Anaplasmosis

2023 Case Definition

CSTE Position Statement Number: 23-ID-01

Clinical Presentation

Anaplasmosis typically presents 5 to 14 days after a tick bite with a combination of nonspecific clinical symptoms such as fever, fatigue, and headache. Illness is often accompanied by laboratory abnormalities including leukopenia, thrombocytopenia, and mildly elevated liver enzymes.

Clinical Criteria

Objective clinical evidence: fever as reported by patient or healthcare provider, anemia, leukopenia, thrombocytopenia, any hepatic transaminase elevation, or elevated C-reactive protein

Subjective clinical evidence: chills/sweats, headache, myalgia, or fatigue/malaise

Laboratory Criteria

Confirmatory Laboratory Evidence:

Detection of A. phagocytophilum DNA in a clinical specimen via amplification of a specific target by polymerase chain reaction (PCR) assay, nucleic acid amplification tests (NAAT), or other molecular testing, OR
Serological evidence of a four-fold change1 in IgG-specific antibody titer to A. phagocytophilum antigen by indirect immunofluorescence assay (IFA) in paired serum samples (one taken in the first two weeks after illness onset and a second taken two to ten weeks after acute specimen collection)2, OR
Demonstration of anaplasmal antigen in a biopsy or autopsy sample by immunohistochemical methods, OR
Isolation of A. phagocytophilum from a clinical specimen in cell culture with molecular confirmation (e.g., PCR or sequencing)

Presumptive Laboratory Evidence:

Serological evidence of elevated IgG antibody reactive with A. phagocytophilum antigen by IFA at a titer ≥1:128 in a sample taken within 60 days of illness onset, OR
Microscopic identification of intracytoplasmic morulae in leukocytes in a sample taken within 60 days of illness onset.

1 A four-fold change in titer is equivalent to a change of two dilutions (e.g., 1:64 to 1:256).

2 A four-fold rise in titer should not be excluded as confirmatory laboratory criteria if the acute and convalescent specimens are collected within two weeks of one another.

Case classifications

Confirmed**:

Meets confirmatory laboratory evidence AND at least one of the objective or subjective clinical evidence criteria.

Probable**:

Meets presumptive laboratory evidence with fever as reported by patient or healthcare provider AND at least one other objective or subjective clinical evidence criterion (excluding chills/sweats), OR
Meets presumptive laboratory evidence without a reported fever but with chills/sweats AND
- at least one objective clinical evidence criterion, OR
- two other subjective clinical evidence criteria.

Suspect**:

Meets confirmatory or presumptive laboratory evidence with no or insufficient clinical information to classify as a confirmed or probable case (e.g.,a laboratory report only).

**Patients should not be classified as cases for both anaplasmosis and ehrlichiosis based on serologic evidence alone.

Criteria to Distinguish a New Case of Anaplasmosis from Reports or Notifications which Should Not be Enumerated as a New Case for Surveillance

A person previously reported as a probable or confirmed case-patient may be counted as a new case-patient when there is an episode of new clinically compatible illness with confirmatory laboratory evidence.

Table VII.A. Classification Table: Criteria for Defining a Case of Anaplasmosis

Criterion	Confirmed	Probable			Suspect
Clinical Evidence
Objective Clinical Evidence
Fever as reported by patient or healthcare provider	O	N
Anemia	O	O	O
Leukopenia	O	O	O
Thrombocytopenia	O	O	O
Hepatic transaminase elevation	O	O	O
Elevated C-reactive protein	O	O	O
Subjective Clinical Evidence
Chills/sweats	O		N	N
Headache	O	O
Myalgia	O	O
Fatigue or malaise	O	O
At least two of the following Subjective Clinical Evidence criteria: Headache Myalgia Fatigue or malaise				N
No or insufficient clinical information to classify as a confirmed or probable case					N
Laboratory Criteria
Detection of A. phagocytophilum DNA in a clinical specimen via amplification of a specific target by polymerase chain reaction (PCR) assay or nucleic acid amplification test (NAAT), or other molecular testing	O				O
Serological evidence of a four-fold change1 in IgG-specific antibody titer to A. phagocytophilum antigen by indirect immunofluorescence assay (IFA) in paired serum samples (one taken in the first two weeks after illness onset and a second taken two to ten weeks after acute specimen collection)2	O				O
Demonstration of anaplasmal antigen in a biopsy or autopsy sample by immunohistochemical methods	O				O
Isolation of A. phagocytophilum from a clinical specimen in cell culture with molecular confirmation (e.g., PCR or sequencing)	O				O
Serological evidence of elevated IgG-specific antibody reactive with A. phagocytophilum antigen by IFA at a titer ≥1:128 in a sample taken within 60 days of illness onset		O	O	O	O
Microscopic identification of intracytoplasmic morulae in leukocytes in a sample taken within 60 days of illness onset		O	O	O	O
Epidemiological Link Criteria
N/A

Notes:
N = All “N” criteria in the same column are NECESSARY to classify a case.
O = At least one of these “O” (ONE OR MORE) criteria in each category (categories=clinical evidence, laboratory
evidence, and epidemiologic evidence) in the same column—in conjunction with all “N” criteria in the same
column—is required to classify a case.
1 A four-fold change in titer is equivalent to a change of two dilutions (e.g., 1:64 to 1:256).
2A four-fold rise in titer should not be excluded as confirmatory laboratory criteria if the acute and convalescent specimens are collected within two weeks of one another.

Date Posted: January 9, 2025