Resistance to ivermectin in the gastrointestinal nematodes of horses from Mexico

Evidence of resistance to ivermectin in the gastrointestinal nematodes of horses from Mexican southeast
Vet Parasitol Reg Stud Reports. 2023 Sep;44:100907. doi: 10.1016/j.vprsr.2023.100907. Epub 2023 Jul 19.
Authors
Gabriela Janett Flota-Burgos 1 , José Alberto Rosado-Aguilar 2 , Roberto Rojas-Becerril 3 , Roger Iván Rodríguez-Vivas 1 , Iris Trinidad-Martínez 1
Affiliations

1 Departamento de salud animal, Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma de Yucatán, Km. 15.5 carretera Mérida-Xmatkuil, CP 97100 Mérida, Yucatán, Mexico.
2 Departamento de salud animal, Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma de Yucatán, Km. 15.5 carretera Mérida-Xmatkuil, CP 97100 Mérida, Yucatán, Mexico. Electronic address: ja.rosado@correo.uady.mx.
3 Asociación Mexicana de Médicos Veterinarios Especialistas en Equinos A.C., Calzada México Tacuba # 213, esquina Maestro Rural, Col. Un lugar para nosotros, Del. Miguel Hidalgo CP 11330, D.F, Mexico.

PMID: 37652626
DOI: 10.1016/j.vprsr.2023.100907

Abstract

Gastrointestinal nematodes (GIN) are a major impediment to breeding and exploitation of horses. Traditional control of GIN has generated resistance to main anthelmintics, including ivermectin. An analysis of five ranches with a history of IVM use was done to determine the efficacy and resistance of GIN to IVM treatment in horses from the Mexican southeast. Predesigned questionnaires were applied to collect information on previous treatment protocols. The fecal egg count reduction test (FECRT) was applied to determine resistance. Before IVM application, a McMaster test was used to diagnose GIN infection in horses, and feces cultures were done to identify L3 larvae for Strongylida eggs. Pre-treatment samples showed that 72.7% (80/110) of horses were GIN positive, with cyathostomins being the most frequent (91.8%), followed by Oxyuris equi (7.0%), Parascaris equorum (1.0%), and Strongylus vulgaris (0.2%). Based on the results, the horses at each ranch were divided in control (CG) and experimental (EG) groups with similar eggs per gram of feces (EPG). The EG (40/80) was dewormed with IVM (0.2 mg/kg orally) and the CG (40/80) remained untreated. After 14 days, EPG were measured, and feces cultures were done again to identify L3 larvae. After treatment of EG, 40% (16/40) of horses were positive, the most frequently identified GIN were cyathostomins (98.6%), followed by P. equorum (1.0%) and S. vulgaris (0.2%). Three of the five ranches were classified as resistant, according to the FECRT, with a percentage of reduction from 53 to 68%, all of which used IVM ≥4 times annually. This is the first evidence of resistance in cyathostomins to IVM treatment in horses from the Mexican southeast, adding to the current problem of anthelmintic resistance in equine GIN.

Keywords: Cyathostomins; Equine; Macrocyclic lactones; Resistant worms.

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