Treatment and Control of Equine Strongyle Infections

The large and small strongyles are considered to be the "bread and butter" parasites of control programs because of their widespread occurrence throughout the world  and potential for causing serious disease. In North America, control of equine strongyles forms the foundation of all parasite control programs and effective measures against other internal parasites such as bots, lungworms and stomach worms are built on this initial foundation.

The use of anthelmintics to control strongyle infections


The first safe and effective anthelmintics against adult large and small strongyles
(benzimidazoles) were first introduced in the mid 1960s. Following this, it was widely recommended that horses should be treated year-round with anthelmintics at 4-6 week intervals. This recommendation was based not only on the known prepatent periods of 4-8 weeks (depending on the species) for the small strongyles, but also that strongyle eggs could be 
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detected year-round in the feces of horses. This recommendation was reflected in the advertising campaigns mounted by the drug manufacturers during the 1960s and 1970s as reflected in this print advertisement ( use your browser back button to return to this page). The accompanying graph illustrates the rebound of strongyle eggs in the feces of grazing horses 4-6 weeks after treatment with Pyrantel pamoate (PRT).

More modern anthelmintics such as the macrocyclic lactones, Ivermectin and Moxidectin are much more effective against the equine strongyles because they kill larval stages of large and small strongyles as well as the lumen-dwelling adults. As a result it takes longer for egg counts of grazing horses to rebound following treatment and this is shown in the graphs below.

This graph shows the effects of treating horses  with strongyle infections using Ivermectin at a dose of 0.2mg/kg. This dose is highly effective and eliminates not only cyathostome  larvae and adults in the lumen of the large intestine but also the mucosal dwelling L4s. The result is a longer interval for the gut to be repopulated by adults developing from unaffected encysted L3s and  Slide77.gif (13162 bytes)
newly ingested L3s. In addition,  Ivermectin has a two week residual effect from drug residues that are sequestered in body fat. If  Ivermectin  is used in a year-round control program its frequency of use can be reduced to once every 10-12 weeks. One significant advantage to using Ivermectin is that it is also highly effective against adult Strongylus equinus plus migrating stages  and adults of   Strongylus vulgaris and Strongylus edentatus.   However, Ivermectin is not effective against cyathostome L3s (hypobiotic early L3s or developing L3s)  in the mucosa.


A similar result is obtained when horses are treated orally with Moxidectin at a dose of 0.3 mg/kg. It is effective against all stages of cyathostomes except for encysted hypobiotic early L3s. It has the same range of activity as Ivermectin against the large strongyles and appears to have a longer residual effect (of 12-16 weeks) than Ivermectin. Slide75.gif (16568 bytes)



Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000