Genomic evolution, patterns of global dissemination, and interspecies transmission of human and simian T-cell leukemia/lymphotropic viruses.

Using both env and long terminal repeat (LTR) sequences, with maximal representation of genetic diversity within primate strains, we revise and expand the unique evolutionary history of human and simian T-cell leukemia/lymphotropic viruses (HTLV/STLV). Based on the robust application of three different phylogenetic algorithms of minimum evolution-neighbor joining, maximum parsimony, and maximum likelihood, we address overall levels of genetic diversity, specific rates of mutation within and between different regions of the viral genome, relatedness among viral strains from geographically diverse regions, and estimation of the pattern of divergence of the virus into extant lineages. Despite broad genomic similarities, type I and type II viruses do not share concordant evolutionary histories. HTLV-I/STLV-I are united through distinct phylogeographic patterns, infection of 20 primate species, multiple episodes of interspecies transmission, and exhibition of a range in levels of genetic divergence. In contrast, type II viruses are isolated from only two species (Homo sapiens and Pan paniscus) and are paradoxically endemic to both Amerindian tribes of the New World and human Pygmy villagers in Africa. Furthermore, HTLV-II is spreading rapidly through new host populations of intravenous drug users. Despite such clearly disparate host populations, the resultant HTLV-II/STLV-II phylogeny exhibits little phylogeographic concordance and indicates low levels of transcontinental genetic differentiation. Together, these patterns generate a model of HTLV/STLV emergence marked by an ancient ancestry, differential rates of divergence, and continued global expansion.     

Use of a generic polymerase chain reaction assay detecting human T-lymphotropic virus (HTLV) types I,II and divergent simian strains in the evaluation of individuals with indeterminate HTLV serology

In countries with a low prevalence of human T-lymphotropic virus (HTLV) infection, indeterminate HTLV serologies are a major problem in blood bank screening because of the uncertainties about infection in these cases. The recent discovery of two new types of simian T-lymphotropic virus (STLV), which give an HTLV-indeterminate serology, raises the question whether indeterminate serologies in humans may be linked to new types of HTLV. Starting from a Tax sequence alignment of all available primate T-cell lymphotropic virus strains (PTLV), including the two new types STLV-PH969 and STLV-PP1664, we developed generic and type-specific nested polymerase chain reactions (PCRs). The generic PCR proved to be highly sensitive and cross-reactive for all four types of PTLV, while the discriminatory PCRs had a high sensitivity and a specificity of 100%. There was no cross-reactivity with human immunodeficiency virus (HIV), ensuring correct interpretation of results from coinfected patients. Among the 77 serologically indeterminate samples tested, 6 were found to be HTLV-IPCR positive and 1 was HTLV-II PCR positive. Sequencing of one of the HTLV-I PCR positives excluded PCR contamination, and revealed a divergent type of HTLV-I. The majority of the seroindeterminate samples (91%) were however HTLV-PCR negative, and no new types of HTLV were found. This new assay can identify otherwise undetected HTLV-I or HTLV-II infections and is a useful tool of screening for new types of HTLV among seroindeterminate samples. J. Med. Virol. 52:1–7, 1997.© 1997 Wiley-Liss, Inc.     

Human T-cell leukemia virus type I isolates from Gabon and Ghana: comparative analysis of proviral genomes

Two isolates of human T-cell leukemia virus type I (HTLV-I) were obtained from lymphocyte cultures of a healthy carrier in Gabon and another in Ghana. Their proviruses were analyzed by Southern blot hybridization and compared with prototypical HTLV-I isolated in Japan and the United States. The provirus genomes of both strains were highly homologous to the prototype HTLV-I along the whole viral genome. The restriction endonuclease sites of the Ghanian isolate were almost identical with those of the prototype HTLV-I, but 10 of 26 sites of the Gabonese isolate were different from those of the prototype. Furthermore, the restriction map of the Gabonese isolate resembled those of a simian T-cell leukemia virus (STLV-I) isolated from a chimpanzee from Sierra Leone and a variant of HTLV-I from Zaire (HTLV-Ib) more closely than those of any other known HTLV-I. These results indicated the existence of some unique strains of HTLV-I transmitted among African people, and the importance of clarifying the origin and transmission of HTLV group viruses.     

