Evaluation of vaccination of horses as a strategy to control equine monocytic ehrlichiosis

Objective: To determine whether preferentially vaccinated horses were at risk for exposure to Ehrlichia risticii, whether horses with equine monocytic ehrlichiosis (EME) were likely to have been nonvaccinated, and whether clinical severity and financial costs associated with care and treatment of EME were less for vaccinated horses with EME than for nonvaccinated horses with EME. Design: Cross-sectional and case-control studies. Procedure: Information on usage of E. risticii bacterins to control EME was collected for 2,587 horses located on 511 farms throughout New York. Each horse was tested for serum antibodies directed against E. risticii. Data on efficacy of vaccination to reduce the prevalence and clinical severity of EME and monetary losses associated with EME were collected from 68 horses with EME and 132 clinically normal horses. Results: A correlation was not detected between the county seropositive proportion and the proportion of horses vaccinated against EME. Among horses diagnosed for EME, median date of diagnosis was not delayed for vaccinated horses. Mean cost per case was not significantly different for nonvaccinated horses, compared with that for vaccinated horses ($1,082 and $1,001, respectively). Vaccination was not associated with a reduction in prevalence or in severity of EME-related clinical signs. Clinical implications: Administering killed E. risticii bacterin once a year to control EME in New York appears to have limited success. Among horses in which EME was diagnosed, severity of illness and financial costs attributable to EME were indistinguishable for vaccinated and nonvaccinated horses.