D o rp e rn uus • D o rp ernews 2 0 1 2 - Dorper SA

Dorpernuus • Dorper news 2012
are useful to decide: 1, if treatment is necessary, 2,
if previous treatment were successful, 3, to show
worm resistance and 4, to assess the level of worm
contamination on the pasture. In case off high
rainfall and high infestation the EPG count is just
an indication of how successful the dosing program
are.
- Grazing strategy: An understanding of seasonal
changes in pasture larvae numbers is inherent to a
successful control program. Many larvae can survive
on pastures through the winter or even longer. If
sunny, dry conditions prevail, larvae numbers may
decline dramatically. Larvae eaten by sheep as
they begin the grazing season go through the twoweek
development and begin to produce eggs.
Since one larva can result in an adult that produces
thousands and thousands of eggs, a multiplication
in parasite numbers occurs. This is particularly true
if moist, warm weather conditions are favorable to
the development of larvae from eggs. From midsummer
on, if weather conditions are appropriate, a
large number of larvae accumulate on the pasture.
This is referred to as the “midsummer explosion”
in larval numbers. If hot, dry weather conditions
prevail, larval numbers will decrease due to the
killing effect of drying.
Avoid grazing contaminated pasture with parasitesusceptible
animals such as weaners or lambing ewes.
- Maintaining good nutrition and good animal health
care: Worm management strategies will have little
impact if a general animal health program is not
observed. Internal parasites tend to predispose
their hosts to other infections; it is therefore a good
practice to maintain flock health in general through
proper housing, adequate sanitation (clean water)
and appropriate vaccination against diseases.
Nutrient supplementation can greatly offset the
delirious effects of parasitism and hence improve
the production performance of parasitised animals.
- Building worm resistance in the flock: Sheep and
lamb vary in their susceptibility to internal parasites.
Sheep more susceptible are weaned animals, late
born lambs, yearlings, high producing female
and thin animals, periparturient ewes, unadapted
breeds and stress animals. More resistant animals
are mature sheep, dry sheep, sheep in good body
condition and fat sheep.
Older sheep have the ability to develop some
immunity to parasites. However, this resistance is
not complete and may break down during times
when sheep are challenged with high numbers
of infective larvae of Haemonchus. Treatment of
mature ewes is, important in an attempt to prevent
infection of young animals. Resistance will vary
within breeds as well. Some animals, regardless of
breed, will be more resistant to parasites than others.
Having parasite-resistant animals will decrease the
need for dewormers. Additionally, knowing when to
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treat your sheep is essential.
- Biosecurity: measures for new sheep arrivals on the
property or any outbreak of worm disease in your
flock. Internal parasite control starts with good
management and common sense. Sheep should not
be fed on the ground. Feeders, which cannot easily
be contaminated with feces, should be utilized for
grain, hay, and minerals. Water should be clean and
free from fecal matter. Pastures and pens should not
be overstocked.
- Safe pastures: Clean or safe pastures are pastures,
which are not contaminated with the worm larvae
that affect sheep. Examples of clean pastures
include pastures that have not been grazed by
sheep or goats for the past 6 to 12 months; pastures
which have been grazed by horses or cattle; pasture
fields in which a crop has been removed; pasture
fields which have been rotated with field crops;
and pastures that have been recently established
or renovated by tillage. Decreasing the stocking
rate decreases the number of worms spread on a
pasture. The more animals you have on one pasture,
the more densely the worms are deposited. Animals
on densely stocked pastures are more likely to have
parasite problems. Sheep (and goats) are generally
not affected by the same internal parasites as cattle
and horses. Consequently, pastures grazed by cattle
and horses are safe(r) for sheep (and goats) and
conversely.
- Proper anthelmintic use: Anthelmintics are still an
important part of parasite control. However, they
must be used properly to ensure effectiveness of
treatment and slow down the rate by which worms
develop drug resistance. Weights of sheep and lambs
must be known or approximated accurately in order
to calculate the proper dosage of medicine. Under
dosing results in the survival of worms, which are,
resistant to the anthelmintic used. If an anthelmintic
is more slowly absorbed in the gut, drug levels are
prolonged and the treatment may be more effective.
Thus, fasting sheep for up to 24 hours may improve
efficacy of dewormers. Using the same anthelmintic
or drugs from the same chemical family will increase
the rate at which worms become resistant to the
drugs. To lessen the effect, anthelmintics (or drug
groups) should be rotated on an annual basis
- Minimising the risk of drench resistance developing
on your farm: Small numbers of parasites within
a population are not affected by any of the drugs
and an increase in the occurrence of their genes
contributes to the development of resistance to
these drugs. There are some factors which magnify
the rate of development of resistance in worms:
timing of use of anthelmintics, climatic factors,
drenching frequency, treatment of all the animals in
the flock at any one time and under dosing.
There is no “one size fits all” in gastro intestinal
parasite control. Worms eat into the pockets of small