Improved definition of risk factors for equine colic is necessary to develop effective interventions to reduce colic incidence. A one-year prospective study was conducted to estimate colic incidence and to identify risk factors. Farms with greater than 20 horses were randomly selected from two adjacent counties of Virginia and Maryland. Management information was recorded by questionnaire for 31 farms with 1427 horses. Owners kept calendars to record occurrence of specified events. Colic was reported by the owner when a horse exhibited signs of abdominal pain.

The incidence of colic was 10.6 colic cases per 100 horse-years, based on 104 cases per 983.5 horse-years. Twenty-five deaths occurred from all causes, the proportional mortality rate of colic was 7/25 (28%).

Risk factors were analyzed by logistic regression at the farm-level and the horse-level with farm as a random effects variable. No farm-level variables were significant. Significant horse variables were: age 2-10 years, odds ratio (OR)=2.8 (95% confidence interval, 1.2-6.5); previous colic, OR=3.6(1.9-6.8); changes in concentrate feeding during the year, OR=3.6(1.6-5.4); more than one change in hay feeding during the year, OR=2.1(1.2-3.8); feeding high levels of concentrate (>2.5 kg/day dry matter, OR=4.8(1.4-16), >5 kg/day dry matter, OR=6.3(1.8-22)); and vaccination with monocytic ehrlichiosis vaccine during the study, OR=2.0(1.8-22). Feeding whole grain with or without other concentrates had less risk than diets without whole grain included. Variables related to concentrate feeding frequency or concentrate type could be substituted for the concentrate level variable.

A nested analysis examined risk for the time period following an event. The odds ratio was determined for the proportion of cases with an event within 14 days prior to the colic-date, relative to the proportion of horses without colic with an event within 14 days of a date chosen at random from the observation time. Weather events were analyzed for the three days before the colic or assigned date. Foaling was analyzed for three time periods: before, 0-60 and 60-150 days post-foaling. Significant events were recent vaccination, OR=3.31(1.9-6.0); recent transport, OR=3.3(1.2-5.5); 60-150 days post-foaling, OR=5.9(1.8-13); and recent fever, OR=20(2.5-169). Snow on the day of the colic, OR=2.8(1.0-7) and humidity <50% the day before the colic OR=1.6(1.0-2.9) were marginally significant.