Etiology and Epidemiology of Anaplasmosis in Dogs
Canine granulocytic anaplasmosis is caused by Anaplasma phagocytophilum. The main vectors for this pathogen are the black-legged tick (also called the deer tick, Ixodes scapularis) and the western black-legged tick (Ixodes pacificus).
Although anaplasmosis has a worldwide distribution, there is a higher prevalence in the northern Midwest and Northeast of the US.
Canine cyclic thrombocytopenia is reported worldwide; it is caused by A platys and is transmitted by the brown dog tick (Rhipicephalus sanguineus).
As with all other bloodborne infections, anaplasmosis can also be transmitted via blood transfusions.
Clinical Findings of Anaplasmosis in Dogs
Infection with either A phagocytophilum or A platys is usually subclinical. When present, clinical signs most commonly include fever, lymphadenomegaly, splenomegaly, and bleeding tendencies (petechiae, ecchymoses). Patients infected with A phagocytophilum can also develop lameness.
Coinfection with other vector-borne disease-causing organisms should be considered in seropositive dogs that show more severe clinical signs.
The most common abnormality noted on CBCs of patients with anaplasmosis is thrombocytopenia, which occurs 1–2 weeks after infection. In cases of cyclic thrombocytopenia, the platelet counts wax and wane with a cycle of 1–2 weeks in the absence of treatment.
Review of blood smears can reveal the presence of morulas in neutrophils (A phagocytophilum) or platelets (A platys). If intraneutrophilic morulas are found, equal consideration should be given to infection with Ehrlichia ewingii.
Lesions of Anaplasmosis
Anaplasmosis lesions in dogs are nonspecific. Most commonly, they include lymphadenomegaly and splenomegaly secondary to lymphoid hyperplasia and erythrophagocytosis.
Diagnosis of Anaplasmosis in Dogs
Clinical signs
Cytological evaluation
Serological testing
PCR assay
Serological tests based on point-of care enzyme immunoassays and immunofluorescence assays (IFAs)are available for anaplasmosis. Because anaplasmosis is usually an acute disease, serological testing within 1 week after infection can yield negative results, underlining the need for paired titers performed 2–4 weeks later.
Serological testing does not enable differentiation between Anaplasma spp. Circulating antibodies can persist for many months after infection.
PCR assay of blood samples or organ aspirates can be very useful for diagnosing anaplasmosis soon after infection, because genetic material can typically be detected within the first week. In addition, PCR assay is the only method that enables speciation of the infecting organism.
Treatment of Anaplasmosis in Dogs
Doxycycline
The treatment of choice for infection by either Anaplasma sp is doxycycline (5 mg/kg, PO or IV, every 12 hours; or 10 mg/kg, PO or IV, every 24 hours). Although the optimal treatment course is unknown, experimental infection data suggest that the drug should be administered for 28 days (1).
If doxycycline is not available, minocycline (5–10 mg/kg, PO, every 12 hours for 28 days) can be substituted.
Clinical signs of anaplasmosis resolve rapidly, in most cases within 1 week after antimicrobial therapy is initiated.
Because Anaplasma infections typically result in mild clinical signs, supportive care is rarely required.
Prevention of Anaplasmosis in Dogs
Anaplasmosis can be prevented by controlling tick exposure of dogs. Dogs in areas infested by ticks should be restricted from walking off-trail, in certain grassy areas, and where there might be ticks in leaf litter. Dogs should be examined after walks, and any ticks should be removed carefully (to prevent human exposure). It is unrealistic, however, to expect that owners will find all ticks in the coat of a dog.
Multiple topical medications are available to prevent tick bites; these should be used in compliance with labeling before bringing dogs into areas infested with ticks. Formulations containing fipronil, isoxazoline, pyrethroid, or amitraz are approved for this indication and are the most effective against ticks when used as labeled.
Transfusion-associated Anaplasma transmission can be decreased by using seronegative screened blood donors; however, new donors with a negative screen cannot be presumed free of infection for several weeks, because they could be incubating infection.
Zoonotic Risk of Anaplasmosis in Dogs
Anaplasmosis is a zoonotic disease, and both A phagocytophilum and A platys infections have been reported in humans. However, direct transmission does not occur, because the presence of a tick vector is required.
Key Points
Clinical signs related to anaplasmosis are usually mild; they are similar to signs of other vector-borne diseases.
Infection with A phagocytophilum is cytologically indistinguishable from infection with E ewingii.
Minimizing exposure to ticks is crucial in managing exposure risk.
For More Information
Canine anaplasmosis prevalence map. Companion Animal Parasite Council.
Ehrlichia spp. and Anaplasma spp. Companion Animal Parasite Council.
Sykes JE. Canine and Feline Infectious Diseases. Elsevier/Saunders; 2014.
Also see pet owner content regarding ehrlichiosis and related infections in dogs.
References
Yancey CB, Diniz PPVP, Breitschwerdt EB, Hegarty BC,Wiesen C, Qurollo BA. Doxycycline treatment efficacy in dogs with naturally occurring Anaplasma phagocytophilum infection. J Small Anim Pract. 2018;59(5):286-293. doi:10.1111/jsap.12799