The human HTLV-3 and HTLV-4 retroviruses: new members of the HTLV family

Human T cell leukemia/lymphoma virus Type 1 and 2 (HTLV-1 and HTLV-2), together with their simian counterparts (STLV-1, STLV-2), belong to the Primate T lymphotropic viruses group (PTLV). HTLV-1 infects 15 to 20million people worldwide, while STLV-1 is endemic in a number of simian or ape species living in Africa or Asia. The high percentage of homologies between HTLV-1 and STLV-1 strains, led to the demonstration that most HTLV-1 subtypes arose from interspecies transmission between monkeys and humans. STLV-3 viruses belong to the third PTLV type and are equally divergent from HTLV-1 than from HTLV-2. They are endemic in several monkey species that live in West, Central, and East Africa. In 2005, we and others reported the discovery of the human homolog (HTLV-3) of STLV-3 in two asymptomatic inhabitants from South Cameroon whose sera exhibited HTLV indeterminate serologies. More recently, we reported a third case of HTLV-3 infection in Cameroon suggesting that this virus is not rare in the human population living in Central Africa. Together with STLV-3, these three human viral strains belong therefore to the PTLV-3 type. A fourth HTLV type (HTLV-4) was also discovered in the same geographical area. Current studies are aimed at determining the prevalence, distribution and modes of transmission of these viruses as well as their possible association with human diseases. Furthermore, molecular characterization of their viral transactivator Tax is ongoing in order to look for possible oncogenic properties.     

Detection and characterization of simian retroviruses homologous to human T-cell leukemia virus type I

Lymphoid cell lines were established from five different species of monkeys which were positive in antibodies cross-reactive with human T-cell leukemia virus type I (HTLV-I) and were shown to contain provirus sequences homologous to HTLV-I. Gene-specific probes of HTLV-I, gag, pol, env, pX, and LTR, hybridized efficiently with monkey DNAs from these cell lines under stringent conditions, indicating that the proviruses are very similar to HTLV-I along with whole viral genomes. However, the preliminary restriction mapping turned out the difference between simian retroviruses and HTLV-I and also among simian retroviruses. These findings suggest a common ancestor of simian and human retroviruses, but exclude the recent interspecies transmission between monkeys and humans.     

Identification and molecular characterization of new STLV-1 and STLV-3 strains in wild-caught nonhuman primates in Cameroon

Humans and simian species are infected by deltaretroviruses (HTLV and STLV respectively), which are collectively called primate T-cell lymphotropic viruses (PTLVs). In humans, four types of HTLV have been described (HTLV-1 to -4) with three of them having closely related simian virus analogues named STLV-1, 2 and 3. In this study, our aim was to search for a simian HTLV-4-related virus and to document and characterize further the diversity of STLV infections in wild primate populations. We screened 1297 whole blood samples from 13 different primate species from southern Cameroon. Overall, 93 samples gave HTLV-1, HTLV-2 or dual HTLV-1/-2 INNOLIA profiles, 12 were HTLV positive but untypeable and 14 were indeterminate. Subsequently, we performed generic and specific (STLV-1 to -3) tax-rex PCRs to discriminate the different PTLV types, completed with phylogenetic analysis of 450-bp LTR sequences for STLV-1 and 900 bp pX-LTR sequences for STLV-3. We show for the first time that Lophocebus albigena and Cercopithecus cephus carry both STLV-1 and a divergent STLV-3. We also identified a new STLV-1 lineage in one C. cephus. Finally, we identify relative divergence levels in the tax/rex phylogeny suggesting that additional types of PTLV should be defined, particularly for the highly divergent STLV-1(MarB43) strain that we provisionally name STLV-5.     

Prevalence of infection by human T-cell leukemia virus types I and II in southern Spain

To assess the spread of human T-cell leukemia virus (HTLV) type I and II in different population groups at potential risk of infection in Spain, a total of 756 subjects were studied: 453 belonging to groups at risk for retrovirus infection, 255 with diseases potentially linked to HTLV-I/II infection and 48 immigrants from endemic areas. An HTLV-I viral-lysate enzyme immunoassay (EIA) with a recombinant transmembrane envelope protein incorporated was used to screen serum samples. Reactive specimens were confirmed by Western blot strips spiked with recombinant proteins that differentiated HTLV-I from HTLV-II. Infection was then verified by the polymerase chain reaction (PCR). Serum samples from 19 of the 756 subjects analyzed (2.5 %) were reactive for HTLV by EIA. One of these was from an intravenous drug user (IVDU) in whom HTLV-II infection was confirmed by Western blot and PCR; a specimen from another IVDU showed Western blot reactivity for both retroviruses, but PCR results were negative. Lastly, Western blot confirmed the presence of HTLV in one of the immigrant subjects. Western blot did not verify HTLV infection in the remaining 16 cases, indicating a high rate of nonspecific anti-HTLV reactivity when a second-generation EIA screening test was applied. These results suggest that HTLV is present in Spain among populations at high risk for HTLV, although at a very low rate and restricted to intravenous drug users and individuals immigrating from endemic areas.     

