Abstract
In the United States, Dermacentor spp. are common vectors of Francisella tularensis subspecies (ssp.), while Ixodes scapularis is not, though the geographic distribution and host range of pathogen and tick overlap. To examine if differences in infection competence at the cellular level underpin these ecological differences, we evaluated the competence of D. andersoni (DAE100) and I. scapularis (ISE6) cell lines to support F. tularensis ssp. novicida (F. novicida) infection. Importantly, D. andersoni is a vector for both F. tularensis spp. tularensis, and F. novicida. We hypothesized F. novicida infection would be more productive in D. andersoni than in I. scapularis cells. Specifically, we determined if there are differences in F. novicida i) invasion, ii) replication, or iii) tick cell viability between DAE100 and ISE6 cells. We further examined the influence of temperature on infection kinetics. Both cell lines were permissive to F. novicida infection; however, there were significantly higher bacterial levels and mortality in DAE100 compared to ISE6 cells. Infection at environmental temperatures prolonged the time bacteria were maintained at high levels and reduced tick cell mortality in both cell lines. Identifying cellular determinants of vector competence is essential in understanding tick-borne disease ecology and designing effective intervention strategies.
Highlights
Tick-borne diseases are the most common vector-borne diseases of humans in the United States with the number of reported cases steadily increasing and the distribution of tick vector species and tick-borne pathogens continuing to expand and overlap
Novicida as a model to examine if the ecological relevance of D. andersoni and I. scapularis for F. tularensis ssp. transmission is mirrored at the cellular level, we compared the competence of the DAE100 and the ISE6 cell lines to become infected with and support F. novicida replication
Tick cell cultures were inoculated with F. novicida and bacterial infection levels were measured at defined time points to determine cell line infection competence and bacterial infection kinetics
Summary
Tick-borne diseases are the most common vector-borne diseases of humans in the United States with the number of reported cases steadily increasing and the distribution of tick vector species and tick-borne pathogens continuing to expand and overlap. In the United States, the number of reported cases of tick-borne disease increased from ~17,000 cases in 2001 to >40,000 cases in 20141. With the exception of Borrelia burgdorferi, an extracellular spirochete, the determinants of vector competence for tick-borne pathogens, at the cellular and molecular level, are largely unknown. Tick-borne pathogens of humans, including Rickettsia rickettsii, Francisella tularensis, and Anaplasma phagocytophilum are intracellular pathogens and the determinants of vector competence for these www.nature.com/scientificreports/. For intracellular tick-borne bacterial pathogens, limited experimental models and genetic tools have hampered identification of cellular and molecular determinants of vector competence[7]. F. novicida serves as non-hazardous laboratory model for F. tularensis spp. tularensis in ticks
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