Ethnic composition, age and sex, together with location and standard of housing as determinants of HLTV-I infection in an urban Trinidadian community

High prevalence of HTLV-I infection in Mashhad, Northeast Iran: a population-based seroepidemiology survey

BACKGROUND

Mashhad, in the northeast of Iran has been suggested as an endemic area for human T cell lymphotropic virus type I (HTLV-I) infection since 1996.

OBJECTIVES

We performed a community-based seroepidemiology study to examine the prevalence and risk factors for HTLV-I infection in the city of Mashhad.

STUDY DESIGN

Between May and September 2009, overall 1678 subjects from all the 12 geographical area of Mashhad were selected randomly by multistage cluster sampling for HTLV antibody. The study population included 763 males and 915 females, with the mean age of 29.1 ± 18.5 years. 1654 serum samples were assessed for HTLV antibody using ELISA and reactive samples were confirmed by Western blot and PCR.

RESULTS

The overall prevalence of HTLV-I infection in whole population was 2.12% (95% CI, 1.48-2.93) with no significant difference between males and females (p = 0.093) and the prevalence of HTLV-II seropositivity was 0.12% (95% CI, 0.02-0.44). The HTLV-I Infection was associated with age (p<0.001), marital status (p<0.001), education (p = 0.047), and history of blood transfusion (p = 0.009), surgery (p<0.001), traditional cupping (p = 0.002), and hospitalization (p = 0.004). In logistic regression analysis, age was the only variable that had a significant relation with the infection (p = 0.006, OR = 4.33).

CONCLUSIONS

Our results demonstrated that Mashhad still remains an endemic area for HTLV-I infection despite routine blood screening. Thus, further strategies are needed for prevention of the virus transmission in whole population.

Global epidemiology of HTLV-I infection and associated diseases

Epidemiologic aspects of human T-lymphotropic virus type I (HTLV-I) infection have been thoroughly studied over the course of approximately 25 years since its first description. The geographic distribution of the virus has been defined, with Japan, Africa, Caribbean islands and South America emerging as the areas of highest prevalence. The reasons for HTLV-I clustering, such as the high ubiquity in southwestern Japan but low prevalence in neighboring regions of Korea, China and eastern Russia are still unknown. The major modes of transmission are well understood, although better quantitative data on the incidence of transmission, and on promoting/inhibiting factors, are needed. Epidemiologic proof has been obtained for HTLV-I's causative role in major disease associations: adult T-cell leukemia (ATL), HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP), HTLV-associated uveitis and infective dermatitis. However, more and better studies are needed for other apparent disease outcomes such as rheumatologic, psychiatric and infectious diseases. Since curative treatment of ATL and HAM/TSP is lacking and a vaccine is unavailable, the social and financial cost for the individual, his/her family and the health system is immense. For this reason, public health interventions aimed at counseling and educating high-risk individuals and populations are of paramount importance.

A case-control study of risk factors associated with human T-cell lymphotrophic virus type-I seropositivity in blood donors from Guadeloupe, French West Indies

BACKGROUND AND OBJECTIVES

An age- and gender-specific distribution characterizes human T-cell lymphotrophic virus type-I (HTLV-I) seropositivity in Guadeloupe (French West Indies). Further epidemiological studies are required to identify other possible risk factors associated with this retroviral infection.

MATERIALS AND METHODS

A nested case-control study was conducted between 1997 and 1999 among blood donors. A total of 102 HTLV-I-positive subjects were matched (at a ratio of 1 : 3) by gender, age (+/-5 years) and donor status (new or regular) to 306 HTLV-I-negative controls. Information was obtained through a questionnaire assessing both environmental and behavioural variables.

RESULTS

Factors independently associated with HTLV-I infection included a low level of education [odds ratio (OR) 6.61, confidence interval (CI) 2.89-15.15], black ethnicity (OR 3.28, CI 1.01-10.65), two or more sex partners in the previous 3 years (OR 2.43, CI 1.16-5.10), early age at first sexual intercourse (0.84 risk reduction per additional year, CI 0.76-0.93), a history of sexually transmitted diseases (OR 2.29, CI 1.0-5.34) and positive Chlamydia serology (OR 1.95, CI 1.03-3.68).

CONCLUSION

These data provide a wide spectrum of features associated with HTLV-I seropositivity, especially sexual risk factors. It strongly suggests that heterosexual intercourse is an important route of HTLV-I transmission in Guadeloupe, even among low-risk populations such as blood donors.

