The prevalence of parasites (95.16%) in Yili horses was higher than previously reported in equines from Lesotho (88.2%) [16], Nicaragua (94%) [17], and Ethiopia (69.4%) [18]. The high prevalence observed in the current study may have been due to differences in management practices and habitat between the current horse population and those previously studied. In our study, H. muscae was recorded in 63.71% of infected Yili horses, while previously it was recorded in 56.67% of horses [8], 55–90% of donkeys [19], 2–72% of horses [19,20], or 0.7% of horses [21]. These data indicate that the distribution of this parasite among horses worldwide is quite serious, and also reflects the wide distribution of the intermediate host in China. The disease is associated with seasons when flies are abundant, and lesions commonly regress during the winter in temperate climates.

Draschia megastoma is maintained in the environment by muscid flies, which act as intermediate hosts [22]. In the present study, D. megastoma was reported in 25% of horses while it was reported in 0.69–47% of donkeys [23,24] and 5–66% of horses [19,20,25]. No other study has recorded D. megastoma in any Equidae group [26]. Clinical signs of parasites vary from mild to severe depending on the developmental stage and localisation site of the parasite. Habronematidosis is responsible for significant economic losses, mostly when sport horses are affected, because their performance is impaired and the infection can be unaesthetic [27]. A significantly higher prevalence of D. megastoma lesions has been observed from late winter through early summer with a second peak in autumn in horses from Western Australia [28]. However, no association could be established between the prevalence of adult D. megastoma and the age of the horses [28].

The prevalence of O. equi in the present study was 46.77% which was higher than the previously recorded prevalence rates of 8.53% [26], 6.4% [29] and 2.5% [30], 7–26% [19,31], 0.4% [9], and 11% [21]. This may be due to the differences in management systems or climatic conditions between the study areas [32]. Additionally, the development of O. equi appears to be favoured by high rainfall, as higher infection rates with adult worms have been observed in horses from areas with uniformly high rainfall rates [19,25]. A low prevalence (12.10%) of S. westeri has been previously reported in Yili horses, but this was still higher than in foals from Queensland (6%)[19]. The limited number of very young foals in this study could be a reason for the reported low prevalence, as most foals become resistant to the infection by 16 weeks of age [19,33]. However, the parasite may be more widespread that recognised, as it is anecdotally regarded as a common parasite in horses from serval regions of the country.

The prevalence of P. equorum (86.29%) was higher than previous rates of 29.26% [26], 17.3% [34], 43% [35], and 27.8% [30] recorded in Hungary and 13.7% [9], 5.3% [21], and 5–58% recorded in Australia [19,36]. This may be due to differences in grazing areas among horses and the lack of awareness of the health of animals. Older horses (> 3 years) were less likely to be diagnosed with P. equorum infection in the current study. It is widely acknowledged that horses develop age-related immunity to P. equorum, making infection a problem only for younger horses [37,38], with maximum ascarid egg shedding reported in foals [39]. However, several studies have reported the lack of a relationship between age and P. equorum infection in working horses [16] and donkeys [40,41]. Differences in infection patterns between horses in developed and developing countries could be due to compromised immunity as a result of underlying disease or malnutrition [16]. Higher prevalence rates are observed during summer, in paddocked rather than ST horses [19], and in males compared to gelding and female horses [25].

The EPG prevalence (proportion in the community) of large strongyles (89.52%) fell between the proportions of 99.5% [26], 96.77% [29], 92% [43], 53.8% [9], and 44.7% [21] cited in previous studies. These differences might be due to differences in the climate, agroecological conditions, variations in sample size, or different sampling methods [32]. In addition, horses might be neglected in these areas and receiving less attention from their owners [44]. Large strongyle infection is common in grazing horses, as daily access to pasture for 30 days before parasitological examination is associated with a greater risk of egg shedding [45]. By contrast, horses that are stall managed with limited access to pasture are at a reduced risk of developing large strongyle infection [46]. Management practices of ST Yili horses in the study area have been previously reported and showed no access to pasture grazing and limited contact between horses [8,9], which could explain why most horses had low infection intensities in the current study. The prevalence of large strongyle infection varied significantly in Zhaosu. This may have been caused by differences in availability of veterinary services, horse owners’ knowledge about the importance of preventive veterinary care, type of anthelmintic commonly used, or other unknown factors between the study areas. Some studies have shown no associations between the variations in site preference and age, sex, horse breed, or the environment [19,21,25,47]. However, significant differences in the level of horse infections by S. equinus between sexes and management strategies were found in the present study (P < 0.05). At the same time, this study also found that seasonal variations may affect the distribution of parasites at different sites, and there could be an increase in the proportion of parasites in regions anterior or posterior to the preferred site during a particular season. This result has been confirmed by related research [19].

