Discovery and In-vivo characterization of the new Bluetongue virus strain BTV-28

Bluetongue virus (Reoviridae; Orbivirus, BTV) is transmitted by biting midges (Culicoides spp.), and causes the bluetongue disease, which affects wild and domestic ruminants worldwide. This disease has devastating economic consequences and can severely damage affected populations of wild small ruminants. We recently discovered and characterized a new strain (serotype) of this virus, termed BTV-28, and demonstrated that unlike other BTV strains, it may be transmitted directly from host to host. BTV-28 was isolated from a contaminated vaccine batch and showed significant genomic difference, compared with other BTV serotypes. Experimental infection by injecting BTV-28 to 6-months old ewes caused clinical signs in all injected animals, lasting from 2-3 days to several weeks post injection. Clinical signs were characteristic of bluetongue disease, including fever, conjunctivitis, nasal discharge, coronitis, depression and tongue cyanosis, accompanied by the presence of viral RNA in the blood and tissues. Naïve control animals, placed together with the injected sheep, displayed clinical signs and were positive for viral RNA, but their acute disease phase was shorter than that of BTV-28-injected ewes. All the injected and one of the two contact-infected animals displayed BTV-specific antibody response. These findings suggest that BTV-28 is virulent and, contrary to other BTV strains, can be transmitted directly from sheep to sheep, sparing the need for an intermediate vector. To our knowledge, this study is unique in the sense that it shows that direct transmission of BTV can elicit clinical disease in sheep.