Investigators Discover How Ticks Reproduce Powassan Virus in Salivary Glands

Investigators at the National Institute of Allergy and Infectious Diseases of the National Institutes of Health have reported new discoveries on black-legged tick transmission of Powassan virus, a re-emerging disease in the United States.

Powassan is one of a growing number of tickborne diseases in the United States, and according to the US Centers for Disease Control and Prevention, case counts have gone up nationally from 2 in 2008 to 33 in 2017. The recent study, published in the journal mBio, examines the emergence of Powassan virus lineage 2—also known as deer tick virus—the only endemic tick-borne flavivirus in North America. Although most people who become infected with the virus do not develop symptoms, more serious cases can result in encephalitis and meningitis, and about 10% of the encephalitis cases are fatal.

“Similar viruses in Europe and Asia cause tens of thousands of cases, even in the face of effective vaccines,” study co-author Marshall E. Bloom, MD, said in a recent interview with Contagion®. “It is uncertain why Powassan/deer tick cases seem so rare. In animal models, it induces a very severe encephalitis so the virus itself has that potential, so other factors must be at play. Since humans acquire the virus from the bite of an infected tick in as little as 15 minutes, we decided to examine in detail the nature of virus infection in the tick itself.”

In the new study, investigators studied this rapid transmission from Ixodes scapularis ticks to mammals and found that flaviviruses multiplication was higher in salivary glands from fed ticks than in those from unfed ticks. They also found that flaviviruses reproduce in specific locations in tick salivary gland cultures and identified a tick gene involved in infection. This demonstrates that the virus is indisputably reproducing in the salivary gland cultures and that infectious virus is present, as if it were “locked and loaded,” said Dr. Bloom. “This is an important observation and may very well explain why virus can be transmitted to people or other mammals so quickly.”

The study team worked with ex vivo salivary gland cultures to avoid the safety issues of working with Powassan virus in live ticks, allowing them to examine the process in a highly controlled fashion and look for choke points to attack the virus’ life cycle. Dr. Bloom also noted that study lead Jeffrey M. Grabowski, PhD, has shown that it is possible to disrupt Powassan replication in the cultures by inactivating certain tick cell genes.

“This is another very important observation for 2 reasons,” said Dr. Bloom. “First, it demonstrates that inactivation of tick genes may prove an effective countermeasure against Powassan, and second, that the utilization of the organ cultures is likely a bona fide way to rapidly assess the efficacy of gene inactivation methods as well as drugs and small molecule inhibitors as antivirals.”

For now, Dr. Bloom says the best countermeasure to avoid Powassan virus infection is to avoid tick bites in the first place by wearing light colored, long-sleeved clothing, using effective tick repellents like DEET, and removing the ticks before they bite.