I n the United States, tickborne Rickettsia parkeri, R. philipii (Rickettsia 364D), and R. rickettsii, causative agents of Rocky Mountain spotted fever (RMSF), are well-documented human spotted fever group (SFG) rickettsioses (1). R. rickettsii is the only known cause of SFG rickettsioses in dogs (2). The extent to which other SFG Rickettsia are pathogenic in dogs is unclear; however, SFG Rickettsia seroprevalence is high among dogs in the United States and Mexico (3,4). The increased R. rickettsii seroprevalence in humans in the United States during the past decade has been attributed to SFG Rickettsia cross-reactivity (1,5). We report 3 dogs with febrile illness located in different US states. Samples from the dogs were R. rickettsii seroreactive. Identical Rickettsia DNA gene sequences were obtained from each dog's blood specimen and used to investigate Rickettsia spp. The Cases On May 15, 2018, a 10-year-old male neutered mixed breed dog (case 1) from Tennessee was examined by a veterinarian for lethargy and hyporexia. The owner reported removing a tick (species unknown) within the previous 2 weeks. On physical examination, the dog had fever (39.8°C) and possible hepatomegaly. Radiographic imaging results were unremarkable. Thrombocytopenia was the only abnormality noted on complete blood count (CBC). Serum biochemistry panel (SBP) abnormalities included hyperglobulinemia, increased serum alkaline phosphatase activity, hypoglycemia, and hyponatremia (Table 1). Results of urine dipstick and sediment examination were unremarkable. The dog's samples were R. rickettsii seroreactive and PCR positive for Rickettsia (Table 2). Clinical abnormalities resolved after treatment with doxycycline, and the dog remained healthy during the 1-year follow-up period. On May 8, 2019, a 9-year-old male neutered Boston terrier (case 2) from Illinois was examined by a veterinarian for lethargy, difficulty walking, and painful elbows. Clinical signs developed 3 days after returning from a tick-infested area in Arkansas. Abnormalities noted on physical examination included fever (40.1°C), dehydration, joint effusion, elbow pain, and shifting leg lameness. Thrombocytopenia and mild leukocytosis were the only CBC abnormalities (Table 1). SBP abnormalities included hypoalbuminemia, increased alanine amino transferase activity, alkaline phosphatase activity, hypercholesterolemia, and hypocalcemia (Table 1). Mild microalbuminuria was noted. Neutrophilic inflammation was documented by synovial fluid cytology in the right and left stifle joints, right tarsus, and left elbow joint. The left carpus contained moderate, chronic inflammation with very rare extracellular cocci; however, culture resulted in no bacterial growth. The dog experienced cardiorespiratory arrest during sedated arthrocentesis but recovered after CPR and sedative reversal. Thoracic radiographs were unremarkable. The dog's samples were R. rickettsii seroreactive and PCR-positive for Rickettsia and convalescent titers demonstrated 4-fold seroconversion (Table 2). Most clinic...
