Disclosed are compositions and methods related to the isolation and identification of the primate T-lymphotropic viruses, HTLV-3 and HTLV-4. The diversity of HTLVs was investigated among central Africans reporting contact with NHP blood and body fluids through hunting, butchering, and keeping primate pets. Herein it is shown that this population is infected with a variety of HTLVs, including two retroviruses; HTLV-4 is the first member of a novel phylogenetic lineage that is distinct from all known HTLVs and STLVs; HTLV-3 falls within the genetic diversity of STLV-3, a group that has not previously been seen in humans. The present disclosure also relates to vectors and vaccines for use in humans against infection and disease. The disclosure further relates to a variety of bioassays and kits for the detection and diagnosis of infection with and diseases caused by HTLV-3 and HTLV-4 and related viruses.