Antibodies to Ehrlichia equi in dogs from the northeastern United States

Objective - To determine whether dogs living in tick-infested areas of the northeastern United States had been exposed to Ehrlichia equi, an etiologic agent of granulocytic ehrlichiosis. Design - Analyses of dog sera. Animals - 106 ill dogs and 12 clinically normal dogs. Procedure - Antibodies to E equi were detected by indirect fluorescent antibody staining methods and western blot analyses. Results - 10 of 106 (9.4%) sera tested from ill, privately owned dogs living in tick-infested areas of Connecticut and New York state had antibodies to E equi, a member of the E phagocytophila genogroup. Titration end points ranged from 1:80 to 1:1,280. Immunoblots revealed antibodies to proteins of E equi having molecular masses of predominantly 29, 40, 44, 105, 120, and 160 kd. There was good agreement between results of serologic testing methods, but use of the human isolate (NCH-1 strain) in western blot analyses detected 2 additional seropositive dogs found to be negative by indirect fluorescent antibody staining methods with the MRK strain. Clinical Implications - Dogs living in areas where Ixodes scapularis is abundant may be exposed to multiple pathogens, such as E equi or Borrelia burgdorferi. Although mild or subclinical infections with E equi may develop, dogs with marked leukopenia, thrombocytopenia, or anemia should be viewed as possibly having ehrlichiosis. Laboratory diagnosis should include examinations for morulae in granulocytes or monocytes in addition to serologic analyses.