Simian T-cell lymphotropic viruses (STLV) are nonhuman primate retroviruses closely related to the human T-lymphotropic virus (HTLV). Types I, II, and III of HTLV have been found in humans and are believed to have originated from cross-species transmission of STLV from infected nonhuman primates. The HTLV viruses are known to cause leukemia, lymphoma, and neurological disorders.

CDC researchers discovered a strain of simian T-cell lymphotropic virus type 3 known as STLV-3 subtype D variant. STLV-3 may be widespread in primates hunted in West-Central Africa, including the monkey Cercopithecus mona, which has a known geographic habitat range from Ghana to Cameroon. This increases the risk to hunters and persons in contact with primate bushmeat for infection with STLV-3-like viruses. Thus, the discovery of the highly divergent STLV-3 subtype D variant implies that a similar virus (HTLV-3) subtype D variant could be spreading undetected in humans.