Simulation of the influence of Danish cattle markets on a Foot-and-Mouth epidemic

During the epidemic of Foot-and-Mouth disease (FMD) in the United Kingdom in 2001, live animal markets had large influence on the spread of the disease. The culture of and behavior around markets are expected to be different between countries. During the last decade, the number of animals traded through markets in Denmark has decreased and only few cattle markets are left.

The purpose of this study was to investigate, whether cattle markets would influence the duration, size and economic consequences of a potential FMD epidemic in Denmark. The spread of FMD was simulated using the stochastic and spatial disease-spread InterSpread Plus, version 2.001.11. From Danish databases, we collected data movements of animals.

These were used to model movements of animals for each individual herd. For movements of cattle to and from markets, we modeled the frequency of movements to markets for the individual herd and categorized herds that could receive contacts from markets. The epidemics were initiated in herds with market contacts.

In a basic market scenario, we used the individual herds’ probability of moving animals to markets, while in a control scenario we reduced all probabilities of movements to markets to zero, to reflect a situation with no markets. Each scenario was initiated in 100 different herds (index), and for each index herd, the model was run 100 times.

The number of extra contacts generated through a market was set to 3.5 and the probability of transmission from markets was modeled as a normal distribution with a mean of 0.415 and a standard deviation of 0.06. This probability was a combination of the risk from purchase of animals from markets and the indirect contact from visitors on markets.

Danish markets would be closed as soon as FMD is detected. Therefore, markets were only active during the first three weeks of the epidemic, as time from infection to first detection was modeled as 21 days. The results was described as epidemic duration (from first detection day to last detection day), number of detected herds and infected area, direct costs (costs of surveillance visits, tests, slaughter etc.) and indirect losses due to the epidemic (export losses).

The results show an effect of markets on the size and duration of a FMD epidemic (Table 1); the median duration of epidemics is 1 week longer, and 28 more herds are detected with FMD. In the scenarios with markets, the epidemics included a larger area compared to scenarios without markets. Economic results will be described in the final paper.

Markets can influence spread of other diseases as well. Little is known about the influence of markets on spread of other diseases. Even though FMD is more contagious than many other diseases, the markets effect on spread of other diseases might be more prominent, as the activity of markets is not influenced by the presence of endemic diseases.