Retained efficacy of ivermectin in horses with selective anthelmintic treatment

Here is a large study that tests the idea of selective anthelmintic treatments to prevent ivermectin resistance in small strongyles. By selective it is meant that the decision to deworm is based on an elevated fecal egg count. Though this idea has been proposed for some time and smaller studies covering less time have supported it, this is the most complete report covering years of practice. The results support the idea that areas still practicing routine and frequent deworming are showing signs of increasing resistant populations of parasite to the dewormers used. A practical program of selective anthelmintic treatment is presented in the article associated with this post.
DrO

Retained efficacy of ivermectin against cyathostomins in Swedish horse establishments practicing selective anthelmintic treatment

Vet Parasitol. 2023 Aug 14;322:110007. doi: 10.1016/j.vetpar.2023.110007. Online ahead of print.
Authors
Ylva Hedberg Alm 1 , Eva Osterman-Lind 2 , Frida Martin 3 , Rebecca Lindfors 4 , Nina Roepstorff 5 , Ulf Hedenström 6 , Isabelle Fredriksson 7 , Peter Halvarsson 3 , Eva Tydén 3
Affiliations

1 Department of Biomedical Science and Veterinary Public Health, Parasitology Unit, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden. Electronic address: ylva.hedberg.alm@slu.se.
2 Department of Microbiology, Section for Parasitology, National Veterinary Institute (SVA), SE-751 89 Uppsala, Sweden.
3 Department of Biomedical Science and Veterinary Public Health, Parasitology Unit, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden.
4 Ambulatory Clinic, University Animal Hospital, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden.
5 Ridskolan Strömsholm AB, Stallbacken 6, Knytpunkten, SE-734 94 Strömsholm, Sweden.
6 WångenAB, Vången 110, SE-835 93 Alsen, Sweden.
7 Flyinge AB, Kungsgården, SE-247 93 Flyinge, Sweden.

PMID: 37639919
DOI: 10.1016/j.vetpar.2023.110007

Abstract

Cyathostominae are ubiquitous to grazing horses and regarded the most prevalent internal parasite in the horse. Unfortunately, decades of indiscriminate use of anthelmintic drugs have resulted in the development of resistance in cyathostomins to all currently available drug groups, the most recent being a documented lack of efficacy to the macrocyclic lactones (ML). In vivo determination of anthelmintic resistance in horses most often utilises the faecal egg count reduction test (FECRT). Further, a shortened egg reappearance period (ERP) can indicate a change in response to the applied treatment and suggest an upcoming reduction of efficacy. Although both true resistance as demonstrated by the FECRT and shorter ERPs after ML treatment have now been shown in cyathostomins worldwide, the efficacy of ML as regards to cyathostomins in Sweden is currently unknown. The aim of the present study was therefore to determine FECRTs and ERPs after ivermectin (IVM) treatment in Swedish horses. Sixteen equestrian establishments with a minimum of six horses excreting at least 150 eggs per gram faeces (EPG) at screening were selected. For each establishment, FECRTs and ERPs were determined by collecting faecal samples prior to and 14 days after IVM treatment (200 µg/kg), and thereafter at weekly intervals for a total of eight weeks. All participants responded to a questionnaire detailing pasture management methods and anthelmintic routines.Questionnaire results showed that the majority of establishments (69%) only treated horses with anthelmintic drugs if indicated by faecal diagnostics and all of the establishments had a mean FECRT exceeding 99.0% and ERPs ranging from six to over eight weeks. The ERP was shown to increase with age as young individuals were shown to excrete cyathostomin eggs earlier after treatment compared with older horses (R = 0.21, p = 0.015). Riding schools, stud farms and those declaring not to use separate summer and winter paddocks had significantly shorter ERPs (p <0.01).In conclusion, retained ERPs and no confirmed resistance to IVM were found in Swedish equine establishments practising selective anthelmintic treatment, and supports the use of selective deworming regimens as a means of reducing the risk of anthelmintic resistance development.

Keywords: Anthelmintic resistance; Cyathostominae; Cyathostomins; ERP; FECRT; Ivermectin; Macrocyclic lactones; Small strongyles.