Modelling – progress update Stephen Catterall, BioSS 28 th November 2007.
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Transcript of Modelling – progress update Stephen Catterall, BioSS 28 th November 2007.
Contents
• Sheep flock model– Refinements
• JSRV infection model– Progression to clinical disease– Transmission
• Conclusion
Sheep flock model
• Status: it has now been implemented in ‘C’• Very fast!• Three versions: hill, upland, lowland• Refinements:
– Lamb mortality modelling– Incorporate variability between farms
240 250 260 270 280 290 300
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time
she
ep
sheeplambshoggsbreeding ewes
sample output for lowland farm
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she
ep
sheeplambshoggsbreeding ewes
sample output for upland farm
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time
she
ep
sheeplambshoggsbreeding ewes
sample output for hill farm
JSRV infection dynamics
• Status: the JSRV infection model has been implemented in ‘C’
• However, more data is needed so as to better estimate some of the parameters
• Assume:– All sheep initially susceptible
– Some sheep acquire infection (without being infectious)
– At some point later on, the sheep becomes infectious
– After some time the sheep then develops clinical symptoms
JSRV infection dynamics
SNot
infected
EInfected
Not infectious
Not clinical
IInfected
InfectiousNot
clinical
STANDARD MORTALITY
voluntary/involuntary culling
CLINICALremoval from
flock
JSRV infection dynamics
• Modes of transmission– Horizontal transmission
– Vertical transmission? close contact between the ewe and her lamb
– Indirect transmission via the environment?? not very important but cannot be excluded?
• All three modes of transmission have been implemented within the model
JSRV infection dynamics• Simulation of transmission…
– When initialising a susceptible sheep, take U from
– and compute
– set H=0
UH lnmax
)1,0(U
JSRV infection dynamics
• At each discrete timestep…– Increment H by
– the sheep acquires infection when
n
n
t
t
indirectdirect dttcfRI1
))(()(
maxHH
Sheep
E
IR
300 320 340 360 380 400 420
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01
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time
she
ep
sample model output