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During the last 35 years the pharmaceutical industry has produced a
succession of highly
effective, broad spectrum anthelmintics, and veterinarians and livestock
producers have come to
expect that worm control is easy, either by drenching or injecting cattle,
sheep and goats with
these products. This has made helminth control easy but has not fostered
conservative use of the
products. The following are strategies for the use of chemical
anthelmintics.
Suppressive (systematic) treatments
This is a strategy that has been widely applied, particularly for parasites
of small ruminants in the
tropics and sub-tropics, where epidemiological knowledge is limited or
absent. Without this
knowledge owners of sheep and goats have been forced to treat regularly to
keep their animals
alive.
Principle: Regular treatments at intervals at or near the length of the
pre-patent period of the
parasite, or if drugs with residual effect are used, the length of the
effective persistence of the
drug, whichever is greater.
Prerequisites: Availability of the chosen drugs at affordable prices.
Advantages: This approach is very effective in the short term in minimising
parasite
populations and production losses.
Disadvantages: Numerous examples from the field and modelling have clearly
demonstrated
that this strategy selects inexorably for drug resistance in the parasites.
It is also not necessarily
cost effective.
Epidemiological consequences: This strategy will initially lead to reduced
contamination of
pastures with parasite eggs and a subsequent lower challenge with infective
larvae. However,
resistance develops quickly because of the small refugia (parasites not
exposed to the chemical
agent) and consequent high selection rate. As resistance develops, the
parasite epidemiology will
change and control is lost.
Possible combination with other strategies: Suppressive strategies should
not be promoted.
Curative treatments
Treatment based on clinical diagnosis was a common practice in the past.
With the introduction
and promotion of strategic treatments, this method went out of use in most
of the areas with
industrialized livestock production. It was however still used by many
small-scale farmers. Due
to the problems of anthelmintic resistance, tissue residues and possible
negative impact of
chemicals in the environment, this strategy is being re-evaluated.
Principle: Animals are treated therapeutically, whenever production losses
and/or
uncontrolled disease is considered to be significant. The trigger for
treatment has been based on
different criteria according to knowledge or interest and availability of
support services. The
triggers include:
Any clinical signs or evidence of sub-clinical disease.
Rise in faecal egg count. Attainment of threshold levels of EPG in groups of
monitored
animals indicates a need for treatment. The threshold will vary according to
the composition of
the parasite population, host type and geo-climatic conditions.
Anaemia in sheep and goats identified using systems such as the FAMACHA
method (Van
Wyk and Van Schalkwyk, 1990) (see below for more details).
In order to best utilize 2. and 3. it is necessary to know the composition
of the parasite
population on the farm. In cases where treatment has been initiated using 1.
or 2. as indicators of
infection level, two different approaches have been applied:
Treat all animals in the herd or flock.
Treat only those animals that are perceived to need treatment.
The use of FAMACHA specifically aims to differentiate between the animals in
a group that
need treatment and those that do not.
Prerequisites: A regularly applied monitoring system such as clinical
examination, faecal
egg counts or FAMACHA.
Advantages: Reduced expenses for anthelmintics as number of annual
treatments will be
lower and, if selective treatment is practised, the number of animals
treated will be lower. The
possibility of selecting for resistance is significantly reduced, and the
risk of selection is delayed
if only some animals are treated, as this will ensure the presence of a
susceptible parasite
population. Regular monitoring in the context of improved animal production
and health
management.
Disadvantages: Regular monitoring needs labour input.
Epidemiological consequences: These will depend on the variables. This
strategy may not,
however, reduce the overall contamination level and subsequent numbers of
infective larvae on
pasture. With EPG or FAMACHA monitoring, the sub-clinical effect should be
controlled (Van
Wyk and Schalkwyk, 1990).
Possible combination with other strategies: No validated, integrated
strategies have yet been
developed.
Monitoring of Haemonchus infections using the FAMACHA system
Based on the strong correlation that exists between the coloration of the
mucous membranes of
the conjunctival sac of sheep, and the degree of anaemia (packet cell volume
(PCV)) caused by
the blood-sucking parasite Haemonchus contortus, a standardized test known
as the FAMACHA
system has been developed by South African scientists (Van Wyk and Van
Schalkwyk, 1990).
FAMACHA ANAEMIA GUIDE
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Principle: Based on the above-mentioned correlation this assay uses a
standardized colour
chart showing illustrations of sheep eyes with colour variations from bright
pinkish red to almost
white. Treatment is recommended when the colour of the mucous membranes of
sheep matches a
tint that is correlated with anaemia.
Advantages: The method is easy and cheap to apply for continuous monitoring
and it is
easily taught to farmers. The use of pictures and signs make it suitable for
illiterate sheep owners.
There is a substantial reduction in the costs of drenching. A lower rate of
selection for
anthelmintic resistance is expected.
Disadvantages: Currently the method only applies to infection with
Haemonchus contortus
and the assay is only validated for sheep.
Possible combination with other strategies: Based on records that identify
which animals
require repeated treatment, it is possible for the sheep owner to cull these
and breed from the most
resistant animals, increasing the overall resistance of the flock.
Current strategies based on modified use of anthelmintics
The Worm Kill principle
As a consequence of widespread anthelmintic resistance in sheep parasites in
Australia, the Worm Kill programme was developed. The main aim of this programme was
to reduce the
number of treatments, while maintaining effective control of parasites. This
was done by the use
of a narrow spectrum drug, closantel, in combination with a minimum number
of treatments with
broad spectrum anthelmintics. Closantel is particularly effective against
Haemonchus and has a
persistent effect for 2 to 3 months.
Strategic treatments based on epidemiology
Clinical parasitic disease in ruminants usually occurs at or shortly after
times of peak larval
availability. The timing of peak larval availability on pasture is of
crucial importance in
understanding the population dynamics of the parasite population, because
this is when the largest
worm burdens are acquired. It is in order to prevent these seasonal peaks
from developing that
strategically timed control measures are implemented. Thus treatments are
often administered at
times when the larval challenge on pasture is low and the majority of the
parasite population is in
the host. This reduces the pool of susceptible parasites and may increase
selection pressure for
resistance. Due to the effect of the climate and weather on development,
survival and
transmission of free-living stages, weather conditions play a dominant role
in determining the
timing of strategic treatments. Geographical differences in the seasonal
availability of infective
larvae from the pastures have similarly been a key factor for determining
the timing of strategic
treatments. In temperate climatic zones, sequential treatments at the
beginning of the grazing
season, using intervals similar to the pre-patent period or pre-patent
period plus the length of the
residual effect of the drugs, have been used. Similar sequential treatments
have been applied at
the beginning of the rainy season in tropical zones, with one treatment
added during the dry
season when pastures would be almost sterile.
Principle: Strategic treatments are administered, not only for therapeutic
purposes to rid the
animals of worms, but also as a prophylactic measure to prevent future
contamination of pasture
and reduce the risk of future re-infection (Barger et al., 1999; Barger et
al., 1994a).
Prerequisites: Knowledge of local epidemiology of relevant parasite species.
Advantages: Proven record of reducing contamination of pastures with
parasite eggs, and
subsequent challenge of grazing animals with infective larvae. This has
resulted in significant
increases in productivity.
Disadvantages: Some of the strategies are associated with strong selection
pressure for
anthelmintic resistance.
Epidemiological consequences: Significant reductions in egg excretion and
pasture larval
contamination (Barger et al., 1994b).
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