Nonrandom Distribution of Vector Ticks ( Dermacentor variabilis) Infected by Francisella tularensis

Abstract

The island of Martha's Vineyard, Massachusetts, is the site of a sustained outbreak of tularemia due to Francisella tularensis tularensis. Dog ticks, Dermacentor variabilis, appear to be critical in the perpetuation of the agent there. Tularemia has long been characterized as an agent of natural focality, stably persisting in characteristic sites of transmission, but this suggestion has never been rigorously tested. Accordingly, we sought to identify a natural focus of transmission of the agent of tularemia by mapping the distribution of PCR-positive ticks. From 2004 to 2007, questing D. variabilis were collected from 85 individual waypoints along a 1.5 km transect in a field site on Martha's Vineyard. The positions of PCR-positive ticks were then mapped using ArcGIS. Cluster analysis identified an area approximately 290 meters in diameter, 9 waypoints, that was significantly more likely to yield PCR-positive ticks (relative risk 3.3, P = 0.001) than the rest of the field site. Genotyping of F. tularensis using variable number tandem repeat (VNTR) analysis on PCR-positive ticks yielded 13 different haplotypes, the vast majority of which was one dominant haplotype. Positive ticks collected in the cluster were 3.4 times (relative risk = 3.4, P<0.0001) more likely to have an uncommon haplotype than those collected elsewhere from the transect. We conclude that we have identified a microfocus where the agent of tularemia stably perpetuates and that this area is where genetic diversity is generated.

Author Summary

We present evidence, for the first time, that the agent of tularemia persists in microfoci for at least four years. The existing literature alludes to the natural nidality of this bacterium and the importance thereof in its long-term survival in nature, but “natural foci” that have been described to date have been very nebulously defined, poorly analyzed, and at a scale much larger than what we describe. In addition, we demonstrate such a focus by using modern GIS methods as well as by genetic cluster analysis of bacterial DNA. The work, therefore, contributes to our understanding of how the agent of tularemia perpetuates over the long term. Furthermore, this paper serves as a paradigm for analyzing the ecology of other vector-borne infections and, in particular, demonstrating the mode of their long-term persistence in the environment.