Evidence on horizontal transmission of bovine leukemia virus due to blood-sucking tabanid flies

Detection of genotype 1 bovine leukemia virus from a C.schultzei pool: Do Culicoides spp. have a role on the transmission of bovine leukemia virus?

Bovine leukemia virus (BLV) is known as the etiological agent of Enzootic bovine leukosis (EBL), which is the most common neoplastic disease of cattle. While the major route of virus transmission is believed to be iatrogenic, BLV proviral DNA has been identified in biological materials, including nasal secretions, saliva, milk, colostrum, and semen, and in several insect species, including horses flies. However, insects' role in the natural transmission of BLV has not been clearly demonstrated. This study assessed the possible role of midges - Culicoides spp. - in BLV transmission. BLVs were genetically characterized and BLV infection seroprevelance was determined in 224 cattle sampled from 27 different small family herds in five different districts in Hatay province, southern Turkey. Out of the 25 Culicoides spp. pools, one (4.0%; 1/25) was a C.schultzei pool while 2.67% (6/224) of the sampled cattle were positive for BLV nucleic acid. The seroprevalance rates for the sampled herds and all sampled cattle were 7.40% (2/27) and 1.33% (3/224), respectively. According to the phylogenetic analysis, the sequences of the BLVs from the cattle (n = 6) and the one BLV-positive C.schultzei pool clustered on genotype 1 (G1) BLVs. Although these results do not reveal the exact role of Culicoides spp. or other midges flies in BLV transmission, the simultaneous presence of same substitions in BLVs from both cattle and a C.schultzei pool is noteworthy. Further studies on the env gene and other BLV gene regions detected from cattle and C.schultzei pools are ongoing to understand the possible epidemiological relationship between cattle and flies.

Bovine Leukemia Virus Infection in Neonatal Calves. Risk Factors and Control Measures

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis (EBL). Although efficient eradication programs have been successfully implemented in most European countries and Oceania, BLV infection rates are still high worldwide. BLV naturally infects cattle, inducing a persistent infection with diverse clinical outcomes. The virus infects lymphocytes and integrates a DNA intermediate as a provirus into the genome of the cells. Therefore, exposure to biological fluids contaminated with infected lymphocytes potentially spreads the virus. Vertical transmission may occur in utero or during delivery, and about 10% of calves born to BLV-infected dams are already infected at birth. Most frequently, transmission from dams to their offspring occurs through the ingestion of infected colostrum or milk. Therefore, although EBL is not a disease specific to the neonatal period, during this period the calves are at special risk of becoming infected, especially in dairy farms, where they ingest colostrum and/or raw milk either naturally or artificially. Calves infected during the first week of life could play an active role in early propagation of BLV to susceptible animals. This review discusses the main factors that contribute to neonatal BLV infection in dairy herds, as well as different approaches and management practices that could be implemented to reduce the risk of BLV transmission during this period, aiming to decrease BLV infection in dairy herds.

Vector control efficacy of fly nets on preventing bovine leukemia virus transmission

Bovine leukemia virus (BLV) is horizontally transmitted among cattle through infected blood. This 3-year field study (2013-2016) aimed to confirm the potential of the blood-sucking stable fly as a risk factor of BLV transmission and to determine the efficacy of vector control on preventing the transmission of BLV. The BLV-positive conversion rate during summer was higher than that during winter in a model dairy farm, where many stable flies were observed during the summer. After fly nets were fixed onto the barn to prevent fly invasion, the BLV-positive conversion rate during the summer was significantly decreased compared with that in the absence of fly nets (P<0.01). These findings suggest that vector control using a fly net may inhibit BLV transmission.

