Geographical variation of seropositivity to Ehrlichia risticii (equine monocytic ehrlichiosis) of horses in New York state.

This study examines the geographical variation of horses’ seropositivity to Ehrlichia risticii, a bacteria causing equine monocytic ehrlichiosis (EME), in New York state. The results showed varying EME-disease rates across counties, with the highest risk regions located primarily along river valleys.

Study Methodology and Findings

• The study was based on a total of 2,579 serum samples collected from horses in New York state during 1985-1986, to test for seropositivity to Ehrlichia risticii, the bacterium that causes EME.
• The researchers employed the indirect fluorescent antibody technique to identify the presence of E. risticii antibodies in the horse serum samples.
• Using a statistical method called cluster analysis, researchers grouped the counties into four different categories based on their EME-disease rates, represented by the proportion of sampled horses that tested seropositive. The four groupings were high, medium, medium-low, and low, with corresponding seropositive rates of 86%, 66%, 47%, and 6%, respectively.
• The logistic regression statistical technique was used to investigate the possibility of other EME-risk factors influencing the geographic distribution of E. risticii seropositivity.

Results Analysis and Interpretation

• The results of the logistic regression analysis showed that known risk factors for EME, including age, breed, sex, and month of sample submission, did not have a significant impact on the observed geographical variation of the disease in New York state.
• Additionally, the density of horse populations across counties and the presence of a major equine racetrack were not found to be correlated with the geographic variation in E. risticii seropositivity.
• The high risk regions for EME were predominantly located along river valleys, suggesting that there are other contributing factors that need to be further investigated to fully understand the geographical variation of the disease. As of the time of the study, however, specific reasons explaining why EME activity varies geographically remained speculative.