Spatial distribution of the Human T-Lymphotropic Virus types I and II (HTLV-I/II) infection among blood donors of Hemominas Foundation, Belo Horizonte, Minas Gerais State, Brazil, 1994-1996

We conducted a cross-sectional study of the spatial distribution of HTLV-I/II infection among blood donors of Hemominas Foundation, living in Belo Horizonte, from 1994 to 1996. Study population (1,022) was composed by 533 cases (positive Western Blot (WB), indeterminate WB and ELISA positive without WB result) and a random sample of 489 non-cases (HTLV-I/II serum negative). Cases and non-cases were georeferenced using the exact or an approximation of the household address reported at the blood donation interview. Using multivariate analysis, cases with WB result are less likely to be reposition blood donors compared to voluntary ones (OR = 0.70; CI 95%: 0.50-0.99). Using the difference between univariate K functions, we found no evidence that cases and non-cases differ in their spatial distribution. We found no evidence that cases with and without WB result differ in the distance between their residence and Hemominas Foundation. No donors without WB result were georeferenced by the exact address. These donors could not have received the Hemominas letter inviting them to return to collect the second blood sample.

On the etiology of tropical spastic paraparesis and human T-cell lymphotropic virus-I-associated myelopathy

The purpose of this review is to present some concepts on the etiology of tropical spastic paraparesis or human T-cell lymphotropic virus-I (HTLV-I)-associated myelopathy (TSP/HAM). The large number of syndromes that have been associated with HTLV-I (60 to date), the existence of TSP/HAM cases associated with other retroviruses (human immunodeficiency virus-2 [HIV-2], HTLV-II), the existence of many TSPs without HTLV-I, and the evidence of clear epidemiologic contradictions in TSP/HAM indicate that the etiopathogenesis of TSP/HAM is not yet clear. Tropical spastic paraparesis/HAM affects patients of all human ethnic groups, but usually in well localized and relatively isolated geographic regions where HTLV-I has been endemic for a long time. Environmental factors and geographic locations appear to be critical factors. Because the neuropathology of TSP/HAM suggests a toxometabolic, rather than a viral cause, it is proposed that an intoxication similar to neurolathyrism could account for some of TSP/HAM cases, mainly in tropical and subtropical countries. If this were the case, HTLV-I could be a cofactor or act as a bystander. it is possible that co-infection with another agent is necessary to produce TSP/HAM and most of the syndromes associated with HTLV-I.

Human T-lymphotropic virus type I among blood donors from Guadeloupe: donation, demographic, and biologic characteristics

BACKGROUND

Epidemiologic data on human T-lymphotropic virus type I (HTLV-I) in Guadeloupe (French West Indies) are scant.

STUDY DESIGN AND METHODS

From January 1989 to December 1996, 59,426 blood donors were screened by enzyme immunoassay for antibodies to HTLV-I. All repeatedly reactive samples were confirmed by Western blot. Temporal trends in HTLV-I seropositivity rates were examined during the study period. A multivariate analysis of donation, demographic, and biologic characteristics was performed.

RESULTS

Of the screened blood donors, 195 were confirmed as seropositive, for an overall prevalence of 0.33 percent (95% CI 0.28-0.38). A marked decrease in overall HTLV-I prevalence with time (from 0.47% in 1989 to 0.13% in 1996) was observed, which can be explained mainly by the decreasing percentage of recruited new donors during the study period. Four independent risk factors for HTLV-I were identified: new donor status (odds ratio [OR] 12.5), female sex (OR 1.7), increasing age (30-39 years: OR, 2.4; 40-49 years: OR, 3.7; >50 years: OR 6.6), and positive antibodies to hepatitis B virus core antigen (OR, 1.7). Selection of specific locations for blood collection was inversely associated with HTLV-I (OR 0.5).

CONCLUSION

New donor status, advancing age, female sex, and positivity for hepatitis B virus core antibodies were the major factors associated with HTLV-I infection in Guadeloupe.

