Detection by ELISA of bluetongue antigen directly in the blood of experimentally infected sheep.

An antigen-capture ELISA (Ag-ELISA) was developed to detect bluetongue virus (BTV) antigen directly from blood samples. Four blood preparations [whole blood, buffy coat, washed red blood cells (RBC) and plasma] taken pre-inoculation and on days 6 to 9 post-inoculation (PI) were used in the ELISA to study antigenaemia in forty sheep, each experimentally infected with one of 20 South African BTV serotypes. Seventeen of the 20 serotypes were detected and 27 of the 40 sheep were at some stage Ag-ELISA positive. Over the period of sampling, Ag-ELISA positive results were most frequently returned from whole blood taken on days 6 and 7 PI. However in some instances the quantity and/or duration of BTV antigenaemia was greater in buffy coat and washed RBC preparations. In a selection of samples examined, positive Ag-ELISA results were generally obtained when samples had an infectious virus titre in eggs of > 10(3.2) egg lethal doses (ELD50/ml). The appearance and duration of detectable antigenaemia was compared with the development of clinical signs and antibody responses of infected sheep. On days 6 and 7 PI the presence of fever (> 40 degrees C) was indicative to the likelihood of detectable antigenaemia. After day 5 PI antigenaemia declined and clinical signs of swollen face and inflamed feet appeared together with the first detectable antibody response. The Ag-ELISA, when used in conjunction with clinical observations and serologic data, should be useful as a rapid diagnostic procedure for bluetongue disease.