These are closely related retroviruses somewhat distantly related to HIV-1. Transmission is similar to that of HIV-1 (contaminated blood products, less frequently by sexual intercourse or breast feeding). HTLV-I is found predominantly in Southern Japan, some of the Caribbean islands, parts of Central and South America, and sub-Saharan Africa. HTLV-I has been detected in U.S. intravenous drug abusers (20%-25%; range 7%-49%) and female prostitutes (7%; range, 0-25%) and in Native Americans in the United States (1%-13%) and Central and South America (8%-33%). HTLV-I is associated with adult T-cell leukemia (also called T-cell leukemia/lymphoma), involving peripheral blood and lymph nodes with large malignant cells having a multilobated (monocyte-shaped) nucleus and having a short clinical course. HTLV-I is less frequently associated with a neurologic condition called tropical spastic paraparesis. HTLV-II currently has no definite disease association, although several have been suggested.

Serologic tests for HTLV-I antibody are mostly ELISA methods based on whole virus antigen. In general, these tests also detect most patients with HTLV-II. However, some reports indicate a significant number of HTLV-II patients are missed. Western blot methods are used to confirm and differentiate positive test results, and these procedures also have shown inconsistent results. Several new ELISA tests are based on several recombinant viral proteins and are said to reliably detect and differentiate the two viruses. At present, nucleic acid probe with PCR enhancement is the most sensitive and reliable way to differentiate HTLV-I and II.

Idiopathic CD4 T-cell lymphocytopenia (ICL)

This syndrome is being defined as CD4 T-cell counts below 300/mm3 (µL) or less than 20% of the total number of lymphocytes, no serologic evidence of HIV or HTLV infection, and no other known cause for CD4 depression. The main clinical findings are infection and other conditions associated with immunosuppression. Only a few cases have been reported as of 1994. Thus far, there has not been any strong evidence of blood-borne or sexual transmission. Retrovirus etiology has been suspected but not proven (to date).