A new rabies test developed at the Centers for Disease Control and Prevention (CDC) could mean people exposed to potentially rabid animals could forego the weeks-long regimen of shots to prevent the deadly disease.
The new test, designed for use in animals, can more easily and precisely diagnose rabies infection, according to a study published today in PLOS One. The new LN34 test is simpler and easier to use than current tests. During the pilot study, it produced no false negatives, fewer false positive, and fewer inconclusive results. It could allow doctors and patients to make better informed decisions about who needs treatment for rabies, which is nearly always fatal once symptoms start.
The LN34 test can also be run on testing platforms already widely used in the U.S. and worldwide, without any extra training. And it yields results even from decomposing animal brain tissue. The current gold-standard for rabies testing in animals is the direct fluorescent antibody (DFA) test, which can only be interpreted by laboratory workers with special skills, extensive training, and a specific type of microscope.
The new test could help improve rabies testing in the United States and in resource-poor countries. Currently, testing facilities in many countries in Africa and Asia most affected by rabies are not able to easily rule out the disease in animals that have bitten someone. In these countries, equipment for testing and rabies vaccine supplies are often held in centralized urban areas, several days’ travel from where someone is bitten – and rabies vaccine can cost several months’ salary. So knowing if an animal that bit someone is rabid is valuable information.
“Many of the areas hardest hit by rabies are also the areas least prepared to run current tests to diagnose it,” said Crystal Gigante, a microbiologist in CDC’s Division of High-Consequence Pathogens and Pathology and the study’s first author. “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.”
In the recent study, staff at 14 labs worldwide assessed nearly 3,000 animal brain samples from the Americas, Europe, Africa, and Asia, of which more than 1,000 were known to be infected with rabies virus. The samples came from more than 60 mammal species that get rabies, including dogs, raccoons, skunks, foxes, and bats.
LN34 correctly identified all DFA-positive samples as positive. In addition, it produced definitive findings for 80 samples that were inconclusive or untestable by the DFA test – and 29 of those were positive for rabies. Of the 3,000 samples tested, the LN34 identified one false negative and 11 false positive DFA test results. Only one sample was indeterminate using both tests. This study is the largest ever to validate usage of this type of test (a real-time RT-PCR) to diagnose rabies in animals.
LN34 testing uses PCR, a testing platform that labs worldwide already use to test for flu, HIV, and tuberculosis. In contrast, DFA testing requires a fluorescent microscope, an instrument not always available in settings with less public health infrastructure. The LN34 test 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 DFA test can be run only on fresh brain tissue samples that have been kept cold, which can be difficult in areas without reliable electricity. Surprisingly, researchers found that LN34 was able to detect miniscule amounts of rabies virus genetic material, even in samples so old they had liquefied. Moreover, the condition of the sample did not affect the test’s accuracy.
Rabies kills about 60,000 people annually, mostly in Africa and Asia. The disease can take months to develop following a person’s contact with a rabid animal. Once symptoms appear, rabies is nearly always fatal, so identifying cases and starting treatment early is critical to a patient’s survival. Having a quick, easy-to-run and accurate test to tell if an animal that bit someone is rabid could help doctors decide whether someone needs preventive treatment.
LN34 could also offer advantages for testing in the United States. Currently, if rabies tests are inconclusive, the bitten person usually is given rabies vaccine as part of post-exposure prophylaxis, at an individual cost often over $3,000. LN34 could decrease the number of inconclusive and false positive tests, thus eliminating unnecessary vaccine use. Experts estimate that rabies testing, prevention, and control cost $245 to $510 million annually in the United States. About 40,000-50,000 people get rabies PEP in the United States annually.
Currently, the DFA test is the only internationally approved test to confirm rabies in animals. However, 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 (meaning they could be used as stand-alone tests to confirm rabies, rather than being used with the DFA test).
CDC is working with the Association of Public Health Laboratories to develop rabies testing guidance that will help clinicians and laboratory staff decide which tests to run in different scenarios and which tests can be used to confirm rabies, either singly or in combination with other tests.
For more information, visit www.cdc.gov/rabies.