Geographical clustering of human T-cell lymphotropic virus type I in Guadeloupe, an endemic Caribbean area

Between January 1989 and December 1996, 59,426 blood donors from Guadeloupe (French West Indies) were screened for antibodies to human T-cell lymphotropic virus type I (HTLV-I). Of these, 195 were confirmed as being positive for HTLV-I, yielding an overall prevalence of 0.33% [95% confidence interval (CI) 0.28-0.38]. On multiple logistic regression analysis, risk factors for HTLV-I were female gender [odds ratio (OR) 1.8; CI 1.3-2.4], increasing age (30-39 years, OR 2.2, CI 1.4-3.4; 40-49 years, OR 3.1, CI 2.1-4.7; > or =50 years, OR 5.6, CI 3.6-8.6) and positive hepatitis B core antibodies (OR 2.0; CI 1.5-2.8). HTLV-I seropositivity was also significantly associated with current residence in certain areas, highlighting microgeographic clustering: individuals living along the Atlantic Facade of Guadeloupe, which is a traditional sugar cane plantation area where Africans were brought during slave trading, were at increased risk for HTLV-I infection (OR 1.9; CI 1.3-2.7) compared with other areas in Guadeloupe devoted to other activities. Our report of HTLV-I cluster identification in Guadeloupe probably reflects both its low spread and its highly intrafamilial restricted transmission within this endemic Caribbean population.

Among all human T-cell leukemia virus type 1 proteins, tax, polymerase, and envelope proteins are predicted as preferential targets for the HLA-A2-restricted cytotoxic T-cell response

The human T-cell leukemia virus type 1 (HTLV-1) is a human retrovirus associated with two diseases for which no successful treatment is yet available; the development of a vaccine is therefore an important issue. Since HTLV-1 is a persistent virus, an efficient vaccine will probably require a cytotoxic T-lymphocyte (CTL) response in addition to the production of antibodies. To identify potential CTL epitopes, we have selected, within all of the HTLV-1 proteins, nonapeptides containing anchor residues required for association with HLA-A2 molecules (residues at positions 2 and 9), which is the most frequently occurring A allele in all human populations. A set of 111 peptides was synthetized and tested in vitro in two assembly assays using processing-defective T2 cells. Anchor motifs selected were those containing two major anchor residues (L2/M2/12-V9/L9/I9) (one letter amino-acid code) and those including tolerated anchor residues (V2/A2/T2 and/or A9/M9/T9). The analysis of the binding capacity of the peptides confirms the high efficiency of the L2-V9 anchor motif and shows that a systematic research of potential binding peptides should exclude peptides containing known detrimental residues rather than select only peptides with known favored residues. We show that 39 peptides representative of all the HTLV-1 proteins are able to bind to HLA-A2 molecules. Strong binder peptides which are very likely good CTL epitopes were identified in three HTLV-1 proteins, Tax, envelope, and polymerase. Three of the strong binder peptides correspond to previously described HLA-A2-restricted CTL epitopes in the Tax protein, and two others are localized in a domain of the viral envelope recognized by natural neutralizing antibodies. This latter result has important implications for the development of an anti-HTLV-1 vaccine.

Role of HTLV-I in development of non-Hodgkin lymphoma in Jamaica and Trinidad and Tobago. The HTLV Lymphoma Study Group

Human T-cell lymphotropic virus type I (HTLV-I) has been implicated in the aetiology of adult T-cell leukaemia/lymphoma in Japan and elsewhere, particularly the Caribbean. We have carried out parallel case-control studies in Jamaica and in Trinidad and Tobago to quantify the role of HTLV-I in the development of non-Hodgkin lymphoma (NHL). 135 cases of NHL were enrolled in Jamaica and 104 in Trinidad and Tobago. Controls were selected from patients treated in the same wards or clinics at the same time as the cases. Overall, patients with NHL were 10 times more likely than were controls to be seropositive for HTLV-I (Jamaica odds ratio 10.3 [95% CI 6.0-18.0], Trinidad and Tobago 14.4 [7.6-27.2]). In both countries the association between NHL and HTLV-I was greatest for T-cell lymphomas (18.3 [9.5-35.6] and 63.3 [25-167]). Among T-cell lymphomas especially, there was no significant difference between men and women in the association between NHL and HTLV-I, but there was a significant inverse relation between age and likelihood of HTLV-I seropositivity. B-cell lymphomas were predominant in the older age groups and were not associated with HTLV-I seropositivity. These findings are consistent with the hypothesis that early life exposure to HTLV-I is important for risk of subsequent ATL. Prevention of vertical transmission of HTLV-I could reduce by 70-80% cases of NHL in people under 60 years in this region.

