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Equine Parasite Control
Based on AAEP Parasite Control Guidelines
Introduction
Monthly deworming of horses for parasite control is a method developed more than 40 years
ago when Strongylus vulgaris (the large strongyle) was the most prevalent parasite type found in the
equine gut. However, times have changed and small strongyles are more prevalent in adults while
roundworms (Parascaris spp.) remain more prevalent in foals and weanlings. Resistance against
dewormers (anthelmentics) is common in small strongyles and roundworms. Adult horses vary greatly
in their susceptibility to parasitic infection and in their level of egg shedding. All these things must be
incorporated together in order to find the best deworming program for your horse.
Goals of Parasite Control
The goal of parasite control is NOT to get rid of all parasites but to limit parasite infections so
animals remain healthy and clinical illness does not develop. Not only is eradication impossible, but it
will also lead to development of drug resistance, which is an increasing concern. Treatments are only
necessary when the environmental conditions are conducive to egg and larval development and
survival. Adverse environmental conditions such as hot summers will rapidly kill developing parasites on
pasture.
Strategic Deworming
All horses should receive one or two deworming treatments per year to target large strongyles,
tapeworms, pinworms, bots, and nematodes which are responsible for causing summer sores.
Deworming treatments should be given during seasons of peak shedding (usually spring and fall).
Administration of ivermectin (Zimectrin) in the fall and moxidectin + praziquantel (Quest Plus) in the
spring as well as one fecal egg count per year is adequate for most adult horses in this area. All further
treatments should target horses that are moderate- high egg shedders.
Fecal Egg counts
Fecal samples may be obtained within 6 hours and should be immediately refrigerated.
Refrigerated samples can be tested up to 7 days later. Fecal samples should be obtained 16 weeks after
moxidectin administration, 12 weeks after ivermectin administration, and 9 weeks after pyrantel
administration.
Classification
low
moderate
high
Fecal Egg Count
0-200 eggs/gram
200-500 eggs/gram
>500 eggs per gram
Environmental Control
Some environmental control strategies exist for controlling parasite burden on pastures.
Composting will obtain high enough temperatures to eliminate parasites from manure. Non-composted
manure should not be spread on pastures. Rotating animals from pasture to pasture and allowing the
pasture to be rested for weeks in the summer and up to six months during winter months can decrease
the parasite burden. Additionally, allowing livestock to graze on “rested” pasture can also decrease
parasite burdens.