Generally what seems to happen is that a larval tick acquires the spirochete by taking its blood meal from an infected host—a mouse, a shrew, a whatever. It molts to become a nymph and then, if it gets its next meal from an uninfected host, the nymph passes the infection to that animal, by drooling spirochetes into the wound along with its anticoagulant saliva. “If mammals didn’t make ticks sick,” Ostfeld said, “ticks wouldn’t make mammals sick later on.” Such reciprocal infectivity helps keep the prevalence of B. burgdorferi high in both tick populations and hosts.
