Human Granulocytic Anaplasmosis
A Systematic Review of Clinical Manifestations, Diagnosis, and Treatment Approaches Reported in Canadian Case Reports and Series
PHOTOGRAPHY: KPIXMINING/ADOBE STOCK
Abstract
Anaplasmosis, a tick-borne disease caused by the bacterium Anaplasma phagocytophilum, is emerging as a significant public health issue in Canada. This systematic review synthesizes case reports and series on Canadian anaplasmosis patients to better understand their clinical presentation and diagnostic challenges. A comprehensive search was conducted in PubMed, CINAHL and EMBASE for case reports and case series on anaplasmosis in Canadian patients. Eligible studies contain detailed individual or grouped cases with clinical features and diagnostic information. Data was analyzed for common clinical characteristics and diagnostic approaches used. The review included 10 case reports and one case series, encompassing 37 patients, primarily from Ontario and Québec. Common symptoms include fever, headache, malaise and myalgia. Laboratory findings frequently showed leukopenia, thrombocytopenia and elevated liver enzymes. Polymerase chain reaction (PCR) and serological tests were the main diagnostic tools, with doxycycline being the most prescribed antibiotic. This review underscores the clinical variability and diagnostic challenges of anaplasmosis in Canada. Early detection and treatment with doxycycline are crucial. Raising awareness among health care providers in endemic areas can help reduce diagnostic delays and improve outcomes. Further research is needed to better understand epidemiological trends and refine diagnostic and therapeutic approaches.
Introduction
Tick-borne diseases are a significant public health concern worldwide. Ticks transmit multiple pathogens, including bacteria and viruses, through their bites. In Canada, more than 40 tick species and several tick-borne diseases have been identified, including Lyme disease (Borrelia burgdorferi), Rocky Mountain spotted fever (Rickettsia rickettsii), babesiosis (Babesia duncani and Babesia microti), Borrelia miyamotoi, ehrlichiosis (Ehrlichia muris eauclairensis) and Powassan virus disease.1 An emerging disease in Canada is human granulocytic anaplasmosis (HGA),2 caused by the obligate intracellular bacterium A. phagocytophilum (formerly Ehrlichia phagocytophilum).3 HGA is transmitted by black-legged ticks (Ixodes scapularis) east of the Rockies and by western black-legged ticks (Ixodes pacificus) west of the Rockies.4
Disease Manifestation
HGA symptoms appear one to two weeks after a tick bite.2 Most patients experience mild illness with fever, fatigue and myalgia, but severe cases can result in pancytopenia, multi-organ failure and death.5 The U.S. mortality rate of untreated HGA is 0.6 - 1%.5 Unlike Lyme disease, HGA lacks the characteristic erythema migrans rash, leading to potential under-recognition.6
Diagnosis
Diagnostic tools vary depending on the time since infection and possible co-infections. Methods include PCR, serology and microscopy.7 PCR of whole blood, which detects the msp2 gene, is the most accurate test, though sensitivity declines after two weeks.7,8 Serology using indirect immunofluorescence assay (IFA) is the diagnostic reference standard, but may be negative during the first week of symptoms.7 Peripheral blood smear microscopy can identify A. phagocytophilum morulae, deep blue to grey-blue coccobacilli forming clusters within granulocyte cytoplasm (Figure 1).7,9,10 Although the presence of morulae strongly suggests HGA, they may also appear in Ehrlichia infections.7
Figure 1. Peripheral blood smear (Wright–Giemsa stained, 100×) demonstrating a neutrophil containing morulae within a vacuole (arrow).
