Serosurvey of horses with evidence of equine monocytic ehrlichiosis

Abstract

Summary In August 1986, an extensive serosurvey for prevalence of IgG and IgM antibodies against Ehrlichia risticii, the causative agent of equine monocytic ehrlichiosis (eme), was performed at 2 Ohio racetracks, River Downs (rd) and Beulah Park (bp). Of 840 horses at rd and 574 at bp, 13 and 20%, respectively, were IgG antibody-positive (by indirect fluorescent antibody test results), with antibody titer ranging from 1:20 to 1:10,240. The titer observed at highest frequency at both racetracks was 1:80. A higher proportion of horses was ill at rd (operating during the summer months) than at bp (winter track). Of ill horses, 41% (24/58) at rd and 58% (11/19) at bp were seropositive. At rd, 70% (589/840) of all horses and 95% (102/107) of IgG seropositive horses had been stabled only at rd during the month prior to testing. Analysis of these sera by use of an elisa to detect IgM antibody against E risticii antigen indicated that at rd, 42% (57/137) of the seropositive horses were IgM seropositive. At bp, 17% (20/120) of seropositive horses were IgM seropositive. The larger number of IgM seropositive horses at rd indicates that more horses were recently infected at rd than at bp (P = 0.0001). Therefore, at least half the seropositive horses at rd seemed to have acquired the infection at rd. These serosurvey data also indicate that at bp and rd, 78% (85/109) and 91% (111/122) of IgG seropositive horses, respectively, had subclinical infection. At ≤ 1:40 titer, there was no difference in seropositive rates between healthy and ill horses. However, the higher the titer, the greater the difference in seropositive rates between healthy and ill horses. Thus, the higher the titer, the more reliable the serodiagnosis of eme seems to be. In contrast, sera submitted from 285 horses in Ohio for serologic testing for E risticii had a higher (63%) positivity rate than sera from horses of the racetrack serosurvey. Of 120 horses with clinical signs compatible with eme, 69% were seropositive. Typical clinical signs of eme in seropositive horses were those of enteric disease (72%; 60/83), with laminitis being a secondary sign (12%; 10/83). Results indicate that seroprevalence is much greater than prevalence of clinical disease. Combination of IgG and IgM testing with horse movement history should help to identify the location of actual infection.