PVMA Keystone Veterinarian Fall 2021 - 23

Diagnosing Acute
Anaplasmosis in Dogs
By Holly Brown, DVM, PhD, DACVP
Anaplasmosis is a tickborne disease
caused by the bacteria Anaplasma
phagocytophilum. Here in Pennsylvania,
infection is transmitted by Ixodes
scapularis (the Eastern black-legged tick
or deer tick) and transmission can occur
in less than 24 hours of tick attachment.
Clinical Presentation
The exact pathogenesis of disease is still
unknown, and it is unclear why some
dogs develop clinical signs of disease and
others do not. Most commonly, dogs
experience subclinical infection followed
by clearance of the organism. When
sick, dogs with anaplasmosis present
with acute onset of symptoms, generally
developing 1-2 weeks after infection.
Non-specific signs of fever, lethargy, and
inappetence are common, as well as
stiffness and lameness consistent with
musculoskeletal pain. Vomiting, diarrhea,
difficulty breathing, cough, enlarged
lymph nodes, and neurologic signs (i.e.,
seizures, ataxia) have also been reported.
Diagnostic Testing
In dogs presenting with clinical signs
consistent with acute anaplasmosis,
bloodwork commonly reveals a low
platelet count along with a superimposed
glucocorticoid (or " stress " ) response
reflected as decreased lymphocytes
and eosinophils. Practices using IDEXX's
reference lab for their complete blood
counts (CBCs) or the in-clinic ProCyte
Dx hematology analyzer will also
commonly note decreased reticulocyte
hemoglobin (RETIC-HGB) as a reflection
of iron sequestration in the face of
intense inflammatory/infectious disease.
Biochemistry and urinalysis abnormalities
are mild and nonspecific.
There is frequent confusion about rapid
antibody testing commonly used in
practice to screen for anaplasmosis.
IDEXX's SNAP 4Dx Plus test, Zoetis's
VETSCAN Anaplasma Rapid test or FLeX4
Rapid Test, and Antech's AccuPlex®4 all
detect antibodies to A. phagocytophilum.
However, the antibody assay results are
almost invariably and falsely negative
in cases of acute anaplasmosis due
to insufficient time for the patient to
mount a detectable antibody response.
In endemic areas, antibodies are present
in up to 50% of tested dogs and their
antibody levels may be detectable for
several months or even years. As such,
Anaplasma-positive rapid antibody results
are much more likely to reflect previous
exposure than acute anaplasmosis, and
other diagnostics should be performed
to identify the underlying cause of a
patient's illness.
Figure 2. Feathered edge of a stained
canine blood film with numerous
Anaplasma phagocytophilum morulae
(arrows) within neutrophils.
In Summary
The diagnosis of anaplasmosis in an
acutely ill dog can be challenging due
to the lack of positive rapid antibody
testing that veterinarians are used
to seeing in cases of Lyme disease.
Diagnosis of anaplasmosis is supported
by CBC changes that include a verified
thrombocytopenia and, commonly, a
glucocorticoid/stress response with low
RETIC-HGB. A. phagocytophilum infection
can be confirmed by stained blood film
review and the identification of morulae
within neutrophils.
Figure 1. Stained canine blood film
with two Anaplasma phagocytophilum
morulae (arrows) within a neutrophil.
A more reliable diagnosis of acute
anaplasmosis is made by the detection
of A. phagocytophilum morulae within
neutrophils on stained blood film review
(Figure 1). As a tip for those looking, a
higher concentration of infected cells
is commonly noted near the feathered
edge of the smear (Figure 2). Also notable
will be significant and often atypical
lymphocyte reactivity.
About the Author: Holly Brown, DVM,
PhD, DACVP, is a Board-certified clinical
pathologist with a unique position as
an on-staff pathologist at VCA Metzger
Animal Hospital, a 12-doctor general,
referral, and emergency hospital in State
College, PA. There, she contributes her
expertise patient-side, allowing for in-time
diagnostic testing to improve patient care.
Dr. Brown satisfies her love for teaching
by providing continuing education to
practitioners and technicians and serves
as a consultant for several companies in
the veterinary diagnostics industry.
Pennsylvania Veterinary Medical Association | 23
PVMA Keystone Veterinarian Fall 2021

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