Multiple resistance to benzimidazole (Group I), imidazothiazole
(Group II) and macrocyclic lactone (Group III) anthelmintics
(wormers) has been reported in sheep flocks as far apart as the
south-east of Scotland, Devon and Northern Ireland. In some flocks,
the problem was first identified during the investigation of very
poor growth in lambs, highlighted by their failure to reach
finished weights by late autumn, while in other flocks the problem
was identified by routine monitoring by veterinary
Fig 1: Multiple
resistance to all anthelmintics has been reported in sheep flocks
as far apart as the south-east of Scotland, Devon and Northern
Fig 2: The
problem was first identified during the investigation of very poor
growth in lambs.
A recent document released by
the British Veterinary Association (BVA guidance on the use of
anthelmintics in grazing animals ) states that "in some areas the
emergence of multi-drug resistance is such that the only options
remaining to clients are to both clear the land and restock later
or to diversify into other business areas. Changes in the climate
with warm and wet seasons are likely to increase the problem. The
survival of these resistant populations has been enhanced by
ineffective treatment from underdosing the affected animals, the
misdiagnosis of signs leading to overuse and inappropriate use of
anthelmintics where they are not indicated".
Strategies must be
implemented now to slow down the inevitable emergence of
anthelmintic resistance, and for the control of helminth parasites
in the face of anthelmintic resistance. These issues led to the
establishment of new sheep worming guidelines, which have been
widely promoted through the publication of a Sustainable Control of
Parasites in Sheep (SCOPS) technical manual (available at www.nationalsheep.org.uk)
for sheep farmers.
Strategies must be implemented now to slow down the inevitable
emergence of anthelmintic resistance
Selection for anthelmintic
resistance occurs whenever helminths in their sheep host are
exposed to anthelmintics. Genes expressing resistance to a
particular anthelmintic or combination of anthelmintics are
believed to be present at a very low frequency in unselected
helminth populations, but with no survival advantage over
susceptible genes. When anthelmintic treatment kills most of the
susceptible helminths in the sheep, the faeces will contain mostly
eggs of the surviving resistant parasites, which consequently make
up a much greater proportion of succeeding generations.
Fig 4: When
anthelmintic treatment kills most of the susceptible helminths in
the sheep, the faeces will contain mostly eggs of the surviving
pressure is influenced by:
anthelmintic concentration the helminth is exposed to;
frequency and timing of anthelmintic treatment;
efficacy of the drug;
life expectancy, fecundity (egg production) and generation time of
proportion of the helminth population exposed to the anthelmintic
compared with that on pasture.
The SCOPS guidelines are
summarised as -
1. Work out a
control strategy with your veterinarian or advisor
2. Use effective
quarantine strategies to prevent introduction of resistant
3. Test for
anthelmintic resistance on your farm.
anthelmintics only when necessary.
6. Select the
appropriate anthelmintic for the task.
strategies to preserve susceptible worms on the farm.
dependence on anthelmintics.
Anthelmintics are an
essential treatment option and highly effective for PGE when used
correctly and in the proper circumstances. They must not however be
used without careful thought being given to the risk that each use
brings to the potential emergence of resistance. Their use must be
detailed in the veterinary flock health plan.
Not all farms have resistant
worms so quarantine treatments are vital to ensure any in-coming
sheep don't bring resistant worms with them.
Quarantine treatments are vital to ensure any in-coming sheep don't
bring resistance with them
Quarantine treatments are vital for rams to ensure any in-coming
sheep don't bring resistant worms with
Follow these 3
A - Drench all in-coming sheep with a
levamisole (Group II; yellow) drench and give them an ML (Group
III; clear drench or injectable). The use of two products minimises
the risk of any resistant worms surviving. Ensure administration is
over the back of the tongue, rather than into the mouth.
B - Keep them off pasture for 24-48 hours so
that all the worm eggs have been passed.
C -Turn them out on to dirty pasture to make
sure any eggs from worms that may have survived treatment are
diluted by worm eggs already on the pasture.
Fig 7: Turn
purchased sheep on to dirty pasture to make sure any eggs form
worms that may have survived anthelmintic treatment are diluted by
worm eggs already on the pasture
Recommendations aimed at ensuring that helminths
are exposed to an effective anthelmintic
Underdosing due to inaccurate
judgment of sheep bodyweights and faulty dosing guns remains
commonplace. Doses must be based upon the heaviest animals in the
group (lambs, ewes and rams classified as different groups). Poor
drenching technique, miscalculation of the correct dose volume and
use of inaccurate weigh scales compounds the problem. Most drenches
have a wide safety margin so if necessary, it is preferable to
overestimate the required dose volume. Accurate estimation of lamb
bodyweights can prove to be particularly difficult in ram breeding
flocks. The problem is compounded by the fact that pedigree sheep
are generally dosed more frequently than commercial animals, in an
attempt to maximise production. Underdosing may also arise
following incorrect storage of anthelmintic drugs, use of expired
product, mixing incompatible drugs or chemicals before dosing, or
use of products of dubious origin.
