Ultramorphological And Molecular Characteristics Of The Larval Stages Of The Horse Nasal-Myiasis Fly, Rhinoestrus Sp (Diptera: Oestridae) From Mongolian Horse In China

Rhinoestrus sp. (Diptera: Oestridae) is an economically important parasite that can cause severe nasal myiasis in equids and can also affect humans. The ultrastructure of all Rhinoestrus sp. larval instars from Mongolian horse was examined by light and scanning electron microscopy to characterize the features of Rhinoestrus. The structure of the anterior region, posterior region, and the spines of the third segment was analyzed for 10 specimens in each larval stage. Additionally, 34 third-instar (L3) larvae of Rhinoestrus sp. from Mongolian horse were subjected to molecular characterization by polymerase chain reaction (PCR). Partial sequencing of the mitochondrial cytochrome oxidase subunit I gene (COXI) was performed. Phylogenetic relationships for Rhinoestrus sp. from Mongolian horse were reconstructed based on COXI sequences and COXI homologs from 12 oestrid species available in GenBank, and inter-and intraspecies variations in nucleotide sequences were analyzed. The cephalopharyngeal skeleton of Rhinoestrus sp. from Mongolian horse at immature stages was composed of "H"-shaped hypopharyngeal sclerites and "Y"-shaped pharyngeal sclerites. Pairs of papillary sensillae in the cephalic region maintained the same number and location in all three instars, but the third instar additionally included two pairs of verruciform sensillae. COXI sequences of Rhinoestrus sp. larvae collected from Mongolian horses were separated from those of Cephenemyia sp. and other species of Rhinoestrus (R. phacochoeri and R. usbekistanicus). Our sequence analyses demonstrate that all samples greatly resembled Rhinoestrus purpureus but were not R. purpureus. Further studies will be needed to confirm the existence of a new species. The pairs of papillary sensilla in the cephalic region retained the same number and location in all three instars, but included two pairs of verruciform sensilla in the third-instar larvae collected from Mongolian horses were more closely related to those of Oestrus ovis and Pharyngomyia picta.