This document summarizes a model developed to understand how climate change may impact parasitic worms (helminths) that infect livestock. The model examines the lifecycle of these parasites, which have free-living stages affected by climate, and infectious stages that infect hosts. The model shows that higher temperatures can increase parasite burden by speeding larval development. Incorporating spatial heterogeneity and grazing behavior into the individual-level model indicates these farm-level processes also influence outbreak intensity under climate change scenarios. The model can be used to predict parasite risk from climate change and assess control strategies.

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Leading the way in Agriculture and Rural Research, Education and Consulting
Modelling macro- parasite risk in a changing climate
Naomi.Fox@sruc.ac.uk Disease Systems Team, Animal and Veterinary Sciences Group, SRUC,Edinburgh, UK

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Background
General helminth lifecycle
Non-infectious stage
Infectious stage
Parasitic worms (helminths) offer one of the most pervasive challenges to livestock
They cause weight loss, decreased production efficiency and even death of the host
The free-living stages of the lifecycle are heavily influenced by climatic conditions
Climate change is already affecting the prevalence and distribution of
macro-parasites
Adult parasite

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Need to understand the potential impacts of future climate change to prepare for changing risk, implement control measures and set up long term management strategies.
Aim:
To determine how changes in climate sensitive parameters affect parasite dynamics
Background

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Adults(AkImmunity(r) )
Infective
larvae (L3)
Non infective
qL1 Larvae(L1)
ωL1
ρL3
 rA
r  (r)A
Non-spatial, population level model
L3
Mechanistic transmission model of gastrointestinal helminth in
grazing livestock, including survival and development of the
parasite’s free-living stages

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Results: General infection pattern
Adults parasites in the host
Infectious larvae ingested per day
Hosts resistance to infection
Increase in adult parasites & host resistance as infectious larvae are ingested
Host resistance then impedes parasite establishment and fecundity
Parasite burden decreases as adult parasites die off and are not replaced
(Fox et al., 2013)

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Results: Impacts of climate
Higher temperatures  increased development rate  higher parasite burden Climate change can lead to non-linear increases in parasite risk, as small temperature changes around critical thresholds can lead to sudden increases in parasite intensity.

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Transmission is a complex process
•Macro-parasite transmission is influenced by:
–Host behaviour
–Spatial effects
–Larvae survival
–Larvae development
–The hosts immune response
Climate change will impact on many stages of the transmission process.
Aim
To incorporate wider elements of transmission to test the robustness of results to farm-level processes

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Grass height
Faecal contamination
Infective Larvae
Distance from cow
Parasite burden
Immunity
Grazing decisions
The spatial, individual level model

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i(k)
Stomach
contents (sk)
Faeces(fi)
J(k)
Adults(AkImmunity (r ) k)
Infectious
larvae (L3i)
Larvae
(L1i)
Eggs(Ek)
qL1i
ωL1i
patch cow
φfi
ρL3i
fi
i h h e    (  ) 0
k r k k (r )A εEk
Grass
height(hi)
The spatial, individual level model

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Results: Spatial heterogeneity and grazing behaviour
Day
Faecal contamination
L3 larvae
Sward height

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Influence of other factors
•Our model outputs are robust to the inclusion of more complex, farm level processes.
•These farm-level factors (e.g. spatial heterogeneity in infection risk and resource distributions, host grazing behaviour, host immunity) influence outbreak intensity under different temperature change scenarios.

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Conclusions
•Climate influences parasite risk e.g. by changing development rates of the free-living stages
•Small changes in climate around critical thresholds can lead to sudden increases in parasite intensity
•Climate change will affect broader elements of the system, and there will be farm level variation in the response to climate change
•This model can be used to predict the impacts of climate change on parasite risk, and asses the efficiency of different control strategies

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Thanks
Mike Hutchings (SRUC)
Ross Davidson (SRUC)
Glenn Marion (Biomathematics & Statistics Scotland)
Piran White (University of York)
This work was funded by the Scottish Government
Questions?
Naomi.Fox@sruc.ac.uk