Evaluation of strategies to control transmission of foot and mouth disease virus in a high-density livestock region of California by use of an epidemic simulation model

The present research was carried out with the goal of evaluating strategies to prevent transmission of foot and mouth disease virus (FMDV), should it be introduced into a high-density livestock region of California. The specific objectives included evaluating methods to identify geographic herd locations, quantifying the numbers of direct and indirect animal contacts occurring among livestock facilities and distance traveled between herd contacts, assessing alternative means of eradicating FMD, and evaluating cost-effectiveness of the eradication strategies. Geographic locations, as obtained from geocoding and from datasets containing the geographic centers of 5- and 9-digit zip code regions, census tracts, and towns, were compared to herd locations obtained from a hand-held GPS receiver. Mean spatial measurement errors were 0.6, 8.0, 1.9, 9.2, and 9.2 km, respectively. Results from a survey undertaken to quantify direct and indirect herd contacts and distances livestock traveled indicated that the mean monthly direct contacts varied from 0.2 to 17, depending on herd size and type. Mean indirect contact rates on dairies ranged from 234 to 743 contacts/mo. and distance traveled by personnel and vehicles during a 3-day period ranged from 58.4 to 210.4 km. Results from an epidemic model considering alternative eradication strategies indicated that increasing the size of the infected area from 10 to 20 km decreased the expected epidemic size from 46 to 24 herds. Supplemental strategies to slaughter herds with the highest FMDV exposure risks, to slaughter herds in a ring around infected herds, or to vaccinate herds had statistically significantly (P < 0.010) lower median epidemic sizes of 28 to 40 herds than that of the baseline strategy (46 herds). A 50 km ring-vaccination strategy had the lowest eradication cost of {dollar}3.1 million per herd infection averted, whereas a 3 km ring-slaughter strategy had the highest cost of {dollar}56.4 million per herd infection averted. Results suggest vaccination was the most cost-effective supplemental eradication strategy and should be considered if it were possible to quickly vaccinate herds. Preemptive slaughter of the highest-risk herds also was found to statistically significantly decrease the epidemic size and duration, but the cost per herd infection averted was higher than that for vaccination.