Cross-sectional evaluation of environmental, host, and management factors associated with risk of seropositivity to Ehrlichia risticii in horses of New York state.

OBJECTIVES

To locate counties within New York state with a high seroprevalence among the equine population, to determine host, management, and environmental factors that were associated with seropositivity to Ehrlichia risticii, and to determine evidence for arthropod- or helminth-mediated transmission of E risticii to horses.

DESIGN

Cross-sectional study.

SAMPLE POPULATION

A random sample of 3,000 of the 39,000 equine operations in New York state was selected, and 2,587 horses from 511 operations were tested.

PROCEDURE

Blood samples were collected from horses and tested for seropositivity, using the indirect fluorescent antibody technique. Data on each horse and each farm's management were obtained by personal interview. The significance of each factor on the risk of seropositivity was evaluated, using mixed-effect logistic regression.

RESULTS

The seroprevalence among E risticii-nonvaccinated horses was 7.3%. The county-specific seroprevalence ranged from 0 to 27%, with higher-risk counties located at low elevation. Farms at higher risk for having seropositive horses were located predominately at low elevation with no bodies of water nearby. Risk of seropositivity was associated with time spent in a stall or run-in shed, with frequency of application of fly spray, and, depending on duration of residency at the farm, with frequency of deworming with benzimidazole and pyrantel. Standardbreds were 2 to 3 times more likely to have been exposed, compared with Thoroughbreds. Depending on duration of residency at the farm, male and middle-age horses were at higher risk. Up to 32% of the variance for a horse to test seropositive for E risticii on the logit scale was attributable to farm-level random effects, but the nested social group random effect was not significant.

CONCLUSIONS

Arthropods and helminths may have a role in the transmission of this disease. Several management factors may directly or indirectly modify the risk of exposure to E risticii, allowing for the possibility of additional control measures besides traditional vaccination strategies.