Background: Human granulocytic anaplasmosis (HGA) is a potentially severe tick-borne
infection caused by the bacterium Anaplasma phagocytophilum (A. phagocytophilum) of
the genus Rickettsia. Here, we describe the epidemiological and clinical characteristics of an
unusual cluster of HGA cases detected in the Estrie region in Québec, Canada, during the 2021
Methods: Confirmed cases of HGA were defined as individuals with typical clinical
manifestations and a positive polymerase chain reaction assay. The cases were interviewed
using a structured questionnaire and clinical data was obtained from medical records.
Results: A total of 25 confirmed cases were identified during the 2021 transmission season,
thus constituting the largest known cluster of HGA in Canada. The most common symptoms
reported were fever, fatigue and headaches. Laboratory investigations found that 20 (80%)
of the patients had thrombocytopenia and 18 (72%) had leukopenia at presentation. Almost
half of the patients required hospitalization (n=11, 44%), with a median duration of four days
(interquartile range [IQR] 2.5–5 days), including one patient who required intensive care. No
deaths were recorded during the study. Epidemiological investigation found that all cases were
domestically acquired, and yard maintenance was the most prevalent at-risk activity identified.
Only seven (28%) cases had been aware of a tick bite in the previous two weeks.
Conclusion: Detection of this unusual cluster of HGA cases provides further evidence that
A. phagocytophilum may now be established along the southern border of Québec. Clinicians
should consider HGA when assessing patients with typical symptoms and recent exposure to
high-risk environments for tick bite.