Enzyme-linked immunosorbent assay (ELISA)
In the rabies field the enzyme-linked immunosorbent assay (ELISA) was initially developed for the titration of rabies virus-neutralizing antibodies. The technique was applied to the quantification of rabies antigen by Atanasiu et al using fluorescein-labelled IgG to the purified nucleocapsid. Subsequently, Perrin et al developed an ELISA called rapid rabies enzyme immunodiagnosis (RREID), which was based upon the detection of rabies virus nucleocapsid antigen in brain tissue. In this test, microplates are coated with purified IgG and an IgG-peroxidase conjugate is used to react with immunocaptured antigen.
This technique was compared with the FA test in a collaborative study involving six laboratories in Europe and North America. The study showed a good correlation between the FA test and RREID, although the latter test was less sensitive. A further study was organized to evaluate the RREID under conditions prevailing in rabies laboratories in developing countries. The study found over 96% agreement between the FA test and RREID. Similar results were obtained in a study on more than 3000 specimens. It should be noted, however, that in view of its lower sensitivity, RREID should not replace FA in laboratories where FA is already performed.
RREID is a simple and relatively cheap technique, which can be especially useful for epidemiological surveys. It may be used to examine partially decomposed tissue specimens for evidence of rabies infection, but it cannot be used with specimens that have been fixed in formalin. Since the antigen can be visualized with the naked eye, the test can be carried out in laboratories that do not have the necessary equipment for FA tests.