An immunocompromised mouse model to infect Ixodes scapularis ticks with the relapsing fever spirochete, Borrelia...

The hard tick-borne relapsing fever spirochete, Borrelia miyamotoi, has recently gained attention as a cause of human illness, but fundamental aspects of its enzootic maintenance are still poorly understood. Challenges to experimental studies with B. miyamotoi-infected vector ticks include low prevalence of infection in field-collected ticks and seemingly inefficient horizontal transmission from infected immunocompetent rodents to feeding ticks. To reliably produce large numbers of B. miyamotoi-infected ticks in support of experimental studies, we developed an animal model where immunocompromised Mus musculus SCID mice were used as a source of B. miyamotoi-infection for larval and nymphal Ixodes scapularis ticks. Following needle inoculation with 1 × 10⁵ spirochetes, the SCID mice developed a high spirochetemia (greater than 1 × 10⁷ copies of B. miyamotoi purB per mL of blood) that persisted for at least 30 d after inoculation. In comparison, immunocompetent M. musculus CD-1 mice developed transient infections, detectable for only 2-8 d within the first 16 d after needle inoculation, with a brief, lower peak spirochetemia (8.5 × 10⁴ - 5.6 × 10⁵purB copies per mL of blood). All larval or nymphal ticks fed on infected SCID mice acquired B. miyamotoi, but frequent loss of infection during the molt led to the proportion infected ticks of the resulting nymphal or adult stages declining to 22-29%. The ticks that remained infected after the molt had well-disseminated infections which then persisted through successive life stages, including transmission to larval offspring.