Immune recognition of genetically diverse simian T-cell lymphotropic virus type I isolates

Summary Nucleotide sequence analysis of selected regions of the gag, pol, env and pX genes of simian T-cell lymphotropic virus type I (STLV-I) strains indicated that African isolates were more closely related to human T-cell lymphotropic virus type I (HTLV-I) than Asian isolates. Despite these recent comparative studies on nucleotide sequence homology between HTLV-I and STLV-I isolates, only limited information is available regarding the influence of genetic differences on antigen-antibody recognition of distinct STLV-I strains. In this study, we demonstrated that sera from STLV-I-infected yellow baboons (Papio cynocephalus) reacted strongly with env gp62/68 from HTLV-I-infected cell lines MT-2 and C10/MJ. In contrast, sera from Japanese macaques (Macaca fuscata) naturally infected with Asian STLV-I had weak reactivity to env gp62/68 of these prototypic HTLV-I strains. Pst-1 restriction enzyme analysis of proviral DNA indicated that the baboon virus isolates were more closely related to HTLV-I than the Japanese isolates. These results indicate that nucleotide sequence diversity, correlates with variations in proviral restriction enzyme sites and antibody recognition of viral envelope proteins. These differences in immunoreactivity may have important implications for serologic diagnosis, as well as epidemiological and vaccine studies of STLV-I infection.     

Primary isolation of human T-cell leukemia-lymphoma virus types I and II: use for confirming infection in seroindeterminate blood donors.

We describe the use of an immunofluorescence assay and coculture to confirm human T-cell leukemia-lymphoma virus (HTLV) infection. Peripheral blood mononuclear cells from 32 of 32 seropositive donors were positive in the immunofluorescence assay, and 63% of their cocultures produced p24 antigen. Specific antibodies distinguished HTLV type I (HTLV-I) from HTLV-II. HTLV-I or HTLV-II was isolated from donors with indeterminate serologic test results.     

HTLV-Associated Diseases: Human Retroviral Infection and Cutaneous T-Cell Lymphomas

An array of neurologic, oncologic, and autoimmune disorders are associated with infection with the human pathogenic retroviruses human T-cell leukemia virus types I and II (HTLV-I, II), as well as the human immunodeficiency viruses (HIV). The cutaneous T-cell lymphomas, mycosis fungoides (MF) and its hematogenous variant Sezary Syndrome (SS), share similar clinical and pathological features to HTLV-I-associated adult T-cell leukemia (ATL) and speculation of a retroviral link to MF and SS, especially in areas non-endemic for ATL, has lead to an intensified search for HTLV- and HIV-like agents in these diseases. To further explore a potential role for human retroviruses in MF and SS, skin biopsy-derived or peripheral blood mononuclear cell-derived DNA from 17 patients (MF, n=7; erythrodermic MF (EMF), n=5; SS, n=5) from the North Eastern United States were screened using gene amplification by PCR and a liquid hybridization detection assay. Previously published primers and probes for HTLV-I (LTR, gag, pol, env, and pX), and our own primers and probes for HTLV-I (gag, pol, and env), HTLV-II (pol and env) and HIV-I (gag and pol) were employed. Serum antibodies to HTLV-I were negative in all but one EMF patient. The single HTLV-I seropositive patient carrying a diagnosis of EMF generated positive amplified signals for all of the eight HTLV-I regions tested. Ultimately, this individual evolved to exhibit clinical manifestations indistinguishable from ATL. The other 16 patients were negative for all 12 HTLV and HIV retroviral regions. Our findings suggest that none of the known prototypic human retroviruses are associated with seronegative MF and SS. The uniformly positive results for HTLV-I in the seropositive patient suggests that this patient initially presented with a smoldering form of ATL and illustrates the difficulty that sometimes may be encountered in the differential diagnosis of MF, SS, and ATL based solely on clinical and histopathological criteria.     

