Abstract:
We estimated the diagnostic sensitivity (DSe) and specificity (DSp) of an
immunohistochemistry (IHC) protocol compared to the direct fluorescent antibody test (DFAT),
which is the gold standard test for rabies diagnosis. We obtained brain samples from 199
domestic and wild animal cases (100 DFAT-negative, 99 DFAT-positive), by convenience
sampling from 2 government-accredited rabies virus (RABV) testing laboratories in South
Africa, between February 2015 and August 2017. Tissues that had been stored at 4–8°C for
several days to weeks at the 2 accredited laboratories were formalin-fixed and paraffin-embedded. Nighty-eight cases tested IHC-positive using a polyclonal anti-RABV nucleoprotein
antibody and a polymer detection system. The overall DSe and DSp for the RABV IHC test were
98% (95% CI: 93–100%) and 99% (95% CI: 95–100%), respectively. Domestic dogs accounted
for 41 of 98 RABV IHC–positive cases, with the remainder in 4 domestic cats, 25 livestock, and
28 wildlife. Herpestidae species, including 7 meerkats and 9 other mongoose species, were the
most frequently infected wild carnivores, followed by 11 jackals. Three cases in domestic dogs
had discordant test results; 2 cases were IHC–/DFAT+ and 1 case was IHC+/DFAT–.
Considering the implications of a false-negative rabies diagnosis, participating in regular interlaboratory comparisons is vital, and a secondary or confirmatory method, such as IHC, should be
performed on all submitted specimens, particularly negative cases with human contact history.