Prevalence in Canada
Established populations of black-legged ticks carrying A. phagocytophilum exist in all provinces, though transmission cycles remain poorly defined in BC, AB, SK, NL, and PEI.11,12 Climate change and milder winters have increased tick distribution and activity.11,12 Black-legged ticks spread north at 35 to 55 km/year, primarily introduced via migratory songbirds.13–15 Reliable prevalence data for HGA in Canada remain scarce. HGA became nationally notifiable in 2024,16 which will eventually enable better tracking. Prior to this, reporting requirements varied across provinces, and surveillance was passive.17 To provide context, we reviewed peer-reviewed published literature on Canadian HGA cases: The first confirmed Canadian case occurred in AB in 2009,14 with subsequent reports from MB, ON, NS, and QC.2,12,18,19 No published cases exist for NL, PEI, SK, BC, NB, or the Territories.17 CIHI data on HGA hospitalizations, diagnostics, treatment and outcomes were inconsistent and comprehensive clinical data was primarily limited to case reports and series. These findings highlight the importance of recent national reporting requirements.
Existing Reviews
Two systematic reviews have synthesized global HGA case report data. Dumic et al. (2022) analyzed 110 patients from 19 countries, providing the first international overview of clinical features, diagnosis and treatment.5 Acosta-España et al. (2025) reviewed 73 case reports across continents, summarizing epidemiology and clinical presentation.20 However, no synthesis exists for Canadian cases specifically.
Study Objectives
Given the emergence of HGA in Canada, timely and accurate diagnosis is critical, yet diagnostic approaches may differ from those described in more established endemic regions due to variations in clinician familiarity, diagnostic availability, co-circulating tick-borne pathogens, and environmental influences on disease presentation. In addition, limited Canadian incidence data may hinder recognition of clinical and laboratory patterns that support diagnostic decision-making. This review therefore asks the question: Among published Canadian case reports and case series of human granulocytic anaplasmosis, what diagnostic approaches have been used, how do clinical and laboratory characteristics present in the Canadian context, and what treatment strategies have been reported? The objectives of this systematic review were to: Use published Canadian case reports and case series as evidence to identify and synthesize diagnostic approaches for HGA, including PCR, serology and microscopy; describe the clinical and laboratory features reported in Canadian cases that support timely diagnostic recognition; and summarize reported treatments, particularly the use of doxycycline, as described in the Canadian case literature.
Methods
We conducted a systematic review of published case reports and case series on Canadian patients diagnosed with Human Granulocytic Anaplasmosis (HGA), following PRISMA guidelines.21 Searches were performed in PubMed, CINAHL and EMBASE from database inception to March 21, 2025.
Eligibility Criteria
We included case reports and case series describing Canadian patients with confirmed HGA, published in English or French. These studies provided detailed clinical, diagnostic, or treatment information. Reviews, editorials, non-primary research and studies without sufficient patient data were excluded.
Search Strategy
The search aimed to identify all Canadian HGA cases. PubMed searches combined MeSH and keywords: (“Anaplasmosis”[MeSH] OR Anaplasmosis OR “Anaplasma phagocytophilum”) AND (“Case Reports”[Publication Type] OR “Case Series”[Publication Type] OR “case report” OR “case series”) AND (“Canada”[MeSH] OR Canada OR Canadian). Comparable strategies, adapted to database-specific headings, were used in CINAHL and EMBASE.
Study Selection
Three reviewers (MS, JZ, RG) independently screened titles and abstracts, retrieving full texts for eligible studies. Disagreements were resolved by consensus. Reference lists of included papers were also searched. The PRISMA flow diagram (Figure 2) outlines the selection process.
Data Extraction and Quality Assessment
Data were manually extracted using a standardized form capturing author, year, province/territory, study type, case numbers, demographics (age, gender), clinical features (symptoms, laboratory findings), diagnostic methods and treatments. Study quality was appraised using the Joanna Briggs Institute (JBI) Critical Appraisal Checklist for Case Reports and Series.22
Data Synthesis
A narrative synthesis summarized findings, with quantitative data tabulated to identify key trends and patterns.
Figure 2. Systematic literature search results.
Results
The systematic literature search identified 11 unique studies, including 10 case reports (n=12 patients) from NS, ON, MB, AB, and QC (Figure 2). Additionally, one study, published by a physician in the United Kingdom (UK), described a Scottish traveller who contracted the infection in ON. The search also identified one case series involving 25 patients from QC. These studies, published between 2009 and 2023, collectively document 37 cases of HGA in Canada. Notably, no peer-reviewed case series or reports have been published on patients residing in BC, Yukon (YT), Northwest Territories (NT), Nunavut (NU), SK, PEI, NB, or NL.