Characterization of the Temporal Trends in the Rate of Cattle Carcass Condemnations in the US and Dynamic Modeling of the Condemnation Reasons in California With a Seasonal Component

Based on the 2016 National Cattlemen's Beef Association statistics, the cattle inventory in the US reached 93.5 million head, from which 30.5 million were commercial slaughter in 2016. California ranked fourth among all the US states that raise cattle and calves, with 5.15 million head and approximately 1.18 million slaughtered animals per year. Approximately 0.5% of cattle carcasses in the US are condemned each year, which has an important economic impact on cattle producers.In this study, we first described and compared the temporal trends of cattle carcass condemnations in all the US states from Jan-2005 to Dec-2014. Then, we focused on the condemnation reasons with a seasonal component in California and used dynamic harmonic regression (DHR) models both to model (from Jan-2005 to Dec-2011) and predict (from Jan-2012 to Dec-2014) the carcass condemnations rate in different time horizons (3 to 12 months).Data consisted of daily reports of 35 condemnation reasons per cattle type reported in 684 federally inspected slaughterhouses in the US from Jan-2005 to Dec-2014 and the monthly slaughtered animals per cattle type per states. Almost 1.5 million carcasses were condemned in the US during the 10 year study period (Jan 2005-Dec 2014), and around 40% were associated with three condemnation reasons: malignant lymphoma, septicemia and pneumonia. In California, emaciation, eosinophilic myositis and malignant lymphoma were the only condemnation reasons presenting seasonality and, therefore, the only ones selected to be modeled using DHRs. The DHR models for Jan-2005 to Dec-2011 were able to correctly model the dynamics of the emaciation, malignant lymphoma and eosinophilic myositis condemnation rates with coefficient of determination (Rt2) of 0.98, 0.87 and 0.78, respectively. The DHR models for Jan-2012 to Dec-2014 were able to predict the rate of condemned carcasses 3 month ahead of time with mean relative prediction error of 33, 11, and 38%, respectively. The systematic analysis of carcass condemnations and slaughter data in a more real-time fashion could be used to identify changes in carcass condemnation trends and more timely support the implementation of prevention and mitigation strategies that reduce the number of carcass condemnations in the US.

The molecular epidemiological study of bovine leukemia virus infection in Myanmar cattle

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis, which is the most common neoplastic disease of cattle. BLV infects cattle worldwide and affects both health status and productivity. However, no studies have examined the distribution of BLV in Myanmar, and the genetic characteristics of Myanmar BLV strains are unknown. Therefore, the aim of this study was to detect BLV infection in Myanmar and examine genetic variability. Blood samples were obtained from 66 cattle from different farms in four townships of the Nay Pyi Taw Union Territory of central Myanmar. BLV provirus was detected by nested PCR and real-time PCR targeting BLV long terminal repeats. Results were confirmed by nested PCR targeting the BLV env-gp51 gene and real-time PCR targeting the BLV tax gene. Out of 66 samples, six (9.1 %) were positive for BLV provirus. A phylogenetic tree, constructed using five distinct partial and complete env-gp51 sequences from BLV strains isolated from three different townships, indicated that Myanmar strains were genotype-10. A phylogenetic tree constructed from whole genome sequences obtained by sequencing cloned, overlapping PCR products from two Myanmar strains confirmed the existence of genotype-10 in Myanmar. Comparative analysis of complete genome sequences identified genotype-10-specific amino acid substitutions in both structural and non-structural genes, thereby distinguishing genotype-10 strains from other known genotypes. This study provides information regarding BLV infection levels in Myanmar and confirms that genotype-10 is circulating in Myanmar.

Risk factors associated with increased bovine leukemia virus proviral load in infected cattle in Japan from 2012 to 2014

