Helminthosis in horses in the Amur region: correlation and regression analysis of the influence of housing system and weather factors

The purpose of the research is to evaluate the influence of housing system, season, and weather conditions on the fecal egg count of gastrointestinal nematodes in horses in the Amur Region of the Russian Far East from 2021 to 2024.

Materials and methods. A total of 1,494 fecal samples were collected from 212 horses kept on permanent pasture (three farms) or in stalls with daily grazing (four farms). Eggs of Strongylata spp., Parascaris equorum, and Oxyuris equi were counted using the McMaster method. Average monthly air temperature, relative humidity, and precipitation were obtained from the Hydrometeorological Center of Russia. Egg counts were log-transformed [log₁₀(EPG + 1)]. Preliminary relationships were examined using Spearman's rank correlation. Determinants of Strongylata spp. egg shedding were estimated using ordinary least squares regression with cluster-robust (HC1) standard errors for the farm factor (n = 7).

Results and discussion. Strongylata spp. nematodes were nearly ubiquitous (99% of horses), while P. equorum and O. equi were present in 24% and 8% of horses, respectively. Pasture grazing increased the log FEC of Strongylata spp. By 0.29±0.06 (1.9 times increase; P < 0.001) and autumn sampling by 0.22±0.05 (1.6 times increase; P < 0.001) compared to spring sampling. Each 1 оC increase in mean monthly temperature added 0.035±0.009 log units (P < 0.001). Humidity and precipitation showed no independent effects after adjustment. No significant relationship with weather conditions was found for P. equorum or O. equi. Therefore, continuous grazing combined with warm weather conditions is the main risk factor for Strongylata spp. infection. To improve control and reduce unnecessary winter treatments, strategic deworming in April and September, combined with selective therapy, is recommended.

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