G. Guyver-Fletcher - A model exploration of carrier transmission

A MODEL EXPLORATION OF CARRIER TRANSMISSION SUFFICIENT FOR MAINTENANCE OF ENDEMIC FMD

There are many factors which might contribute to the persistence of FMD as an endemic disease in many regions. One such factor is persistently infected livestock – animals from whom virus can be isolated more than 28 days after infection, also known as carriers. Despite anecdotal evidence of transmission from carrier animals, and recent evidence that oropharyngeal fluid from such animals can transmit infection, experiment after experiment has consistently failed to demonstrate transmission via contact-challenge. It is clear that if carrier animals can infect naïve animals, the per-capita probability of such an event must be very low.

We investigate the required intensity of transmission from carrier animals to sustain transmission of FMD, and whether this minimum intensity can be consistent with the repeated failure to demonstrate transmission from carrier animals in many experimental settings. Using a realistic model of FMD transmission explicitly modelling transmission within and between farms, as well as real shipment records and farm locations from the Republic of Turkey, we simulate the spread of FMD with different intensities of carrier spread, as well as combinations of other relevant parameters, and measure how likely the persistence of FMD over the duration changes as these parameters vary.

The model finds that it is theoretically possible for carrier animals to contribute towards persistence even at very low transmission values, and that the average duration of the carrier state is the parameter with the greatest effect on FMD persistence, as well as the average duration of disease immunity. However, more work needs to be done to understand the dynamics of FMD spread with and without carrier animals.

G. Guyver-Fletcher1,
I. Keskin2, A. Bulut2, K. Shea3, M. Ferrari3, M. Huran3, X. Li3, C. Jewell4, E. Gorsich1, M. Tildesley1

1 SBIDER, The Zeeman Institute, The University of Warwick, Coventry, United Kingdom
2 Ministry of Agriculture and Forestry, Republic of Turkey
3 Penn State University, Pennsylvania, USA
4 CHICAS, Lancaster Medical School, Lancaster University, United Kingdom

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