  Market trade-routes can support infectious-disease transmission, impacting
biological populations and even disrupting causal trade. Epidemiological models
increasingly account for reductions in infectious contact, such as
risk-aversion behaviour in response to pathogen outbreaks. However, market
dynamics clearly differ from simple risk-aversion, as are driven by different
motivation and conditioned by trade constraints, known in economics as
friction, that arise because exchanges are costly. Here we develop a novel
economic-market model where transient and long-term market dynamics are
determined by trade friction and agent adaptation, and can influence disease
transmission. We specify the participants, frequency, volume, and price in
trade transactions, and investigate, using analytical insights and simulation,
how trade friction affects joint market and epidemiological dynamics. The
friction values explored encompass estimates from French cattle and pig
markets. We show that, when trade is the dominant route of transmission, market
friction can be a significantly stronger determinant of epidemics than
risk-aversion behaviour. In particular, there is a critical friction level
above which epidemics do not occur. For a given level of friction, open
unregulated markets can boost epidemics compared with closed or tightly
regulated markets. Our results are robust to model specificities and can hold
in the presence of non-trade disease-transmission routes. In particular, we try
to explain why outbreaks in French livestock markets appear more frequently in
cattle than swine despite swine trade-flow being larger. To minimize contagion
in markets, safety policies could generate incentives for larger-volume,
less-frequent transactions, increasing trade friction without necessarily
affecting overall trade flow.
