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
| 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
| 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)