The oncogenic potential of the virus is linked to the regulatory gene tax , which transactivates transcription of proviral LTR, cellular oncogenes, and the cellular gene encoding the growth factor inerleukin-2 receptor. This can cause a cascade of events that has the potential to lead to cancer by inducing autocrine stimulation of T-cell proliferation. Most HTLV infections remain asymptotic. Less is known about HTLV-2, but is common among intravenous drug users and linked to a form of atypical hairy cell leukemia. Most HTLV-1 infected individuals are asymptomatic carriers of the virus.
Malignant cells are pleomorphic with large distorted nuclei.
Immunosuppression can lead to a variety of opportunistic infections. This is a neurological disease where the virus infects the central nervous system in addition to the blood. Characterized by progressive demyelination of the long motor neuron tracts of the spinal cord Clinical manifestations start with lumbar back pain which radiates down the legs, leading to dysethesia, urinary frequency or retention, and sometimes mild sensory loss. Areas with the highest prevalence of HTLV-I infections include southern Japan, the Caribbean, equatorial Africa, parts of South America, eastern Siberia, and the Pacific islands. HTLV-II predominates in Native American populations and among intravenous drug users.