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WORM
CONTROL AT THE GRASS ROOTS
by Helen Barnes BSc, DMM, DipM, MCIM, Equine Business
Unit Manager, Fort Dodge Animal Health
| Pasture
management is an essential part of any worm control
programme, and plays a vital role in ensuring the
health and well being of your horse. Most life cycles
of equine parasites involve a period of development
outside the horse and on the pasture; with eggs
produced by the adult worms in the horse's gut being
passed out in the horse's dung. These eggs then
develop into infective larvae on the pasture. |
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A
single horse is capable of passing tens of thousands
of small redworm eggs in its dung each day, with the
majority of worm eggs in the horse's dung and larvae
on the pasture being small redworm1.
Because
a horse is mainly infected by ingesting infective larvae
as it grazes, reducing its exposure to infective larvae
on the pasture is paramount.
Horses' grazing
behaviour
The ways horses are kept dictate their feeding habits.
Horses are fastidious feeders and prefer to eat young,
immature plants and will graze some areas of a pasture
down to the bare ground. In other parts of the pasture,
plants are avoided and allowed to grow to maturity,
which lessens palatability and nutrient availability.
In addition, horses will not graze around droppings
so pasture plants around dung are also mature and less
palatable. A typical horse, weighing 450kg produces
5-12 pats or approx. 24kg of dung a day, that's 10 tonnes
a year! As such up to 50% of grazing can be lost due
to fouled areas, resulting in the characteristic "lawns"
and "roughs" as well as an increase in weed infestation.
In the wild,
horses are free to graze over very large areas so they
can easily avoid eating from pasture that has been contaminated
with potentially infective droppings. For domestic horses,
however, grazing is often limited thereby increasing
their exposure to infection; as such action is required
to reduce the number of infective larvae on the pasture.
This reduction of pasture contamination is achieved
by a combination of worming and pasture management.
| Pasture
management Pasture management involves a range of
actions, including: |
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not over stocking
pastures, i.e. no more than 1-2 horses per acre,
as horses lower down the pecking order will be forced
to graze the "rough" pasture where worm burdens
will be higher; |
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·grazing horses alongside
sheep, goat or cattle since parasites that affect
horses are host specific and cannot survive in other
species, therefore any larvae eaten by sheep/goat/cattle
are destroyed and so reducing worm larval contamination
of the pasture. In addition sheep and cattle will
help to improve pasture quality by eating the "rough"
grass rejected by horses; |
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dividing paddocks
into smaller areas so they can be alternatively
grazed and rested to reduce the pressure on the
pasture, as well as making it easier to remove droppings |
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resting pasture for
at least 5 months, although a good idea is not always
an option. Besides which worm larvae can live for
many years both on pasture and in horses and so
simply resting a pasture does not guarantee it will
be worm free; |
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worming horses 48
hours before moving to new pasture; |
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not turning out young
stock onto small turn out paddocks as pasture will
develop extremely high larval counts particularly
if droppings are not removed each day; |
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avoiding grazing foals
alongside older horses, as foals are a major source
of pasture contamination; |
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only harrow in very
dry conditions, as in damp conditions harrowing
simply spreads worm eggs and larvae over the pasture
and so increasing the risk of being ingested by
horses; and |
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most importantly regularly
removing dung at least twice weekly during the grazing
season, and once a week between November and March. |
The
impact of the climate change
Climate changes have also had an effect on pasture contamination.
Although harsh frosts (and hot dry conditions) will
kill infective larvae on the pasture, the milder and
wetter climate now experienced in the UK has meant high
levels of infective larvae can occur on the pasture
at any time of year2. Thus there is an increased likelihood
of grazing animals acquiring parasite burdens throughout
the year. Therefore if the autumn and winter remain
mild and wet the twice-weekly removal of dung will be
necessary all year round.
The role of worming in reducing pasture contamination
In addition to pasture management, worming as previously
mentioned also reduces pasture contamination. Asides
from the fact that the main reason we worm our horse
is to keep it healthy; worming kills the adult worms
in the horse's gut and so the number of eggs that appear
in its droppings are reduced. Also a feature unique
to EQUEST, the only wormer to contain the active ingredient
moxidectin, is that any small redworm larvae eaten by
the horse as it grazes are killed for a further two
weeks after having been dosed with EQUEST, thereby cleaning
the pasture of this parasite and so reducing horse's
exposure to re-infection.
By
a combination of worming and pasture management, a horse's
exposure to infection while it grazes can be reduced,
thereby helping to ensure its health and welfare.
 1.
Mair TW et al 1990 Veterinary Record 126, 479
2 . Love S. Vet Clin. Equine 19 (2003) 791-806
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