The seroepidemiology of human T-lymphotropic viruses: types I and II in Europe: a prospective study of pregnant women

BACKGROUND: Up to 20 million persons are infected with the human retroviruses human T-lymphotropic virus (HTLV)-I and HTLV-II globally. Most data on the seroprevalence of HTLV-I and HTLV-II in Europe are from studies of low-risk blood donors or high-risk injection drug users (IDUs). Little is known about the general population. METHODS: A prospective anonymous study of HTLV-I and HTLV-II seroprevalence among 234,078 pregnant women in Belgium, France, Germany, Italy, Portugal, Spain, and the United Kingdom was conducted. Maternal antibody status was determined by standard methods using sera obtained for routine antenatal infection screens or eluted from infant heel prick dried blood spots obtained for routine neonatal metabolic screens. RESULTS: Anti-HTLV-I/II antibodies were detected and confirmed in 96 pregnant women (4.4 per 10,000, 95% confidence interval [CI]: 3.5-5.2). Of these, 73 were anti-HTLV-I, 17 were anti-HTLV-II, and 6 were specifically anti-HTLV but untyped. The seroprevalence ranged from 0.7 per 10,000 in Germany to 11.5 per 10,000 in France. CONCLUSIONS: Pregnant women better reflect the general population than blood donors or IDUs. The seroprevalence of HTLV-I and HTLV-II in Western Europe is 6-fold higher among pregnant women (4.4 per 10,000) than among blood donors (0.07 per 10,000). These data provide a robust baseline against which changes in HTLV-I and HTLV-II seroprevalence in Europe can be measured.     

Comparative analysis of the HTLV-I Rex and HIV-1 Rev trans-regulatory proteins and their RNA response elements

The Rex proteins of types I and II human T-cell leukemia viruses (HTLV-I, HTLV-II) are required for expression of the viral structural gene products, gag and env and, thus, are essential for the replication of these pathogenic retroviruses. The action of Rex is sequence specific, requiring the presence of a cis-acting Rex response element located in the 3' long terminal repeat. This element corresponds to a predicted RNA secondary structure and functions in an orientation-dependent but position-independent manner. Rex acts through this response element to stimulate the nuclear export of the unspliced or singly spliced viral mRNA species encoding the virion structural proteins that are normally excluded from the cytoplasm. Although the Rex proteins of HTLV-I and HTLV-II can also function via the related Rev response element present in the env gene of the type I human immunodeficiency virus (HIV-1), the analogous HIV-1 Rev protein is unable to act on the HTLV-I Rex response element. This nonreciprocal pattern of genetic complementation by Rex and Rev suggests that these viral trans-regulators may interact directly with their RNA response elements.     

Human T-cell lymphotropic virus types I and II infections in patients with leukaemia/lymphoma and in subjects with sexually transmitted diseases in Nigeria

Summary Serological assays that distinguish antibodies to human T-cell lymphotropic virus types I (HTLV-I) and type II (HTLV-II), and polymerase chain reaction (PCR) tests were used to investigate association of these two human retroviruses with several well-defined clinical conditions in Nigeria. We compared the frequency of HTLV-I and HTLV-II infections among patients with lymphoproliferative disorders (n=65), individuals with various sexually transmitted diseases (n=40), patients with genital candidiasis (n=25) and apparently healthy individuals (n=60). Serological analysis of blood samples from all four groups showed that 10 of the 190 (5.3%) individuals tested were confirmed positive for the presence of antibodies to HTLV-I (6) or HTLV-II (4). Using the PCR technique, specific HTLV-I or HTLV-II sequences were amplified from the genomic DNA of 4 of 6 HTLV-I seropositive and 3 of the 4 HTLV-II seropositive individuals respectively. However, sequences of both viruses were amplified from the genomic DNAs of the remaining 3 seropositive individuals. Since one of the 5 sets of primer pairs [(SK110 (II)/SK III (II)], which is used for specific identification of HTLV-II did not amplify the target sequence from the genomic DNAs of any of the 4 HTLV-II-confirmed seropositive individuals in this study, it suggested sequence diversity of these viruses in Nigeria. The virus-infected individuals identified in this study were one (1.5%) of the 65 patients with leukaemia/lymphoma (HTLV-I), 6 of 40 (15.0%) individuals (HTLV-I=1, HTLV-II=3, HTLV-I/II=2) with sexually transmitted diseases (STD), one of 25(4.0%) subjects with genital candidiasis for HTLV-I, and 2 of 60 (33.3%) healthy individuals (one for HTLV-I and one for HTLV-I/II). There was a significant difference (P<0.025) between the prevalence of HTLV-I/II infections among patients with lymphoma/leukaemia and those who attended STD clinic in Ibadan, Nigeria. This study also suggests that while HTLV-I and HTLV-II may be important sexually transmitted viruses, they may not be specific aetiological agents of the common lymphoproliferative disorders in Nigeria.     

