BT Serogroup diagnosis: First step
Duration of viraemia in BTV-infected ruminants is prolonged, and even BTV nucleic acid can be detected by RT-PCR assay in the absence of infectious virus in the blood of ruminants, during a time considerably longer. This higher sensitivity of detection and shortening the time required for the diagnosis are the reasons why RT-PCR is the election method for the confirmation of clinical cases, and EDTA blood and organs from died animals, such as spleen, lung or lymph nodes, are the election samples for agent diagnosis.
Several BTV gel-based RT-PCR and real-time RT-PCR methods targeted at viral conserved segments have been published at date. In the Proficiency Tests annually organized by the EURL since 2006, a wide variety of RT-PCR methods have been used by the EU-NRLs and other participant laboratories around the word. Hoffman et al. 2008a y 2008b, targeted to the segment-10, have demonstrated to detect all 24 notifiable BTV serotypes and BTV atypical strains appeared so far, showing enough sensitivity and specificity. This is the real-time RT-PCR method described in the OIE Manual.
After detection, virus isolation should be tried to dispose of the strain for further studies of characterization.
BTV serological response develops 7–14 days after infection, and it is usually considered long-lasting. At the present, there are not developed and validated any method to differentiate infected of vaccinated animal (DIVA). Although serological tools can be used to confirmation of clinical case or demonstrate individual animal freedom from infection prior to movement, the main purposes of serological methods are: prevalence of infection studies (including surveillance programs) and post-vaccination assays. Among a wide range of different assays developed along the years for the same purpose, ELISA is the most used technique for serogroup determination. Competitive/blockingELISA (VP7) is a prescribed test for international trade by the OIE terrestrial manual. In addition, a different kind of ELISA, Double recognition ELISA (VP7), has shown efectively complement c/bELISA.