Human T-Cell Lymphotropic Viruses

Epidemiology

Human T-cell lymphotropic viruses (HTLV) include four types (HTLV-1, HTLV-2, HTLV-3, and HTLV-4). HTLV-1 and HTLV-2 were first reported in 1980 and 1982, respectively, and are the two most clinically important HTLVs. ^, Infections with HTLV-3 and HTLV-4 were identified in Cameroon in 2005, although the prevalence, transmissibility between humans, and clinical significance are not well defined. ^,

There is significant geographic variation in the prevalence of HTLV-1 and HTLV-2. HTLV-1 is most prevalent in southwestern Japan, the Caribbean, Melanesia, Central Australia, and parts of West and Central Africa and generally is less common in Europe and North America. Around the world, at least 5–10 million people are estimated to be infected with HTLV-1. The prevalence of HTLV-2 has been less studied but appears to have a more restricted distribution in West and Central Africa and specific subpopulations in the Americas. In the US, approximately 1 in 5000 (0.02%) of first-time blood donors had antibodies to HTLV-1 or HTLV-2. However, foci of HTLV-1 infection have been reported from the Gulf Coast states and from New York City among people who were born in the Caribbean or had sexual contact with people from that area. Endemic foci of HTLV-2 infection exist among the Guayani Indians of Panama and some Native Americans living in the southwest. In the US, Europe, South America, and Asia, HTLV-2 infection is four times more common than infection with HTLV-1 among injection drug users.

Genomic similarities between HTLV and simian T-lymphotropic viruses (STLVs) suggest that the human viruses are derived from viruses transmitted to humans from nonhuman primates. Human populations that are exposed to nonhuman primates through hunting, butchering, or keeping primates as pets appear to be at increased risk for HTLV infections. STLVs do not appear to be transmitted to humans through occupational exposure to nonhuman primates (e.g., employment in primate centers or laboratories).

HTLV-1 and HTLV-2 are transmitted between people via sexual, parenteral (blood transfusion and intravenous drug use), or mother-to-child transmission. Sexual transmission is an efficient mode of transmission. A 10-year study of serodiscordant sexual partners suggest women have a substantially higher risk of being infected (60%) versus men (0.4%). Parenteral exposure to HTLV-1-contaminanted blood was a common route of transmission. However, since 1988, blood donors in the US have been screened for HTLV-1 antibodies and excluded if seropositive. HTLV-3 and HTLV-4 appear to be serologically indistinguishable from HTLV-1 and HTLV-2. Because the viruses are infectious only when present in lymphocytes, there is thought to be minimal risk for infection from acellular products such as serum or clotting factors, and people with hemophilia do not have high rates of seropositivity. Mother to child transmission is responsible for almost all HTLV-1 and HTLV-2 infections in childhood. Rates of 4%–22% have been reported for mother-to-child transmission, and most cases are thought to occur through breastfeeding. The risk for transplacental or perinatal transmission of viruses from an infected mother is about 5% for an infant who is not breastfed, whereas the risk for infection is about 25% for the breastfed child. Infected women should not breastfeed. HTLV-1 is not transmitted by casual contact. Medical workers are not at risk for contracting infection if they observe universal precautions.

Clinical Manifestations