Gasterophilus (Diptera, Gasterophilidae) infestation of equids in the Kalamaili Nature Reserve, China

We investigated infections with Gasterophilus spp. in three equids within the Kalamaili Nature Reserve (northern China). We conducted necropsies on 6 Przewalski’s horses (Equus ferus przewalskii) and 6 Mongolian wild asses (Equus hemionus) and administered ivermectin to 10 overwintering domestic horses to expel parasites during winter periods. All 22 equids studied (100%) were infested with Gasterophilus spp. and a total of 17,225 larvae were collected. These included six species: G. haemorrhoidalis, G. inermis, G. intestinalis, G. nasalis, G. nigricornis, and G. pecorum. The mean intensity of Gasterophilus spp. larvae was 1904 in Przewalski’s horses, 780 in Mongolian wild asses, and 113 in domestic horses. Gasterophilus pecorum was the most abundant species in all three equids. Przewalski’s horses, a reintroduced species, had a significantly higher intensity of Gasterophilus spp. than the Mongolian wild ass, indicating greater susceptibility to parasites in its ancestral home.

More than 150 species of internal parasites infect horses [5]. Gasterophilus spp. are obligate parasites that infest the gastrointestinal tracts of equids, affecting the horses' health by absorbing nutrients and secreting toxins [15,19]. They may cause host death when the infestation is severe [4]. Gasterophilus spp. consists of nine species distributed worldwide [21]. In China, six of them are present, namely G. haemorrhoidalis, G. inermis, G. intestinalis, G. nasalis, G. nigricornis, and G. pecorum [6,18]. All of them have been reported in wild populations of Przewalski's horses in KNR, China [9].
The present study was carried out on Przewalski's horses and Mongolian wild asses that died accidentally and with preserved corpses and feces of domestic horses following antiparasitic treatment during the winter. The aim of the study was to investigate the epidemiological features of Gasterophilus spp. in the three equid species.

Study area
The KNR is located in the southeast corner of the northeast Junggar Basin, Xinjiang (Fig. 1). It is dry and cold in winter and hot during the summer. Mean annual precipitation is 159 mm, and mean annual evaporation 2090 mm, which is characteristic of a typical temperate continental arid climate [2].

Larvae collection
Six Przewalski's horses and six Mongolian wild asses that died accidentally during the winter from 2010 to 2015 were preserved and necropsied. All Gasterophilus spp. larvae were collected from the digestive tracts.
A total number of 10 domestic horses (with no repeat samples) were randomly selected and treated once with ivermectin at the conventional dose of 0.2 mg/kg orally during the winter from 2013 to 2015. Following ivermectin treatment, we collected Gasterophilus spp. larvae that were eliminated in all feces of each horse three times a day until there were no larvae for three consecutive days. The domestic horses had received no other anti-Gasterophilus drug treatment prior to the survey.

Statistical analysis
The infestation prevalence, intensity, and abundance intensity were estimated according to Margolis and Schad [11].
Differences among the mean intensities of Gasterophilus spp. in the three equids were tested by the Kruskal-Wallis test. The statistical analysis and graphics were performed using SPSS version 20.0. A significant difference was assumed when p 0.05.
Gasterophilus pecorum was the most common species (100%) in the Przewalski's horse and the Mongolian wild ass. G. pecorum, G. nasalis, and G. nigricornis were found in every domestic horse (Table 1).
Abundance intensity of Gasterophilus spp. larvae G. pecorum was the most abundant species of Gasterophilus spp. in the three equids. The abundance intensity  (Table 3).

Discussion
All three equids were infected with Gasterophilus spp. larvae. This may reflect a wide distribution of Gasterophilus spp. in KNR. The high prevalence of Gasterophilus spp. larvae (100%) in the three equids is comparable to that reported for horses in Kazakhstan (100%) [8]. In contrast, this prevalence is much higher than reported in Sanliurfa, Turkey (9.82%) [7].
The mean intensities of Gasterophilus spp. larvae in Przewalski's horses (1904) and Mongolian wild asses (780) were higher than some reports from donkeys and horses in Asia [7,12]. The lack of anti-parasitic treatment in wild equids in KRN may be the main reason for this situation, but it may also be due to the different climate and environment. However, the domestic horse had the lowest intensity of Gasterophilus spp. compared to the other two equids. This difference was partly attributed to the difference of habitats between the domestic horse and the other two equids. The summer pastures were mountain meadow with a lower temperature. The vegetation and water resources were more abundant than in KRN. These different environmental conditions may affect the activity of Gasterophilus spp. On the whole, our results indicate that the equids in KNR are severely affected by Gasterophilus spp. and the winter is not the main infection period for Gasterophilus spp. here.
G. pecorum was the most abundant species of Gasterophilus spp. in the three equids, which differs from studies in other regions of the world where it was reported that G. nasalis and G. intestinalis are the abundant species [8,13,14,16]. Our results suggest that G. pecorum is more adaptable to the local environment in KNR. G. pecorum was the only Gasterophilus spp. species that oviposits on grass [21]. The association with this unique behavior and the desert steppe ecosystem may help explain the situation.
Water availability restricts the activity area of wild animals in a territory such as KNR which has low precipitation, high evaporation, and limited surface runoff. A previous study showed that the oviposition sites of G. pecorum are often near a water source [10]. This suggests that the water locations may be the important ''epidemic'' areas of G. pecorum. Przewalski's horses seem to drink daily [17]. For wild asses, it is often assumed that they can ''regularly do without water'' [1]. Frequent drinking at water sources may increase the risk of G. pecorum infection. Thus, the equids in arid desert grasslands have a higher intensity of Gasterophilus spp., and the intensity in Przewalski's horses is higher than in Mongolian wild asses.