Progress in the development of a direct rapid immunohistochemical test for diagnosing rabeies

At present, the direct fluorescent antibody test (FAT) is an OIE-prescribed and WHO-recommended test and is considered the ‘gold standard’ for sensitive and specific rabies diagnosis. The FAT provides consistent results on fresh brain specimens in more than 95% to 99% of cases. As plans for the global elimination of canine rabies advance, the need for additional relevant diagnostic testing is crucial to improve laboratory-based surveillance and monitor programme success, especially in developing countries. A direct rapid immunohistochemical test (DRIT) for rabies diagnosis has been developed as one alternative to meet this need. Similarly to the FAT, the DRIT involves the examination of brain impressions but, rather than immunofluorescence, employs light microscopy and biotin-labelled antibodies. Using the DRIT, tens of thousands of suspect animal brains have been examined since 2008 by diagnosticians and researchers in the Americas, Africa, Asia and Europe. Despite widely varying conditions by locale, species tested, sample quality, viral type, and background conditions, test sensitivity and specificity have approached 100%. All currently recognised lyssavirus species have been identified in naturally or experimentally infected mammals using the DRIT. In the United States, it is applied to laboratory confirmatory testing of equivocal FAT results and to enhance surveillance under field conditions. For example, in support of oral rabies vaccination programmes, more than 60,000 specimens have been tested within North America and, as with the FAT, the DRIT is included in routine proficiency testing. Moreover, preliminary inter-laboratory comparisons, using different monoclonal and polyclonal conjugates, with antibodies tested against a wide variety of viral variants, support the basic premise of the DRIT as a relevant test for future validation and OIE consideration as a prescribed test for rabies diagnosis.