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Evidence of reduced activity in one of the big hitters among equine wormers was found in a study on three farms in the Netherlands.
The research examined the performance of ivermectin and moxidectin, both members of the anthelmintic family known as the macrocyclic lactones.
Frans Kooyman and his colleagues, in an article accepted for publication in the journal Veterinary Parasitology, noted that moxidectin and ivermectin were the most commonly used anthelmintics against small strongyles.
However, reduced activity – observed as shortening of the egg reappearance period (ERP) in fecal testing – has been described worldwide.
The ERP is the time between treatment and the reappearance of eggs in the feces. The ERP for ivermectin at the time of introduction was reported to be eight to nine weeks, and moxidectin was reported to be 12-25 weeks.
The study team said shortening of the ERP may be caused by a decreased susceptibility of immature worms to macrocyclic lactones, or immature worms may possibly develop faster into egg-producing adults as a result of repeated treatments.
The study team used three Dutch farms with a known history of shortened ERPs, with eight horses per farm selected.
The researchers devised an experiment in which horses, divided into two groups, were dosed with either ivermectin or moxidection twice, with a dose of pyrantel in-between.
In all cases, the feces were monitored for the reappearance of eggs, with DNA-based testing undertaken to identify the species present.
Both ivermectin and moxidectin were both 100 percent effective 14 days after treatment, and pyrantel ranged from 98% to 100% effective at the same mark.
However, the researchers reported that evidence of a shortening of the ERP was found on all three farms after the ivermectin and moxidectin treatments.
The third larval stage (L3) of Cylicocyclus dominated in larval cultures obtained after moxidectin treatment.
The shorter the egg reappearance rate, the more small strongyle species were found shortly after moxidectin treatment.
Decreased moxidectin susceptibility was the most plausible cause of shortening of the ERP, they said.
“The high numbers of L3s of Cylicocyclus species 42 days after moxidectin treatment and the low numbers 42 days after pyrantel treatment can be explained by reduced susceptibility of the immature worms to moxidectin, but not by a faster development,” they wrote.
“In conclusion, this study provides evidence that a shortening of the ERP is a process in which an increasing number of immature worms from an increasing number of cyathostomin species are becoming less susceptible to moxidectin and possibly ivermectin, although this was not clear in the present study.
“Monitoring shortening of the ERP as an early sign of resistance development can help in evaluating old and new treatment strategies in order to be able to use the still effective macrocyclic lactones for an extended period of time.”
The research was centered at Utrecht University’s Department of Infectious Diseases and Immunology, which is part of the Faculty of Veterinary Medicine.
Kooyman, F.N.J., van Doorn, D.C.K., Geurden, T., Mughini-Gras, L., Ploeger, H.W., Wagenaar, J.A.,
Species composition of larvae cultured after anthelmintic treatment indicates reduced moxidectin
susceptibility of immature Cylicocyclus species in horses. Veterinary Parasitology