A simple guide to the new rules.
First, Some Key Pointers:
4 The goal of modern equine deworming is not to
kill all parasites within the horse; rather it is to limit
their numbers so that the horse remains healthy.
4 Not all horses have the same worm burdens and
therefore should not all be dewormed in the
same way. Fecal egg counts (FECs) done by your
veterinarian will help determine how frequently
you should deworm your horse.
4 The old eight-week rotational-dewormer plan is
out the window. Most adult horses only need to be
dewormed twice yearly, unless they are high egg
shedders, which your vet can determine with a
The New Way to Deworm
Once you and your vet have determined it’s time to
deworm, here are some tips:
4 Choose a dewormer with your veterinarian’s
help. Lots of parasites are resistant to drugs like
fenbendazole throughout the U.S., but in certain
locations, it’s still effective against ascarids.
Ivermectin and moxidectin are generally effective
against small strongyles and bots, as well as
Habronema, the cause of summer sores.
4 Determine your horse’s weight with a weight tape so
you administer the proper dose of dewormer. Many
tubes only go to 1,200 pounds, so large warmbloods
and draft crosses may need part of a second tube.
4 Make sure your horse actually receives his full
dose of dewormer. This means he can’t spit it
out and leave half of the tube on the ground.
Administering less than the full dose of dewormer
only encourages faster development of parasite
resistance. (Flushing hay and grass particles out of
his mouth with the hose beforehand works well.)
4 Write down what dewormer you gave and when;
records are key for proper parasite management.
The American Association of Equine Practitioners
has an excellent handout providing further details on
the sometimes-confounding subject of deworming
horses at www.aaep.org.
» BYANNA O’BRIEN, DVM
arasite control is an important part of
your horse’s health management. Since
parasites develop resistance to commonly
used deworming drugs, rotation is no longer
the recommended strategy. Here’s what you need
to know about the new world of deworming.
ONLINE Find more deworming help at