Bluetongue: laboratory diagnosis

Definitive diagnosis of bluetongue virus (BTV) infection, often subclinical in domestic and wild ruminant relies heavily on laboratory techniques for BTV isolation and demonstration of BTV antigens, viral nucleic acids and antibodies. The virus can be isolated from blood components, mainly the erythrocyte fraction, collected from affected animals during the period of febrile response. Semen collected from male animals at the peak of viremia and tissues from affected animals and fetuses may also be used for BTV isolation. The primary procedure for BTV isolation is inoculation of embryonated chicken eggs with a subpassage onto cell cultures (e.g. BKH-21, Vero cell lines). In addition to the conventional techniques such as fluorescent antibody staining and virus neutralization procedures for sero-grouping and serotyping of BTV isolates, immunohistochemical, immunoenzymatic and immunoelectron microscopic techniques, using monoclonal antibodies (MAb), offer more rapid, specific and sensitive approaches for BTV identification and antigen detection. The progress of molecular biology, especially the development of genetic probes for hybridization analysis and polymerase chain reaction techniques for detection of BTV nucleic acids hold the promise of most efficient diagnostic assays. Among the various serogroup-specific assays for antibody detection, the agar gel immunodiffusion (AGID) and competitive (C) ELISA are the most widely used tests. Because of its limitations (i.e. anticomplementary serum and complexity of the procedure) the complement fixation (CF) test is virtually abandoned and is used in only a few laboratories. Although the AGID test is simple to perform and rapid, it is not highly sensitive or quantitative and has limitations in its specificity. Sera containing antibodies to other group of Orbiviruses (e.g. epizootic hemorrhagic disease) may result in non-specific reaction in the AGID test. Among several ELISAs that have recently been developed, the C.ELISA in which a group-specific MAb to BTV is used, has proved to be the most sensitive and specific assay for detection of antibodies to BTV. Following extensive national and international validation, the C.ELISA is gradually replacing the AGID as a universal test to certify ruminants for trade purposes and to diagnose BT infection in domestic and wild animals. The cell culture-based microtiter serum neutralization (MTSN) is the most commonly used assay for the detection of serotype-specific antibodies to the recognized BTVs in animal sera. The MTSN may be used to type virus isolates and also to monitor animal population for specific serotypes of BTV in epidemiological investigations.