Vertical Transmission: A Vector-Independent Transmission Pathway of Babesia microti in the Natural Reservoir Host Peromyscus leucopus.

Background: Babesia microti, a malaria-like pathogen, is increasing in mammal and human populations in endemic areas and is unlikely to be the sole result of horizontal pathogen transmission.

Methods: Peromyscus leucopus mice, natural reservoir hosts, were infected via Ixodes scapularis nymphs. Infected parental females (n = 6) produced F1 offspring (n = 36) that were screened for B. microti using quantitative PCR. Xenodiagnostic larvae were fed on infected offspring to determine horizontal transmission and pathogen viability. Fifty engorged larvae were screened; the rest were allowed to molt and then screened to determine transstadial transmission. Infected F1 generation offspring were placed in breeding groups, producing 34 F2 offspring and screened for B. microti infection. Chronic infection was monitored in parental females since time of initial vector infection.

Results: Vertical transmission of B. microti was 74% efficient in offspring born in the first 6 months. Horizontal transmission occurred in larvae (61% prevalence) and molted nymphs (58% prevalence); these nymphs were able to infect susceptible hosts. F2 generation offspring infection prevalence was 38%. Chronic infection persisted for 1 year in some adults.

Conclusions: These results demonstrate that vertical transmission is an important nonvector-mediated pathway of B. microti transmission in the natural reservoir host.