Potential early rabies infection detected in two raccoon cases by LN34 pan-lyssavirus real-time RT-PCR assay in Pennsylvania

During 2017 – 2019, the Pennsylvania Department of Health Bureau of Laboratories (PABOL) tested 6,855 animal samples for rabies using both the gold standard direct fluorescent antibody (DFA) test and LN34 pan-lyssavirus reverse transcriptase quantitative PCR (RT-qPCR). Two samples (0.03 %) were identified as LN34 RT-qPCR positive after failure to detect rabies virus antigen during initial DFA testing: an adult raccoon collected in 2017 and a juvenile raccoon collected in 2019. After the positive PCR result, additional tissues were collected and re-tested by DFA, where very sparse, disperse antigen was observed. Tissues from both animals were submitted to the Centers for Disease Control and Prevention (CDC) for confirmatory testing, and were confirmed positive.

At both PABOL and CDC, rabies virus antigen and RNA levels were much lower than for a typical rabies case. In addition, rabies virus antigen and RNA levels were higher in brain stem and rostral spinal cord than cerebellum, hippocampus and cortex. Cross-contamination was ruled out in the case of the 2019 juvenile raccoon by sequencing, as nucleoprotein and glycoprotein gene sequences displayed >1% nucleotide differences to sequences from all positive samples processed at PABOL within two weeks of the juvenile raccoon.

Taken together, the low level of rabies virus in the central nervous system combined with presence in more caudal brain structures suggest the possibility of an early infection in both cases. These two cases highlight the increased sensitivity and ease of interpretation of LN34 RT-qPCR in rabies diagnostics for the identification of low positive cases.