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Stevie Hay
SPRING 2011 ISSUE
S P R I NG F E AT U R E
LIVESTOCK MATTERS
8
There has been little or no reported evidence
of resistance in the Nematodirus species and I
found no evidence in the samples I examined.
Strongyloides spp. are not generally thought
to be very clinically significant and unless we
find large numbers associated with a scour
we don't specifically recommend treating.
I bear in mind that a strongyloides count can
be much more unreliable as there may have
been larval hatching. I therefore decided to
look at the results specifically relating to the
strongyle counts as these have much more
clinical importance.
I was expecting to find evidence of white
drench resistance as historically we have been
told this has been around for a long time -
and I was mindful that there has been triple
resistance found in North Devon. What
concerned me most was that on 9 groups of
sheep the post treatment worm egg count was
unchanged or higher then the pre treatment
sample - sometimes considerably higher.
One particular farm with a flock of around
700 ewes has three distinct groups of sheep.
This farmer marked the sheep that he sampled
so that he could take the second sample from
the same animals. In two out of three groups
the egg count increased after treatment with
an avermectin. The third group had only a
43% reduction. He was advised to change
his wormer! Later in the season he brought a
faeces sample from lambs dosed three weeks
previously with a levamisole; this had an
egg count over 2,000 epg. I would be
reasonably sure that this farm is likely to have
triple resistance.
Looking at the responses to the questionnaire -
it's not surprising that there is so much
resistance around. About half the farmers
quarantine dose incoming sheep, but generally
it is not being done according to SCOPS
(Sustainable Control of Parasites in Sheep)
recommendations. Several of the worm egg
counts early in the season were low and the
flocks did not require dosing. Very few farmers
(other than organic farms) regularly have
faecal egg counts done to determine if dosing
is needed and most are treating their ewes
more than once a year, which in the majority
of cases is not necessary.
In total, only half the flocks sampled achieved
over 75% egg reduction after dosing and
more worryingly a third are getting less than
50% egg reduction. If this applies to all flocks
in the area that's a lot of resistant worms!
On the whole SCOPS principles are not
being followed in full on the majority of farms.
A lack of effective quarantine dosing and
over-treatment seems to be allowing resistance
to continue to spread. Most farmers also get
their worming advice from the merchants
where they buy their wormer, rather than
asking their vet, so often we are only
approached if they perceive that the wormer
‘hasn't worked’. Now I've got some idea
of the resistance levels in our area hopefully
I can encourage our clients to involve us in
their worming management plans.
OV E RA L L R E S U LT S
Overall egg reduction (of all three
egg types):
Benzimidazole (15 treatments)
l
73% had less than 50% reduction
l
20% had over 75% reduction
Levamisole (10 treatments)
l
30% had less then 50% reduction
l
60% had over 75% reduction
Avermectin (16 treatments)
l
25% had less than 50% reduction
l
50% had over 75% reduction
Samples were examined
under the microscope in
our own laboratory
S TRONGY L E TY P E EGG
R EDUCT I ON
Benzimidazole
l
46% less than 50% reduction
l
40% over 75% reduction
Levamizole
l
40% less than 50% reduction
l
50% over 75% reduction
Avermectin
l
25% less than 50% reduction
l
63% over 75% reduction
Strongyle type eggs - the main cause of PGE
in sheep
These store lambs purchased at market were carrying a high worm burden