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Potential impact of an introduction of foot-and-mouth disease into the California State Fair

Tim E. CarpenterDepartment of Medicine and Epidemiology and the Center for Animal Disease Modeling and Surveillance (CADMS), School of Veterinary Medicine, University of California, Davis, CA 95616

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Lasse E. ChristiansenDepartment of Medicine and Epidemiology and the Center for Animal Disease Modeling and Surveillance (CADMS), School of Veterinary Medicine, University of California, Davis, CA 95616

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Bradley F. DickeyDepartment of Medicine and Epidemiology and the Center for Animal Disease Modeling and Surveillance (CADMS), School of Veterinary Medicine, University of California, Davis, CA 95616

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Clair ThunesDepartment of Medicine and Epidemiology and the Center for Animal Disease Modeling and Surveillance (CADMS), School of Veterinary Medicine, University of California, Davis, CA 95616

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Pamela J. HullingerCalifornia Department of Food and Agriculture, 1220 North St, Sacramento, CA 95814

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Abstract

Objective—To estimate the potential spread of foot-and-mouth disease (FMD) if infected livestock had been exhibited at the 2005 California State Fair.

Design—Epidemic model.

Animals—Dairy cattle, dairy goats, and pygmy goats exhibited between August 24 and August 28 by 195 exhibitors.

Procedures—2 stochastic models were used to simulate epidemics of FMD that might originate from 1, 3, 5, 7, or 10 index cases at the state fair. Data obtained from state fair exhibitors were used to determine the spatial distribution and types of herds to which livestock visiting the state fair returned.

Results—Mean estimated numbers of latently infected animals on day 5 were 12.3 and 75.9, respectively, when it was assumed that there were 1 and 10 index cases. Regardless of the number of index cases, mean estimated numbers of subclinically infected and clinically infected animals were low throughout the 5-day study period. Mean estimated duration of the resulting epidemic ranged from 111 to 155 days, mean number of infected premises ranged from 33 to 244, and mean probability that at least 1 animal that became infected with FMD would subsequently leave the state ranged from 28% to 96% as the number of index cases increased from 1 to 10, respectively.

Conclusions and Clinical Relevance—Results suggested that following introduction of FMD at the California State Fair, infection would likely go undetected until after animals left the fair and that the subsequent outbreak would spread rapidly.

Abstract

Objective—To estimate the potential spread of foot-and-mouth disease (FMD) if infected livestock had been exhibited at the 2005 California State Fair.

Design—Epidemic model.

Animals—Dairy cattle, dairy goats, and pygmy goats exhibited between August 24 and August 28 by 195 exhibitors.

Procedures—2 stochastic models were used to simulate epidemics of FMD that might originate from 1, 3, 5, 7, or 10 index cases at the state fair. Data obtained from state fair exhibitors were used to determine the spatial distribution and types of herds to which livestock visiting the state fair returned.

Results—Mean estimated numbers of latently infected animals on day 5 were 12.3 and 75.9, respectively, when it was assumed that there were 1 and 10 index cases. Regardless of the number of index cases, mean estimated numbers of subclinically infected and clinically infected animals were low throughout the 5-day study period. Mean estimated duration of the resulting epidemic ranged from 111 to 155 days, mean number of infected premises ranged from 33 to 244, and mean probability that at least 1 animal that became infected with FMD would subsequently leave the state ranged from 28% to 96% as the number of index cases increased from 1 to 10, respectively.

Conclusions and Clinical Relevance—Results suggested that following introduction of FMD at the California State Fair, infection would likely go undetected until after animals left the fair and that the subsequent outbreak would spread rapidly.

Contributor Notes

Dr. Christiansen's present address is Informatics and Mathematical Modelling, Technical University of Denmark, DK-2800 Lyngby, Denmark. Dr. Dickey's present address is Operations Evaluation Group, Center for Naval Analyses, 4825 Mark Center Dr, Alexandria, VA 22133. Dr. Hullinger's present address is Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550.

Supported by the Department of Homeland Security and the National Center for Foreign Animal and Zoonotic Disease Defense.

Presented in part at the 2005 meeting of the Council of Research Workers in Animal Diseases, St Louis, December 2005.

Address correspondence to Dr. Carpenter.