Managing Complexity: Simplifying Assumptions

for FMD models for swine

Amy Kinsley, DVMKimberly VanderWaal, PhD

Meggan Craft, PhDRobert Morrison, DVM, PhD

Andres Perez, DVM, PhD

Vesicular lesions on feet of pigs experimentally infected with Senecavirus A.

Vesicular lesions on feet and snout of experimentally infected with Senecavirus A.

Recent outbreaks of vesicular disease

FMD: Timing of clinical signs and transmission

• Affected animals develop lameness, decreased appetite, vesicles in and around the mouth and on the feet-varies by species

• Mortality in adults is low but can reach up to 100% in young

• Transmission occurs through direct contact, indirect contact, and aerosol spread

Distribution of Livestock in the United States

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1 Dot = 20,000 Hogs and Pigs

1 Dot = 10,000 Cattle and Calves

1 Dot = 1,000 Sheep and Lambs

Individual HerdProduction system

Why use models?

To predict the outcome of a particular action on a system

To test, verify, and improve our understanding of a system

To explore the effects of population scales

To characterize transmission dynamics of infectious diseases to guide policy decisions on disease outbreak management.

Each farm has an equal chance of mixing with every other farm in study.

Homogenous mixing within a defined geographical area.

(Pomeroy et al., 2015)

Transmission is scaled inversely with distance from infected farm.

All means of contact that can cause FMDV transmission that is spatially independent.

Current models: Methods used to represent contact

Each farm has an equal chance of mixing with every other farm in study.

Homogenous mixing within a defined geographical area.

(Pomeroy et al., 2015)

Transmission is scaled inversely with distance from infected farm.

All means of contact that can cause FMDV transmission that is spatially independent.

Current models: Methods used to represent contact

Objective: To develop a within-herd simulation model to estimate the spread of FMD on a swine farm.

• Because FMD control strategies are implemented at the herd-level (Pomeroy, 2015).

• FMDV is highly transmissible, therefore immediate or rapid spread through the population (Keeling, 2001).

• Within-herd dynamics may vary depending on farm structure which can have rippling effects on down the transmission chain, which is particularly important in swine.

Demographic and structural model representing the number of animals in each room on a swine farm on a daily basis

Conceptual transmission model

A stochastic, state-transition, frequency-dependent, transmission model (within- and between-room transmission)

Two strains: O/TAW/97 and O/NET/01.

Accounted for piglet death in the O/TAW/97 model (farrow to finish and farrow to wean only)

LS Is RIc

Methods and Approach

• We ran 1,000 simulations to account for the stochastic nature of an outbreak and account for biological uncertainty.

• Integrated the models using R to simulate the spread of FMDV within a herd.

Data Analysis

• The mean time to detection was calculated based on the proportion of animals displaying clinical signs in

each of the stages of production.

Data Analysis

• The mean time to detection was calculated based on the proportion of animals displaying clinical signs in each of the stages of production.

• The mean duration of the epidemic was calculated at the time from the introduction of the index case to the time when the number of infected pigs reached zero.

• The mean time to the peak of the epidemic and themean number of infected pigs was calculated for the duration of the epidemic.

Within-herd model conclusions

Most practitioners feel confident that FMD will be detected in a herd when less than 15% is demonstrating clinical signs, which is at about 11-12 days after introduction into the herd.

Sow farms may experience FMD persistence or a longer period of infectiousness to other farms.

However, the time to detection will likely we influenced by the mortality rate in piglets which may vary by FMD strain. Identifying the cause of piglet mortality is important.

Dr. Andres Perez and Dr. Meggan Craft

Dr. Kim VanderWaal

Dr. Bob Morrison

Experts and veterinarians involved in the development of parameter values

Acknowledgements

Questions?