HTLV-1 (human T-lymphotropic virus type 1)

HTLV-1 was the first human retrovirus shown to cause a cancer. It infects CD4-positive T cells and integrates into their DNA, where its proteins Tax and HBZ drive proliferation and block the repair of damage. Most infections are acquired in infancy through breast milk; sexual transmission, transfusion of cellular blood products and shared needles account for the rest.

The great majority of people infected never develop a lymphoma, and infection is lifelong. The two diseases it causes are adult T-cell leukaemia/lymphoma, after a latency usually measured in decades, and a progressive neurological disease, HTLV-1-associated myelopathy or tropical spastic paraparesis. Infection also suppresses cell-mediated immunity, which is why strongyloides hyperinfection and opportunistic infection are common complications.

Where it matters clinically: HTLV-1 serology belongs in the work-up of any T-cell lymphoma in a person who comes from, or whose parents come from, an endemic region, because adult T-cell leukaemia/lymphoma is treated differently from every other T-cell lymphoma and is easy to mistake for peripheral T-cell lymphoma not otherwise specified. A systematic review of HTLV-1 among immigrants and refugees worldwide found a pooled prevalence of 1.28 per cent, highest among people from the Western Pacific region at 7.27 per cent, which is the practical argument for testing rather than assuming.

There is no vaccine and no treatment that clears the virus. Prevention rests on antenatal screening and avoidance of breastfeeding where that is safe, screening of blood donations, and barrier contraception.