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Updated: Jun 4, 2026
Helminth Collection and Identification from Wildlife
Published on: December 14, 2013
Shrews and Mice Are Associated with Genetically Distinct Anaplasma phagocytophilum Variants in Northeastern United
Heidi K Goethert1, Alanna O'Callahan1, Richard Johnson2
1Department of Infectious Disease and Global Health, Cummings School of Veterinary Medicine at Tufts University, Grafton, Massachusetts, USA.
Abstract:
Anaplasma phagocytophilum (Ap) is a tick-borne bacterium that causes human granulocytic anaplasmosis (HGA) in the United States. There are distinct genotypes that seem to differ in capacity to infect humans and may be maintained in separate enzootic cycles. Anaplasma phagocytophilum-var1, which apparently does not infect humans, is found primarily in deer, and Ap-ha isolated from HGA cases is associated with mice. Genetic analyses demonstrate that Ap-ha comprises two lineages; whether these are maintained by the same hosts is not known. We clarified the Ap-ha enzootic cycle by genotyping Ap from host-seeking nymphal deer ticks collected from three sites in northeastern United States. Each genotype was associated with host species using our published tick blood-meal identification assay targeting mammalian retrotransposons. Anaplasma phagocytophilum-var1 was more common than Ap-ha (5.8% versus 3.6% of 2,799 ticks tested), and the two Ap-ha genotypes were unevenly distributed among sites. Although both Ap-ha genotypes were found on Martha's Vineyard, Ap-ha-H1 was the primary type on Nantucket, and Ap-ha-H2 was the only type detected in Rhode Island. Blood-meal analysis confirmed that deer were significantly associated with Ap-var1 infection (odds ratio [OR] = 11.7, 95% CI = 7.7-18). However, host associations for Ap-ha depended upon the genotype. Anaplasma phagocytophilum-ha-H1 was significantly associated with ticks that had fed on shrews (OR = 3.6, 95% CI = 1.8-7.2), whereas Ap-ha-H2 was associated with ticks that had fed on mice (OR = 8.3, 95% CI = 4-17.3). Spatial heterogeneity of host associations may be a basis for local HGA risk.
Insights
Anaplasma phagocytophilum (Ap) genotypes show distinct host associations, with Ap-var1 linked to deer and Ap-ha genotypes linked to shrews (H1) or mice (H2). This clarifies the enzootic cycle and informs local human granulocytic anaplasmosis (HGA) risk.
Area of Science:
- Veterinary Entomology
- Microbial Ecology
- Zoonotic Disease Epidemiology
Background:
- Anaplasma phagocytophilum (Ap) causes human granulocytic anaplasmosis (HGA) in the U.S.
- Distinct Ap genotypes may infect humans differently and exist in separate enzootic cycles.
- Ap-var1 is found in deer and does not infect humans, while Ap-ha from HGA cases is linked to mice.
Purpose of the Study:
- To clarify the enzootic cycle of Ap-ha, a causative agent of HGA.
- To determine host associations for different Ap genotypes.
- To understand the spatial distribution of Ap genotypes and their hosts.
Main Methods:
- Genotyping of Ap from host-seeking nymphal deer ticks across three northeastern U.S. sites.
- Tick blood-meal identification assay targeting mammalian retrotransposons to link genotypes to hosts.
- Statistical analysis of host associations using odds ratios and confidence intervals.
Main Results:
- Ap-var1 (5.8%) was more common than Ap-ha (3.6%) in 2,799 ticks.
- Ap-ha genotypes showed uneven distribution across sites (e.g., Nantucket vs. Rhode Island).
- Deer were strongly associated with Ap-var1 (OR=11.7). Ap-ha-H1 associated with shrews (OR=3.6), and Ap-ha-H2 with mice (OR=8.3).
Conclusions:
- Host associations for Ap-ha genotypes differ significantly (shrews for H1, mice for H2).
- Deer are the primary hosts for the non-pathogenic Ap-var1.
- Spatial heterogeneity in host associations likely influences local HGA risk.
