Immunity reduces reservoir host competence of Peromyscus leucopus for Ehrlichia phagocytophila

Infection with Ehrlichia phagocytophila in white-footed mice is transient a nd followed by a strong immune response. We investigated whether the presen ce of acquired immunity against E. phagocytophila precludes whitefooted mic e from further maintenance of this agent in nature. Mice were infected with E. phagocytophila via tick bite and challenged either 12 or 16 weeks later by Ixodes scapularis nymphs infected with the same agent. Xenodiagnostic l arvae fed upon each mouse simultaneously with challenging nymphs and 1 week thereafter. Ticks were tested for the agent by PCR, and the prevalence of infection was compared to that in ticks that fed upon nonimmune control mic e. Only 30% of immunized mice sustained cofeeding transmission of E. phagoc ytophila between simultaneously feeding infected and uninfected ticks, comp ared to 100% of control mice. An average of 6.3% of xenodiagnostic ticks ac quired Ehrlichia from previously immunized mice when fed 1 week after the c hallenge, compared to 82.5% infection in the control group. Although an imm une response to a single infection with E. phagocytophila in white-footed m ice provided only partial protection against reinfection with the same agen t, the majority of mice were rendered reservoir incompetent for at least 12 to 16 weeks. Immunity acquired by mice during I. scapularis nymphal activi ty in early summer may exclude a large proportion of the mouse population f rom maintaining E. phagocytophila during the period of larval activity Late r in the season.