More than 50 species of cyathostomins have been recognised [48] and 10 species are most prevalent. The mean number of cyathostomin species expelled per horse was 11 (> 10) in this study. This result corresponds with those obtained in studies in Ukraine [1], Poland [49], the United States [50], Australia [19,25], Ethiopia [41], Sweden [51], and Britain [47]. The important epidemiological risk factors for cyathostomin infection are age, season, and time since last deworming [52]. Interestingly, access to grazing and shared grazing with other horses were only weakly associated with cyathostominosis. More than 90% of the cyathostomin burden may be in the mucosa at specific times of the year, as they are not affected by anthelmintic treatments [53,54,55,56]. A remarkable similarity in the predominant species is observed, regardless of geography. For example, C. calicatus, C. nassatus and C. longibursatus are among the most prevalent species in France, Ukraine, the United States, Brazil, and Australia. The results of this study are in good agreement with those findings. Other species that were not recognised in this study included Cylicostephanus minutus and Cylicocyclus insigne. No associations were found between horse age and the prevalence rates of cyathostomins or large strongyles, including S. vulgaris, S. westeri, and S. edentatus. This result is consistent with a previous study [57]. By contrast, cyathostomins showed an increasing worm burden with age, peaking in 2–7-year-old horses and decreasing in older horses [28]. Intensive anthelmintic use in the last few decades has led to a lower frequency of large strongyle infections in many places, whereas infections with small strongyles (cyathostomins) have increased. This seems to be because small strongyles have become resistant to many anthelmintics, whereas large strongyles have not. Horses that have been exposed to small strongyle infections may acquire resistance, but rather slowly and often not to the point that they become immune. Damage may be less dramatic but still causes clinical signs as the horse will continue shedding eggs and contaminating the pasture [14]. This is particularly significant for the transmission of worms to young foals through infective larvae produced by their mothers in shared pastures. Cyathostominae are the most relevant horse strongyles, mainly due to the diversity of genera and species, prepatent periods, host specificity, and biological cycles with hypobiotic larval stages.

Four species of Gasterophilus were identified, with the following proportions of overall parasite abundance: Gasterophilus pecorum 2.34%, Gasterophilus nasalis 13.53%, Gasterophilus haemorrhoidalis 7.21%, and G. intestinalis 11.93%. Furthermore, the 96.77% occurrence rate of Gasterophilus spp. larvae in this study exceeded previously reported rates of 9.9% [58], 2.25% [59], 0.72% [60], 28.57% [61], 43% [62], 53% [63], 85.90% [64], 86.60% [65], and 95.2% [66] but was lower than the 98.3% [67] and 100% [67,68,69] occurrence rates of several other studies. The widespread prevalence of G. nasalis and G. intestinalis in the present examination was similar to previous investigations [25,50,70]. G. nasalis and G. intestinalis were the most abundant Gasterophilus spp. in Yili horses in Zhaosu, which differed from studies in other regions of Xinjiang, China (Kalamaili Nature Reserve, KNR) where it was reported that G. pecorum is the most abundant species [71,72]. The oviposition site selection behaviour and genetic diversity of G. pecorum provide better opportunities for successfully infesting a suitable host after hatching and reduce the chance of offspring dying due to the lack of a host, explaining why it is the dominant species in the KNR [69,72]. The difference in Gasterophilus infection rates between Zhaosu and the KNR region may be related to habitat differences. The Zhaosu region is a humid climate of Alpine meadows, whereas the KNR region is an arid desert steppe climate.

Helminth parasites were generally more frequently found in females than in males, possibly because males are less exposed to infection because they tend to be more solitary [73]. In addition, female horses have higher infestation rates because they have lower immunity due to gestation, lactation, and stress during those times [42]. It is assumed that sex is a determining factor influencing the prevalence of parasitism [74].