Clinical and laboratory findings
Most dogs with naturally occurring infection have mild clinical disease, although more severe clinical signs of fever, lethargy,
pale mucous membranes, petechial hemorrhages, epistaxis, and lymphadenopathy can occur. As with other tick-transmitted diseases,
coinfection of A. platys with other infectious agents such as Ehrlichia, Bartonella, or Borrelia species can result in more severe clinical manifestations.
Experimentally, clinical signs appeared between eight and 14 days after inoculation.18 The organism appears in circulating platelets, resulting in a severe thrombocytopenia, typically < 20,000/µl. Platelet counts
usually remain below 20,000/µl for only one or two days. The organisms disappear rapidly from the peripheral blood, and platelet
counts rise quickly, reaching a normal value in three or four days. Subsequently, organisms will reappear at one- to two-week
intervals, and the process repeats, resulting in a cyclic thrombocytopenia. The percentage of parasitized platelets and the
severity of the thrombocytopenic episodes decrease with subsequent cycles of parasitemia.
Anaplasma platys infection can be diagnosed by light microscopic identification of morulae in circulating platelets. An IFA test is also commercially
available, and serologic testing has indicated that seropositivity in dogs can be high in the Southeast and Midwest.23 There appears to be some cross-reactivity between A. platys and the A. phagocytophilum with the SNAP 4Dx assay. With this assay, many animals infected with A. platys will have positive test results for A. phagocytophilum. The organisms are closely related and evidently share epitopes used in this ELISA. PCR testing for A. platys, which can distinguish between the two organisms, is typically done in experimental laboratories for research purposes, but
a PCR test is also commercially available (e.g. Zoologix).
Treatment and prognosis
Doxycycline at the dosage described above for A. phagocytophilum is apparently effective in treating A. platys infection.12 However, animals with circulating A. platys organisms that have moderate to severe clinical disease or that do not rapidly respond to doxycycline therapy should be tested
for other tick-borne diseases.
Anaplasmosis causes one of two clinical syndromes: Fever, lethargy, polyarthritis, and, less often, gastrointestinal, respiratory,
or neurologic signs, which are associated with A. phagocytophilum infection. Signs due to thrombocytopenia are associated with A. platys infection.
Since the two organisms share the same tick vector, the diseases have a similar geographic distribution. Anaplasma phagocytophilum infection is an emerging tick-borne infection in dogs, endemic to several areas of the United States and expanding in distribution.
The organism typically causes acute clinical disease similar to Lyme disease, resulting in a suppurative polyarthritis. In
addition to causing clinical disease, A. phagocytophilum may persistently infect dogs, causing a subclinical carrier state associated with positive serologic test results in a healthy
dog. These dogs should be monitored for evidence of clinical disease because there may be potential for future clinical manifestations,
particularly if they become coinfected with other tick-borne agents or become immunosuppressed. Since animals can have subclinical
infections, it is important to recognize that clinically ill animals may have positive test results for A. phagocytophilum but be experiencing the clinical manifestations of an unrelated disease process. Therefore, seropositivity alone does not
indicate a causal relationship to the clinical disease. Clinical signs must be consistent with those found in animals with
anaplasmosis and should rapidly resolve in most cases shortly after doxycycline therapy is initiated.
Dogs infected with A. platys experience a cyclic thrombocytopenia. Clinical disease is often mild in naturally infected dogs, but some animals may have
clinical evidence of bleeding (epistaxis or petechiation), particularly during the initial bacteremic phase when platelet
counts may be reduced to < 20,000 cells/µl. Platelet counts are less dramatically affected in subsequent cycles of bacteremia
and clinical signs become milder or unapparent. As with A. phagocytophilum infection, animals with clinical disease resulting from A. platys infection respond rapidly to treatment with doxycycline.
Finally, A. phagocytophilum is a zoonotic pathogen. However, direct transmission from animals to people or animal to animal is highly unlikely and, to
our knowledge, has never been documented. Even so, alert owners of pets with known exposure to A. phagocytophilum to potential human exposure from infected ticks in the environment.
Editors' note: IDEXX Laboratories has sponsored presentations by both Dr. Alleman and Dr. Wamsley at various continuing education symposia.
A. Rick Alleman, DVM, PhD, DABVP, DACVP
Heather L. Wamsley, DVM, DACVP
Department of Physiological Sciences
College of Veterinary Medicine
University of Florida
Gainesville, FL 32608