Emerging Tick-Borne Viruses
“Most of us think of TBDs as fairly badly neglected in the current environment, and tick-borne viruses are neglected even among these neglected pathogens,” said Gregory D. Ebel, ScD, from Colorado State University, Fort Collins. According to Dr. Ebel, Powassan virus (a flavivirus) is the tick-borne virus about which scientists know the most. It is maintained in nature in 3 distinct transmission cycles that involve woodchucks, squirrels, and deer ticks. Recent cases in humans seem to be related to the deer tick-driven transmission cycle, and this is partly why it is considered an emerging health threat in the United States.
Since 2006, 68 cases of Powassan encephalitis have been reported, said Dr. Ebel. Cases have occurred in the northeastern United States and upper Midwest, consistent with transmission by deer ticks. Severe disease occurs due to neurological involvement which arises secondary to either inflammation in the brain or direct neuronal injury, he noted.
First recognized in 2009 in Missouri, the Heartland virus, a phlebovirus transmitted by the bite of an infected lone star tick, is another emerging tick-borne virus, said Dr. Ebel. And one case of infection with Bourbon virus, a thogotovirus, was also reported in 2015. This virus remains poorly understood, but is probably tick-borne.2,3
Many knowledge gaps persist about how new tick-borne viruses emerge, and Dr. Ebel highlighted several factors that hinder tick-borne virus research. In particular, the tick’s long life cycle—which often exceeds the length of a research grant—is an extremely difficult barrier to overcome, he concluded. (Learn more about Powassan virus by reading, “Another Tick-Borne Disease Growing in the United States: Powassan Virus,” available at contagionlive.com/link/3
Laboratory Detection Methods
Bobbi Pritt, MD, MSc, DTM&H, from Mayo Clinic in Rochester, Minnesota, discussed testing methods for diagnosing TBDs. Indirect methods measure the host’s immune response to an infecting organism, she said, and detect IgM or IgG class antibodies in serum. Serology is the method of choice for detecting many TBDs, but Dr. Pritt stressed that it is also insensitive in the first few days of an infection before antibody levels begin to rise. But by the second or third week, its sensitivity significantly increases, she said.
In contrast, direct methods detect the organism itself or some component of it. These include nuclei acid amplification tests, such as polymerase chain reaction to detect the organism’s DNA or RNA. Although nucleic acid amplification represents one of the earliest methods for detecting some organisms, it can be insensitive if nucleic acid is no longer detectable when the patient presents for evaluation. Nevertheless, it is especially useful for such organisms as the Ehrlichia
species because large amounts of their DNA and RNA are present during the symptomatic stage of infection.
Several next-generation technologies are also emerging, said Dr. Pritt. Multiplex molecular panels can detect multiple bacterial, viral, and parasitic pathogens in a single test. In addition, broad range sequencing uses common targets and allows targeted gene amplification with subsequent sequence identification. “The last category of emerging technologies that I find particularly exciting is metagenomics,” she said, which allows amplification of all nucleic acid in a specimen, whether bacterial, fungal, viral, parasitic or human in origin. “This is currently very expensive and time consuming, but I think this is bound to change in the future,” she concluded.
Dr. Parry, a board-certified veterinary pathologist, graduated from the University of Liverpool in 1997. After 13 years in academia, she founded Midwest Veterinary Pathology, LLC, where she now works as a private consultant. Dr. Parry writes regularly for veterinary organizations and publications.
- Drexler N, Miller M, Gerding J, et al. Community-based control of the brown dog tick in a region with high rates of Rocky Mountain spotted fever, 2012-2013. PLoS One. 2014;9(12):e112368.
- Centers for Disease Control and Prevention. Bourbon virus. CDC website. https://www.cdc.gov/ncezid/dvbd/bourbon/. Updated February 19, 2015. Accessed April 3, 2017.
- Kosoy OI, Lambert AJ, Hawkinson DJ, et al. Novel thogotovirus associated with febrile illness and death, United States, 2014. Emerg Infect Dis. 2015;21(5):760-764. doi: 10.3201/eid2105.150150.