Page 1499 - Small Animal Internal Medicine, 6th Edition

CHAPTER 95 Polysystemic Rickettsial Diseases 1471

an assay is being used that detects antibodies against both A. infections by serology and PCR assay; the American College
phagocytophilum and A. platys, antibodies that are detected of Veterinary Internal Medicine (ACVIM) optimally rec-
VetBooks.ir in a dog in a region that is not endemic for Ixodes spp. are ommends using only dogs that are negative in both tests
(Wardrop et al., 2016). However, if PCR assay cannot be
likely against A. platys. Polymerase chain reaction (PCR)
assays performed on blood collected in ethylenediamine tet-
only seronegative dogs as donors.
raacetic acid (EDTA) can be used to confirm infection and performed, the minimal standard recommended was to use
can be used to differentiate A. phagocytophilum infection
from other infections. In one study of 19 dogs infected by A.
phagocytophilum after infestation by wild-caught I. scapu- FELINE GRANULOCYTOTROPIC
laris, PCR results were positive before detection of antibod- ANAPLASMOSIS
ies in any of the three antibody assays tested (Moroff et al.,
2014). Etiology and Epidemiology
Most dogs infected by A. phagocytophilum have subclini- As in dogs, A. phagocytophilum is transmitted by Ixodes
cal infections and do not need to be treated. Most dogs ticks, so infections of cats are likely to be most common
with granulocytic anaplasmosis only have an acute phase, in these areas. Cats can be infected by A. phagocytophilum
exposure rates in endemic areas are high, and the disease infection after being infested with wild-caught I. scapularis
syndromes associated with infection have multiple other from Rhode Island (Lappin et al., 2015). Although rodents
causes. Thus antibody test results and PCR assay results are commonly infected with A. phagocytophilum, whether
alone cannot be used to prove clinical disease associated ingestion or direct contact with rodents plays a role in A.
with A. phagocytophilum infection. For example, although A. phagocytophilum infection of cats is currently unknown.
phagocytophilum is known to cause thrombocytopenia and DNA of A. phagocytophilum can be amplified from ticks
polyarthritis in some dogs, one study failed to show an asso- collected from cats (Duplan et al., 2018) and has been ampli-
ciation between A. phagocytophilum PCR assay or serologic fied from blood in naturally exposed cats with or without
test results in dogs with polyarthritis or thrombocytopenia clinical illness in multiple countries (Adaszek et al., 2013;
(Foley et al., 2007). Veterinarians should recognize that a Bjoersdorff et al., 1999; Galemore et al., 2018; Hegarty et al.,
clinically ill dog positive for A. phagocytophilum antibodies 2015; Lappin et al., 2004; Savidge et al., 2016). Cats living in
or DNA but is not responding to appropriate therapy could endemic areas are commonly seropositive (Galemore et al.,
have another cause of the clinical syndrome, and the diag- 2018; Hoyt et al., 2018). Although the pathogenesis of disease
nostic workup should be continued. associated with A. phagocytophilum in cats is unknown, it is
likely similar to that for dogs and human beings.
Treatment
Doxycycline administered at 5 mg/kg orally (PO), q12h or Clinical Features
at 10 mg/kg PO q24h for at least 10 days is recommended Fever, anorexia, and lethargy are the most common clinical
by most clinicians for the treatment of canine granulocyto- abnormalities in cats with granulocytic anaplasmosis. Ticks
tropic anaplasmosis. Most dogs respond to therapy within may or may not currently infest infected cats. Overall, clini-
hours to days of initiating therapy. In one group of experi- cal signs associated with A. phagocytophilum infection in cats
mentally infected dogs, administration of doxycycline did are mild and resolve quickly after initiating doxycycline or
not result in decreasing antibody levels and so following minocycline therapy. Subclinical infections are the most
antibody titers as an indication of response to therapy is common manifestation, like in dogs. In one study of feral
likely not of clinical benefit (Chandrashekar et al., 2017). cats in Massachusetts, there was no association being posi-
After 28 days of doxycycline administration, there was no tive for A. phagocytophilum infection and anemia or throm-
evidence of persistent infection in one study (Yancey et al., bocytopenia in cats (Galemore et al., 2018).
2018).
Diagnosis
Zoonotic Aspects and Prevention Approximately 50% of cats with proven clinical infections
Anaplasma phagocytophilum infects people and dogs, and induced by A. phagocytophilum have a mild thrombocytope-
so is zoonotic. Human infections are most likely acquired nia (66,000-118,000/µL). Neutrophilia with a left shift, lym-
by direct tick transmission, but handling infected blood phocytosis, lymphopenia, and hyperglobulinemia have been
and carcasses can also lead to infection. Care should also detected in some cats. Morulae (see Fig. 95.1) can be detected
be taken when handling ticks. Infection can be avoided by in experimentally and naturally exposed cats (Lappin et al.,
tick control or prophylactic use of tetracyclines when vis- 2015; Savidge et al., 2016). Clinical abnormalities resolve
iting endemic areas. In at least two studies, use of acari- quickly after doxycycline treatment is initiated. Biochemical
cides prevented transmission of A. phagocytophilum to dogs and urinalysis abnormalities are uncommon. Some commer-
(Honsberger et al., 2016). Dogs appear to be susceptible to cial laboratories offer serologic testing. Infected cats are neg-
reinfection, so tick control should be maintained at all times ative for antibodies against E. canis, so A. phagocytophilum
in endemic areas. Dogs used for blood donors that reside in IFA slides should be used. Approximately 30% of cats with
endemic areas should be screened for A. phagocytophilum proven clinical infections induced by A. phagocytophilum are