Various small mammal hosts are involved in the transmission cycle of zoonotic Anaplasma phagocytophilum in Southeastern Canada

Human granulocytic anaplasmosis (HGA) is a tick-borne disease emerging in North America, caused by the zoonotic variant Ap-ha (human active) of the bacterium Anaplasma phagocytophilum. This research aimed to understand the reservoir competency of small mammals for Ap-ha in the province of Québec, Canada. To achieve this aim, wild small mammals were sampled from June to August in 2022 and 2023 and tested for Ap and its variants (Ap-ha, Ap-v1) using qPCR targeting the msp2 and 16S genes. In total, 547 small mammals from nine different taxa were live captured across two years. Nearly half (48.2 %) of small mammals carried at least one Ixodes scapularis tick larva and 29.3 % carried at least one I. scapularis nymph. The odds of Ap infection were three times higher in eastern chipmunks (OR = 2.98, p < 0.05) and five times lower in red squirrels (OR = 0.21, p < 0.01) when compared to Peromyscus spp. mice. When infected, Peromyscus mice and eastern chipmunks transmitted the infection on average to 6.8 % (CI95: 24.9–48.7) and 52.9 % (CI95: 16.7–89.1) of feeding larvae, respectively. An Ap variant could be identified in 90 % (109/121) of infected small mammals, all of which were the Ap-ha strain. In conclusion, eastern chipmunks and Peromyscus mice should be considered as reservoir hosts contributing to the transmission and emergence of anaplasmosis in Canada. This study also confirms the circulation of the Ap-ha variant within host and tick communities in Southeastern Canada.