Epidemiologic and economic considerations regarding persistently infected cattle during vaccinate-to-live strategies for control of foot-and-mouth disease in FMD-free regions

Authors: YADAV, SHANKAR - Food And Agriculture Organization Of The United Nations (FAO); DELGADO, AMY - Animal And Plant Health Inspection Service (APHIS); HAGERMAN, AMY - Oklahoma State University; Bertram, Miranda; MORENO-TORRES, KARLA - Kansas State University; STENFELDT, CAROLINA - Kansas State University; HOLMSTROM, LINDSEY - Animal And Plant Health Inspection Service (APHIS); Arzt, Jonathan

Submitted to: Frontiers in Veterinary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2022
Publication Date: 10/22/2022
Citation: Yadav, S., Delgado, A.H., Hagerman, A.D., Bertram, M.R., Moreno-Torres, K.I., Stenfeldt, C., Holmstrom, L., Arzt, J. 2022. Epidemiologic and economic considerations regarding persistently infected cattle during vaccinate-to-live strategies for control of foot-and-mouth disease in FMD-free regions. Frontiers in Veterinary Science. 9:1026592. https://doi.org/10.3389/fvets.2022.1026592.
DOI: https://doi.org/10.3389/fvets.2022.1026592

Interpretive Summary: Foot-and-mouth disease (FMD) is one of the most important constraints in the international trade of livestock and their derived products. Although vaccines are available to prevent the disease, outbreaks in FMD-free countries like USA are typically controlled with mass depopulation (slaughter) of animals. Even when animals are vaccinated to control outbreaks, they are usually depopulated later (vaccinate-to-kill policy). The objective of this study was to use modeling techniques to investigate the economic impacts of vaccination-to-live strategy for FMD outbreaks in USA, with emphasis on consideration of the long-term FMD carrier state. In general, larger outbreaks occurred when depopulation was carried out through longer periods, and the onset of the vaccination was late. These findings contribute to improved preparedness of USDA for the eventuality of an FMD outbreak in USA.

Technical Abstract: Development of a foot-and-mouth disease (FMD) carrier state following FMD virus (FMDV) infection is a well-established phenomenon in cattle. However, the proportion of cattle likely to become carriers and the duration of the carrier state at a herd or population-level are incompletely understood. The objective of this study was to examine the epidemiologic and economic impacts of vaccination-to-live strategy in a disease-free region or country. We developed and simulated scenarios of FMD spread and control in the U.S. livestock population, which included depopulation for a limited period, followed by a vaccinate-to-live strategy with strong biosecurity and movement restrictions. Six scenarios of FMD spread and control were simulated in the InterSpread Plus (ISP) modeling tool. Overall, 1.82 million livestock farms including bison, cattle, goat, swine, and sheep operations were included in the simulation model. Data on the number of infected and depopulated cattle (by operation types) from ISP model runs were used to estimate the monthly number of infected but not depopulated (potential carrier) cattle after the infection. Using available literature data on the FMD carrier state, we estimated the monthly proportion of carrier cattle (from infected but not depopulated cattle) over time following infection. Among the simulated scenarios, the median (25th, 75th percentile) number of infected cattle ranged from 43,217 (42819, 55274) head to 148,907 (75819, 205350) head, and the epidemic duration ranged from 20 (11, 30) to 76 (38, 136) days. The length of time that depopulation was carried out and the timing of the onset of vaccine deployment were varied. In general, larger outbreaks occurred when depopulation was carried out through longer periods, and the onset of the vaccination was late (p>0.05). Over the duration of the simulations, the estimated proportion of surviving cattle which were infected and not depopulated and had the potential to become persistently infected ranged from 14% to 35% of total infected cattle. Production losses in beef and dairy sectors were higher when outbreaks started in multiple states simultaneously, but production losses were small compared to trade losses and consumer avoidance losses. These results can be used to inform the consideration of a vaccinate-to-live strategy for FMD outbreaks and the development of appropriate post-outbreak management strategies. Furthermore, this output will enable a more detailed examination of the epidemiologic and economic implications of allowing convalescent cattle to survive and remain in production chains after FMD outbreaks in FMD-free regions. With the development of vaccines designed to support differentiating infected from vaccinated animals (DIVA), implementation of testing for determining carrier status, and an improved understanding of the risks associated with carrier animals, an opportunity exists to refine trade embargo guidelines and regionalization agreements to account for alternative response strategies to FMD outbreaks such as vaccinate-to-live.