The epidemiology of myelopathy associated with human T-lymphotropic virus 1

A progressive spastic myelopathy is one of the principal manifestations of the human T cell lymphotropic virus type 1. Recent research is reviewed on the geography and epidemiology of this specific form of tropical spastic paraparesis. First recognized in the Caribbean, Colombia and Japan, it is now also confirmed as a major neurological problem in areas of eastern Brazil, western equatorial Africa, Natal and Seychelles, with other scattered foci world-wide. Accurate surveys call for sophisticated techniques including polymerase chain reaction amplification. The only defined modes of spread are by breast feeding, sexual contact and blood cell infusion. The onset of neurological disease is associated with high antibody titres and a high rate of spontaneous T lymphocyte proliferation. Molecular analysis has revealed no nucleotide sequence variation in cases with or without myelopathy. In non-transfusion cases the clinical attack rate is low with a very long latent period, but there are unexplained regional differences and familial cases are an important exception. Current research is focused on possible local, ethnic, or genetic co-factors.

Seroprevalence and cofactors of HTLV-I infection in Tumaco, Colombia

This study presents an analysis of the factors associated with HTLV-I seroprevalence in the endemic area of Tumaco, Colombia. During June to August 1988, 1,077 individuals were selected at random from a population of 45,594. The overall prevalence rate of HTLV-I antibodies was 2.8%. Among females prevalence was significantly higher (p less than 0.02) than among males. Rates increased substantially with age. HTLV-I prevalence among individuals with history of use of intravenous medications was significantly higher (p less than 0.001) than in those without such history. Logistic regression analysis included age in years, indicators for male gender, and for i.v. injections, and their interactions. Age was very strongly associated to HTLV-I infection among females. At early ages prevalence was not different between sexes, but females presented a significantly higher rate than males after age 42. History of i.v. administered medications was very strongly associated in the univariate analysis and, although significance was borderline in the multivariate analysis, it had the effect of doubling the odds of HTLV-I infection.

The epidemiology of HTLV-I infection

It has been 10 years since the discovery of the human T-cell lymphotropic virus type I (HTLV-I), the first human retrovirus. During the past decade, significant progress has been made in understanding the transmission of the virus and defining its geographic distribution. It has been shown conclusively that HTLV-I is a causal factor in the induction of both adult T-cell leukemia/lymphoma and HTLV-I-associated myelopathy. However, the pathogenesis of each of these conditions is not clear, and in the light of the evidence of immune dysfunction seen among carriers of the infection, it is likely that other associated diseases will be identified. The challenge in the next decade will be to develop and implement therapeutic interventions among carriers to prevent such diseases as well as to curtail transmission within endemic populations.

Human T-cell lymphotropic virus-type I

HTLV-I is a retrovirus now identified as the etiologic agent of two diverse disease processes: ATLL, an aggressive T-cell malignancy; and TSP/HAM, a chronic progressive myelopathy. Transmission can occur horizontally through blood transfusions, IV drug abuse and sexual intercourse. Vertical transmission may also occur. Available diagnostic modalities are serologic in nature and include the EIA and the more specific confirmatory assays WIB and RIPA. These studies are thus far suboptimal in terms of sensitivity and specificity, and await refinement. DNA amplification by the polymerase chain reaction seems to hold the most immediate diagnostic promise for the future. AZT apparently is not useful clinically, and current treatment is only palliative in nature. The diverse diseases caused by HTLV-I underscore the insidious nature of the Retroviridae family. These subtle cell-associated pathogens will undoubtedly be shown to play a significant role in other disease processes of uncertain etiology.

Human T-Cell Lymphotropic Virus-Type I

Human T-cell lymphotropic virus type-1 (HTLV-I) is a recently recognized retrovirus identified as the cause of adult T-cell leukemia-lymphoma (ATLL) and HTLV-I-associated myelopathy (TSPI HAM). HTLV-I, a member of the Retroviridae family of viruses, was first described in 1980 after the isolation of the virus from a patient with a T-cell lymphoma. These pathogenic retroviruses are typically divided into the Oncovirinae and Lentivirinae. The oncovirus group, including HTLV-I, HTLV-II and bovine leukemia virus (BLV), is generally associated with tumors. The lentiviruses are associated with immune deficiency and/or neurologic disease, and include agents such as the visna virus of sheep and the human immunodeficiency virus type-1 and -2 HIV-1 and HIV-2).

Screening for prolonged incubation of HTLV-I infection in British and Jamaican relatives of British patients with tropical spastic paraparesis

OBJECTIVE

To compare the prevalence of antibody to and proviral DNA of the retrovirus HTLV-I in relatives of 11 British patients with tropical spastic paraparesis who had migrated from Jamaica before they developed symptoms, and to examine factors possibly related to transmission of HTLV-I.

