Small ruminant lentiviruses (SRLVs) represent a group of viruses infecting sheep and goats worldwide. Despite the high heterogeneity of genotype A strains, which cluster into as many as ten subtypes, genotype B was believed to be less complex and has, so far, been subdivided into only two subtypes. Here, we describe two novel full-length proviral sequences isolated from Sarda sheep in two Italian regions. Genome sequence as well as the main linear epitopes clearly placed this cluster into genotype B. However, owing to long-standing segregation of this sheep breed, the genetic distances that are clearly .15 % with respect to B1 and B2 subtypes suggest the designation of a novel subtype, B3. Moreover the close relationship with a gag sequence obtained from a Turkish sheep adds new evidence to historical data that suggest an anthropochorous dissemination of hosts (small ruminants) and their pathogens (SRLV) during the colonization of the Mediterranean from the Middle East.

Synthetic peptides were generated, corresponding to SU5 domain of envelope glycoprotein of Italian SRLV isolates It--561 and It--Pi1, belonging respectively to MVV--and CAEV--like genotypes. The peptides, encompassing an N--terminal variable and a C--terminal conserved antibody--binding site, were used in an ELISA assay to analyse the sera of two groups of sheep experimentally infected with these isolates. The kinetics and specificity of the humoral response to the homologous and heterologous antigen and the affinity maturation of the sera were evaluated. Seroconversion occurred between week 3 and 8. The response to SU5 antigen was mostly type--specific. The few broadly reacting sera may reflect the production of antibodies directed to the SU5 constant antibody--binding site. All sera underwent with time avidity maturation, resulting in the appearance of high affinity antibodies. This study suggests constant monitoring of the circulating viral variants to develop a panel of diagnostic peptides representative of local genotypes.

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