Thelazia lacrymalis in horses from Romania: epidemiology, morphology and phylogenetic analysis

Background: Equine thelaziosis is a neglected vector-borne parasitic disease in modern veterinary medicine, lacking recent reports. It is transmitted by Musca autumnalis, and potentially other Muscidae species, by ingesting the lachrymal secretions of its equine host. The distribution of both Thelazia lacrymalis and its intermediate hosts remains largely unknown throughout Europe, with most studies dating back 20 years. The aim of this study was to assess the presence, prevalence and distribution of T. lacrymalis in horses from Romania. Methods: The eyes of 273 horses, slaughtered at two abattoirs from the Northwestern and Western regions of Romania, were examined for the presence of T. lacrymalis between March and November 2021. Upon detection, the nematodes were collected and morphologically identified using the keys from literature. Following identification, one specimen from each animal was selected for molecular analysis while the rest underwent detailed morphometric measurements. Mapping and distribution, according to ecoregions, was done using the QGis 3.20 software, while sequences obtained were compared to those available in GenBank through BLAST analysis using the MEGA X software. Results: Of the 273 animals sampled, 12 (4.39%) were positive for Thelazia spp. infection. Eighty-seven nematodes were recovered, all morphologically identified as T. lacrymalis. The intensity of infestation varied between one and 33 nematodes/animal while five animals presented a bilateral infestation and seven a unilateral one. The highest prevalence was encountered in Pannonian ecoregion (12.12%) while the lowest was in the Alpine ecoregion (0%). Seventy-five intact specimens underwent detailed morphometric analysis, of the 18-20 parameters, resulting in notable differences in striation lengths compared to the data available in other reports. BLAST analysis identified a 96.46-98.60% similarity to the only other COI gene sequence available for T. lacrymalis. Conclusions: The current study represents the first report of T. lacrymalis in horses in Romania. The low prevalence rates are probably linked to the wide use of macrocyclic lactones. Graphic Abstract

Automated summary

This study assessed the presence, prevalence, and distribution of Thelazia lacrymalis in horses in Romania. The results showed a low prevalence rate of the infection, which may be attributed to the widespread use of macrocyclic lactones.