Serologic testing of horses for granulocytic ehrlichiosis, using indirect fluorescent antibody staining and immunoblot analysis.

OBJECTIVE: To diagnose granulocytic ehrlichiosis in horses, compare results of indirect fluorescent antibody (IFA) staining procedures with those of immunoblot analysis, and compare serologic test findings with polymerase chain reaction (PCR) results. ANIMALS: 69 horses with high rectal temperatures (> or = 39 C) and lethargy, anorexia, or limb edema. PROCEDURE: 43 convalescent serum samples obtained from 38 horses 2 to 18 weeks after onset of illness were analyzed by use of immunoblot procedures and IFA staining methods, using the NCH-1 or BDS ehrlichial strains. Blood samples from 69 acutely ill horses were tested by PCR to detect ehrlichial DNA. RESULTS: Antibodies to Erlichial equi were detected in serum samples obtained during all seasons; seropositivity rates ranged from 50 to 93%. In IFA assays using the BDS or NCH-1 strain, seropositivity rates were 70 and 79%, respectively, whereas in immunoblot analyses using the NCH-1 strain, a seropositivity value of 79% was recorded. By immunoblot analysis, all serum samples of all seropositive horses were reactive to a protein having molecular mass of about 44 kd. Blood samples from 29 of 69 (42%) acutely ill horses contained ehrlichial DNA. CONCLUSION AND CLINICAL RELEVANCE: Results of the various serologic testing procedures were in close agreement with each other. All serologic testing methods are suitable for laboratory diagnosis of equine granulocytic ehrlichiosis.