A seven-legged tick: Report of a morphological anomaly in Ixodes scapularis (Acari: Ixodidae) biting a human host from the Northeastern United States.

Cases of morphological anomalies in the blacklegged tick, Ixodes scapularis (Acari: Ixodidae), have recently been reported from the Northeastern and upper Midwestern United States, potentially complicating identification of this important vector of human disease-causing pathogens. We hereby report a case of a morphological anomaly in I. scapularis, biting a human host residing in Norwich, Connecticut. Using a dichotomous morphological key, high-resolution and scanning electron microscopy images, as well as DNA sequencing, the tick was identified as an adult female I. scapularis with three legs on the left side of the abdomen versus four on the right side, which we believe is the first case of ectromely in an adult I. scapularis. Using diagnostic genes in polymerase chain reaction, the specimen tested positive for Borrelia burgdorferi sensu lato and Anaplasma phagocytophilum, the causative agents for Lyme disease and anaplasmosis, respectively, and also showed evidence of a rickettsial endosymbiont. Here we discuss recent reports of morphological anomalies in I. scapularis, and emphasize the significance of additional studies of teratology in this important tick species and its potential implications.