While the total number of new cases in the United States is down from 2008 (1,356), the number of reported cases of meningitis or encephalitis associated with WNV continues to rise, says Natalie Prow, PhD, Australian Research Council, or ARC, Postdoctoral Research Fellow in the Centre for Infectious Disease Research, University of Queensland, Australia. “If you look at the percentage of meningitis/encephalitis cases compared to total cases over the last four years, it does seem like in 2008-2009 a greater percentage of all cases reported to the CDC were associated with neurological involvement,” she says.
- 2009: 50.5% present with meningitis/encephalitis (335/663)
- 2008: 50.6% (687/1,356)
- 2007: 33.5% (1,217/3,630)
- 2006: 34.2% (1,459/4,269)
Prow adds, “But it is unclear whether this increase is due to the change in the genotype of the virus or better reporting and/or diagnosis of symptoms associated with cases.
Interestingly, virulent strains of WNV still have not been detected in Australian mosquitoes.
“If you compare WNV to other encephalitic flaviviruses, WNV is able to consistently produce a high percentage of cases with neurological involvement. For example, Japanese encephalitis virus causes approximately 50,000 cases of disease each year; and about 25% to 50% of cases have neurological involvement.”
Interestingly, virulent strains of WNV still have not been detected in Australian mosquitoes. In October 2007, however, one case of Kunjin virus, a subtype of WNV, was reported in Victoria, notes Prow. “Further investigations resulted in the reclassification of the diagnosis of WNV,” she says. “This is the first report of a lab confirmed WNV(NY99) infection imported into Australia.”
Given the rise of highly neuroinvasive strains of WNV, virus genotyping may become increasingly important in diagnosing the illness. Researchers have been focusing on tests based on lateral flow detection of IgM and real-time polymerase chain reaction (PCR) that will simultaneously detect and genotype the virus. Lateral flow detection, however, has been found to be most suitable only when a small number of specimens is being tested.
Still, work is progressing to improve these tests, says Prow. “Current developments of more rapid, sensitive, and specific assays to detect IgM to a range of flaviviruses and the same for detection of viral RNA by multiplexed real-time PCR may allow for a definitive diagnosis from a single specimen in the near future,” she says.
And while molecular testing holds promise, it is not without its problems. Because serum levels of WNV RNA usually peak before the appearance of symptoms and then decline rapidly over several days as a result of antibody formation, false-negatives can occur for some infections. Therefore, a combination of viral RNA and IgM tests, while targeting multiple viral proteins to improve sensitivity of serology, is recommended as the most accurate way to diagnose WNV, Prow says. “Traditionally, most serological tests target the major envelope protein, but recent publications have focused on alternative viral targets such as NS5. We are also pursuing this avenue by investigating the potential of prM as a strain-specific diagnostic marker for WNV,” she adds.
There are other challenges as well, Prow says. “The challenges with the molecular tests is cost (usually RNA extraction/RT is required, which can be expensive); preparation of samples; need for a cold chain; and, most importantly, whether the virus will still be present at time of presentation for sera collection. It is known that most flaviviruses are cleared from the immune system in the first week of infection, so it can be difficult to get sera samples in this acute phase of infection to reliably use molecular techniques for diagnosis. Molecular techniques, complemented with serological analysis, are much more reliable.”
To date, the only vaccine commercially available is for horses. But Prow says researchers are still trying to develop a vaccine for humans by focusing on live attenuated chimeras, standard DNA vaccines, and single-round infectious particles-based, or SRIPs-based, technologies with needle-free delivery systems.
Richard R. Rogoski is a freelance journalist based in Durham, NC. Contact him at [email protected].
