Transcript Parasite population models Parasite control

Parasite control
Objectives
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Describe the principles of control
Describe types of anthelmintic usage
Be aware of organised control schemes
Understand the value of models in
predicting the outcome of treatments
Parasite control
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Flock nutrition
Grazing management
Stock management
Using anthelmintics
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Suppressive
Curative
Strategic
Quarantine drench
• Testing for resistance
Nutrition
• Feed young stock and ewes well. Under-nourished stock
have a reduced capacity to resist parasites
• Immunity to parasites is related closely to bodyweights.
It is essential that target weights should be established
and stock monitored & fed accordingly
• Supplementary feeding of weaner/hogget sheep with
protein feeds will increase both bodyweight and parasite
immunity
• Feeding in troughs preferable
Grazing Management
• Use of cattle to prepare weaner/hogget pastures
with a 6 month rotation
• Match young stock DSE with cattle DSE
• Use of dry sheep older than 3 years of age to aid
decontamination of weaner/hogget pastures
• The 3 year + dry sheep have well developed
parasite resistance if maintained in good condition
• Cell or strip grazing in some conditions
Stock Management
• Tops & tail strategy. Weaner supplementation targeted
at tail of mob. Monitor weights of young stock and
feed accordingly
• Culling sheep that are “worm factories” where possible
• Consideration given to leaving 5% mob [heaviest]
untreated, in mobs where natural immunity is
established
• Consideration could also be given to destocking sheep
from pasture for 15 months and running cattle on that
portion of the property, to create a safe/clean paddock
Some sheep have more worms than others.
Which ones would you sell?
Drench usage
• Know your drenches
• Some companies sell “solutions” for
resistance
• Mixtures are useful
• Treating only heavily infected animals is
being trialed
• There are Government sponsored schemes
• You can use drenches in different ways
Using drenches –
Quarantine Drenching
• Stock purchased onto a property should
receive a quarantine drench and be isolated
on a contaminated paddock before being
run with the main flock
• The cleanout treatment should consist of
ML [preferably moxidectin], BZ & LEV
• “Q drench” is marketed for this purpose
Suppressive (Systematic) Treatments
• Regular treatments at intervals equal to the prepatent period of the parasite, or the ppp plus
persistence period.
• Advantages: Very effective, in the short, term
in minimising parasite populations and
production losses.
• Disadvantages: Rapidly selects for drug
resistance.
• This strategy should not be promoted
• Temporal refugia low
Curative Treatments
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Treatment based on clinical diagnosis.
Only animals perceived to need treatment
are treated
Trigger mechanisms include:
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Signs of sub-clinical and clinical symptoms
Faecal egg count monitoring.
Using bio-assays (e.g. FAMACHA)
Weight loss
Benefits: it slows selection for resistance.
But: production losses occur
Temporal (& spatial) Refugia high
FAMACHA
• Way to select anaemic sheep with haemonchosis for
treatment and culling
• Curative treatment
• Costly in Australia (developed in RSA)
• Reduces treatment frequency so reduces R selection
• Various references
Strategic Drenching
• Aims to prevent contamination of
pastures with eggs and larvae
• Requires knowledge of local conditions
for egg development and larval survival
• Has a proven record in control
• May select strongly for resistance
• Treatment is applied when larval pasture
levels are low (Spatial Refugia low)
Summary - Control of resistance
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Be aware of refugia issues, do not treat all sheep onto a
“clean” pasture) (ie drench and move is not best)
Administer drugs correctly, give full & effective doses,
starve sheep before a ML or BZ drench, yard 24h post
treatment see http://www.csiro.au/WormTreatment
Use mixture of drugs with similar half lives, if this is not
feasible rotate annually between drugs with a different
mode of action
Where possible use non-persistent chemicals
Provide a quarantine treatment/regimen for all imports
Reduce drench frequency by using strategic control &
developing a flock structure with immune hosts
Leave some sheep untreated.
Wormkill
• Integrated, strategic worm control originally based
on closantel
• Northern Ranges of NSW in summer rainfall
zones
• Flock management plus monitoring
• Highest compliance worm control approach in the
world
• Closantel resistance emerged now rotations reduce
pressure on R
• NSW Ag
Drenchplan
• Integrated strategic control relying on 1-2 summer
drenches (Nov/Dec & Feb) also drench weaners
• Drenches OK but pasture management is complex
• Combinations to delay R
• Treat in summer so low refugia and R selection.
Drench lambs at risk
• NSW Ag
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This is a model
What is a model? What is a good model
Computer models
Host/parasite
systems are
complex
especially when
environmental
and
management
aspects are
superimposed.
Examples of models for parasite
control
• PARABAN (cattle)
• WORM WORLD (Sheep)
• TROPICAL WORM WORLD (sheep
in tropical climates)
WormWorld Simulation Model
Negative Binomial Distribution
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Low k means more aggregation
Typically k  0.3 - 2
As k   distribution tends to Poisson.
Models
need to
account for
stochastic
distributions
proportion of flock
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0.40
0.35
negative binomial
k = 0.3 mean = 10
0.30
0.25
0.20
0.15
Poisson mean = 10
0.10
0.05
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2
4
6
8 10 12 14 16 18 20 22 24
number of worms
Input parameters
• Flocks
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Number of sheep
Age
Sex
Initial worm burden
Immunity
Lethal burden
Lambing dates and data
Input parameters
• Pasture
– Number and size of paddocks
– Initial contamination
• Management
– Dates
– Paddock moves
– Treatments
• Genetics
– Genes for R
– Gene frequency
– Efficacy against phenotypes (ie. dominant, recessive)
Outputs
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Larvae on pasture
Eggs in faeces
Worms in sheep
Deaths
Drug resistance gene frequencies
More about models
• Models must be validated
• They allow us to ‘test’ parasite control
measures before we use them
• They can tell us what is important in using
drugs sustainably
• They can tell us about the vulnerability of
different parts of the life cycle
lamb deaths per 2000
Examples:
50
40
30
20
100
10
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60
80
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60
90
% efficacy
100
%
responding
Now a demonstration of
Tropical Worm World
Rotate 22 days
Rotate 4 days
Rotate 22 plus a drench
Challenge
Sloths
Bots
Scapegoats
Pumas
Moose
Brumbies
Tigers
PP