Genotypic diversity of Francisella tularensis infecting Dermacentor variabilis ticks on Martha's Vineyard, Massachusetts.

Martha's Vineyard, Mass., has been the site of two outbreaks of tularemia (1978 and 2000). Although most patients from both outbreaks presented with pneumonic disease and although aerosol transmission has been suggested, the bite of a dog tick and exposure to rabbits remain the only proven modes of transmission. The factors that precipitated the tularemia outbreaks or the proximal determinants of human risk remain undescribed. We sought to test the hypothesis that the ongoing outbreak is due to a recent introduction event as opposed to amplification of a cryptic enzootic cycle. From 2001-2003, we collected 4,246 dog ticks and tested them in pools for evidence of tularemia by PCR. We then measured the genetic diversity of Francisella tularensis by using multiple-locus variable-number tandem repeat analysis. The prevalence of F. tularensis in dog ticks averaged 0.7%. From 29 positive pools, we identified 10 unique genotypes, which was an unexpectedly large degree of diversity (Simpson's index = 0.86). This degree of genetic diversity is inconsistent with a recent introduction event. We conclude that there has been long-standing enzootic transmission of tularemia on the island.