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Anthelmintics
for use against parasitic
gastrointestinal worms
There are four main groups of broad
spectrum wormers:-
WORMING SHEEP
Benzimidazoles (BZ)
‘white drenches’
These are effective against all the gut
worms, but some vary in efficacy against
N battus.
Macrocyclic lactones (ML)
‘clear drenches’
Includes avermectins (ivermectin/
doramectin) and milbemycins (moxidectin).
Can be given by drench or injection and
due to their storage in fat tissue tend to
have longer meat withdrawal periods.
Monepantel
This is a relatively new wormer on the
market so resistance to it should be
negligible at present. However it should
be used as part of a worm control regime
that incorporates some of the other
wormers, rather than a total conversion to
this wormer. This will allow it reduce the
build up of resistant populations, but at the
same time delay any build up of resistance
to the drug itself. This wormer has to be
prescribed by your vet so its use should
be accompanied by advice on when
and where to use it.
Levamisole/morantel (LM)
‘yellow drenches’
Can also be given by injection, but
tend to be high volume doses and can
be irritant. Overdosing can produce
symptoms of hyperactivity and salivation.
Worm egg counts can be used to
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Diagnose a worm problem
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Estimate the level of infection
(with some worms)
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Allow targeted timing of dosing
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Monitor efficacy of treatment or the
emergence of resistance
Use of faecal worm egg counts
Worm egg counts
are a useful
tool in a worm control programme.
10 sheep from a group should be
sampled. This is best done by loosely
holding the group of sheep in a corner
of a yard or field for 10 minutes then
letting them go. The fresh dung samples
can then be easily picked up and sent
for analysis.
Guidelines for controlling worms and
avoiding development of resistance
Quarantine treatments
These should be used for all new arrivals
on the farm.
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Assume the sheep are carrying resistant
worms (BZ & LM).
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Treat sequentially with an ML product
followed by an LM product.
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Hold sheep off pasture for 24-48hrs to
allow eggs in the gut to pass out.
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Turnout on to dirty pasture that is
contaminated with eggs and larvae.
This allows the remaining potentially
resistant worms in the gut to be 'diluted'
by infection with the 'home' population
of worms.
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Consider using Monepantel if your vet
advises this.
Test for anthelmintic resistance
This is done by post-dosing faecal egg
counts from a sample of 10 sheep. Timing
depends on the type of wormer used.
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LM - 7 days
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BZ - 10-14 days
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ML - 14-16 days
Administer the wormer effectively
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Dose at the recommended level for the
heaviest sheep in the group. Actually
weigh some of the heavier sheep.
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Check the dosing gun is working
properly and calibrated correctly.
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Use the correct technique. Dose onto
the back of the tongue not just into
the mouth.
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Restricting feed (where possible)
enhances the activity of BZ & ML
wormers. Do
NOT
do this with heavily
pregnant ewes.
Use wormers only when necessary
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Pre-tupping dosing of ewes is not usually
required as ewes at this time of year
have a low worm burden. Dosing will
tend to select for AR. Treat only gimmers
and thin ewes.
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Dosing ewes after lambing.
There is
a dilemma here as dosing may select
for AR but failure to dose will result
in contamination of the pasture for
the lambs.
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Dosing of lambs.
The risk period can be
very weather dependent. Make use of
worm egg counts to accurately determine
the time for dosing.
Select the appropriate wormer
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Avoid combination (fluke) wormers unless
this is indicated.
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Rotate the type of wormer used. An ML
wormer should be rotated with either a
LM or BZ. However rotation should not
take precedence over other factors such
as the need to treat quarantined sheep.
Preserve susceptible worms on
the farm
The traditional advice of dose then move is
good for keeping fields with low levels of
infections low for longer periods, however
it does tend to select for AR by allowing
the build up of resistant strains on the clean
pasture. Research in this area is ongoing
and possible solutions are to dose only part
of the flock or to delay the move after the
dosing to allow low levels of infections to
re-establish. Consultation with your vet is
essential before embarking on such policies.
SUMMARY
For effective control of worms on
the farm, now and in the future, it
is essential to develop a planned
worming strategy, that is regularly
reviewed and monitored.
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Consult with your vet to develop a
control programme.
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Quarantine sheep on arrival and
treat appropriately.
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Use worm egg counts.
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Administer the wormer effectively
and only when necessary.
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Carefully select the type of wormer
used.
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Use techniques that preserve
susceptible worm populations on
the farm.
SUMMER 2011 ISSUE
LIVESTOCK MATTERS
6