Analysis of Strongyle Egg Shedding Consistency in Horses and Factors That Affect It

Strongyle egg shedding consistency in horses and factors affecting consistency were investigated. Fecal samples were collected from 26 equine populations over one grazing season. Samples were collected on four “screening” occasions (S1-S4) and fecal egg count (FEC) performed (to 1 egg per gram [epg] egg detection limit). On each screening occasion, FEC was assigned an egg shedding category: 1 (<50 epg) to 7 (>500 epg); and a treatment category: <200 epg (no treatment) or ≥200 epg (treatment). Rank changes in shedding and treatment categories between S1 and subsequent screening occasions were calculated. Factors affecting the likelihood of an individual changing shedding or treatment category were assessed using multivariable logistic regression of FEC data from horses that had not received anthelmintic during the study. In total, 573 horses were sampled at S1, 468 at S2, 417 at S3, and 83 at S4. Results showed that between S1 and S4, 73.5% (61/83) horses remained in the same egg shedding category and 94.0% (78/83) in the same treatment category. For horses that did not receive anthelmintic (n = 304), 90.4% (225/304) remained in the same shedding category. Horses under 5 years old were more likely to change egg shedding (odds ratio [OR] = 3.3; 95% confidence interval [CI]: 1.22-8.46) and treatment (OR = 2.8; 95% CI: 1.1-6.3) categories compared to older horses. These results suggest a high level of consistency in strongyle egg shedding in individuals within one season, and withholding anthelmintics from horses with negative/low (i.e., <50 epg) FEC does not appear to lead to significant increases in egg shedding.