CDC researchers have developed a real-time PCR-based rabies test that the agency said is simpler and more precise than direct fluorescent antibody, or DFA, testing, which is the current gold-standard for rabies diagnosis.
The new LN34 assay is designed to identify rabies infection in animals and can run on testing platforms that are widely available in the United States, according to the agency. During a pilot study, LN34 produced no false-negative results and fewer false-positive and inconclusive results compared with DFA testing.
Unlike the DFA test, which requires specialized training and equipment, LN34 can be easily used in resource-limited settings, where rabies diagnostics and treatment supplies are often stored in urban areas that may take people several days to reach, according to the CDC. The agency reported that its accuracy and feasibility could reduce unnecessary use of a rabies vaccine that is given as part of postexposure prophylaxis to patients who are bitten by animals with confirmed rabies or those with inconclusive test results. The weeks-long regimen, which is administered to about 40,000 to 50,000 people in the U.S. each year, can cost Americans more than $3,000. For patients in low-income countries, the cost may be several months’ salary.
“Many of the areas hardest hit by rabies are also the areas least prepared to run current tests to diagnose it,” Crystal Gigante, PhD, microbiologist in CDC’s Division of High-Consequence Pathogens and Pathology, said in a press release. “The LN34 test has the potential to really change the playing field. Quickly knowing who needs to receive rabies treatment — and who does not — will save lives and families’ livelihoods.”
For the pilot study — the largest ever to validate a real-time PCR-based test for rabies — laboratory staff at 14 sites in seven countries examined 2,978 animal brain samples in the Americas, Africa, Asia, Europe and the Middle East. The samples were taken from more than 60 mammal species, including dogs, racoons, skunks, foxes and bats. Among them, 1,049 were known to be positive for rabies.
Gigante and colleagues reported in PLoS One that LN34 accurately identified all samples that were positive with DFA testing. It also produced results for 80 samples that were indeterminate or untestable by DFA, according to the researchers. Of those, 29 had evidence of rabies. LN34 identified one false-negative DFA result and 11 false-positive DFA results. Overall, it was associated with a sensitivity of 99.9% and specificity of 99.7%.
In contrast to the DFA test, which can use only fresh brain tissue samples that are kept cold, LN34 detected rabies in animal tissues that were fresh, frozen and decomposed, including those so old that they liquified. According to the CDC, LN34 also worked on tissue samples that were fixed into paraffin blocks to deactivate the virus. Gigante and colleagues reported that the condition of the samples did not compromise the accuracy of the test.
“The international evaluation presented here revealed the robustness, low variability, and excellent diagnostic specificity and sensitivity of the LN34 assay,” they concluded. “Together, these data provide evidence that the LN34 assay is a reliable test.”
WHO and the World Organization for Animal Health are now considering PCR-based assays, such as LN34, as a primary tool for rabies diagnosis, according to a news release. In addition, the CDC is coordinating with the Association of Public Health Laboratories to develop its own guidance on which type of rabies tests should be used in certain scenarios in the U.S. – by Stephanie Viguers
Gigante C, et al. PLoS One. 2018;doi:10.1371/journal.pone.0197074.
Disclosures: The authors report no relevant financial disclosures.