Seroprevalence of HTLV-1 and HTLV-2 among intravenous drug users and persons in clinics for sexually transmitted diseases.

Background. The human T-cell lymphotropic virus Type I (HTLV-I) is associated with adult T-cell leukemia and myelopathy, whereas HTLV-II infection has uncertain clinical consequences. We assessed the seroprevalence of these retroviruses among intravenous drug users and among patients seen at clinics for sexually transmitted diseases (STD clinics). METHODS. We used serum samples that were collected in eight cities in 1988 and 1989 during surveys of human immunodeficiency virus infection among intravenous drug users entering treatment and persons seen in STD clinics. The serum samples were tested for antibodies to HTLV, and positive specimens were tested further by a synthetic peptide-based enzyme-linked immunosorbent assay to differentiate between HTLV-I and HTLV-II. RESULTS. Among 3217 intravenous drug users in 29-drug-treatment centers, the median seroprevalence rates of HTLV varied widely according to city (range, 0.4 percent in Atlanta to 17.6 percent in Los Angeles). Seroprevalence increased sharply with age, to 32 percent in persons over 44 years of age. HTLV infection was more common among blacks (15.5 percent) and Hispanics (10.7 percent) than among whites (4.1 percent), and it was strongly associated with a history of heroin injection (P less than or equal to 0.001). Among 5264 patients in 24 STD clinics, the median rates of HTLV infection were much lower (range, 0.1 percent in Atlanta and Newark to 2.0 percent in Los Angeles). Again, this infection was more common among intravenous drug users (7.6 percent) than among non-drug users (0.7 percent). Eighty-four percent of the seropositive samples from drug-treatment centers and 69 percent of those from STD clinics were due to HTLV-II infection (P = 0.03). CONCLUSIONS. HTLV infections are common among intravenous drug users and are primarily caused by HTLV-II. Among patients seen at STD clinics, HTLV is strongly associated with intravenous drug use, but the retrovirus is also prevalent among non-drug users.     

Association of primate T-cell lymphotropic virus infection of pig-tailed macaques with high mortality

Natural infection of humans with human T-cell lymphotropic virus type I (HTLV-I) and of old world nonhuman primates with the simian counterpart, STLV-I, is associated with development of neoplastic disease in a small percentage of individuals after long latent periods. HTLV-I is also the etiologic agent of a more rapidly progressive neurologic disease, HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Macaques have been used experimentally in studies to evaluate HTLV-I candidate vaccines for efficacy, but no evidence of disease was observed. Here we report experimental infection of pig-tailed macaques with STLV-I(sm) and HTLV-I(ACH), both of which were associated with a disease syndrome characterized by rapid onset, hypothermia, lethargy, and death within hours to days. Other pathologic sequelae included diarrhea, rash, bladder dysfunction, weight loss, and, in one animal, arthropathy. Both retroviruses were detected in the central nervous systems of some animals, either by culture or by direct antigen capture for p19 Gag in cerebrospinal fluid. Although virus was recovered throughout infection from peripheral blood mononuclear cells (PBMC), all infected macaques maintained low antiviral antibody titers and stable proviral burdens, which generally ranged between 10 and 100 copies per 10(6) PBMC. However, of 13 macaques infected with HTLV-I(ACH) or STLV-I(sm), seven animals (54%) died between 35 weeks and 412 years after infection. This unexpected high mortality within a relatively short time suggests that infection of pig-tailed macaques might be a useful model for studying immune responses to and pathologic events resulting from HTLV-I infection.     

The Origin and Evolution of Human T-Cell Lymphotropic Virus Types I and II

Studies on human T-cell lymphotropic virus types I (HTLV-I) and II (HTLV-II) are briefly reviewed from the viewpoint of molecular evolution, with special reference to the evolutionary rate and evolutionary relationships among these viruses. In particular, it appears that, in contrast to the low level of variability of HTLV-I among different isolates, individual isolates form quasispecies structures. Elucidating the mechanisms connecting these two phenomena will be one of the future problems in the study of the molecular evolution of HTLV-I and HTLV-II.     