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis, a malignant B cell lymphoma. BLV has spread worldwide and causes serious problems. After infection, the BLV genome is integrated into the host DNA and can be amplified during periods of latency. We previously designed degenerate primers using the Coordination of Common Motifs (CoCoMo) algorithm to establish a new quantitative real-time PCR method (BLV-CoCoMo-qPCR-2) of measuring the proviral load of both known and novel BLV variants. Here, we aimed to examine the correlation between proviral load and risk factors for BLV infection, such as breeding systems, parousity, and colostrum feeding. Blood and serum samples were collected from 83 BLV-positive farms in 22 prefectures of Japan, and the BLV proviral load and anti-BLV antibody levels were measured. BLV was detected in 73.3% (1039/1,417) of cattle by BLV-CoCoMo-qPCR-2 and the provirus was detected in 93 of 1039 antibody-negative samples. The results showed that the proviral load increased with progression of lymphocytosis. Next, the risk factors associated with increasing BLV infection rate were examined along with any association with proviral load. The proviral load was higher in cattle with lymphocytosis than in healthy cattle, and higher in multiparous cows than in nulliparous cows. Finally, proviral loads were higher in contact breeding systems than in non-contact breeding systems. Taken together, these findings may help to formulate a plan for eliminating BLV from contaminated farms. This is the first nationwide study to estimate BLV proviral load in Japanese cattle.

Horizontal transmission and phylogenetic analysis of bovine leukemia virus in two districts of Miyazaki, Japan

Horizontal transmission is recognized as a major infection route for bovine leukemia virus (BLV), and cattle with high viral loads are considered to be a major infectious source in a herd. However, a correlation between viral loads and the risk of infection has been insufficient to use as a foundation for BLV control strategies. In this report, we examined the epidemiology of BLV infection and the infectious source in a local area. In 2013–2014, BLV infection was investigated in 1,823 cattle from 117 farms in two adjacent districts, Miyazaki, Japan. Seropositive samples for BLV were detected with 88 cattle and in 14 farms. Phylogenetic analysis revealed that 94% of the isolates clustered into genotype I and the remaining isolate into genotype III. Among genotype I, genetically distinct strains were spread at each farm, and cattle infected with less than 3 copies/100 cells did not transmit BLV to other cattle for more than thirty months. This is the first report of concrete data of viral load in relation to viral horizontal transmission under the field condition. The data facilitate farmers and veterinarians understanding the status of BLV infected cattle. This research contributes to BLV infection control and the development of effective BLV eradication programs.

The role of neighboring infected cattle in bovine leukemia virus transmission risk

A cohort study was conducted to evaluate the risk of bovine leukemia virus (BLV) transmission to uninfected cattle by adjacent infected cattle in 6 dairy farms. Animals were initially tested in 2010-2011 using a commercial ELISA kit. Uninfected cattle were repeatedly tested every 4 to 6 months until fall of 2012. The Cox proportional hazard model with frailty showed that uninfected cattle neighboring to infected cattle (n=53) had a significant higher risk of seroconversion than those without any infected neighbors (n=81) (hazard ratio: 12.4, P=0.001), implying that neighboring infected cattle were a significant risk factor for BLV transmission. This finding provides scientific support for animal health authorities and farmers to segregate infected cattle on farms to prevent spread of BLV.

Preventive and therapeutic strategies for bovine leukemia virus: lessons for HTLV

Bovine leukemia virus (BLV) is a retrovirus closely related to the human T-lymphotropic virus type 1 (HTLV-1). BLV is a major animal health problem worldwide causing important economic losses. A series of attempts were developed to reduce prevalence, chiefly by eradication of infected cattle, segregation of BLV-free animals and vaccination. Although having been instrumental in regions such as the EU, these strategies were unsuccessful elsewhere mainly due to economic costs, management restrictions and lack of an efficient vaccine. This review, which summarizes the different attempts previously developed to decrease seroprevalence of BLV, may be informative for management of HTLV-1 infection. We also propose a new approach based on competitive infection with virus deletants aiming at reducing proviral loads.

The recent prevalence of bovine leukemia virus (BLV) infection among Japanese cattle

