When we physicians hear the term “retrovirus,” we instinctively think of HIV. That is understandable. HIV/AIDS has infected and killed tens of millions of people worldwide. But there are other retroviruses out there. One such virus is called Human T-lymphotropic virus type 1, or HTLV-1. HTLV-1 was the first cancer-causing retrovirus to be discovered, in 1980, by Robert Gallo and colleagues. HTLV-1 causes a dreadful, rare disease called adult T-cell leukemia/lymphoma (which I have only seen once as a physician). That patient was left debilitated and unable to communicate. HTLV-1 is found around the world and 5-10 million people are infected. It apparently originated from a primate retrovirus called simian T-lymphotropic virus type 1 (STLV-1). STLV-1 infects many species of apes and monkeys. HTLV-1 is thought to have originated from STLV-1 because the viruses have similar genetic sequences. Interestingly, HTLV-1 is very prevalent in some areas of the world, like Japan, where there is virtually no contact between people and primates. In those areas, it seems to spread via mother-to child or sexual transmission. In areas of Sub-Saharan Africa such as Cameroon, however, the situation is markedly different. In rural Cameroon, many hunters collect and consume bush-meat from apes and monkeys and that may lead to HTLV-1 transmission. The authors of a recent study in Clinical Infectious Diseases set out to investigate the risk factors for HTLV-1 acquisition in humans via interspecies transmission of STLV-1. The researchers studied 269 individuals (mostly men) living in the southern Cameroonian rainforest. Most lived in the general vacinity of Lomie, seen on the map below. The investigators compared individuals who were bitten by a gorilla, chimpanzee, or small monkey to those who had not been bitten. Interestingly, they found that individuals with severe bites (particularly bites by gorillas) were more likely to be infected with HTLV-1. Their results suggested ongoing direct transmission of STLV-1 in humans through severe bites during hunting activities. More studies are now being undertaken in hunters. What are the implications of this study?
In the context of HIV/AIDS (also a retrovirus of zoonotic origin), Ebola in West Africa, MERS-Coronavirus in the Middle East, and H7N9 influenza in China, there is an increasing recognition of the importance of the transmission of microbes between animals and humans. There is a concept called “One Health” which recognizes that the health of humans is connected to the health of animals and the environment. Unfortunately, as far as I am aware, “One Health” are barely taught in medical schools. That needs to change, quickly. Habitats are being disrupted all around the world and disease transmission is occurring as a result.