DIAGNOSIS

A presumptive diagnosis of BT in disease-susceptible species, based on clinical signs and lesions, can be made. However, in most ruminants BTV infection is subclinical, and laboratory confirmation by virus isolation/identification or serology is required.

Confirmation of BT or a BTV infection is via antigen detection and identification and/or serology.

ANTIGEN DETECTION AND IDENTIFICATION

• Identification of viral antigen directly from samples by group-specific conventional or real-time RT-PCR assays or the antigen detection enzyme-linked immunosorbent assay (ELISA)(³²). PCR assays are preferred over ELISA because of their increased sensitivity, and real-time over conventional PCR assays for the same reason.

• Isolation of infectious virus via intracerebral inoculation of suckling mice or intravenous inoculation of embryo- nating hens’ eggs, followed by adaptation to cell culture. Virus identification is attempted, first using group­specific conventional or real time RT-PCR assays or an antigen detection ELISA, and then using type-specific RT-PCRs or virus neutralization tests (VNT))³³). PCR assays are preferred over VNT, as they do not require virus adaptation to cell culture.

Despite the high sensitivity, specificity and speed of PCR- based assays, a major disadvantage is that they detect viral RNA, not infectious virus. Virus isolation should be used in addition to PCR to confirm the presence of live virus.

SEROLOGY

• Identification of BTV antibodies is undertaken using a group-specific, antibody detection ELISA followed by the serotype-specific serum neutralization test)³⁴).