Modelling – progress update

Stephen Catterall, BioSS

28th 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

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02

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40

06

00

80

01

00

01

20

0

time

she

ep

sheeplambshoggsbreeding ewes

sample output for lowland farm

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10

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15

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time

she

ep

sheeplambshoggsbreeding ewes

sample output for upland farm

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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

02

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40

06

00

80

01

00

01

20

0

time

she

ep

sample model output

In summary

• Sheep flock model– Complete subject to a few refinements

– A paper describing the model is being written

• JSRV infection transmission model– This has been implemented in ‘C’

– Three modes of transmission modelled

– ‘Estimation’ of key parameters is still required