An epidemic model was developed to estimate the potential spread of foot-and-mouth disease (FMD), if infected livestock had been exhibited at the 2005 California State Fair.

The model used dairy cattle, dairy goats, and pygmy goats exhibited by 195 exhibitors in August 24-28, 2005. Two (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.

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 sub-clinically infected and clinically infected animals were low throughout the 5-d study period. Mean estimated duration of the resulting epidemic ranged from 111-155 d, mean number of infected premises ranged from 33 to 244, and mean probability that at least one animal infected with FMD would subsequently leave the state ranged from 28-96% as the number of index cases increased from 1 to 10, respectively.

Conclusions and Clinical Relevance Results of this model study 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.

Reference: Carpenter et al, J Am Vet Med Assoc 231:1231-1235, 2007.