCV) infection (73). HTLV co-infections are more common than assumed, since HTLV infection is not routinely tested for in people who are diagnosed with other blood-borne/sexually transmissible infections (74, 75). It is estimated that rates of HTLV-1 or HTLV-2 co-infection in HIV-1-infected individuals are significantly higher than in the general population, especially in people who inject drugs and/or have condom-less sex (13, 76). In some geographic regions, 5–74% of people who inject drugs and are infected with HIV-1 may be coinfected with HTLV-1 or HTLV-2; in a recent study about 27% of people with syphilis also tested positive for HTLV-1 (13, 18, 77-80).

While there are only a few studies of HTLV/HIV-1 co-infection, and no definitive studies, the general consensus is that the natural history of HIV-1 infection in people co-infected with HTLV-1 differs from that in patients with HTLV-2 co-infection. HTLV-2 co-infection seems to have no adverse, and a possibly beneficial, effects on immunological and survival outcomes in people with HIV-1 infection independently of the effects of antiretroviral therapy (ART) (81-83). However, this is disputed and other investigators have reported higher HIV viral loads and faster progression to AIDS with HTLV-2 co-infection (84, 85).

HTLV-1 co-infection carries no benefit and may be detrimental to the health of people with HIV-1 infection, even if taking ART. Due to HTLV-1 driven cellular proliferation, CD4+ T cell counts may be misleadingly normal or higher than expected compared to the observed HIV-1 clinical stage of patients with only HIV-1 infection. This increase in CD4+ T cell count occurs without immunological benefit. Co-infected patients may experience a faster clinical progression and a shorter survival time, especially in the absence of ART (86-88). Due to HIV-1-induced immunosuppression, patients with HIV-1/HTLV-1 co-infection may have an increased risk of developing HTLV-1 diseases, such as HAM/TSP and ATL (89, 90).