Fig 8: Doses
must be based upon the heaviest animals in the group (lambs, ewes
and rams classified as different groups).
Drug bioavailability may be
reduced due to rapid flow of digesta through the intestines of
scouring lambs, or due to the effects of liver disease, for example
associated with subacute fluke, on drug metabolism.
The rate of selection for
anthelmintic resistance by a helminth species is influenced by the
proportion of its total population which is exposed to the drug.
The greater the proportion of the helminth population exposed to
the drug in its sheep host compared to that on pasture at the time
of anthelmintic treatment, the faster the selection for resistance.
Thus, the rate of selection for anthelmintic resistance is highest
on those farms which achieve the most effective helminth control.
Some of the updated SCOPS worming guidelines, therefore, inevitably
involve a compromise between achieving adequate helminth control
and reducing the rate of selection for anthelmintic
Extend the interval between anthelmintic
Suppressive control of
helminth parasites aimed at preventing pasture contamination with
eggs and developing larvae, involves conventional anthelmintic
treatment of susceptible sheep at intervals close to the interval
between ingestion of the parasite as a larva and it producing eggs
(around 3 to 4 weeks in summer). This high frequency treatment
strategy has been an important factor in the emergence of
anthelmintic resistance. Anthelmintic usage and selection for
resistance could be reduced by extending the interval between
treatments although this strategy may not prevent production losses
due to parasitic gastroenteritis. This dilemma may be addressed by
basing the timing of anthelmintic treatments on faecal worm egg
counts (FWEC) which are regularly monitored. During spring and
early summer on heavily stocked farms, lamb FWECs can increase
rapidly, so substantial pasture contamination may already have
occurred by that time that intervention targets are identified.
These intervention targets must be based on previous knowledge of
the epidemiology of parasitic gastroenteritis in individual flocks
so seek veterinary advice.
Anthelmintic treatments should be based upon faecal worm egg counts
(FWEC) which are regularly monitored
Avoid unnecessary anthelmintic
Reduce the reliance on
anthelmintics by removing unnecessary treatments. In many flocks,
ewes are treated with an anthelmintic before mating in the belief
that the practice might improve lambing percentages. The benefits
of this strategy are that if the ewes are in poor body condition,
anthelmintic treatment before mating can lead to improved lambing
percentages, provided that they are subsequently well fed. However,
the disadvantages associated with anthelmintic treatment of ewes
before mating are that it does not lead to improved lambing
percentages where ewes are in good body condition, incurring
unnecessary expense and effort. Furthermore the practice possibly
selects for anthelmintic resistance by exposing parasites to
anthelmintics at a time when a significant proportion are in an
arrested, hard-to-kill state, and by affording surviving resistant
helminths with a prolonged period during which they dominate egg
production until ewes are treated again at lambing time.
Treatment at flushing (before mating) possibly selects for
anthelmintic resistance by exposing parasites to anthelmintics at a
time when a significant proportion are in an arrested, hard-to-kill
Treatment at lambing time is important to reduce the
peri-parturient increase in egg output
It is therefore difficult to
make general recommendations about anthelmintic treatment of ewes,
apart from treatment at lambing time to reduce the peri-parturient
increase in egg output. Risks must be assessed for individual
flocks and included in the veterinary flock health plan. In some
cases, the best strategy may be to treat only lean ewes before
mating. This strategy avoids unnecessary anthelmintic usage and
permits limited pasture contamination by eggs of susceptible
helminths shed by the untreated ewes.
Treatment of lambs may be unnecessary when they are grazed on safe
Treatment of lambs may be
unnecessary when they are grazed on safe pasture, or later in the
year when they have acquired some immunity. In these situations,
the decision to treat lambs or not, can be supported by monitoring
Treatment of lambs later in the year should be based upon
Integrated helminth parasite
Reliance on anthelmintics may
be reduced by the use of safe grazing. On many farms, the
opportunity exists to introduce safe grazing for a proportion of
the sheep flock by alternating annually between arable cropping or
cattle grazing and sheep grazing.
All of the new
recommendations concerned with anthemintic treatments before a move
onto safe pasture involve a compromise between helminth control and
selection for anthelmintic resistance. This risk of selection for
anthelmintic resistance may be reduced by leaving a proportion of
the flock untreated. It may be possible to target anthelmintic
treatments to scouring or lighter weight lambs, leaving faster
growing animals untreated. Alternatively, the move onto safe
grazing can be delayed for about 5 days after conventional
anthelmintic treatment. When anthelmintic treatment is considered
essential before a move to safe grazing, sheep grazing of the
pasture should be avoided during the following season.