Benefit-cost analysis of vaccination of horses as a strategy to control equine monocytic ehrlichiosis

Objective - To determine whether horses in New York should be vaccinated against equine monocytic ehrlichiosis (EME). Design - Decision-tree analyses of data from a cross-sectional study and a case-control study. Sample Population - Horses in New York. Procedure - Annual expected monetary loss per horse attributable to EME was calculated for vaccinated and nonvaccinated horses in New York. Because risk of being seropositive was dependent on county in which the horse was located, farm elevation, and use of each horse, decision-tree analyses were stratified by these factors. Results - Annual expected monetary loss per horse attributable to EME for horses vaccinated by veterinarians ranged from $21 to $21.83/horse/y; for horses vaccinated by owners ranged from $10 to $10.83/horse/y; and for nonvaccinated horses ranged from $0 to $4.03/horse/y. Assuming 78% of vaccinated horses were protected and mean losses associated with EME included costs for horses that died, annual incidence density at which expected monetary loss for vaccinated horses was equal to that for nonvaccinated horses was 12 cases/1,000 horses/y and 25 cases/1,000 horses/y for horses vaccinated by owners or by veterinarians, respectively. Clinical Implications - Annual vaccination minimizes monetary losses attributable to EME only when the annual incidence density exceeds 12 to 25 cases/1,000 horses/y. In New York, expected monetary losses are minimized when horses are not vaccinated because of the low annual incidence density in most regions.