A seroepidemiological survey of bovine leukemia virus (BLV) infection was conducted in Japan in 2007 using an enzyme-linked immunosorbent assay (ELISA) and an agar gel immunodiffusion (AGID) test. A total of 5420 cattle (dairy, 3966; breeding beef, 797; fattening beef, 657) from 209 farms in seven prefectures in Japan were tested. The overall prevalence of BLV infection was 28.6%. The prevalence of BLV infection in dairy cattle (34.7%) was higher than for both fattening beef cattle (7.9%) and breeding beef cattle (16.3%). Age-specific prevalence showed that BLV prevalence increased with age in all types of cattle and was notably different between dairy and beef cattle under 1 year of age. Among 207 farms, 141 herds (68.1%) had one or more positive animals. The proportion of these positive farms was significantly higher among dairy farms (79.1%) than among beef breeding farms (39.5%) and beef fattening farms (51.9%) (P<0.001). Dairy farms (40.5%) also showed a significantly higher within-herd prevalence than beef breeding (27.4%) and fattening (14.9%) farms (P=0.001). This study indicated that BLV is more widely spread in dairy cattle than in beef breeding cattle in Japan. Given the prevalence of BLV infection in dairy and beef cattle was 8- and 1.7-fold higher, respectively, than rates previously found in 1980-1982, BLV appears to be spreading particularly among the dairy cattle population during the last two decades. Further investigation is required to determine the risk factors necessary to control BLV infection that take into account the different farming practices that exist between dairy and beef sectors.

Estimation of the within-herd transmission parameter of bovine leukemia virus

In this study, we estimated the transmission parameter of bovine leukemia virus (BLV) infection within herds by following up the serological status of cattle in beef and dairy herds. Two consecutive serological tests using ELISA were conducted for cattle at four beef breeding farms and nine dairy farms with an interval of 5 months. Assuming that sero-converted animals were newly infected, transmission parameters were estimated using a hierarchical Bayesian Poisson model. All tested herds had at least one positive animal at the first testing, but sero-prevalence considerably varied between herds. It is estimated that 0.62 animals were infected by one infected animal introduced into a fully susceptible population for both beef breeding and dairy farms during the 5-month interval. The 95% credible interval of the transmission parameter for the combined herds was 0.37-0.89 over this period. Asymptomatically infected animals with BLV can become the source of lifelong infection in herds. These estimated transmission parameters indicated that the early removal of infected animals with periodical tests could reduce the dissemination of BLV infection within herds.

Risk factors associated with within-herd transmission of bovine leukemia virus on dairy farms in Japan

Background

Although several attempts have been made to control enzootic bovine leukosis (EBL) at the local level, a nationwide control program has not been implemented in Japan, except for passive surveillance. Effective control of EBL requires that the transmission routes of bovine leukemia virus (BLV) infection should be identified and intercepted based on scientific evidence. In this cross-sectional study, we examined the risk factors associated with within-herd transmission of BLV on infected dairy farms in Japan. Blood samples taken from 30 randomly selected adult cows at each of 139 dairy farms were tested by enzyme-linked immunosorbent assay (ELISA). Information on herd management was collected using a structured questionnaire.

Results

Infected farms were defined as those with more than one ELISA-positive animal and accounted for 110 (79.1%) of the 139 farms in the study. Completed questionnaires obtained from 90 of these 110 farms were used for statistical analysis. Seroprevalence, which was defined as the proportions of animals that tested positive out of all animals tested on the farm, was 17.1%, 48.1%, and 68.5% for the 25th, 50th, and 75th percentiles, respectively. A mixed logistic regression analysis implicated a loose housing system, dehorning, and a large number of horseflies in summer as risk factors (coefficient = 0.71, 1.11, and 0.82; p = 0.03, < 0.01, and 0.01, respectively) and feeding of colostrum to newborn calves from their dams as a protective factor (coefficient = -1.11, p = 0.03) against within-farm transmission of BLV on infected farms.

Conclusion

Control of EBL in infected dairy farms in Japan will be improved by focusing particularly on these risk and protective factors.

