This study aimed to assess the efficacy of pyrethroid-based insecticides against Gasterophilus spp. egg-laying activity and to establish the seasonality patterns in the specific climatic area of Transylvania, Romania. To fulfil the aims of the study, a total of 40 horses were treated every 4 weeks and inspected every 2 weeks between 11 May and 6 November (Pilo et al. Parasitol Res 114:1693–1702, 2024). Through this period, eggs were found between 24 June and 2 November. The eggs were found on 34 out of 40 horses (prevalence = 85%, CI 95% = 73.43–96.56%). Overall, a total of 50,029 Gasterophilus spp. eggs were counted during this study, and all were morphologically identified as G. intestinalis. Statistical analysis revealed no significant difference in prevalence and intensity between treated and control horses (p > 0.05). The number of laid eggs varied significantly across body regions (p < 0.05). The risk factors associated with increased egg-laying activity were significantly (p < 0.05) associated with dark-color coats, outside husbandry regime, and the month of exposure. No significant differences in egg numbers and prevalence were found across gender or age groups (p > 0.05). The applied statistical tests were as follows: the non-parametric Mann–Whitney U test, the Friedman test, Wilcoxon test corrected by the Bonferroni method, the Kruskal–Wallis test, and the Bonferroni correction for multiple comparisons. These results highlight gasterophilosis as an important parasite in horses. In conclusion, we recommend that, in order maintain a low prevalence of Gasterophilus spp., treatment should include the use of larvicidal drugs and the mechanical removal of the eggs.
