The Modern Equine Vet
April 2024
Vol 14 Issue 4 2024
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News Notes

New Testing Approach Improves Detection of Rare but Emerging Powassan Virus Spread by Deer Ticks

Researchers at the New England Regional Center of Vector-Borne Diseases at the University of Massachusetts Amherst have come up with a new, more accurate method for detecting the emerging Powassan virus (POWV) in ticks.

This robust, real-time approach reduces the incidence of false-positive results, the NEWVEC researchers found because false-positives can confound surveillance efforts, according to Stephen Rich, MS, PhD, a professor of microbiology at UMass Amherst and the principal investigator and executive director of NEWVEC. “The development of sensitive detection methods for diagnostics and surveillance is critical,” he said.

Named after the town in Ontario, Canada, where it was first identified in 1958 in a 5-year-old boy who died from encephalitis, POWV is a flavivirus related to West Nile, which can affect humans and horses.

Although still rare, POWV is increasing in incidence in the United States, predominantly in the Northeast and Great Lakes region. More than 10% of the record 290 U.S. human cases reported in 2022 (compared with only 1 case per year from 2004 to 2006) resulted in death, and half of the survivors suffered long-term neurological damage.

The virus is transmitted to humans primarily by Ixodes scapularis, which also transmits Lyme disease, babesiosis and other tickborne illnesses.

The team at NEWVEC developed a triplex real-time polymerase chain reaction (PCR) test for the simultaneous and quantitative detection of the Powassan virus and Powassan virus lineage II in I. scapularis.

The NEWVEC team conducted a tick survey in coastal and offshore Massachusetts, focusing on 13 sites from the highly endemic regions of tickborne diseases in Cape Cod and Martha’s Vineyard. They tested the ticks for POWV, comparing their new triplex PCR method to the standard, commercially available Luminex xMap technology.

The new triplex method accomplishes a reduction in false negatives by using a clever quality control. Both tests seek to detect the presence of POWV RNA. “But we also had a paired search for the RNA from the tick, which is present in every tick regardless of whether it has the virus or not,” Dr. Rich said. “If we don’t detect the tick DNA, then we have no hope of being able to detect the virus RNA.

“And before we developed that method, people would be left to wonder whether a negative result meant that the virus wasn’t there or that the sample wasn’t testable. So, we’ve ruled out that latter possibility. And now we know with some assurance that when a tick tests negative, it’s a true negative. It’s not that the sample just isn’t good enough,” he said.

In the areas surveyed, “We found pockets of high incidence of this virus,” Dr. Rich said.

POWV was detected at 4 of 6 sites in Cape Cod and 2 of 7 sites in Martha’s Vineyard. Of 819 ticks collected, 33 (4.03%) tested positive for POWV and 752 tested as Powassan negative, using the new triplex method. Thirty-four ticks (4.15%) failed the quality control tick RNA test. That showed that the standard Luminex method underestimated the overall prevalence of POWV because those 34 ticks were found Powassan negative. And only 30 ticks tested positive using the Luminex method, demonstrating that the triplex technique has a higher sensitivity to detect the virus RNA.

Infection rates reached as high as 10.43% at 1 site in Truro on Cape Cod and were completely absent at 7 other sites. All the ticks that tested positive for the POWV also were positive for the lineage II deer tick virus.

The researchers said they hope this improved triplex PCR test will be useful in transmission studies and as a tool to monitor and prevent POWV infections in areas where the virus has been reported. MeV

For more information:

Xu G, Siegel E, Fernandex N, et al. Active surveillance of powassan virus in Massachusetts Ixodes scapularis ticks, comparing detection using a new triplex real-time PCR assay with a Luminex Vector-Borne Panel. Viruses. 2024;16(2):250 doi: 10.3390/v16020250.