Evaluation of natural transmission of bovine leukaemia virus within dairy herds of Argentina

The purpose of this study was to describe patterns of seroconversion to bovine leukaemia virus and to estimate the main parameters needed for future model building. A longitudinal study was carried out between February 1999 and November 2001 in seven commercial dairy farms in Argentina using 1535 lactating cows. Time-interval parameters were analysed using a parametric survival model with shared frailty, time until infection was analysed using a Bayesian interval-censoring survival model and the infection transmission parameter (beta) was estimated by a generalized linear model. The reproduction ratio (R0) was calculated. In total, 1000 cows tested positive and 494 tested negative. The predicted median age at infection was 4.6 years for seroconverted cows. For infected herds, the proportion of positive calves was as high as for infected cows and showed a large proportion of infected breeding heifers. Peaks in the overall average incidence per season-year were observed during autumn and spring. Results reveal that the period around parturition is a high-risk period. Moreover, heavily infected herds seem to have an increased proportion of young stock infected. The overall beta was estimated as 2.9/year (95% CI 1.9-3.7) and combined with a relatively long infectious period it resulted in a high reproductive ratio (R0=8.9). Therefore, a high effectiveness of control measures needs to be achieved to eradicate the disease.

Use of a polymerase chain reaction assay to detect bovine leukosis virus in dairy cattle

OBJECTIVE

To evaluate the use of a polymerase chain reaction (PCR) assay in detecting bovine leukosis virus (BLV) in adult dairy cows.

DESIGN

Prospective study.

ANIMALS

223 adult dairy cows.

PROCEDURE

Cows were tested for BLV status by use of an ELISA and a PCR assay. Sensitivity, specificity, predictive values of positive and negative tests, and the percentage of cows correctly classified by PCR assay were calculated. Ninety-five percent confidence intervals were calculated for sensitivity and specificity.

RESULTS

Sensitivity and specificity were 0.672 and 1.00, respectively. Prevalence of BLV in this herd was 0.807. Predictive value of a positive test was 1.00, and predictive value of a negative test was 0.421. The percentage of cows correctly classified by PCR assay was 73.5%.

CONCLUSIONS AND CLINICAL RELEVANCE

A positive PCR assay result provided definitive evidence that a cow was infected with BLV. Sensitivity and negative predictive value for PCR assay were low. Consequently, PCR assay alone is unreliable for routine detection of BLV in herds with high prevalence of the disease.

Association between the strength of serologic recognition of bovine leukosis virus and lymphocyte count in bovine leukosis virus-infected cows

OBJECTIVE

To determine whether strength of serologic recognition of bovine leukosis virus (BLV) by use of ELISA is associated with blood lymphocyte counts.

DESIGN

Prospective study.

ANIMALS

161 cows with positive results of ELISA for BLV.

PROCEDURE

Sample-to-positive ratio (S:P), which is the ratio between the test sample and a positive control sample, was compared among lymphocytotic and nonlymphocytotic cows. A regression model was constructed to evaluate the association between blood lymphocyte concentration and S:P, age, and the interaction of these terms.

RESULTS

Mean S:P differed significantly between lymphocytotic (2.58 +/- 0.36) and nonlymphocytotic (2.38 +/- 0.39) cows. Age and S:P were significantly associated with lymphocyte count.

CONCLUSIONS AND CLINICAL RELEVANCE

Sample-to-positive ratio and lymphocyte count were related; however, cows with high S:P were not always lymphocytotic. Culling cows on the basis of S:P will reduce the herd load of infectious virus faster than random culling of ELISA-positive cows; however, culling on the basis of lymphocyte count will eliminate a greater proportion of the reservoir of infection.

Detection of bovine leukemia viruses (BLV) in mammary tissues of BLV antibody-positive cows affected by subclinical mastitis

The mammary tissues of 6 cows with bovine leukemia virus (BLV) antibody and subclinical mastitis were investigated histopathologically, and their organ cultures were ultrastructurally observed. Numerous BLV particles, 110 to 120 nm in diameter, were seen around lymphocytes, which had infiltrated into mammary alveoli and showed blastogenesis under culture. Particles budding from the cell membrane were also found.

