A-103 Epidemiology and origin of HTLV-1 and related viruses infection
Authors: Gessain Antoine 
Published: April 2019

HTLV-1, which is the etiological agent of Adult T cell Leukemia/lymphoma and of Tropical Spastic Paraparesis/HTLV-1 Associated Myelopathy, is not a ubiquitous virus. Indeed, this retrovirus, whose origin is the STLV-1, endemic in many species of Old-World non human primates (NHPs), is present throughout the world with clusters of high endemicity located often nearby areas where the virus is nearly absent. The main endemic areas are the Southwestern part of Japan, Sub-Saharan Africa and South America, the Caribbean area and foci in the Middle East and Australo-Melanesia. The origin of this puzzling geographical or sometimes rather ethnic repartition is probably linked to a founder effect in some groups with the persistence of a high viral transmission rate. Despite different socio-economic and cultural environment, the HTLV-1 prevalence increases gradually with age, especially among women in nearly all highly endemic areas. The number of HTLV-1 infected carriers in the world is estimated to be at least 5–10 millions individuals.1 This is very probably underestimated. In most of the high endemic areas, HTLV-1 is mainly disseminated and maintained in the human population through intra-familial transmission (mother-to-child and by sexual intercourses). More rarely, transmission may also occur by transfusion or intra-venous drug use. In Central Africa, the relative contribution of each of the different transmission routes for HTLV-1/STLV-1 (inter-humans vs inter-species/NHP-Humans) still remains largely unexplored. We have thus performed epidemiological studies in order to get better knowledge on the possible still ongoing STLV-1 retroviral transmission from NHPs to human in a natural setting. These studies were done in populations of high-risk individuals living in villages or settlements in the rain-forest of South Cameroon and Gabon. Bantu and Pygmy groups, who still actively hunt diverse NHP species, inhabit this region. These hunters are thus frequently exposed to the body fluids of these NHPs. Furthermore, many of the NHPs species living in these areas are infected with simian retroviruses, including various STLV-1 strains as well as STLV-3 and STLV-4 strains, and simian foamy viruses. State of the art serologies and different specific and generic PCR studies were performed to detect and characterize the different HTLV infecting these individuals.
The main findings were the following: (1) A severe bite by an ape (gorilla ++) or a monkey is a major risk factor for HTLV-1 infection in hunters, of South Cameroon2 and Gabon.3,4 (2) HTLV-3, a new human retrovirus discovered in 2005 by our team, is present in hunters of NHPs from these areas (Richard et al, in preparation). (3) HTLV-4 infections (the second and third discovered strains) of zoonotic origin were characterized in 2 hunters living in Gabon, severely bitten by a gorilla during hunting activities.5

Interspecies transmission, from apes and monkeys to humans, of different STLV and simian foamy viruses are still ongoing in Central Africa. These studies reinforce the fact that apes, especially gorilla, should be considered as keystone reservoirs for human/disease infections (HIV-1P group, HTLV-1b, origin of P. falciparum…).
1. Gessain A, Cassar O. Epidemiological aspects and world distribution of HTLV-1 infection. Front Microbiol. 2012;3:388.
2. Filippone C, et al, and Gessain A. A severe bite from a nonhuman primate is a major risk factor for HTLV-1 infection in hunters from Central Africa. Clin Infect Dis. 2015;60:1667–1676.
3. Kazanji M, et al, and Gessain A. Origin of HTLV-1 in hunters of nonhuman primates in Central Africa. J Infect Dis. 2015;211:361–365.
4. Doren-Djuicy D, et al submitted 2018.
5. Richard L, et al, and Gessain A. Zoonotic transmission of 2 human T lymphotropic virus type 4 strains in hunters bitten by gorillas in Central Africa. Clin Infect Dis. 2016;63:800–803.