In vivo fluctuation of HTLV-I and HTLV-II proviral load in patients receiving antiretroviral drugs

HTLV-I and HTLV-II infect T lymphocytes. A high HTLV-I proviral load in peripheral blood mononuclear cells (PBMCs) has been associated with a higher risk of neurologic disease. For HTLV-II, large numbers of infected lymphocytes might contribute to accelerate the immunodeficiency and increase the risk of neuropathy in HTLV-II/HIV-1 coinfected people. We have examined the impact of antiretroviral drugs on HTLV proviral load, testing longitudinal samples collected from 1 HTLV-I infected patient suffering HTLV-I-associated myelopathy (HAM), and two HTLV-II/ HIV-1 coinfected subjects. The HAM patient showed a reduction greater than 2 log in the peripheral proviral load after being treated with zidovudine and lamivudine. In contrast, potent antiretroviral treatment in HIV-1/HTLV-II coinfected carriers produced an initial increase in the HTLV proviral load, which was followed by a reduction greater than 1 log thereafter. In conclusion, antiretroviral drugs seem to reduce HTLV proviral load, although in HIV-1 coinfected persons a transient increase in HTLV proviral load could reflect the rapid blocking of HIV-1 replication occurring in response to therapy, thus causing an increase in the number of circulating T lymphocytes carrying HTLV proviral DNA.     

The discovery of two new divergent STLVs has implications for the evolution and epidemiology of HTLVs

We have isolated and characterised two divergent simian T‐lymphotropic viruses (STLV), not belonging to the established human and simian T‐lymphotropic virus lineages HTLV‐1/STLV‐1 and HTLV‐2. STLV‐L, from an Eritrean sacred baboon (Papio hamadryas), has been typed as a third type of simian T‐lymphotropic virus, distinct from HTLV‐1/STLV‐1 and HTLV‐2. The other virus, isolated from Congolese bonobos (Pan paniscus), is a distinct member of the HTLV‐2 clade and has been designated STLV‐2. The isolation of these two simian viruses shows that the spectrum of HTLVs/STLVs is larger than previously expected. Our data indicate that the two lineages STLV‐L and HTLV‐2/STLV‐2 are of African origin, while the HTLV‐1/STLV‐1 lineage has been shown to be of Asian origin. These data, together with our phylogenetic analyses, suggest an African origin of the HTLV/STLV ancestor, which provides new clues about virus dissemination. Furthermore, the atypical serological profiles exhibited by STLV‐L or STLV‐2 infected animals in western blot, raise questions about the efficiency of current screening methods to type highly divergent HTLVs/STLVs. Considering the growing interest in xenotransplantations, more epidemiological and biological knowledge of simian and human T‐lymphotropic viruses is necessary to estimate the risk of interspecies transmissions. Copyright © 1999 John Wiley & Sons, Ltd.     

Prevalence of human T-cell leukemia virus types I and II in Switzerland

The retroviruses human immunodeficiency virus (HIV)-1/2 and human T-cell leukemia virus (HTLV)-I/II share modes of transmission, suggesting that efforts to monitor the current HIV-1 epidemic in Switzerland should be complemented by assessment of HTLV-I/II prevalence. This study presents an updated evaluation of HTLV-I/II infection among groups within the Swiss population polarized towards either low or increased risk of infection. Archived serum and peripheral blood mononuclear cell (PBMC) samples were examined for evidence of HTLV-I/II infection by enzyme-linked immunosorbant assay (ELISA), type-specific Western blot, type-specific polymerase chain reaction (PCR), DNA sequence analysis, and virus culture. Among blood donations obtained from low-risk Swiss donors, we report a complete lack of HTLV-II infection and the occurrence of HTLV-I infection limited to a prevalence of 0.079 per 100,000 (1/1,266,466). Among high-risk HIV-positive persons and HIV-negative persons at increased risk of HIV-infection, we report a focus of HTLV-I and HTLV-II infection at prevalence rates of 62 per 100,000 (1/1,620) and 309 per 100,000 (5/1,620), respectively. The finding of low HTLV-I/II prevalence among Swiss blood donors and containment of HTLV-I/II infection within known risk-groups does not support initiation of HTLV-I/II screening for Swiss blood, tissue, and organ donations.