Quality Assessment of the Body of Evidence
The risk of bias and methodological quality of case reports included in this review were assessed using the Joanna Briggs Institute (JBI) Checklist for Case Reports. Summaries of these assessments are presented in Table 1a. The assessment focused on five key questions as defined in Table 1a. Notably, hematological and biochemical data presented in these reports were based on specimens collected before doxycycline treatment. Additional questions from the JBI checklist did not apply to this study’s research question and were therefore excluded. Overall, the quality of the 10 case report studies included in this synthesis was good. This assessment was primarily influenced by the clarity of patients’ medical histories and the detailed descriptions of patients’ clinical conditions upon presentation. However, it is worth noting that these studies did not provide demographic information regarding the patients’ race. Seven of the 10 case reports included details about patients’ medical history and medications, offering insights into potential underlying immunocompromised conditions that could contribute to abnormal blood test results and prolonged recovery times. The risk of bias and methodological quality of the included case series was conducted using the JBI Checklist for Case Series, summarized in Table 1b. The evaluation focused on eight applicable questions, and the overall quality of the case series was deemed good. A notable strength was the inclusion of Charlson Comorbidity Index (CCI) scores, which provide a standardized measure of patients’ comorbidities and help predict their mortality risk. 23
Table 1a: Quality assessments of included case reports.
Q1: Were the patient's demographic characteristics clearly described? Q2: Was the patient's history clearly described and presented as a timeline? Q3: Was the current clinical condition of the patient on presentation clearly described? Q4: Were diagnostic tests or assessment methods and the results clearly described? Q5: Was the intervention(s) or treatment procedure(s) clearly described? Q= question; ✓= criteria met; ✗=criteria not met. 22
Table 1b: Quality assessment of included case series.
Q1: Were there clear criteria for inclusion in the case series? Q2: Was the condition measured in a standard, reliable way for all participants included in the case series? Q3: Were valid methods used for identification of the condition for all participants included in the case series? Q4: Did the case series have consecutive inclusion of participants? Q5: Did the case series have complete inclusion of participants? Q6: Was there clear reporting of the demographics of the participants in the study? Q7: Was there clear reporting of clinical information of the participants? Q8: Was statistical analysis appropriate? N/A=not applicable; Q= question; ✓= criteria met; ✗=criteria not met 22
Geographic Distribution and Demographic Characteristics of HGA in Published Canadian Studies
Figure 3. Provincial distribution of published case reports and case series in Canada (2009 – 2025).
Figure 3 illustrates the geographic distribution and clustering of anaplasmosis cases in Canada, derived from cases reported in published case reports and case series. Most published case reports were males (n=10; 83%). A similar trend was noted in the case series (n=15; 60%), indicating a higher prevalence of infection among males (Table 2). The median age of infection was 72 years (IQR 67 – 76) in the case reports, which is higher than in the case series at 65 (IQR 53 – 70).
Table 2: Characteristics of included HGA case reports and series
a 11 out of 12 patients received empirical treatment, and one received directed therapy. b Data was found in Campeau et al. 2021 Case Series Study. c patients had objective fever, and one had subjective fever, a sensation of fever without measurement confirmation. d 23 out of 25 patients had objective fever and two had subjective fever, a sensation of fever without measurement confirmation. e Case series data collected during the 2021 season when travel was reduced due to COVID-19. f Not all case reports discussed hospitalizations.
