National case definition: Anaplasmosis

Date of last revision/review: January 2024

On this page

• National notification
• Type of surveillance
• Case classification
• Laboratory criteria
• Clinical evidence
• Exposure
• ICD code(s)
• Comments

National notification

Confirmed and probable cases of disease should be notified to the Public Health Agency of Canada.

Type of surveillance

Routine case-by-case notification to the federal level.

Case classification

Confirmed case

A case that meets confirmatory laboratory results with or without clinical evidence criteria (can include transfusion transmission).

Probable case

A case that meets supportive laboratory results and:

• clinical evidence criteria (can include transfusion transmission); or
• is in a blood donor or recipient epidemiologically linked (see Exposure) to a confirmed or probable anaplasmosis case.

Laboratory criteria

Confirmatory laboratory tests include:

• detection of Anaplasma phagocytophilum DNA in an appropriate clinical specimen by amplification of a specific target by nucleic acid amplification test (NAAT); or
• serological evidence of a four-fold change in immunoglobin G- (IgG) specific antibody titre to A. phagocytophilum antigen by indirect immunofluorescence assay (IFA) in paired serum samples. The first sample taken in the acute phase (in first week of illness) and the second taken in the convalescent phase (2-4 weeks after the first sample); or
• demonstration of A. phagocytophilum antigen in a biopsy/autopsy sample by immunohistochemical (IHC) methods; or
• isolation of A. phagocytophilum from a clinical specimen in cell culture with confirmation by specific PCR.

Supportive laboratory tests include:

• serological evidence of elevated IgG antibody to A. phagocytophilum in a single specimen by IFA where the endpoint titre is four-fold greater than the screening dilution of the assay; or
• identification of typical morulae in the cytoplasm of granulocytes by microscopic examination.

Clinical evidence

Clinical description:

Asymptomatic infections of A. phagocytophilum do occur and may even be more common than symptomatic infections.^{Footnote 2}

Symptoms of anaplasmosis typically begin within 5–21 days after the bite of an infected tick, and commonly manifest as an acute non-specific flu-like illness (e.g. fever, chills, headache, myalgia, arthralgia, malaise, fatigue).^{Footnote 1},^{Footnote 2},^{Footnote 3}

Less commonly noted manifestations include gastrointestinal manifestations, respiratory symptoms, and neurological issues.^{Footnote 1},^{Footnote 2},^{Footnote 3} Skin rash may be present, but could also indicate co-infection with Borrelia spp.^{Footnote 1} or other tick-borne diseases.

Laboratory abnormalities are common in the early phase and can include leukopenia, thrombocytopenia, elevated aminotransferase levels, and anemia.^{Footnote 1},^{Footnote 2}

Patients with comorbidities, who are elderly, immunocompromised, or for whom treatment is delayed are at greater risk for more severe illness, and in rare cases, death.^{Footnote 1},^{Footnote 3},^{Footnote 4}

Severe illness can include renal failure, respiratory distress, hemorrhage, meningitis, encephalitis, and sepsis.^{Footnote 2},^{Footnote 4}

Clinical criteria:

Clinical evidence of infection includes fever and at least one of the following: headache, malaise/asthenia, arthralgia/myalgia, mild anemia, thrombocytopenia, leukopenia, elevated hepatic transaminase concentrations, or elevated numbers of immature neutrophils.^{Footnote 1},^{Footnote 2},^{Footnote 3}

Exposure

Anaplasmosis is a tick-borne illness caused by the intracellular bacterium A. phagocytophilum. The main transmission route for A. phagocytophilum is blacklegged ticks (Ixodes scapularis and Ixodes pacificus).^{Footnote 5},^{Footnote 6}

Although cases of tick-borne illness can occur during any month of the year, most cases occur when ticks are most active, in the spring, summer, and fall. Consideration should be given to patients who have recently spent time in potential blacklegged tick habitats, or who have a history of tick bite (although as many patients have no recollection of a tick bite, lack of a tick bite should not preclude consideration of anaplasmosis).

Evidence suggests transmission of A. phagocytophilum from an infected tick may occur more quickly when compared to Borrelia burgdorferi for Lyme disease and may occur within the first 24 hours of attachment.^{Footnote 11}

The spatial distribution of A. phagocytophilum in Canada is similar, but more restricted, to the geographical distribution of established blacklegged tick populations that correspond to Lyme disease risk areas. However, with the expansion of suitable tick habitats, there may be a risk for transmission outside of these pre-defined areas.

Updated information regarding the distribution of Lyme disease risk areas

A. phagocytophilum can survive for prolonged periods in blood products, and cases of transfusion-transmitted anaplasmosis have been reported in the United States.^{Footnote 7},^{Footnote 8},^{Footnote 9},^{Footnote 10} Currently, no Health Canada or United States of America Food and Drug Administration licensed test exists for the screening of A. phagocytophilum in blood donors.

For the purposes of surveillance, epidemiologic linkage between a transfusion recipient and a blood donor is demonstrated if all of the following criteria are met:

1. Laboratory evidence of A. phagocytophilum infection in the recipient and donor; and
2. Transfusion recipient received one or more red blood cell or platelet unit(s) within one year before the collection date of the recipient's positive specimen; and
3. Transfused unit(s) was/were plausibly infectious based on assessment of donor infectivity at time of donation of implicated unit(s); and
4. Transfusion-associated infection in the recipient is considered at least as plausible as tick-borne transmission.

ICD code(s)

ICD-10-CA code(s)

• A79.8 Other specified rickettsioses (includes Erlichia sennetsu)
• A79.9 Rickettsiosis, unspecified (includes Rickettsial infection not otherwise specified)

ICD-9 code(s)

• 082.9 Unspecified tick-borne rickettsioses
• 083.8 Other rickettsioses
• 083.9 Unspecified rickettsioses

Comments

• These are definitions for surveillance and epidemiologic purposes only and should not be used for clinical diagnostic purposes.
• Diagnostic testing should be performed by provincial public health laboratories and/or appropriate reference diagnostic centres (e.g., National Microbiology Laboratory, National Reference Centre for Parasitology, etc.) which employ conventional diagnostic assays and interpretive criteria when conducting these tests. There are currently no commercially available ELISA kits for A. phagocytophilum in Canada.
• Cell culture or IHC are rarely used and not available in most of the laboratories.
• Antibodies to A. phagocytophilum might remain elevated for many months after the disease has resolved, and in some cases, high titres have been observed up to four years after the acute illness. Comparison of paired, and appropriately timed, serologic assays provides the best evidence of recent infection. Due to the persistence of elevated antibody titres, single or inappropriately timed serologic tests, in relation to clinical illness, can lead to misinterpretation of results.^{Footnote 3}