A circuitous journey of virus characterization and surveillance in North and Central America
Date
2023
Journal Title
Journal ISSN
Volume Title
Abstract
The burden of ticks and the pathogens they carry is increasing worldwide. Powassan virus (POWV, Flaviviridae: Flavivirus), the only known North American tick-borne flavivirus, is of particular concern due to rising cases and the severe morbidity of human disease. In this dissertation we evaluated the recent emergence of POWV from a culmination of field (chapter 2), in vitro (chapter 3), and in-vivo (chapter 4) studies. In addition, we determined the applicability of a vector-enabled surveillance method (xenosurveillance) in Central America (chapter 5). We first used a genetic approach to evaluate the emergence of lineage II POWV, known as deer tick virus (DTV), in parts of North America where human cases occur. We detected DTV-positive ticks from eight of twenty locations in the northeastern United States with an average infection rate of 1.4%. High-depth whole genome sequencing of eighty-four new and archival POWV and DTV samples allowed us to assess geographic and temporal phylodynamics. We observed both stable infection in the northeastern United States and patterns of geographic dispersal within and between regions. Bayesian skyline analysis demonstrated DTV population expansion over the last fifty years. This is concordant with the documented expansion of Ixodes scapularis tick populations and suggests increasing risk of human exposure as the vector spreads. Finally, we isolated sixteen novel viruses in cell culture and demonstrated limited genetic change after passage, a valuable resource for future studies investigating this emerging virus. We then assessed in vitro phenotypes of POWV on human neuronal cells using 16 genetically diverse isolates obtained from a broad geographic and temporal range. We determined over a 10,000-fold range in peak viral titer and significantly decreased cell mortality for two Midwest DTV isolates, though no clear correlation between in vitro phenotype and geo-temporal characteristics could be made. We then performed whole genome sequencing of virus post neuronal cell passage to identify potential residues of interest. Again, no residues could be linked to phenotype, though several interesting residues with increased frequency post-neuronal cell culture were identified. Based on the significant in vitro diversity observed, we sought to assess pathogenesis and tick transmission phenotypes between isolates. We noted neurological disease in mice in both lineages of POWV, with potential low-virulence strains derived from coastal New York. Additionally, we observed an early neuroinvasion phenotype for a Midwest DTV isolate. The ability to infect I. scapularis ticks was determined by feeding on infected host mice (viremic) and through an artificial infection method. Surprisingly, infection rates in ticks via viremic or artificial infection remained consistent between all five isolates tested, resulting in 12-20% infection rate. Taken together, these data demonstrate potential genotype-independent ability to infect ticks and conversely, strain-dependent differences in pathogenesis. In chapter 5, we evaluated a vector-enabled surveillance method ('xenosurveillance') in rural Guatemala. Surveillance methods that permit rapid detection of circulating pathogens are desperately needed. Xenosurveillance is a novel surveillance approach that takes advantage of mosquito feeding behavior to identify blood-borne pathogens that may be circulating in human and animal hosts. This approach circumvents invasive blood sampling of individuals and results in an abundant sample source derived from both humans and animals. In this study, twenty households from two villages (Los Encuentros and Chiquirines) in rural, southwest Guatemala were enrolled and underwent weekly prospective surveillance for 16 weeks. When febrile illness was reported in a household, recently blood-fed mosquitoes were collected from within dwellings and blood samples taken from each member of the household. Mosquitoes were identified to species and blood sources identified by sequencing. Shotgun metagenomic sequencing was used to identify circulating viruses. Culex pipiens (60.9%) and Aedes aegypti (18.6%) were the most abundant mosquitoes collected. Bloodmeal sources were most commonly human (32.6%) and chicken (31.6%), with various other mammal and avian hosts detected. Several mosquito-specific viruses were detected, including Culex orthophasma virus. Human pathogens were not detected. While more intensive sampling may be needed to detect human pathogens, sampling mosquitoes that feed on humans and domestic animals may prove valuable for monitoring pathogens with zoonotic potential.
Description
Rights Access
Subject
emerging disease
surveillance
xenosurveillance
Powassan virus
arthropod-borne disease
tick-borne disease