DESIGN

Migrant, family study. Antibody state was determined by several methods and confirmed by western blotting; the polymerase chain reaction was used to detect proviral DNA.

SETTING

Britain and Jamaica.

SUBJECTS

All available first degree relatives: those born and still resident in Jamaica (group 1); those born in Jamaica who migrated to Britain (group 2); and index patients' children who were born and resident in Britain (group 3). All had been breast fed and none had had blood transfusions.

RESULTS

Of the 66 living relatives, 60 were traced. Seroprevalence among those born in Jamaica (irrespective of current residence) was 22% (10/46; 95% confidence limits 9 to 34%) compared with zero among British born offspring (0/14) and was higher in group 2 at 33% (7/21; 12 to 55%) than in group 1 at 12% (3/25; 0 to 25%). (Patients in group 1 had the greatest mean age.) Proviral DNA was not detected in any subject negative for HTLV-I antibody, making prolonged viral incubation in those negative for the antibody unlikely.

CONCLUSION

In this sample factors related to place of birth and early residence were more important in transmission of HTLV-I than maternal or age effects. In areas with a low to moderate prevalence policies of preventing mothers who are carriers of the virus from breast feeding would be premature.

A cluster of HTLV-1 associated tropical spastic paraparesis in Equateur (Zaire): ethnic and familial distribution

In Lisala, Equateur province, Zaire, 25 patients from 21 pedigrees were identified with human T-lymphotropic virus type 1 (HTLV-1) associated tropical spastic paraparesis (TSP). In the 10 (48%) pedigrees with additional genuine TSP cases established mainly by history, seven of 10 patients' mothers, no fathers or spouses, one of 59 surviving offspring, five of 105 siblings, and six other close blood relatives had TSP. A child may develop TSP before its mother. Three familial cases were in paternal relatives only. In total, 39 cases (11 men, 28 women) were identified in this population of about 50,000. Half were in the Mundunga minority of less than or equal to 10% (p less than 0.001). The data suggest maternal transmission of HTLV-1 and enhanced TSP susceptibility in those infected due to familial, probably genetic factors.

HTLV-1 and tropical spastic paraparesis. 2. The human T-cell lymphotropic virus type 1

Human T-lymphotropic viruses (HTLV) are causally associated with adult T-cell leukaemia and with a progressive form of lower limb paralysis known as tropical spastic paraparesis. HTLV-1 is endemic in parts of Japan, the Caribbean, West Africa and probably South America, and is associated with disease in these areas. Horizontal transmission is probably most common through sexual intercourse which, it is postulated, must be more efficient from male to female because virus carriage is more prevalent in women in endemic areas. Vertical transmission appears to be principally through breast milk. Poor housing and hygiene may facilitate transmission.

A survey of the human T-cell lymphotropic virus type I (HTLV-I) in south-western Colombia

To evaluate altitude of birthplace and residence as factors associated with geographic clustering of HTLV-I infection in Colombia, we sampled a total of 670 current residents of the South Pacific coastal lowland and of upland regions (Cali and environs) of the Valle and Cauca Provinces, located at an altitude of 3,100 ft. Among the 255 lowland study subjects, 4.3% had antibody against HTLV-I, compared to 0.9% of the 415 upland study subjects. A hypothesis emerging from this study is that the lower socio-economic status of lowland residents and associated diseases, particularly untreated syphilis and other sexually transmitted diseases, may explain the increased HTLV-I seropositivity rates in this population.

Human retroviruses

After many unsuccessful years of searching, the first pathogenic human retrovirus, the human T-cell leukaemia lymphoma virus (HTLV-I), was reported as recently as 1980 and since that time has been causally linked to the adult T-cell leukaemia lymphoma syndrome. A second HTLV (HTLV-II) isolated shortly afterwards is less clearly linked to some leukaemic and chronic lymphoid malignancies. The second major family of human retroviruses are the human immunodeficiency viruses (HIV) the first group of isolates (HIV-I) of which cause the acquired deficiency syndrome (AIDS). A second group of these viruses (HIV-II), have recently been identified in West Africa. They appear to be less clearly associated with disease and more similar in molecular structure to the Simian immunodeficiency viruses. AIDS has now become a major global pandemic, and vaccine and therapeutic strategies are urgently being investigated in an effort to control the disease. Unfortunately, current results are not very encouraging. In the meantime, preventative and educational measures are of utmost priority in order to prevent further spread. It is not unlikely that new human retroviruses will be discovered over the next few years.