Exposure, Clinical Indications and Treatment
In the case reports, 90% of patients asked (n=9/10) reported having experienced a tick bite within two weeks of their symptom onset. This differed from the case series where 28% of patients asked (n=7/25) recalled having had a tick bite within two weeks of symptoms. Clinical symptoms reported in the ten case reports (n=12 patients) included fever in all cases (100%). Among the six patients who were asked about fatigue and myalgia, all reported experiencing these symptoms. Of the seven patients asked about headaches, 86% reported having this symptom. These clinical manifestations aligned with those reported for patients in the case series. Five of six case-report patients reported symptoms of confusion (83%); however, patients in the case series were not asked about this symptom. While arthralgia was reported in 48% of case series patients (n=12/25), it was not consistently captured in the case reports. In both case reports and case series, all patients received oral doxycycline, the standard of care for HGA in Canada. Among case report patients, 91% received empirical therapy, while one received directed therapy. However, the case series did not specify whether doxycycline treatment was empirical or directed. Hospitalization was reported in 49% (18 out of 37 cases), with 10% (2 out of 18 hospitalized patients) requiring intensive care. No fatalities were reported. Due to inconsistencies and variations in data reporting, determining the median hospital stay was not possible. Hematological and biochemical results at presentation are summarized in Table 3. Leukocytopenia was reported in 65% of cases (20/31), though not all case reports included data on this parameter. Thrombocytopenia (low platelet count) was observed in 84% of cases (31/37). Median lymphocyte counts for case report and case series patients were 0.3 x109/L and 0.7 x109/L, respectively. These medians represent lymphocyte count below the reference range (e.g., normal range 0.8 to 3.3 x109/L for males). Elevated ALT was reported in 64% of cases (n=21/33). AST was elevated in all assessed case report patients (n=8). The median AST level was 75 IU/L. The acute inflammatory marker, CRP, was elevated in this population. Only two case reports included CRP data, both elevated, while the case series did not report a percentage of patients elevated; however, the median measure was 82 (IQR 35.5-171). The serum ferritin was measured in three case report patients, and all had elevated results, with a median measure of 4740ug/ml and an IQR of 1964- 4740. Anemia was reported in 39 percent of patients assessed (13/33).
Table 3: Hematological and biochemical findings in case reports and series upon presentation
Abbreviation: IQR, interquartile range calculated for all samples assessed; NR = not reported
a Medical Council of Canada.29 b Anemia was defined as hemoglobin levels below 125 g/L for men and 115 g/L for women (Medical Council of Canada).29 c Anemia was defined as hemoglobin levels below 130 g/L for men and 120 g/L for women, following Campeau et al. (2021), 28 at the local hospital.
Diagnostic Screening for Confirmation of Tick-borne Diseases
Polymerase Chain Reaction (PCR), on whole blood, is the best approach for confirming HGA when used within the first two weeks of infection and was the most used approach in the included studies (reported for 34/34 patients, 100% positive) 7,30 (Table 4). Serology, a more traditional approach for HGA diagnosis, has the potential to identify past infections by detecting lingering antibodies; however, it is susceptible to cross-reactivity with antibodies from other tick-borne infections.7 Furthermore, serology is often negative in acute infections. 30 Serology was conducted on 29 confirmed HGA patients and was positive in 41% of cases. Notably, all patients tested in case reports (n=8) had positive HGA serology results, whereas only 4 out of 21 patients tested positive in the case series. In the first week of illness, microscopic examination of peripheral blood smear (PBS) may reveal morulae in granulocytes.7 Identification of morulae is a strong indication of HGA infection.7 Peripheral blood smear examination for the presence of morulae was conducted in 11 patients, with 82% yielding positive results. All patients with detectable morulae were subsequently confirmed to have HGA via PCR. In addition to HGA, some patients in this study were also evaluated for co-infections with other tick-borne pathogens. Positive serology for Lyme disease was reported in 41% of patients tested (n=32). All studies noted that positive serology for Lyme disease may have been from prior infections and not indicative of current co-infection with HGA.1,25,28 One patient tested positive for Powassan virus co-infection through serology.1 A fourfold increase in antibody titres between acute and convalescent phases confirmed acute infection.7 Although one patient initially tested positive for R. rickettsii by serology, convalescent testing did not demonstrate a four-fold increase in antibody titer, ruling out co-infection.12
Table 4: Diagnostic tests for tick-borne diseases: case reports and series data combined
* Edginton et al. (2018) suggest possible false positives from test cross-reactivity or prior infection exposure; no change in IgG titers for R. rickettsii at 1:64.
