Abstract: Background—Testing and empiric treatment for vector-borne infections (VBIs) are commonly pursued in dogs with immune-mediated diseases despite lack of information on regional prevalence. Hypothesis/Objectives—Determine test positivity rate and associated clinicopathologic features of VBIs in dogs with immune-mediated hemolytic anemia (IMHA), immune thrombocytopenia (ITP), and immune-mediated polyarthritis (IMPA) at a teaching hospital in Northern California (2017-2022). Animals—Client-owned dogs meeting inclusion criteria for IMHA (n=94), ITP (n=101), or IMPA (n=100) that underwent testing for VBIs (SNAP 4Dx Plus [IDEXX] with or without PCR or quantitative serology). Methods—Retrospective study. VBI test positivity rate was determined. Clinicopathologic features in test-positive and test-negative dogs were compared. Results—Six (2%) of 295 dogs had VBIs; all responded to specific antimicrobial treatment, 5 without immunosuppression. IMHA-group infections were Ehrlichia canis (4Dx+/PCR+)(1) and concurrent E. canis (4Dx+/PCR+) and Anaplasma platys (4Dx-/PCR+)(1). ITP-group infections were Anaplasma phagocytophilum (4Dx-/PCR+/morulae+)(1) and Babesia conradae (PCR+/piroplasm+)(1). Two IMPA dogs were diagnosed with Lyme polyarthritis (4Dx+/high QuantC6 [125 and 256 U/mL]). Serum globulin concentration was higher in VBI dogs than in test-negative dogs (P = 0.02); 5/6 dogs with VBIs were hyperglobulinemic versus 33% of test-negative dogs. Four dogs with VBIs had travel histories to endemic regions. Conclusions and Clinical Importance—VBIs in this population of dogs were rare. For this population, testing for VBIs should be focused on those with hyperglobulinemia and/or relevant travel history; empiric antimicrobial therapy appears unwarranted. Given their acute nature, serologic testing may have limited utility for diagnosis of rickettsioses when compared with PCR.
