The viruses, which have been named Human T-lymphotropic Virus types 3 and 4 (HTLV-3 and HTLV-4), belong to a genus of viruses known to spread and cause serious illness in humans. The researchers believe the findings demonstrate the need to regularly survey those human populations known to be in contact with animals for new infectious diseases emerging from animals. The study, which was first reported at the 12th Conference on Retroviruses and Opportunistic Infections, is now published in the May 16, 2005, Online Early edition of the Proceedings of the National Academy of Sciences.
"The emergence of HIV from primate origins has cost millions of lives and billions of dollars. The discoveries of HTLV-3 and HTLV-4 show that, far from being rare events, retroviruses are actively crossing into human populations," said the study’s lead author Nathan Wolfe, ScD, assistant professor in the Department of Epidemiology at the Bloomberg School of Public Health.
The research team collected and examined blood samples from more than 900 people living throughout Cameroon. All the individuals studied reported some exposure to blood and body fluids of nonhuman primates, contact mostly due to hunting and butchering of bushmeat, and in some cases to keeping primates as pets. An analysis of the blood samples determined that multiple simian viruses had crossed over from primates to infect the study participants. In two hunters, the researchers identified HTLV-3 and HTLV-4, the two previously unknown viruses.
HTLV-3 is similar to the simian virus STLV-3, and was most likely contracted through direct contact with a primate during hunting. HTLV-4 does not have a known primate counterpart and could have arisen through cross-species transmission from an animal carrying an unknown form of STLV. According to William Switzer who led the CDC laboratory team, researchers were surprised at the amount of HTLV diversity seen crossing into persons exposed to primates, as the identification of HTLV-3 and HTLV-4 essentially doubles the number of human viruses in this group. Switzer believes further research to understand the disease outcomes of these viruses and the potential for human to human transmission is also crucial.
The current study follows an earlier one conducted by Dr. Wolfe and researchers from the Cameroon Ministry of Health and CDC, which described the discovery of another primate retrovirus in hunters, the simian foamy virus (SFV) in the March 20, 2004, edition of The Lancet. While it is still not known if SFV is harmful to humans or can be transferred from person to person, together with the current report, the results show clearly that hunting is an important bridge for the entry of novel retroviruses into humans.
"Ongoing collaboration with hunters in central Africa gives us the potential to predict and prevent disease emergence. Given the incredible potential costs of a new human retrovirus into the general population, the development of sentinel systems for forecasting disease emergence - such as long-term surveillance of hunters - should be seen as a human health imperative," said Dr. Wolfe.