Quantum of infection of Francisella tularensis tularensis in host-seeking Dermacentor variabilis.

The American dog tick, Dermacentor variabilis, is fundamental to the perpetuation of Francisella tularensis tularensis on Martha's Vineyard, Massachusetts, U.S.A. Although infected ticks are relatively common on the island, human cases deriving from tick bite are rare. It may be that the quantum of bacteria within these naturally infected ticks is frequently too small to cause disease. Accordingly, we quantified the amount of F.t. tularensis bacteria in host-seeking ticks from the island. Contrary to our expectations, the majority of the ticks harbor large numbers of bacteria (median 3.3x10(8) genome equivalents/tick). Such a large quantum of infection might suggest that aerosolization of the ticks themselves might comprise the proximal determinant of risk for the inhalational tularemia that is common on Martha's Vineyard. However, the paradox of fewer ulceroglandular tularemia cases than would be expected given the abundance of potentially highly infectious ticks remains to be solved.