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New study finds moxidectin
is tough on worms, not intestines!
Susan Corning BA MSc BVSc
MRCVS FRSH FRIPH
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Just look at a horse cross-eyed, and the next
thing you know, it will be down with colic! Well,
perhaps that is a bit of an exaggeration, but
horses are pretty sensitive when it comes to their
digestive tracts, and horse owners are rightly
concerned about anything that might contribute
to the onset of an episode of colic. Risk factors
known to be related to the incidence of intestinal
disorders include age, sex, breed, and nutritional
status, and worm burden - not to mention other
factors that are just being discovered!
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Although horses may be adversely affected by
large numbers of many different worm species,
the most harmful and life-threatening equine parasite
today is the small redworm ("cyathostomins")1.
Over 90% of a horses small redworm burden can
be attributed to the larvae2. The larvae of this
worm burrow into the mucosa of the horse's gut
wall and become encysted - and up to 85% of these
may also become "inhibited"2, which means that
they can remain dormant in their cysts in the
gut wall for many months or even years. A major
problem can occur when these encysted inhibited
larvae develop and all emerge at the same time
from the horse's gut wall. The related disease
syndrome is known as "larval cyathostominosis",
which can cause a severe colic, with up to 50%
of cases ending in death3.
Due to the seriousness of this disease, it is
essential that a horse's worm control programme
specifically targets the inhibited and developing
encysted small redworm larval stages. There are
only two active ingredients which have been proven
to control both of these stages: fenbendazole
(a member of the benzimidazole chemical family)
and moxidectin (a member of the newer, macrocyclic
lactone chemical family). It is important to note
that no products containing other members of either
of these chemical families are licensed to control
the inhibited and developing encysted stages of
small redworm larvae: only products containing
moxidectin or fenbendazole will do the job.
So the good news is that there are products available
that are proven to help prevent the potentially
devastating larval cyathostominosis - but there
are also other aspects to consider when deciding
which product to use. Worm resistance, treatment
regime, and clinical effects post-treatment are
important considerations when making a decision
to use a product containing either fenbendazole
or moxidectin.
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Worm resistance:
although there is no confirmed small redworm resistance
to moxidectin in horses, resistance to benzimidazoles
is now widespread in the UK4. Therefore, before using
fenbendazole, you should ensure that you do not have
fenbendazole-resistant small redworms on your yard.
Treatment regime:
for the treatment and control of encysted and inhibited
small redworms, it is recommended that fenbendazole
be administered daily for a period of 5 consecutive
days. Moxidectin treats and controls encysted developing
and inhibited small redworm larval stages with one treatment;
i.e. a single standard dose. For both active ingredients,
it is recommended that treatment be given twice a year,
in the autumn and winter.
Clinical effects:
recent research5 has evaluated possible adverse effects
upon the gut wall of small redworm-infected ponies wormed
with fenbendazole and moxidectin. Scientists have found
that either treatment was effective against these encysted
small redworms; however, they observed that the gut
walls of the ponies become inflamed when they are treated
with a five-day course of fenbendazole. This inflammation
began about 4 days after the end of treatment, and by
day 14 post-treatment, this inflammation was frequently
accompanied by ulcerations. The inflammation did not
appear to be caused directly by the fenbendazole, but
by toxins excreted by dying and dead larvae6. Similarly
infected ponies treated with moxidectin did not show
an inflammatory reaction. This was because, different
from the fenbendazole treatment, which can also cause
affected larvae to emerge into the lumen (inner "tube")
of the intestine7, moxidectin caused the larvae to disintegrate
in the gut wall and simply be resorbed (dissolved and
absorbed) without causing severe inflammation of the
gut wall.
The conclusion of this
study was that whilst both fenbendazole and moxidectin
were effective against encysted larvae of small redworms,
there may be different clinical consequences.
Unlike the effect of moxidectin,
the killing of encysted small redworm larvae with fenbendazole
was associated with severe tissue damage, which may
clinically mirror the situation caused by the mass emergence
of encysted larvae. In other words, the effects of fenbendazole
may actually mimic larval cyathostominosis - the very
condition for which the worming programme may be targeted!
Although no clinical signs were exhibited by any of
the ponies in this study, and indeed worming is generally
not associated with colic, the importance of maintaining
a healthy gut wall cannot be underestimated.
When choosing a wormer,
make sure that the product you use has a registered
licence claim against both the inhibited and developing
encysted small redworm larval stages - ie. containing
the active ingredient fenbendazole or moxidectin. And
when deciding which one of these actives will be most
appropriate for your own situation, be sure that you
consider your horse's worm resistance status, your treatment
programme, and the potential effect upon your horse's
gut wall.
EQUEST and new EQUEST
PRAMOX, both of which contain the unique active
ingredient moxidectin, are the only equine anthelmintics
licensed to control the inhibited and developing encysted
larval stages of small redworm with one single standard
dose, with no confirmed resistance in horses the UK,
and no significant tissue damage to your horse's gut
wall. EQUEST and EQUEST PRAMOX - helping you to keep
your horse healthy.
References:
1. Love S et al. Veterinary
Parasitology (1999) 85: 113 - 122.
2. Bairden K et al. Veterinary Record (2001) 148: 138-141.
3. Proudman C et al. In Practice (2000) 90-97.
4. Bairden K et al. Veterinary Record (2006) 766-767.
5. Steinbach T et al. Veterinary Parasitology (2006)
139: 115-131.
6. Lee J. www.TheHorse.com, September 2006 Article #
7461
7. Uhlinger C. Equine Veterinary Journal (1992) 13:
11-18
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