Transstadial transmission and maintenance of Francisella tularensis in the Blacklegged Tick Ixodes scapularis.

Abstract

In the south-central United States tularemia endemic region, approximately 10% of human cases occur during the fall-winter season when the questing activity of its primary tick vectors for this region Dermacentor variabilis and Amblyomma americanum is minimal. It is also been reported that approximately 40% of tularemia infected domestic cats contract the disease during the fall-winter season. To more fully characterize the potential role of I. scapularis, whose host seeking activity peaks during the fall-winter season in the south-central United States, as a vector for tularemia, the biology of F. tularensis subspecies holarctica strain LVS in colony-reared I. scapularis artificially infected with LVS by capillary feeding was studied. Capillary-fed I. scapularis larva and nymph were initially colonized with 1.9 × 103 and 1.1 × 104 CFU/tick, respectively. For nymphs molted from colonized larvae, the percent of molted nymphs colonized declined at the time of molting, but LVS persisted in 15% of molted nymphs up to 175 days post-capillary feeding. In contrast, only 5% of adults molted from infected nymphs maintained LVS colonization through 150 days post-capillary feeding. For capillary-fed adults, LVS initially colonized the gut and disseminated to hemolymph and salivary glands by 14 day post-feeding. Post-capillary feeding larvae and nymphs maintained LVS colonization initially and persisted although in a low percentage in molted nymphs and molted adults. Based on the feeding habits of I. scapularis adults, this tick species has the potential to act as a bridging vector for human tularemia for fall-winter cases of tularemia.