Discussion
Over the past 15 years, 37 Canadian cases of human granulocytic anaplasmosis (HGA) have been described in peer-reviewed literature. These reports offer important insight into the presentation of this emerging tick-borne disease in Canada. Critical appraisal confirmed that the included studies were methodologically sound and transparent, strengthening confidence in the synthesized evidence. Although only 37 cases have been published, this almost certainly underestimates the true burden due to limited reporting and low clinical awareness. Since March 2024, HGA has been nationally notifiable in Canada,16 a key step toward standardized surveillance. Improved reporting will clarify the disease’s impact and guide public health preparedness.
Demographics
Our review found that older males were disproportionately affected, consistent with U.S. data suggesting greater exposure from outdoor work and animal husbandry.31 Females may experience lower exposure due to protective behaviours.31 Acosta-España et al. (2025) similarly reported a median age of 61 years and 71% male cases.20 Younger patients may experience milder or asymptomatic infections, explaining the paucity of cases under age 50.28 Pediatric cases have been documented globally, but none in Canada.20,32 Data on race or ethnicity among Canadian cases are lacking. This gap is significant in a multiethnic country with large Indigenous populations. In the U.S., higher incidence has been reported among American Indians across broader age ranges,33 suggesting a need for focused study in Canada.
Clinical Manifestations
Canadian symptom profiles overlap with international findings but vary in prevalence. Fever was nearly universal, while fatigue, headache, and myalgias were common. Compared to systematic reviews by Dumic et al. (2022) and Acosta-España (2025), Canadian cases showed higher prevalence of fatigue (97% vs. 59% and 9.6%) and headache (88% vs. 37.3% and 20%), but lower rash frequency (10% vs. 17% and 5.5%).5,20 Gastrointestinal symptoms such as diarrhea, nausea, and vomiting were reported but inconsistently.34 No Canadian case reported erythema migrans, underscoring the diagnostic challenge compared to Lyme disease.
Laboratory Findings
Laboratory abnormalities closely mirrored international data. Thrombocytopenia was present in 84% of Canadian cases, compared with 72% globally.5 Its mechanism remains unclear, though in one case bone marrow studies suggest A. phagocytophilum may infect megakaryocytes, reducing platelet production.26 Transaminitis was more frequent in Canadian reports (elevated ALT 64%, AST 100%) compared to global data (48% and 57%).5 The absence of AST data in the Canadian case series limits interpretation, but other studies report elevated transaminases in up to 70% of cases.9 Lymphopenia and CRP data were not consistently reported but may represent important indicators of HGA. Bone marrow suppression could explain the lymphopenia noted in several cases.26 Hyperferritinemia, another potential biomarker, was inconsistently reported but may reflect cytokine-driven inflammation.25 Three Canadian cases developed hemophagocytic lymphohistiocytosis (HLH), a severe hyperinflammatory condition.1,2,25 This highlights the importance of considering HGA in idiopathic HLH, especially in high-risk patients with tick exposure.
