COMPARISON OF PCR AND CULTURE TO THE INDIRECT FLUORESCENT-ANTIBODY TEST FOR DIAGNOSIS OF POTOMAC HORSE FEVER

Potomac horse fever is an acute systemic equine disease caused by Ehrl ichia risticii. Currently, serologic methods are widely used to diagno se this disease, However, serologic methods cannot determine whether t he horse is presently infected or has been exposed to ehrlichial antig ens in the past. The purpose of the present study was to compare the s ensitivities of the nested PCR and cell culture with that of the indir ect fluorescent-antibody (IFA) test for the diagnosis of Potomac horse fever, Blood and fecal specimens serially collected from a pony exper imentally infected with E. risticii Maryland, blood specimens serially collected from mice inoculated with E, risticii Ohio 380, and blood a nd/or fecal specimens collected from 27 horses which had clinical sign s compatible with Potomac horse fever were examined, These horses resi ded in Kentucky, Indiana, Pennsylvania, and Vermont, The IFA test tite r became positive after 6 days postinoculation (p.i.) for the pony, A culture of the blood of the pony was positive for E. E. risticii start ing on day 1 and was positive through day 28 p,i, By the nested PCR, E , risticii was detectable in the blood and feces of the pony starting on day 1 and was detectable through day 32 p,i, E, risticii was detect ed in the blood of subclinically infected mice by the nested PCR, Twen ty-two clinical specimens were seropositive for E, risticii by the IFA test, with titers ranging from 1:20 to 1:1,280, E. risticii was cultu red from 95% (20 of 21) of seropositive clinical blood specimens, E. r isticii was detected in the blood by PCR in 81% (17 of 20) of the cult ure-positive clinical specimens, The study indicated that the nested P CR is as sensitive as culture for detecting infection with E, risticii .