New Rapid Rabies Test Could Transform Testing and Treatment

Megan Brooks

May 18, 2018

Scientists at the Centers for Disease Control and Prevention (CDC) have developed a new rapid test for rabies infection in animals that could mean fewer postexposure prophylaxis shots for people exposed to animals that are possibly rabid.

The LN34 test can more easily and precisely diagnose rabies infection in animals. It may be especially useful in areas where diagnostic tests are limited, the CDC said.

"Many of the areas hardest hit by rabies are also the areas least prepared to run current tests to diagnose it," Crystal Gigante, PhD, a microbiologist in the CDC's Division of High-Consequence Pathogens and Pathology, said in a CDC news 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."

The current gold standard for rabies testing in animals is the direct fluorescent antibody (DFA) test. It can only be interpreted by laboratory workers with special skills, extensive training, and a fluorescent microscope, which is not always available.

The LN34 real-time polymerase chain reaction (PCR) assay "represents a strong candidate for rabies post-mortem diagnostics due to its ability to detect RNA across the diverse Lyssavirus genus, its high sensitivity, its potential for use with deteriorated tissues, and its simple, easy to implement design," Gigante and colleagues say in a report published online May 16 in PLOS ONE.

Global Study

The test was evaluated at 14 laboratories around the world using 2978 animal brain tissue samples from the Americas, Europe, Africa, and Asia. Of those samples, 1049 tested positive for rabies on DFA testing. The samples came from more than 60 mammal species that are susceptible to rabies, including dogs, raccoons, skunks, foxes, and bats. The LN34 assay can be used on animal tissue that is fresh, frozen, decomposed, or that has been fixed in blocks of paraffin to inactivate the virus.

"The LN34 assay exhibited low variability in repeatability and reproducibility studies and was capable of detecting viral RNA in fresh, frozen, archived, deteriorated, and formalin-fixed brain tissue," the authors report.

The LN34 assay had high diagnostic specificity (99.68%) and sensitivity (99.90%) when compared to the DFA test, and no DFA-positive samples tested negative on the LN34 assay, they note. The LN34 assay produced definitive findings for 80 samples that were inconclusive or untestable by the DFA test.

Using the LN34 assay, one false negative DFA test result and 11 false positive DFA test results were identified. For one sample the results of both tests were inconclusive.

"Together, these results demonstrate the reliability and robustness of the LN34 assay and support a role for the LN34 assay in improving rabies diagnostics and surveillance," Gigante and colleagues write.

Rabies kills about 60,000 people each year, mostly in Africa and Asia. The disease can take months to develop following contact with a rabid animal. Once symptoms appear, rabies is almost always fatal. Identifying cases and starting treatment early are key to patients' survival.

Currently in the United States, if rabies tests are inconclusive, the exposed person is usually given rabies vaccine as postexposure prophylaxis, at an individual cost that often tops $3000. It is estimated that rabies testing, prevention, and control cost up to $510 million annually in the United States. About 40,000 to 50,000 people receive rabies postexposure prophylaxis in the United States each year.

The LN34 assay could decrease the number of inconclusive and false positive tests and eliminate unnecessary use of vaccine, the CDC said.

Currently, the DFA test is the only test that has been internationally approved to confirm rabies in animals. The World Health Organization and the World Organization for Animal Health are considering adding PCR-based tests, such as the LN34 test, for primary diagnosis.

PLoS ONE. Published Mary 16, 2018. Abstract

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