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.

West Nile virus (WNV) was isolated from the brains of 3 horses that died from encephalitis in February 2006. The horses were from different farms in central Argentina and had not traveled outside the country. This is the first isolation of WNV in South America.

The utilization of transgenic plants expressing recombinant antigens to be used in the formulation of experimental immunogens has been recently communicated. We report here the development of transgenic plants of alfalfa expressing the structural protein VP1 of foot and mouth disease virus (FMDV). The presence of the transgenes in the plants was confirmed by PCR and their specific transcription was demonstrated by RT-PCR. Mice parenterally immunized using leaf extracts or receiving in their diet freshly harvested leaves from the transgenic plants developed a virus-specific immune response. Animals immunized by either method elicited a specific antibody response to a synthetic peptide representing amino acid residues 135-160 of VP1, to the structural protein VP1, and to intact FMDV particles. Additionally, the immunized mice were protected against experimental challenge with the virus. We believe this is the first report demonstrating the induction of a protective systemic antibody response in animals fed transgenic plants expressing a viral antigen. These results support the feasibility of producing edible vaccines in transgenic forage plants, such as alfalfa, commonly used in the diet of domestic animals even for those antigens for which a systemic immune response is required.

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Previous studies have classified the env sequences of bovine leukemia virus (BLV) provirus from different locations worldwide into between two and four genetic groupings. These different studies gave unique names to the identified groups and no study has yet integrated all the available sequences. Thus, we hypothesized that many of the different groups previously identified actually correspond to a limited group of genotypes that are unevenly distributed worldwide. To examine this hypothesis, we sequenced the env gene from 28 BLV field strains and compared these sequences to 46 env sequences that represent all the genetic groupings already identified. By using phylogenetic analyses, we recovered six clades, or genotypes, that we have called genotypes 1, 2, 3, 4, 5 and 6. Genotypes 1-5 have counterparts among the sequence groupings identified previously. One env sequence did not cluster with any of the others and was highly divergent when compared with the six genotypes identified here. Thus, an extra genotype, which we named 7, may exist. Similarity comparisons were highly congruent with phylogenetic analyses. Furthermore, our analyses confirmed the existence of geographical clusters. INTRODUCTIONBovine leukemia virus (BLV) is a member of the family Retroviridae belonging to the genus Deltaretrovirus. This genus also includes Simian T-lymphotropic virus 1, 2, 3 and 5 (STLV-1, -2, -3 and -5) and Human T-lymphotropic virus 1, 2, 3 and 4 (HTLV-1, -2, -3 and -4). have not yet been associated with any pathology, likely due to their recent identification and to the low number of isolates. Therefore, BLV is considered a model of HTLV-1 and -2 . BLV is recognized as the aetiological agent of enzootic bovine leukosis, a disease that results in significant economic losses for the worldwide cattle industry. The most conspicuous clinical manifestation of bovine leukosis, which only develops in a small fraction of infected animals, is the clonal expansion and local accumulation of B cells that results in the development of lymphoid tumours (lymphosarcoma, LS) (Gillet et al., 2007). The majority of infections are not associated with any clinical signs (AL), and in approximately 30 % of infected cattle, the virus causes a persistent lymphocytosis (PL) (Burny et al., 1987;Mirsky et al., 1996).Analyses of the BLV envelope (env) gene of isolates collected in multiple geographical locations demonstrated significant sequence conservation (Camargos et al., 2002(Camargos et al., , 2007Coulston et al., 1990). Nevertheless, up to seven BLV genotypes can be identified by RFLP analysis (Asfaw et al., 2005;Coulston et al., 1990;Fechner et al., 1997;Kettmann et al., 1981;Licursi et al., 2002). Furthermore, the env sequences of the BLV provirus from different locations worldwide have previously been classified into between two and four genetic groupings (Camargos et al., 2002(Camargos et al., , 2007Felmer et al., 2005;Hemmatzadeh, 2007;Licursi et al., 2003;Mamoun et al., 1990;Monti et al., 2005;Zhao & Buehring, 2007), with similar results...