The Biology of the Goat

Testing for the CAE virus with AGID

The AGID (Agar Gel Immunodiffusion) assay works under the same principle as ELISA but takes advantage of the presence of multiple types of antibodies to regions (epitopes) of the antigen. As discussed earlier the goat immune system makes antibodies to many different epitopes of the envelope protein. Currently, 6 different antibodies have been identified for the transmembrane portion of the envelope protein and 5 antibodies have been identified for the surface component of the envelope protein.

Because of these multiple epitopes, more than one type of antibody can bind to a corresponding antigen and because antibodies have two arms one antibody can sometimes bind to two antigens. If the amount of antigen to antibody is in equilibrium then large complexes of antigen/antibodies can be formed. This causes the complex to precipitate.

In an AGID assay, a disc of agar (which is sort of like stiff jello) is prepared in a petri dish. Holes are punched out and antigen is placed in one hole and the antibody solution is placed in another. The solutions will diffuse out of the holes into the agar. When the antigen meets the antibodies binding will occur. In the area where there is just the right level of antibody to antigen a thin, white line forms which is visible to the unaided eye.

AGID tests are very accurate with respect to specific binding, therefore false positives are rare, but the test does not detect low levels of antibodies. This results in false negatives when the test subject is in the early stages of infection and has a low level of antibodies.

Next: PCR testing for the CAE virus