Diagnostic Methods
Since HGA became nationally notifiable in 2024,16 diagnosis remains based on a combination of microscopy, PCR, and serology.36,37 Figure 4 presents a flow diagram summarizing the recommended laboratory testing approaches for suspected HGA, based on guidance from the Centers for Disease Control and Prevention and the Public Health Agency of Canada.6,7 Microscopy: Peripheral blood smears from whole blood can identify morulae in granulocytes, which appear as deep blue coccobacilli clusters.10 Although highly suggestive, morulae are not pathognomonic, as Ehrlichia species can produce similar findings.7,38–41 Moreover, artifacts may be misinterpreted, emphasizing the need for experienced laboratory technologists. In Canadian cases, morulae were identified in 82% of examined patients, compared with 44% and 69% in global reviews.5,20 Rand et al. (2014) found that counting 200 granulocytes improved sensitivity to 100%, suggesting that microscopy can be a practical and cost-effective tool if properly implemented.10 Gram Stain and Culture: A. phagocytophilum cannot be cultured using routine methods and lacks typical Gram-negative cell wall components, making Gram stain ineffective.3,5,7 PCR: PCR on whole blood remains the most reliable early diagnostic tool,7,8,40,41 though sensitivity declines after antibiotic treatment.7 A Canadian study demonstrated that PCR on serum achieved 84.8–95.8% sensitivity, only slightly lower than whole blood, suggesting serum may be a viable alternative.8 PCR can also detect infection in cerebrospinal fluid, bone marrow, and solid tissues.16 Serology: Indirect fluorescent antibody (IFA) IgG testing from serum is the preferred serological method for detecting HGA.7 A fourfold rise in titers between acute and convalescent samples confirms infection.16 However, IgG antibodies may remain elevated for months or years, and IgM is unreliable due to cross-reactivity and variability.7 Canadian guidelines recommend paired sera, collected during the first two weeks and again 2–6 weeks later.6
Figure 4. Recommended laboratory testing approaches for suspected HGA.
Co-infections and Cross-reactivity
Ixodes scapularis/pacificus can carry A. phagocytophilum, B. burgdorferi, B. microti, and Powassan virus, and may harbour multiple pathogens simultaneously, increasing the risk of co-infection following a single tick bite.1 With co-infection rates for B. burgdorferi and A. phagocytophilum estimated to be between 2.3 and 10% in Canada, clinicians should consider the possibility of concurrent infection.2 Additionally, as cases of babesiosis and Powassan virus infection continue to increase, these pathogens should also be considered, particularly in patients who do not demonstrate clinical improvement following treatment, as doxycycline is not effective against these pathogens. 42, 43 Serologic testing should include both acute and convalescent samples, while PCR testing is most reliable if performed before initiation of antibiotics. The preferred method for the diagnosis of all these potential infections is PCR.6,7,42,43,44 However, for the detection of Powassan virus, serology is typically utilized as PCR requires cerebral spinal fluid or brain tissue for testing.43 Cross-reactivity may also lead to false-positive Lyme serology.24 One PCR-confirmed HGA case tested seropositive for B. burgdorferi but negative by PCR, suggesting cross-reactivity rather than true co-infection.25 Seven additional cases suggested possible co-infection by serology, though PCR confirmation was lacking.
Limitations
This review is limited by reliance on case reports and series, which represent low-level evidence and often focus on severe cases. Retrospective data collection, incomplete laboratory reporting, and limited follow-up constrain generalizability. Most patients were older, potentially biasing findings. These limitations highlight the need for prospective surveillance and systematic data collection under the new national reporting framework.
Conclusion
Anaplasmosis remains less common in Canada than Lyme disease, reflecting its more recent establishment in black-legged tick populations.12 However, as A. phagocytophilum continues to spread northward, incidence will likely rise. Clinicians and laboratory technologists must remain vigilant, as untreated HGA can be life-threatening. Importantly, symptoms often resolve within 48 hours of doxycycline initiation, emphasizing the value of timely diagnosis. This review synthesizes Canadian case data, highlighting demographic patterns, clinical manifestations, laboratory markers, and diagnostic approaches. Findings may inform the development of national diagnostic guidelines and support health care preparedness for this emerging tick-borne disease.
Acknowledgements
The authors would like to thank Danielle Carswell, BHSc, MLT and Dr. Brian Jollymore, MD for their intellectual contributions to this study. The authors declare that they have no conflicts of interest and no external funding was received for this work.
Jiachun Zheng MLT BHSc1,2
Megan Swetnam MLT BHSc MSc1,2
Robert Gilbert PhD1
Stephanie Lea RTR MAHSR PhD1
1 School of Health Science, Faculty of Health, Dalhousie University, Halifax, Nova Scotia 2 Department of Pathology and Laboratory Medicine, Nova Scotia Health, Nova Scotia
Jiachun Zheng MLT BHSc1,2
Megan Swetnam MLT BHSc MSc1,2
Robert Gilbert PhD1
Stephanie Lea RTR MAHSR PhD1