Natural transmission of bovine leukemia virus in dairy and beef cattle

Many potential routes of bovine leukemia virus (BLV) transmission are reviewed in this article. Vertical transmission, in utero, or through colostrum and milk, accounts for a relatively small proportion of infections. Iatrogenic horizontal transmission, through procedures permitting the transfer of blood between cattle, has been shown to be a major route of transmission in most settings. Contact transmission stems from a mixture of natural sources of blood, exudates, and tissues that enter the body through mucosal surfaces or broken skin. Careful analysis of management procedures and environmental conditions present in individual dairy and beef herds affords the greatest opportunity to develop effective BLV prevention programs.

The role of dipterous insects in the mechanical transmission of animal viruses

Animals viruses may be transmitted by arthropods in two ways, either biologically or mechanically. Many different species of Diptera are implicated in mechanical transmission, but haematophagous species are the most important. The insects become contaminated with virus during normal feeding behaviour, and virus persists on their mouthparts or body until the next feed. Some viruses are inactivated rapidly on mouthparts, whereas others survive for many days or weeks, prolonging the potential period of transmission. Some viruses produce high titres in the skin of the infected vertebrate host, which facilitates transmission, whereas other viruses are transmitted even during relatively low levels of viraemia. Mechanical transmission by arthropods is important in the epidemiology of many animal diseases, and may be the major mode of horizontal transmission. In other instances vector spread is merely incidental.

Natural mode of horizontal transmission of bovine leukemia virus (BLV): the potential role of tabanids (Tabanus spp.)

In order to evaluate the potential role of hematophagous insects in the natural spreading of bovine leukemia virus (BLV) infection in cattle, a 2-year survey was carried out involving sequential serological tests on 3328 cattle in three different areas. A parallel entomological study was run over the same period, using continuous trapping, in order to determine both the density and variations of horsefly (Tabanus spp.) populations in the close vicinity of the herds. After statistical analysis, this space-time study showed that: (1) There is a significant positive geographical correlation between the rate of incidence of BLV infection and the density of the horsefly population. (2) Seasonal variations in the incidence rate exist; the highest rates are generally observed during summer (from July of September), and the lowest during winter, spring and early summer (from November to mid-July). (3) There is a time link between the rate of sero-conversion and the variations in activity of the horsefly population. All these data combined would appear to indicate that tabanids play a considerable role in the spread of BLV under natural conditions.

Tabanids as vectors of disease agents

The Tabanidae are considered to be among the major Dipteran pests of man and animals worldwide, but this group is undoubtedly the least studied. There have been at least 137 genera and 4154 species of tabanids described to date. Yet, existing, active research programmes number, at most, 50 in systematics and distribution, 15 in economic entomology, and five in disease transmission. To redress the balance, Lane Foil discusses the entire spectrum of research on the transmission of infections by tabanids, both from the point of view of general factors affecting transmission dynamics, as well as the specific examination of candidate agents, from viruses to filaria.

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.

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

Bovine leukemia virus is the etiological agent of a chronic lymphatic leukemia/lymphoma in cows, sheep, and goats. Infection without neoplastic transformation also was obtained in pigs, rhesus monkeys, chimpanzees, and rabbits, and was observed in capybaras and water buffaloes. Structurally and functionally, BLV is a relative of the human T lymphotropic viruses (HTLV-I and HTLV-II). HTLV-I induces in humans a T cell leukemia, and its type II counterpart has been found in dermatopathic lymphadenopathy, hairy T cell leukemia and prolymphocytic leukemia cases. At variance with HTLV-I, BLV has not been associated with neurological diseases of the degenerative type. BLV, HTLV-I, and HTLV-II show clearcut sequence homologies. The pathology of the BLV-induced disease, most notably, the absence of chronic viremia, a long latency period, and a lack of preferred proviral integration sites in tumors, is similar to that of adult T cell leukemia/lymphoma induced by HTLV-I. The most striking feature of the three naturally transmitted leukemia viruses is the X region located between the env gene and the 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 involved in "genetic instability" of BLV-infected cells of the B cell lineage. The genetic instability puts the cell into a context of fragility and ready to move along a number of stages towards full malignancy. Little is known about these events and their causes; we have presented some theoretical possibilities. BLV 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 hematophageous insects, there are indications of insect-born propagation of the virus.