Anaplasma phagocytophilum infection of domestic cats: 16 cases from the northeastern USA

Anaplasma phagocytophilum is an Ixodes species-transmitted rickettsial organism that is occasionally associated with clinical abnormalities in humans, ruminants, horses, dogs and cats. While serological evidence of A phagocytophilum exposure is common in cats in Ixodes species endemic areas, reports of clinical feline anaplasmosis are few. The objective of this study was to describe the clinical and laboratory abnormalities and treatment responses in 16 cats with A phagocytophilum DNA amplified from blood. Commercial laboratory electronic records were searched to find cats that had A phagocytophilum DNA amplified from their blood. Once cases were identified, the primary care veterinarian was interviewed and the medical records were reviewed. The cats ranged in age from 4 months to 13 years (mean 4.1 years, median 2 years). All cats lived in Ixodes scapularis endemic areas and had potential for exposure. All cats were lethargic, 15 (94%) had elevated body temperature (>39.4°C) and 14 were anorexic on initial physical examination. Other less common clinical findings included hepatosplenomegaly, ataxia, conjunctivitis and elevation of the nictitating membranes. Blood from 11 cats was evaluated by complete blood cell count; abnormalities included lymphopenia in seven (64%) cats, thrombocytopenia in seven (64%), morulae in neutrophils of three (27%), neutropenia in three (27%) and leukopenia in two (18%). Treatment responses were reported for 14 cats, and the clinical abnormalities in these cats resolved when doxycycline was administered. This is the first published report describing A phagocytophilum morulae in neutrophils of naturally infected North American cats with infection confirmed by PCR. A phagocytophilum infection should be considered in cats evaluated for lethargy, anorexia and fever living in Ixodes species endemic areas.