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O
ver the last decade the problem of treating worms in sheep has
been further challenged by two emerging problems, namely
anthelmintic resistance (AR) and changes in the disease patterns
of many of the common parasitic worms. Any worming regime must now
take these factors into consideration if it is to remain effective in both the
short and long term.
Veterinary Surgeon
Bob Norquay
XLVets Practice
Northvet, Orkney
The prevalence of anthelmintic resistance to
certain drugs is now fairly widespread in the
UK, with recent studies reporting resistance to
benzimidazoles (BZ) to be as high as 80% on
lowland farms and 55% on hill/upland farms.
Similarly resistance to levamisole (LM) was 47%
on lowland and 29% on hill/upland farms.
Only a very small amount of resistance to the
ivermectin type drugs has been recorded, but
if these drugs are to remain effective then it is
essential that they are used as part of a
planned worming regime rather than random
indiscriminate dosing.There is therefore a very
real need for farmers to act to slow down the
emergence of resistant worm populations.
The cause of the changes in the disease
patterns of parasitic worms that have
been seen throughout the UK in the last
few years is uncertain, but may be due
to climate change resulting in conditions
that are more favourable to the survival of
these parasites.
Nematodirus battus was traditionally a
spring/early summer problem, but has
now been seen at all times of the year.
Haemonchus contortus 'Barber's pole worm'
has extended its domain from originally
in the South of England,to the far North
of Scotland.
The life cycles of most of the parasitic
gut worms are similar with one or
two variations, which are important
to the control of these worms.
Nematodirus battus eggs can survive over
winter in prolonged cold conditions and
require a climatic stimulus (cold followed
by warmth) before hatching. This allows
Nematodirus infections to pass from one
year's lambs to the next.
Haemonchus is a prolific worm producing
up to 10,000 eggs per day which develop
to the infective larval stage very quickly
in warm wet conditions. The high egg
production rate means that relatively clean
pasture can become highly infective very
quickly leading to significant and
unexpected disease outbreaks.
Worming
regimes for sheep
WORMING SHEEP
BOB NORQUAY
For effective control of worms on the farm, now and in the
future, it is essential to develop a planned worming control
strategy, that is regularly reviewed and monitored.
Pasture stages
Basic Life Cycles
Second stage
larvae
Infective third
stage larvae in
2-12 weeks
Eggs
First stage
larvae (free)
5
LIVESTOCK MATTERS
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