Differential mortality of dog tick vectors due to infection by diverse Francisella tularensis tularensis genotypes.

The factors involved in the long-term perpetuation of Francisella tularensis tularensis in nature are poorly understood. Martha's Vineyard, Massachusetts, has become a site of sustained transmission of Type A tularemia, with nearly 100 human cases reported from 2000 to 2010. We have identified a stable focus of F. tularensis transmission there, where the annual prevalence in host-seeking Dermacentor variabilis is about 3%, suggesting that this tick perpetuates the agent. However, laboratory studies have shown that infection with F. tularensis has a profound negative effect on dog tick mortality, presenting a paradox: how can a vector perpetuate an agent that negatively affects its fitness? It may be that experimental infection does not mimic that of natural transmission. Accordingly, we examined the effects that F. tularensis has on the longevity of field-derived ticks. Of 63 PCR-positive ticks collected in early summer, 89% were dead by December compared to 48% of 214 uninfected ticks collected at the same time and site. However, the quantum of F. tularensis DNA within each tick was not correlated with increased mortality. Instead, ticks with an uncommon genotype were more likely to die early than those with the common genotype. We conclude that the interaction between F. tularensis and its vector is complex and certain bacterial genotypes appear to be better adapted to their arthropod host.