Malignant lymphomas in Japan: epidemiological analysis of adult T-cell leukemia/lymphoma (ATL)

The incidence of malignant lymphomas in Japan is relatively low compared to that in western European countries and the United States. However, in limited areas in Japan a specific type of lymphoid malignancy called adult T-cell leukemia/lymphoma (ATL), which is caused by human T-cell leukemia virus type I (HTLV-I), is highly prevalent, and there are also many healthy carriers of HTLV-I in the same areas. A cross-sectional seroepidemiological study of HTLV-I showed that the age-specific proportion of healthy HTLV-I carriers in these ATL-endemic areas increased with age, especially over 40, and was higher in females than in males. Three main routes of HTLV-I transmission are recognized: 1) vertical transmission from mother to child mainly through breast milk; 2) horizontal transmission from man to woman through semen, and; 3) parenteral transmission from carrier donor to non-carrier recipient. The annual incidence rate of ATL among HTLV-I carriers is estimated at 2.0 in males and 0.5 in females, and the cumulative risk for ATL in HTLV-I carriers during a 70-year life span is 1%-5%. Possible risk factors for ATL in addition to HTLV-I infection were considered, i.e. genetic factors, environmental factors, nutritional condition, thymus involution etc., but none of these were found to be clearly associated with ATL. To determine whether there exist particularly susceptible hosts for ATL in the ATL endemic areas, HLA types were examined, but no conclusive results on the positive relationships between HLA types and ATL manifestation or HTLV-I infection were obtained. From follow-up studies on the age-specific distribution of HTLV-I carriers in Japan, it is now speculated that the HTLV-I infection rate might have decreased naturally in the more recent generational cohort groups, even in the ATL-endemic areas. However, ATL in Japan is an important subject for study in the field of cancer epidemiology, and several trial intervention programs for the prevention of ATL, such as controls of vertical transmission from mother to child through breast milk, are now ongoing in the ATL-endemic areas of Japan.

Bovine leukaemia: facts and hypotheses derived from the study of an infectious cancer

Bovine leukaemia virus (BLV) is the etiological agent of chronic lymphatic leukaemia/lymphoma in cows, sheep and goats. Infection without neoplastic transformation was also obtained in pigs, rhesus monkeys, chimpanzees, rabbits and observed in capybaras and water-buffaloes. Structurally and functionally, BLV is a relative of human T lymphotropic viruses 1 and 2 (HTLV-I and HTLV-II) In humans, HTLV-I induces a T-cell leukaemia and its type 2 counterpart has been found in dermatopathic lymphadenopathy, hairy T-cell leukaemia and prolymphocytic leukaemia cases. At variance with HTLV-I, BLV has not been associated with neurological diseases of the degenerative type. Bovine leukaemia virus, HTLV-I and HTLV-II show clearcut sequence homologies. The pathology of the BLV-induced disease, most notably the absence of chronic viraemia, a long latency period and lack of preferred proviral integration sites in tumours, is similar to that of adult T-cell leukaemia/lymphoma induced by HTLV-I. The most striking feature of these three naturally transmitted leukaemia viruses is the X region located between the env gene and the long terminal repeat (LTR) sequence. The X region contains several overlapping long open reading frames. One of them, designated XBL-I, encodes a trans-activator function capable of increasing the level of gene expression directed by BLV-LTR and most probably is involved in "genetic instability" of BLV-infected cells of the B cell lineage. The "genetic instability" renders the infected cell susceptible to move, along a number of stages, towards full malignancy. Little is known about these events and their causes; we present some theoretical possibilities. Bovine leukaemia virus infection has a worldwide distribution. In temperate climates, the virus spreads mostly via iatrogenic transfer of infected lymphocytes. In warm climates and in areas heavily populated by haematophagous insects, there are indications of insect-borne propagation of the virus.

HTLV-I-associated leukemia: a model for chronic retroviral diseases

Human T-lymphotropic virus type I (HTLV-I) has been associated with adult T-cell leukemia/lymphoma (ATL), a malignancy of mature CD4-positive lymphocytes, and with tropical spastic paraparesis (TSP), a demyelinating neurological syndrome. This article describes the clinical and pathological features of ATL and reviews the epidemiology of this disease and of its putative etiological agent, HTLV-I. From what is known about the molecular biology and epidemiology of HTLV-I, hypotheses on the etiology of TSP are proposed, and strategies for studying the neurological syndrome are suggested.