Canine Granulocytic Anaplasmosis: Dx & Tx

Anaplasma phagocytophilum-a gram-negative intracellular organism previously known as Ehrlichia phagocytophila, Ehrlichia equi, and human granulocytic ehrlichiosis agent- is the causative agent of canine granulocytic anaplasmosis (CGA). CGA is difficult to diagnose, so a prospective study of 18 dogs was conducted to describe the clinical signs, laboratory abnormalities, diagnosis, treatment, and course of disease in naturally infected dogs living in areas with high populations of Ixodes ricinus ticks, which transmit the disease. Cases were diagnosed mainly between April and September (17 dogs), with only 1 case reported in November. All dogs were acutely ill, and CGA was diagnosed on the basis of a positive result on polymerase chain reaction (PCR) for A phagocytophilum with no known coinfections. Common clinical findings included lethargy, inappetence, fever, and splenomegaly. Abnormal laboratory results included thrombocytopenia, anemia, lymphopenia, hypoalbuminemia, and elevated plasma alkaline phosphatase levels. Tick infestation had been noticed by the owner in 14 dogs. Abnormal clinical findings included fever (11 dogs), pale mucous membranes (5 dogs), tense abdomen (5 dogs), diarrhea (3 dogs), vomiting (2 dogs), lameness (2 dogs), petechiae (2 dogs), epistaxis (1 dog), and melena (1 dog). Ventricular tachycardia and tachypnea were noticed in 1 dog each. Splenomegaly was detected radiologically in 17 dogs, and hepatosplenomegaly in 1 dog. All dogs were treated with doxycycline (5 mg/kg PO Q 12 H). Serology alone is not diagnostic of CGA because dogs can be seronegative and clinically ill or vice versa. Season of the year, history, clinical signs, hematology (morulae in neutrophils), clinical biochemistry, and serology can all point to a presumptive diagnosis of A phagocytophilum infection. Diagnosis of CGA should also be based on the exclusion of other infectious diseases, a positive PCR result, and clinical improvement with the administration of doxycycline.

COMMENTARY: The diagnosis of A phagocytophilum infection has, like other tick-borne diseases, become more prevalent as the population of ticks carrying this organism spreads. Annual or more frequent screening of dogs, especially after tick exposure, using the SNAP-4DX test (idexx.com) has become standard protocol in my Wisconsin practice. Even dogs being treated regularly with topical tick control agents are routinely screened because treatment failures in areas of heavy tick populations are common-including in my own dog! Dogs without clinical signs but testing positive for A phagocytophilum on the SNAP-4DX test are evaluated for complete blood count (CBC) abnormalities and are treated with doxycycline if the CBC is abnormal (eg, thrombocytopenia, anemia, lymphopenia). Dogs with clinical signs should also be evaluated by PCR testing because a negative result on the SNAP-4DX test does not rule out this disease; the patient may not have seroconverted at the time of sampling.Clinical features of canine granulocytic anaplasmosis in 18 naturally infected dogs. Kohn B, Galke D, Beelitz P, Pfister K. J VET INTERN MED 22:1289-1295, 2008.