Transmission route efficacy and kinetics of Anaplasma phagocytophilum infection in white-footed mouse, Peromyscus leucopus

Abstract

Anaplasma phagocytophilum was used to infect Peromyscus leucopus mice by three routes of inoculation: infected tick infestation and intraperitoneal (IP) and subcutaneous (SQ) injection of infected tissue culture cells. A set of 12 mice were infected (four tick, four IP, and four SQ), and blood was drawn at 1, 3, 6, 9, 12, 15, 21, 28, 35, and 60 days post-infection and analyzed by use of a quantitative PCR assay to assess the level of infection. An additional set of 108 mice were infected (36 tick, 36 IP, 36 SQ) and euthanized at 1, 3, 6, 9, 12, 15, 21, 28, and 35 days post-infection (four mice/time point), and blood, spleen, bone marrow, and bladder tissue samples were analyzed. Tick infection generally produced the highest average levels of infection and peaked at 9 days post-infestation in blood, spleen, and bone marrow and at 6 days after infestation in the bladder. IP injection resulted in levels of infection that peaked on day 6 (spleen) or 12 (bladder, bone marrow, and blood). A. phagocytophilum injected SQ showed low levels of infection, and the day of peak infection varied. The average level of infection in the blood drawstressed mice was consistently higher and peaked earlier than infection in the non-stressed, euthanized mice. Xenodiagnosis was used to assay a third set of 12 mice (four tick, four IP, and four SQ) on days 7 and 14 post-infection and ticks fed on tick-infected mice showed the highest rate of PCR-positive test results at both time points (day 7, 22.2%; day 14, 17.3%). These data indicate that P. leucopus mice can be infected by tick infestation, IP injection, or SQ injection but that the kinetics and level of infection are quite variable among individual mice, may be influenced by the route of inoculation, and may be further altered by common laboratory procedures such as repeated collection of blood samples.